Mohamed Ruwaid Rafiuddin
email: mohamed-ruwaid.rafiuddin@cea.fr

Anne-Magali Seydoux-Guillaume
email: anne.magali.seydoux@univ-st-etienne.fr

Xavier Deschanels

Adel Mesbah

Cedric Baumier

Stephanie Szenknect

Nicolas Dacheux

An In-Situ Electron Microscopy Study of Dual Ion-Beam Irradiated Xenotime-type ErPO4

Rare-earth phosphates adopting the xenotime (REPO4; RE = Tb -Lu & Y, Sc) structure are proposed as a potential matrix for the confinement of minor actinides. Minor actinides (e.g., Np, Am, Cm) undergo a radioactive decay process in which high-energy recoil atom (70 -100 keV) and energetic alpha particles (4.5 -5.8 MeV) are produced. In this study, the impact of these energetic decay products on the structure of xenotime-type ErPO4 has been investigated via high energy dual ion-beam irradiation of ErPO4 ceramics. Au 2+ (1.5 MeV) and He + (160 keV) ions were used to simulate the effects of recoil atom and α-particles, respectively. Multiple experiments were carried out in which the Au 2+ and He + ions with varying ion-fluences (ions/cm 2 ) and ion-flux (ions/cm 2 /s) were implanted sequentially (Au 2+ followed by He + irradiation) and simultaneously (Au 2+ + He + irradiation) into ErPO4 ceramics. Sequential ion-irradiation experiments have shown that the xenotime structure was amorphized by Au 2+ ions at a relatively lower ion-fluence (5 × 10 13 ions/cm 2 ) in comparison to the monazite structure. Upon irradiation of the amorphous ErPO4 with He + ions, recrystallization of the amorphous xenotime due to α-particles was not observed.

However, simultaneous ion-irradiation experiments on ErPO4 showed that the amorphization of the xenotime structure was prevented upon deposition of higher amounts of electronic energy (Eelectronic) in the lamella. Likewise monazite samples, the α-healing mechanism was also experimentally demonstrated in synthetic xenotime samples.

Introduction

Xenotime (REPO4; RE = Tb to Lu and Y, Sc) is a commercially important rare-earth mineral and is usually found as accessory inclusions in igneous and metamorphic rocks. [START_REF] Anderson | Helium Diffusion in Natural Xenotime[END_REF][START_REF] Meldrum | Displacive radiation effects in the monazite-and zircon-structure orthophosphates[END_REF][START_REF] Boatner | Synthesis, Structure, and Properties of Monazite, Pretulite, and Xenotime[END_REF][START_REF] Harlov | Experimental incorporation of Th into xenotime at middle to lower crustal P-T utilizing alkali-bearing fluids[END_REF][START_REF] Palke | Paramagnetic interactions in the 31 P NMR spectroscopy of rare earth element orthophosphate (REPO4, monazite/xenotime) solid solutions[END_REF][START_REF] Gratz | Monazite-xenotime thermobarometry; experimental calibration of the miscibility gap in the binary system CePO4-YPO4[END_REF][START_REF] Ni | Crystal chemistry of the monazite and xenotime structures[END_REF][START_REF] Vasconcelos | Characterization of Xenotime From Datas (Brazil) as a Potential Reference Material for In Situ U-Pb Geochronology[END_REF] Natural xenotime minerals are rich in yttrium and also contain variable amounts of heavy rare-earth elements (HREE). [START_REF] Anderson | Helium Diffusion in Natural Xenotime[END_REF][START_REF] Spear | Apatite, Monazite, and Xenotime in Metamorphic Rocks[END_REF][START_REF] Seydoux-Guillaume | Experimental determination of Thorium partitioning between monazite and xenotime using analytical electron microscopy and X-ray diffraction Rietveld analysis[END_REF][START_REF] Forster | The chemical composition of REE-Y-Th-U-rich minerals in peraluminous accessory granites of the Erzgebirge-Fichtelgebirge region, Germany; Part II : Xenotime[END_REF] Xenotime mineral crystallizes in the zircon structure type in the tetragonal crystal system with space group: I41/amd (Figure 1). [START_REF] Ni | Crystal chemistry of the monazite and xenotime structures[END_REF] In the structure, the REE atoms are coordinated to eight oxygen atoms and the resulting REEO8 dodecahedra are connected to the PO4 tetrahedra via corner sharing. [START_REF] Ni | Crystal chemistry of the monazite and xenotime structures[END_REF] This mineral is also reported to contain variable amounts of tetravalent actinides such as uranium and thorium occupying the REE site. [START_REF] Anderson | Helium Diffusion in Natural Xenotime[END_REF][START_REF] Van Emden | The incorporation of actinides in monazite and xenotime from placer deposits in Western Australia[END_REF][START_REF] Franz | Crystal chemistry of monazite and xenotime from Saxothuringian-Moldanubian metapelites, NE Bavaria, Germany[END_REF] Xenotime minerals have been exposed to radiation events due to the α-decay of 238 U and 232 Th over geological timescales. During an α-decay event, recoil atom (70 -100 keV) and αparticles (4.5 -5.8 MeV) are produced. The energies of the recoil atoms and α-particles are deposited in the mineral through ballistic and ionization process, respectively. Ballistic effects are primarily responsible for initiating the metamictization (i.e., crystalline to amorphous transformation) events commonly observed in natural minerals. Thus, assessing the impact of radiation on the mineral structure is necessary to adjudge whether synthetic analogues of xenotime mineral could be useful as a host-matrix for the immobilization of minor actinides (e.g., Np, Am, Cm). Yet, to the best of our knowledge, very limited data is available on the radiation response of natural xenotime samples. Anderson et al. have studied the effects of radiation on natural xenotime specimens by analyzing 238 U and 232 Th containing xenotime minerals using Raman spectroscopy. [START_REF] Anderson | Helium Diffusion in Natural Xenotime[END_REF] The α-dose of the xenotime specimen was determined to be 1.59 ± 0.03 × 10 16 αdecay/g. [START_REF] Anderson | Helium Diffusion in Natural Xenotime[END_REF] The similarities of the Raman spectra of natural and synthetic xenotime samples implies the crystalline nature of the xenotime mineral. [START_REF] Anderson | Helium Diffusion in Natural Xenotime[END_REF] Using Raman spectroscopy, Svecova et al. also investigated the effect of radiation damage on various natural xenotime samples. [START_REF] Švecová | Multi-stage evolution of xenotime--(Y) from Písek pegmatites, Czech Republic: an electron probe micro-analysis and Raman spectroscopy study[END_REF] In this study, the α-doses of the analyzed xenotime specimens were higher by three orders of magnitude (0.3 -7.3 × 10 19 α-decay/g) in comparison to those studied by Anderson et al. [START_REF] Anderson | Helium Diffusion in Natural Xenotime[END_REF][START_REF] Švecová | Multi-stage evolution of xenotime--(Y) from Písek pegmatites, Czech Republic: an electron probe micro-analysis and Raman spectroscopy study[END_REF] Svecova et al. showed that the crystallinity of xenotime minerals was found to be either well or poorly crystalline depending on the U and Th contents. [START_REF] Švecová | Multi-stage evolution of xenotime--(Y) from Písek pegmatites, Czech Republic: an electron probe micro-analysis and Raman spectroscopy study[END_REF] The results obtained from natural xenotime minerals are comparable with those obtained from natural monazite samples. [START_REF] Seydoux-Guillaume | An XRD, TEM and Raman study of experimentally annealed natural monazite[END_REF][START_REF] Seydoux-Guillaume | Microstructure of 24-1928 Ma concordant monazites; implications for geochronology and nuclear waste deposits[END_REF][START_REF] Seydoux-Guillaume | Low-temperature alteration of monazite: Fluid mediated coupled dissolution-precipitation, irradiation damage, and disturbance of the U-Pb and Th-Pb chronometers[END_REF] This is primarily because monazite and xenotime minerals, which belong to the same family of phosphate minerals, have the same general chemical formula and differ from one another in terms of crystal structure, size, and coordination number of RE ion. [START_REF] Van Emden | The incorporation of actinides in monazite and xenotime from placer deposits in Western Australia[END_REF] It is worth noting here that natural monazite minerals were never found in a fully amorphous state and have remained crystalline at the geological timescale. [START_REF] Seydoux-Guillaume | Microstructure of 24-1928 Ma concordant monazites; implications for geochronology and nuclear waste deposits[END_REF][START_REF] Ewing | Phosphates as Nuclear Waste Forms[END_REF] However, several investigations carried out on natural and synthetic analogues of monazite mineral showed that the monazite minerals were easy to amorphize but can be brought back to the initial crystalline state through dynamic healing of radiation-induced structural defects. [START_REF] Nasdala | Irradiation effects in monazite--(Ce) and zircon: Raman and photoluminescence study of Auirradiated FIB foils[END_REF][START_REF] Seydoux-Guillaume | Why natural monazite never becomes amorphous: Experimental evidence for alpha selfhealing[END_REF][START_REF] Meldrum | A comparison of radiation effects in crystalline ABO4-type phosphates and silicates[END_REF] Healing of structural defects in monazite is possible via low temperature thermal annealing and/or energy deposition in the mineral due to α-particles. [START_REF] Nasdala | Irradiation effects in monazite--(Ce) and zircon: Raman and photoluminescence study of Auirradiated FIB foils[END_REF][START_REF] Seydoux-Guillaume | Why natural monazite never becomes amorphous: Experimental evidence for alpha selfhealing[END_REF] Such behaviour was not observed for the mineral zircon (ZrSiO4), which is isostructural with the xenotime mineral. [START_REF] Nasdala | Metamictisation of natural zircon: accumulation versus thermal annealing of radioactivity-induced damage[END_REF] Evidently, phosphate-based minerals (monazite and xenotime) are structurally more resilient than silicate (zircon) minerals and the structural resilience of phosphate minerals has been ascribed to their lower critical amorphization temperature (Tc) and lower activation energy for recrystallization process. [START_REF] Meldrum | Displacive radiation effects in the monazite-and zircon-structure orthophosphates[END_REF] Based on these mineralogical evidences, synthetic analogues of monazite and xenotime have been proposed as potential waste forms for the immobilization of minor actinides.

Radiation effects on synthetic xenotime materials were determined using internal and external irradiation experiments. In internal irradiation, short-lived actinides (e.g., 244 Cm [T1/2 = 18.1 years]) were incorporated into the host matrix and the structural response was studied as a function of time. [START_REF] Weber | Radiation effects in crystalline ceramics for the immobilization of high-level nuclear waste and plutonium[END_REF] Luo et al. examined the structure of 1wt% 244 Cm-doped LuPO4 (xenotimetype) crystal using Transmission Electron Microscopy (TEM). [START_REF] Luo | Microscopic effects of self-radiation damage in 244 Cm-doped LuPO4 crystals[END_REF] The radiation dose accumulated in the crystal due to the α-decay of 244 Cm to 240 Pu was determined to be 5 × 10 19 αdecay/g. [START_REF] Luo | Microscopic effects of self-radiation damage in 244 Cm-doped LuPO4 crystals[END_REF] It was shown that the crystallinity of the LuPO4 material remained largely unaffected by the α-decay events of 244 Cm and it was proposed that the rates of damage production and recovery were similar. [START_REF] Luo | Microscopic effects of self-radiation damage in 244 Cm-doped LuPO4 crystals[END_REF] The long-term effects of radiation are simulated in a shorter timescale by employing the external irradiation method in which high energy ions (e.g., Au 2+ , He + ) of varying ion fluences (ions/cm 2 ) are implanted in the solid. In an early study reported by Meldrum et al., crystals of synthetic xenotime were irradiated with heavy ions (800 keV Kr 2+ ) and studied using TEM. [START_REF] Meldrum | Displacive radiation effects in the monazite-and zircon-structure orthophosphates[END_REF] Meldrum et al. argued on the basis of critical amorphization temperature (Tc), that the xenotime materials were more susceptible to radiation damage than the monazite materials. [START_REF] Meldrum | Displacive radiation effects in the monazite-and zircon-structure orthophosphates[END_REF] In another recent investigation on the structural stability of heavy ion-irradiated xenotime, evidence of a partial structural damage recovery was observed in Au -(2 MeV) ion-irradiated YbPO4 (ion fluence -1 × 10 15 ions/cm 2 ) materials using X-ray absorption near-edge spectroscopy (XANES). [START_REF] Rafiuddin | Probing the effect of radiation damage on the structure of rare-earth phosphates[END_REF] Molecular dynamics simulation studies on YPO4 by Urusov et al. also reported the partial recovery of the damaged xenotime structure. [START_REF] Urusov | Radiation resistance of the xenotime YPO4 from the computer simulation data[END_REF] In that study, a 20 keV 232 Th atom was used as the primary recoil atom for initiating a cascade of atomic displacements. [START_REF] Urusov | Radiation resistance of the xenotime YPO4 from the computer simulation data[END_REF] Post-irradiation, some of the partially displaced atoms return to their original atomic sites thereby initiating a partial structural recovery of the xenotime structure. [START_REF] Urusov | Radiation resistance of the xenotime YPO4 from the computer simulation data[END_REF] Lenz et al. in a recent study implanted xenotime-type (YPO4) ceramics sequentially with Au -ions of different energies [START_REF] Weber | The role of electronic energy loss in ion beam modification of materials[END_REF][START_REF] Nasdala | Metamictisation of natural zircon: accumulation versus thermal annealing of radioactivity-induced damage[END_REF][START_REF] Švecová | Multi-stage evolution of xenotime--(Y) from Písek pegmatites, Czech Republic: an electron probe micro-analysis and Raman spectroscopy study[END_REF][START_REF] Ni | Crystal chemistry of the monazite and xenotime structures[END_REF] MeV) and fluences (1.6 × 10 13 -6.5 × 10 13 ions/cm 2 ) and determined the amount of amorphous fraction using photoluminescence spectroscopy. [START_REF] Lenz | The Quantification of Radiation Damage in Orthophosphates Using Confocal μ-Luminescence Spectroscopy of Nd 3+[END_REF] Quadruple high energy Au -ions were chosen in that study in order to create an homogenous thick damage layer of ~5µm. [START_REF] Lenz | The Quantification of Radiation Damage in Orthophosphates Using Confocal μ-Luminescence Spectroscopy of Nd 3+[END_REF] However, based on photoluminescence spectra and TEM results, the damage accumulation in xenotime (YPO4) samples were observed to be heterogenous in the irradiated surface layer and that the amorphous fraction was observed to decrease unto a depth of 5 µm. [START_REF] Lenz | The Quantification of Radiation Damage in Orthophosphates Using Confocal μ-Luminescence Spectroscopy of Nd 3+[END_REF] The heterogenous damage layer observed in the YPO4 ceramics were proposed to be a result of the epitaxial annealing occurring at the crystalline-amorphous interface. [START_REF] Lenz | The Quantification of Radiation Damage in Orthophosphates Using Confocal μ-Luminescence Spectroscopy of Nd 3+[END_REF] To the best of our knowledge, the impact of light-ions such as He + on the structure of pristine/amorphous xenotime has not been studied. In this regard, the present study aims to address the question whether α-particles could induce a healing effect on the structure of amorphized xenotime and/or prevent the xenotime sample from undergoing amorphization. In this study, dual ion-beam irradiation (Au 2+ [1.5 MeV] + He + [160 keV]) experiments were performed on xenotimetype ErPO4 ceramics and the structural response to ion-irradiation was monitored using in-situ TEM. Au 2+ and He + ions were used to simulate the effects of recoil atom and α-particle, respectively. In order to test the α-healing capability of xenotime ceramics, high energy ions with varying ion-fluence (ions/cm 2 ) and flux (ions/cm 2 /s) were bombarded into thin lamella sections of ErPO4 ceramics in sequential (Au 2+ followed by He + ion-irradiation) and simultaneous (Au 2+ + He + ions) irradiation modes. The influence of electron beam on the damage recovery process in amorphized xenotime were also determined and will be discussed in this study.

Experimental Section

2.1.Synthesis and Sintering of ErPO4

ErCl3.nH2O (Sigma Aldrich, 99.9%) and H3PO4 (Carlo Erba Reagents, 85%) were used for the synthesis of ErPO4. Due to the hygroscopic character of ErCl3.nH2O, this solid was dissolved in 1 M HCl. The erbium concentration in solution was then determined using ICP-AES (Spectro Arcos EOP). Stoichiometric amount of ErCl3 was added dropwise to a beaker containing H3PO4 (10 mol % excess) and the solution was stirred for a few minutes before transferring the solution to a Teflon lined acid digestion vessel (Parr). The acid digestion vessel containing the solution mixture of erbium and phosphate ions were placed inside a conventional oven and heated to 250°C for 7 days.

Pink colored precipitate was obtained and separated from the supernatant solution by centrifugation at 14000 rpm. The obtained powder was washed twice with deionized water and once with ethanol before being dried overnight in an oven at 90°C. The phase purity of the assynthesized ErPO4 precipitate was checked using powder XRD data obtained from a Bruker D8 advance diffractometer equipped with a Lynxeye detector and Cu Kα1,2 (λ = 1.54184 Å) X-ray source. The powder XRD patterns were collected over a wide 2θ range from 5° -100° using a step size of 0.019° and the lattice constants of the as-synthesized material were determined using the HighScore Plus software program. [START_REF] Degen | The HighScore suite[END_REF] Rietveld refinement of the powder XRD pattern was performed using the xenotime-type ErPO4 structural model reported by Ni et al. [START_REF] Ni | Crystal chemistry of the monazite and xenotime structures[END_REF] During the refinement, the following parameters were allowed to vary: scale factor, zero-shift, lattice constant, profile variables, and the overall isotropic thermal factor (B).

Prior to sintering, as-synthesized powders of ErPO4 were heated at 600°C for 3 hours to remove adsorbed water. Pre-heated powders of ErPO4 were ground and the fine powders were uniaxially pressed at ~250 MPa to obtain pellets of ~5 mm in diameter. The pellets were sintered at 1600°C in air for 3 hours. The density of the sintered material is 97% of the theoretical density.

The surface of the pellet was initially polished to reach an optical grade using a 5 µm SiC sheet followed by a polishing of the surface using a finer 1 µm diamond paste. The phase purity of the sintered material was determined by performing XRD experiments at the surface of the polished pellets of ErPO4.

2.2.Focussed Ion Beam (FIB): Preparation of Lamellae

Lamellas were prepared by FIB technique using a dual beam Scanning Electron Microscopy (SEM) equipped with a Ga + ion source (FEI Helios 600 NanoLab) located at the Centre Pluridisciplinaire de Microscopie electronique et de Microanalyse (CP2M) facility in Marseille, France. Prior to the lamella preparation, the surface of the sintered ErPO4 pellet was coated with a thin carbon layer to improve the sample conductivity which therefore enabled the collection of microstructural images from the sample using SEM. The region of the pellet from which the lamella was obtained was coated with a layer of Pt in order to protect the lamella from structural damage due to Ga + ion bombardment. The Ga + ion milling was initially carried out at an ion beam voltage of 30 kV and ion beam currents ranging from 9 nA to 90 pA. Thick sections of the lamella were removed using an Omniprobe 200 micromanipulator and transferred to a half copper grid wherein the final ion-milling step was performed at lower Ga + ion acceleration voltage (5 kV) to minimize the ion damage. For the reasons mentioned above, a final cleaning step was also performed using Ga + ions at 2 kV and 1 kV. Structural defects were observed at the surface of the lamella and can be found up to ~500 nm below the surface. These surface defects are likely due to Ga + ion-milling of the pellet and/or due to mechanical polishing of the pellet prior to lamella preparation. Thus, the analysis of the effect of dual ion-beams on the lamella were carried out at regions far from the subsurface locations and in regions, wherein no structural defects were present. Six lamellas (~10.30 × 5.40 µm) of varying thickness (~ 60 -83 nm) were prepared using the above-mentioned procedure were used for the ion irradiation experiments discussed in the following section.

In-situ Dual Ion-Beam Implantation

The ion irradiation experiments were performed at the Joint Accelerators for Nanosciences and Nuclear Simulation (JANNUS-Orsay) facility located at the Centre for Nuclear Sciences and Materials Sciences (CSNSM, Univ Paris-Sud/CNRS, Orsay, France). The JANNUS-Orsay facility is equipped with two ion accelerators (ARAMIS -2 MV and IRMA -190 kV). The ions coming off from these accelerators were transported via beamline to the in-situ TEM end station. In the present study, a 200 keV in-situ TEM (FEI Tecnai G 2 20 Twin) end-station was used to monitor the changes in the material structure due to ion-implantation under in-situ conditions. The thin lamella of pristine ErPO4 was first mounted on a double tilt GATAN sample holder before introducing the sample holder inside the TEM vacuum chamber for ion-irradiation. The ARAMIS and IRMA beamlines which were interfaced with the TEM instrument are separated by an angle of 45° and each of these beamlines are oriented at an angle of 22° with respect to the surface normal of the lamella.

In this study, thin lamellas of ErPO4 were irradiated with 1.5 MeV Au 2+ (ARAMIS) and 160 keV He + (IRMA) ions of varying flux (ions/cm 2 /s) and fluence (ions/cm 2 ) and the structural changes were studied using in-situ TEM. The temperature of the TEM foil was also measured simultaneously during ion-irradiation. The energies deposited by He + and Au 2+ ions into the lamella had contributions from both electronic (Eelectronic in keV/cm 3 ) and nuclear (Enuclear in keV/cm 3 ) process. The values of Eelectronic and Enuclear were derived from the nuclear and electronic stopping powers of Au 2+ and He + ions (expressed in keV/nm) calculated using Stopping and Range of Ions in Matter (SRIM-2013) software (Table 1). The projected range of these ions in the lamella were calculated using the SRIM-2013 software. The ratio of electronic to nuclear stopping powers (ENSP) for single ion-beam (Au 2+ , He + ) and dual ion-beam (Au 2+ + He + ) irradiation are presented in Table 1. The stopping power of Au 2+ and He + ions can be considered as a constant in the entire thickness of the lamella and hence, the ENSP ratio is equivalent to Eelectronic/Enuclear. Similarly, the number of displacements per atom (dpa) in the lamella caused by ion-irradiation were also determined using SRIM-2013 software. The displacement energies of Er, P, and O used for the SRIM simulation are 25 eV, 25 eV, and 28 eV, respectively.

Multiple ion-irradiation experiments were performed in sequential (i.e., Au 2+ ion irradiation followed by He + ion irradiation) and simultaneous (i.e., Au 2+ + He + ) modes. Sequential irradiation experiments were performed to determine whether α-particle irradiation could initiate a structural recovery process in amorphous xenotime. It must be noted that at lower He + ion fluences (3 × 10 13 ions/cm 2 , 5.4 × 10 -4 dpa), the He + ions lose energy largely via ionization process and does not initiate atomic displacements in the structure. However, at higher ion fluences (2.87 × 10 17 ions/cm 2 , 5.1 dpa), the He + ions will lose energy via ionization and ballistic process and can initiate atomic displacements in the structure. On the other hand, simultaneous irradiation experiments were performed to determine whether α-particles could prevent the xenotime sample from undergoing amorphization. In sequential mode, one of the six ErPO4 lamellas, was irradiated with Au 2+ ions up to a fluence of 2 × 10 14 ions/cm 2 followed by irradiation with He+ ions up to a fluence of 1 × 10 17 ions/cm 2 (Table 2). Post-irradiation, BF-TEM images and SAED patterns were collected from the irradiated lamella. In simultaneous mode, four ErPO4 lamellas were irradiated simultaneously with Au 2+ and He + ions with varying ion-fluence, flux, and ratios of Eelectronic/Enuclear (See Tables 3 and4 for experimental details). In all of the above-mentioned experiments, the electron beam was switched off during ion-irradiation in order to prevent the sample from undergoing an electron-induced damage recovery process. The effect of electron beam on the structure of amorphized xenotime was also determined in this study via sequential irradiation (Au 2+ followed by e -beam) of the lamella (See Table 5 for experimental details). The pristine lamella was first irradiated with Au 2+ ions to a fluence of 1 × 10 14 ions/cm 2 followed by exposure to e - beam for ~ 4.3 hours (Table 5). The BF-TEM images and SAED patterns were recorded from the lamella at frequent time-intervals to monitor the damage recovery due to electron beam (Figure 9).

Results and Discussion

Structure: Before ion-irradiation

The powder XRD pattern of the sintered ErPO4 material from which the thin lamellas were obtained are presented in Figure 2. ErPO4 adopted the xenotime-type structure and the lattice constants of ErPO4 obtained through the Rietveld refinement of the powder XRD data are presented in Table 6. Prior to performing the ion irradiation on ErPO4, BF-TEM images and SAED patterns were collected from the pristine lamellas (Figure 3). The BF-TEM image of pristine lamella clearly revealed the presence of grains and grain boundaries (Figure 3). The SAED pattern of the pristine lamella contains sharp diffraction spots thereby indicating the high crystallinity of the material (Figure 3).

Structure: After ion-irradiation 3.2.1. Sequential Irradiation

The pristine ErPO4 lamella was initially irradiated with Au 2+ ions to a lower fluence of 1 × 10 13 ions/cm 2 (0.04 dpa) followed by increasing the fluence to 5 × 10 13 (0.20 dpa), 1 × 10 14 (0.40 dpa), and 2 × 10 14 ions/cm 2 (0.82 dpa). The ion-fluences were increased progressively in order to increase the amount of nuclear energy deposited in the lamella. The BF-TEM images and the corresponding SAED patterns of the lamella irradiated to various ion-fluences were recorded (Figure 4). In comparison to the pristine lamella, the lamella irradiated to 1 × 10 13 ions/cm 2 exhibited little or no sign of radiation damage and retained its crystallinity as indicated by the observation of the grains (and grain boundaries) as well as the presence of sharp diffraction spots in the SAED pattern (cf. Figures 3 and4a). This observation indicated that at a lower fluence of 1 × 10 13 ions/cm 2 , the amount of nuclear energy (Enuclear = 5.3 × 10 20 keV/cm 3 ) deposited in the lamella by the Au 2+ ions was not enough to initiate a large number of atomic displacements in the structure. Upon increasing the Au 2+ ion fluence to 5 × 10 13 ions/cm 2 (Enuclear = 2.3 × 10 21 keV/cm 3 ), the material became amorphous as is evident from the disappearance of grains in the lamella (and the presence of homogenous contrast in the BF-TEM image) and the observation of diffuse rings in the corresponding SAED pattern (Figure 4b). Ion-irradiation using Au 2+ ions would usually result in the production of point defects (e.g., vacancies and interstitials) and the concentration of these defects is expected to increase with increasing Au 2+ ion fluence. The smaller ENSP ratio of Au 2+ ion-irradiation suggests a greater accumulation of point defects and consequently, the number of isolated amorphous domains in the structure would increase and begin to overlap thereby rendering the structure completely amorphous (Table 1 and Figure 4b). [START_REF] Meldrum | Displacive radiation effects in the monazite-and zircon-structure orthophosphates[END_REF] The ErPO4 material continued to remain amorphous upon increasing the Au 2+ ion fluence to 1 × 10 14 ions/cm 2 (Enuclear = 4.7 × 10 21 keV/cm 3 ) and 2 × 10 14 ions/cm 2 (Enuclear = 9.4 × 10 21 keV/cm 3 ) (Figure 4c). It should be noted that under identical ion-irradiation conditions, the monazite (LaPO4) material was found to remain partially crystalline up to 1 × 10 14 ions/cm 2 before becoming completely amorphous at a fluence of 2 × 10 14 ions/cm 2 . [START_REF] Seydoux-Guillaume | Why natural monazite never becomes amorphous: Experimental evidence for alpha selfhealing[END_REF] This comparison showed that the xenotime (ErPO4) structure, which amorphized at a much lower ion-fluence (5 × 10 13 ions/cm 2 ), exhibit a greater ease of amorphization compared to the monazite structure (LaPO4). The susceptibility of a material to amorphization could be explained using the topological model proposed by Hobbs. [START_REF] Hobbs | Topology and geometry in the irradiation-induced amorphization of insulators[END_REF] Hobbs model states that materials having a higher connectivity of the cation polyhedra are structurally constrained and hence, less susceptible to radiation-induced amorphization. [START_REF] Hobbs | Topology and geometry in the irradiation-induced amorphization of insulators[END_REF] In the xenotime structure, the individual REO8 polyhedra are connected to the four neighbouring REO8 polyhedra via edge-sharing whereas in the monazite structure, the REO9 polyhedra are connected to six neighbouring REO9 polyhedra via edge-sharing. [START_REF] Meldrum | Displacive radiation effects in the monazite-and zircon-structure orthophosphates[END_REF][START_REF] Ni | Crystal chemistry of the monazite and xenotime structures[END_REF] Thus, the average connectivity of the REO8 and REO9 polyhedra in the xenotime and monazite structures are 4 and 6, respectively. Based on Hobbs model, the xenotime structure, which has a lower connectivity of the REO8 polyhedra, is structurally less constrained than the monazite structure and hence, can be amorphized at a lower ion-fluence.

As outlined in Section 1, the natural xenotime minerals investigated by Svecova et al. were found to be present in a partially crystalline state and were never found in a fully amorphous state. [START_REF] Švecová | Multi-stage evolution of xenotime--(Y) from Písek pegmatites, Czech Republic: an electron probe micro-analysis and Raman spectroscopy study[END_REF] However, it was shown in this study that the synthetic xenotime (ErPO4) was amorphized upon Au 2+ ion irradiation at a lower ion-fluence. The different radiation response of synthetic and natural xenotime samples could be explained by considering the effects of recrystallization that could occur in natural xenotime minerals. Similar to natural monazite minerals, recrystallization in natural xenotime minerals could be initiated either by thermal annealing or via α-annealing of amorphous regions. Electronic energy deposition during α-particle or other light-ion irradiation could also provide sufficient energy for the recombination of point defects (e.g., vacanciesinterstitial recombination) produced due to α-recoil atom. As is demonstrated for synthetic monazite samples (LaPO4) by Seydoux-Guillaume et al., the electronic energy deposited during α-particle irradiation induced the recrystallization of amorphous regions in the monazite samples. [START_REF] Seydoux-Guillaume | Why natural monazite never becomes amorphous: Experimental evidence for alpha selfhealing[END_REF] Debelle et al. also observed the recrystallization of amorphous SiC ceramics upon irradiation using swift heavy ions in the electronic energy loss regime. [START_REF] Debelle | Swift heavy ion induced recrystallization in cubic silicon carbide: New insights from designed experiments and MD simulations[END_REF] Similarly, in the study reported by Zinkle, the effect of electronic energy deposition on the microstructure of MgAl2O4 spinel oxides was discussed. [START_REF] Zinkle | Microstructure of ion irradiated ceramic insulators[END_REF][START_REF] Zinkle | Fundamental radiation effects parameters in metals and ceramics[END_REF][START_REF] Zinkle | Effect of irradiation spectrum on the microstructural evolution in ceramic insulators[END_REF] Here, dislocation loops were observed in both the heavy-and light-ion irradiated regions of the samples. However, it was shown that the rate of formation and the density of dislocation loops due to heavy-ion irradiation was higher in comparison to light-ion irradiation of spinel oxides and the mechanism for such retardation of formation of dislocation loops was proposed by Yasuda et al. [START_REF] Zinkle | Microstructure of ion irradiated ceramic insulators[END_REF][START_REF] Zinkle | Fundamental radiation effects parameters in metals and ceramics[END_REF][START_REF] Zinkle | Effect of irradiation spectrum on the microstructural evolution in ceramic insulators[END_REF][START_REF] Yasuda | Electron-beam induced dissociation of dislocation loops in magnesia-alumina ceramics[END_REF] In that study, it was shown that the ionizing radiation (light-ion and electron irradiation) promotes the dissociation of dislocation loops into interstitial atoms which then recombine with the vacancies generated during the ion-irradiation. [START_REF] Yasuda | Electron-beam induced dissociation of dislocation loops in magnesia-alumina ceramics[END_REF] In order to determine the effect of electronic energy deposition on amorphous xenotime, the amorphized Au 2+ ion-irradiated ErPO4 was irradiated with He + ions to ion fluences ranging from 3 × 10 13 ions/cm 2 (Eelectronic = 1.2 × 10 20 keV/cm 3 ) to 1 × 10 17 ions/cm 2 (Eelectronic = 3.4 × 10 23 keV/cm 3 ) (See Table 2 and Figure 4d). The ENSP ratio of He + ion irradiation was much higher than those of Au 2+ ion irradiation and as a result, He + ion-irradiation of amorphous ErPO4 would result in a lower accumulation of defects, due to the greater mobility of point defects (Table 1). [START_REF] Meldrum | Displacive radiation effects in the monazite-and zircon-structure orthophosphates[END_REF] The temperature of the TEM foil was measured after each step of He + ion-irradiation and were found to be 25°C (Steps 5 -9, Table 2), 28°C (Steps 10 -12, Table 2), and 55°C (Steps 13 -14, Table 2). It should be noted that the temperature of the foil was cooled down to 25°C prior to the collection of BF-TEM images. As is observed from BF-TEM images and their corresponding SAED patterns, the electronic energy deposited in the lamella due to He + ion irradiation did not bring about the structural recovery of amorphous xenotime (Figure 4d). Unlike xenotime samples, evident signs of recrystallization of the amorphous monazite upon He + ion (160 keV) irradiation were observed at ion-fluences starting from 4 × 10 16 ions/cm 2 and was attributed to the electronic energy loss of He + ions in the sample. [START_REF] Seydoux-Guillaume | Why natural monazite never becomes amorphous: Experimental evidence for alpha selfhealing[END_REF] It is proposed in this study that the activation energy barrier for the transformation of amorphous to crystalline state could be higher for the xenotime structure than those of the monazite structure. [START_REF] Meldrum | Displacive radiation effects in the monazite-and zircon-structure orthophosphates[END_REF] Therefore, the electronic energy deposited in amorphous ErPO4 during He + ion irradiation was not sufficient to overcome the activation energy barrier and hence, the xenotime materials continued to remain in the amorphous state. However, it is also possible that the He + ion irradiation should be performed simultaneously with Au 2+ ion irradiation in order to observe the recrystallization in synthetic xenotime samples. Simultaneous irradiation of synthetic ErPO4 with Au 2+ and He + ions better simulates the radiation conditions experienced by the natural xenotime mineral. Therefore, simultaneous ion-irradiation experiments were performed on synthetic ErPO4.

Simultaneous Irradiation

Simultaneous ion-irradiation of ErPO4 lamella using Au 2+ and He + ions allows for a study of the combined effects of α-decay products (recoil atom and α-particle) on the xenotime structure. In a recent review by Weber et al., the synergistic effects of the electronic and nuclear energy losses on the microstructure of materials has been discussed. [START_REF] Weber | The role of electronic energy loss in ion beam modification of materials[END_REF] In the current study, the primary aim of performing simultaneous ion irradiation experiments was to determine whether α-particles could prevent the xenotime structure from amorphization and in this regard, four experiments were performed in which the lamellas of ErPO4 were simultaneously irradiated using an identical flux of Au 2+ ions (2 × 10 10 ions/cm 2 /s) and a varying flux of He + ions (3 × 10 12 , 5 × 10 12 , 1.4 × 10 13 , and 2.9 × 10 13 ions/cm 2 /s). In this manner, the amount of electronic energy (Eelectronic in keV/cm 3 ) deposited in the lamella was varied in each of the four experiments (See Tables 3 and4). The amount of electronic energy (Eelectronic) deposited in the lamella was increased on going from one experiment to another thereby resulting in an increase in the ratio of Eelectronic/Enuclear.

In the first experiment (Eelectronic/Enuclear = 10), the lamella which was initially irradiated to 1 × 10 13 Au 2+ ions/cm 2 and 1.4 × 10 15 He + ions/cm 2 did not exhibit any sign of amorphization and the material is found to remain in the crystalline state as is evident from their BF-TEM image and SAED pattern (Figure 5b). However, the mottled diffraction contrasts observed in the BF-TEM image indicated the early stages of structural damage (i.e., onset of crystalline to amorphous transition) in the xenotime structure due to the α-recoil atom. [START_REF] Hawthorne | Alpha-decay damage in titanite[END_REF][START_REF] Lumpkin | Alpha-decay damage in minerals of the pyrochlore group[END_REF] The diffraction contrasts could arise primarily from the presence of strain fields and point defects (Frenkel defects) surrounding isolated α-recoil and α-particle tracks (Figure 5b). [START_REF] Hawthorne | Alpha-decay damage in titanite[END_REF][START_REF] Lumpkin | Alpha-decay damage in minerals of the pyrochlore group[END_REF] Subsequently, the Au 2+ and He + ion fluences were increased by an order of magnitude up to 1 × 10 14 ions/cm 2 and 1.5 × 10 16 ions/cm 2 , respectively, in two steps (Table 3). Analysis of the BF-TEM images and SAED patterns recorded from the lamellas irradiated to higher Au 2+ + He + ion fluences revealed a complete loss of crystallinity (See Figures 5c and5d). This structural transformation from crystalline to amorphous state was a result of accumulation and overlap of the α-recoil tracks. [START_REF] Hawthorne | Alpha-decay damage in titanite[END_REF][START_REF] Lumpkin | Alpha-decay damage in minerals of the pyrochlore group[END_REF] The amount of electronic energy deposited in the lamella was increased in the second experiment (Eelectronic/Enuclear = 17 -18) and the lamella was irradiated to a total of 1 × 10 14 Au 2+ ions/cm 2 and 2.78 × 10 16 He + ions/cm 2 in two steps (Figure 6). The lamella irradiated to 1 × 10 13 Au 2+ ions/cm 2 and 2.83 × 10 15 He + ions/cm 2 retained its crystallinity and only, mottled diffraction contrasts due to the presence of point defects were observed in the BF-TEM image (Figures 6b and6c). The lamella that was irradiated to 5 × 10 13 Au 2+ ions/cm 2 and 1.35 × 10 16 He + ions/cm 2 was largely amorphous as indicated by the presence of diffuse halo rings in the SAED pattern (Figure 6d). However, the presence of two diffraction spots (marked by red arrows) in the SAED pattern indicated that the material was not completely amorphous (Figure 6d). The material became completely amorphous upon increasing the Au 2+ and He + ion fluences to 1 × 10 14 ions/cm 2 and 2.78 × 10 16 ions/cm 2 , respectively (Figures 6e and6f). It is worth noting that under similar dual ion-beam irradiation conditions (Eelectronic/Enuclear = 13 -15), the monazite samples did not undergo a structural transformation to the amorphous state and remained crystalline for all the ion fluences studied. [START_REF] Seydoux-Guillaume | Why natural monazite never becomes amorphous: Experimental evidence for alpha selfhealing[END_REF] Based on these observations, it can be inferred that the electronic energy deposited in the ErPO4 lamella was not enough to prevent the material from undergoing a radiation-induced amorphization process.

In the subsequent experiments, higher amounts of electronic energy were deposited in the lamella (Eelectronic/Enuclear = 36 -39 and 54 -57) by increasing the ion fluxes and fluences of He + ions (Figures 7 and8). It must be noted that for higher Eelectronic/Enuclear ratios, the nuclear energy loss component of the He + ion is not negligible and is comparable to those of Au 2+ ions. Hence, He + ion-irradiation could also initiate atomic displacements in ErPO4. Evidence of α-healing (i.e., α-particle assisted healing of structural defects) of the xenotime structure was observed in the third experiment (Eelectronic/Enuclear = 36 -39) in which mixtures of both crystalline and amorphous domains were observed in the lamella irradiated up to 1.5 × 10 14 Au 2+ ions/cm 2 and 1.06 × 10 17 He + ions/cm 2 (Figures 7b -7e). This observation implied that the electronic energy deposited during He + ion-irradiation initiated a partial recovery of the structure. On further increase in the Au 2+ (2 × 10 14 ions/cm 2 ) and He + (1.4 × 10 17 ions/cm 2 ) ion fluences, the material underwent a complete structural transformation to the amorphous state and α-healing of the damaged xenotime structure was not observed (Figure 7f). Consequently, the electronic energy deposition in the lamella was increased in the fourth experiment (Eelectronic/Enuclear = 54 -57) and the material did not exhibit any signs of radiation-induced amorphization for all the ion fluences studied (Figure 8).

Barring the mottled diffraction contrasts in the lamella attributed to radiation-induced lattice strain and point defects, the xenotime-type ErPO4 remained crystalline at all ion fluences thus demonstrating the capability of α-particles in preventing the xenotime structure from undergoing amorphization process (Figures 8b -8i). The underlying reason for the prevention of amorphization of xenotime ceramics at higher Eelectronic/Enuclear ratios was due to α-healing mechanism. The healing of structural defects can be attributed to a multitude of factors such as increase in the local temperature of the material due to α-particles irradiation, ionization-enhanced diffusion of point defects, and vacancy-interstitial recombination. [START_REF] Weber | Radiation effects in crystalline ceramics for the immobilization of high-level nuclear waste and plutonium[END_REF][START_REF] Zinkle | Microstructure of ion irradiated ceramic insulators[END_REF][START_REF] Zinkle | Fundamental radiation effects parameters in metals and ceramics[END_REF][START_REF] Zinkle | Effect of irradiation spectrum on the microstructural evolution in ceramic insulators[END_REF][START_REF] Yasuda | Electron-beam induced dissociation of dislocation loops in magnesia-alumina ceramics[END_REF] It was shown conclusively from the dual-ion beam experiments on xenotime samples, that the α-healing mechanism, which was experimentally demonstrated for monazite samples, was also responsible for the prevention of amorphization in the xenotime samples. [START_REF] Seydoux-Guillaume | Why natural monazite never becomes amorphous: Experimental evidence for alpha selfhealing[END_REF] It should be noted that unlike monazite, the α-healing mechanism in the xenotime structure was evident only upon increasing the amount of electronic energy deposition in the lamella.

Electron Beam Irradiation

Previous studies on in-situ TEM observations of ion-irradiated nuclear wasteforms demonstrated the structural healing ability of electrons. In this study, the electron beams were switched off during the time of ion-irradiation in sequential and simultaneous experiments in order to minimize the impact (if any) of electron beams on the structure of xenotime. [START_REF] Seydoux-Guillaume | Why natural monazite never becomes amorphous: Experimental evidence for alpha selfhealing[END_REF][START_REF] Deschanels | Swelling induced by alpha decay in monazite and zirconolite ceramics: A XRD and TEM comparative study[END_REF] However, electron beams were used for a short period of time to monitor the structural response of post-irradiated samples and it was important to decipher whether electron beams had some contribution to the recrystallization of amorphous xenotime that was observed in the dual ion-beam experiments. In order to address this question, the pristine ErPO4 lamella was completely amorphized using Au 2+ ions, as in previous experiments, and only a small area of the foil was then exposed to the electron beams (200 keV) with varying electron flux (270 -5040 e -nm -2 s -1 ) for a total time-period of 4.3 hours (Figure 9). The electronic stopping power of electrons is 1.5 eV/nm and the electronic energy deposited in the lamella due to electrons varied from 2.18 × 10 24 keV/cm 3 (e -flux: 270 e -nm -2 s -1 ; time: 90 mins) to 1.17 × 10 26 keV/cm 3 (e -flux: 5040 e -nm -2 s -1 ; time: 260 mins). The Eelectronic deposited in the amorphous ErPO4 due to the electron beam is 4 -6 orders of magnitude higher than the Eelectronic deposited during He + ion-irradiation of amorphous ErPO4 (See Tables 2 and5).

Crystallization of the amorphous domains started after exposure to electron beam as evidenced by the appearance of nuclei in the lamella (Figure 9d). Indexing of the electron diffraction pattern conclusively showed that the recrystallized nuclei are xenotime crystallites (See Figure 10 and Table 7). The dhkl values determined in this study were compared with those reported by Ni et al.

(Table 7). [START_REF] Ni | Crystal chemistry of the monazite and xenotime structures[END_REF] Electron beam induced recrystallization occurred after ~ 2 hrs of exposure to the electron beam. This observation clearly demonstrated that the recrystallization observed in dual ion-beam experiments was exclusively due to α-particle irradiation since the TEM images and SAED patterns were collected immediately after the ion-irradiation in less than ~1 hour. It should also be noted that the electron flux used to observe the lamella after dual-ion beam irradiation was much lower than the electron flux used for the electron irradiation experiment (Table 5).

The concentration of the recrystallized nuclei increased slowly as a function of electron beam exposure time and electron flux (Figure 9d -9i). Because of the slower nucleation rate, large crystals of xenotime were observed in the lamella upon exposure to electron beam (See Figure 9i).

In the study reported by Seydoux-Guillaume et al., electron irradiation experiments performed on amorphized LaPO4 (monazite) lamella also yielded similar results that were observed for ErPO4 (xenotime) in this study. [START_REF] Seydoux-Guillaume | Why natural monazite never becomes amorphous: Experimental evidence for alpha selfhealing[END_REF] However, the nucleation rate was faster for monazite samples as indicated by the appearance of the first nuclei after ~25 minutes of exposure to electron beam. [START_REF] Seydoux-Guillaume | Why natural monazite never becomes amorphous: Experimental evidence for alpha selfhealing[END_REF] Due to the faster rate of nucleation, only smaller sized nuclei were observed upon exposure of amorphous monazite samples to electron beam. [START_REF] Seydoux-Guillaume | Why natural monazite never becomes amorphous: Experimental evidence for alpha selfhealing[END_REF] These observations confirmed that the activation energy for the recrystallization of xenotime structure was higher than the monazite structure and is in good agreement with the results reported by Meldrum et al. [START_REF] Meldrum | Displacive radiation effects in the monazite-and zircon-structure orthophosphates[END_REF] Global view of the entire lamella after the electron irradiation experiment clearly reveal the presence of two regions (Figure 11). The region exposed to the electron beam underwent an electron-induced amorphous to crystalline transformation while the region not exposed to the electron beam remained amorphous (Figure 11). This observation clearly showed that the electron beams could also induce recrystallization of amorphous xenotime samples and that the efficiency of electronbeam induced recrystallization process was lower for the xenotime structure in comparison to the monazite structure.

Dual Ion-Beam Radiation Response : Monazite vs Xenotime

Comparison of the dual ion-beam radiation response of monazite and xenotime structures was made by comparing the results from this study with those obtained from a similar study carried out on monazite (LaPO4) samples by Seydoux et al. [START_REF] Seydoux-Guillaume | Why natural monazite never becomes amorphous: Experimental evidence for alpha selfhealing[END_REF] In order to facilitate this comparison, the electronic energy deposited in the ErPO4 and LaPO4 lamellas due to ion-implantation were plotted against the corresponding nuclear energies (Figure 12). For the xenotime structure, complete amorphization of the structure was observed for lower Eelectronic/Enuclear ratios (Eelectronic/Enuclear = 9 -10 & 16 -18, Figure 12) and the metamictization of the xenotime structure was completely prevented upon increasing the amount of electronic energy deposited in the lamella (Eelectronic/Enuclear = 54 -57, Figure 12). Unlike xenotime, the monazite structure was not amorphized for lower Eelectronic/Enuclear ratios (Eelectronic/Enuclear = 13 -15; See Figure 12) and remain in the crystalline state at all the ion-fluences studied. [START_REF] Seydoux-Guillaume | Why natural monazite never becomes amorphous: Experimental evidence for alpha selfhealing[END_REF] Based on these observations, prevention of metamictization of the xenotime structure would require ~ 4 times the electronic energy deposition required for the prevention of amorphization of the monazite structure.

The different structural response to dual ion-beam irradiation could be attributed to the differences in the crystal structures of monazite and xenotime materials. [START_REF] Ni | Crystal chemistry of the monazite and xenotime structures[END_REF] In the monazite structure, the REO9 polyhedra is irregular and distorted. Due to the distorted nature of the REO9 polyhedra, it is proposed in this study that the amorphized monazite samples would require lower amounts of electronic energy to return to its original crystalline state. As a result, the monazite samples did not undergo a radiation-induced amorphization for lower Eelectronic/Enuclear ratios (Eelectronic/Enuclear = 13 -15, Figure 12). On the contrary, the xenotime structure is characterized by a more symmetrical REO8 polyhedra. Considering the symmetric nature of the REO8 polyhedra, it is proposed that the amorphized xenotime samples would require large amounts of electronic energy in order to undergo a structural reorganization. As it was shown in this study, the amorphization of xenotime structure can be completely prevented only upon deposition of large amounts of electronic energy (Eelectronic/Enuclear = 54 -57, Figure 12). Based on these observations, it is proposed that the amorphous monazite sample exhibited a greater ease of structural recovery due to α-particles than the xenotime sample.

Conclusion

The effect of dual ion beam irradiation on the structure of xenotime-type ErPO4 was investigated in this study using in-situ TEM. Multiple ion-irradiation experiments were carried out in which thin lamellas of ErPO4, obtained using the FIB technique, was irradiated with Au 2+ (1.5 MeV) and He + (160 keV) ion beams in sequential (Au 2+ followed by He + ) and simultaneous (Au 2+ + He + ) modes. Sequential ion-irradiation experiments showed that the xenotime structure was amorphized at a lower Au 2+ ion-fluence in comparison to the monazite structure and that the subsequent He + ion-irradiation of amorphized ErPO4 samples did not result in the structural recovery of xenotime.

On the contrary, the simultaneous ion-irradiation experiments showed that the He + ions prevented the xenotime structure from undergoing radiation-induced amorphization events thereby demonstrating the presence of α-healing mechanism in xenotime samples. Unlike the monazite samples wherein the α-healing mechanism was observed for lower Eelectronic/Enuclear ratios (Eelectronic/Enuclear = 13 -15), the α-healing mechanism was observed in xenotime samples only upon deposition of ~ 4 times the amount of electronic energy required for the monazite structure (Eelectronic/Enuclear = 54 -57). Thus, the xenotime structure was less efficient towards α-particle and electron-induced recrystallization events than the monazite structure. The different response of these materials to ion-irradiation are attributed to the different crystal structures of monazite and xenotime materials. The results obtained from this study will contribute to the growing database of candidate ceramic materials proposed for the immobilization of long-lived radionuclides.

Tables Table 1: irradiated simultaneously with Au 2+ and He + ions (Eelectronic/Enuclear = 10) to higher ion-fluences of Au 2+ : 1 × 10 13 ions/cm 2 ; He + : 1.40 × 10 15 ions/cm 2 (B), Au 2+ : 5 × 10 13 ions/cm 2 ; He + : 7.37 × 10 15 ions/cm 2 (C), and Au 2+ : 1 × 10 14 ions/cm 2 ; He + : 1.5 × 10 16 ions/cm 2 (D). ions/cm 2 ; He + : 1.35 × 10 16 ions/cm 2 (D) and Au 2+ : 1 × 10 14 ions/cm 2 ; He + : 2.78 × 10 16 ions/cm 2 (E). The presence of two diffraction spots in the SAED pattern of (D) are indicated by two red arrows. Global view of the ErPO4 lamella after dual ion-beam radiation (F). of ErPO4 irradiated simultaneously with Au 2+ and He + ions (Eelectronic/Enuclear = 36 -39) to ionfluences of Au 2+ : 1 × 10 13 ions/cm 2 ; He + : 7.57 × 10 15 ions/cm 2 (B), Au 2+ : 5 × 10 13 ions/cm 2 ; He + : 3.69 × 10 16 ions/cm 2 (C), Au 2+ : 1 × 10 14 ions/cm 2 ; He + : 7.12 × 10 16 ions/cm 2 (D), Au 2+ : 1.50 × 10 14 ions/cm 2 ; He + : 1.09 × 10 17 ions/cm 2 (E), and Au 2+ : 2 × 10 14 ions/cm 2 ; He + : 1.42 × 10 17 ions/cm 2 (F). of ErPO4 irradiated simultaneously with Au 2+ and He + ions (Eelectronic/Enuclear = 54 -57) to ionfluences of Au 2+ : 1 × 10 13 ions/cm 2 ; He + : 1.52 × 10 16 ions/cm 2 (B), Au 2+ : 5 × 10 13 ions/cm 2 ; He + : 7.12 × 10 16 ions/cm 2 (D), Au 2+ : 1 × 10 14 ions/cm 2 ; He + : 1.41 × 10 17 ions/cm 2 (F), and Au 2+ : 2 × 10 14 ions/cm 2 ; He + : 2.87 × 10 17 ions/cm 2 (H)

Figure 2 :

 2 Figure 2: Powder XRD pattern of the xenotime-type ErPO4 pellet sintered at 1600ºC in air.

Figure 3 :

 3 Figure 3: BF-TEM image of thin foil of pristine ErPO4 samples. The SAED pattern of pristine

Figure 4 :

 4 Figure 4: BF-TEM images and the corresponding SAED patterns (inset) of ErPO4 samples irradiated sequentially with Au 2+ ions to ion-fluences of 1 × 10 13 ions/cm 2 (A), 5 × 10 13 ions/cm 2 (B), and 2 × 10 14 ions/cm 2 (C). The BF-TEM image and the corresponding SAED pattern (inset) of amorphous ErPO4 irradiated with He + ions to an ion-fluence of 1 × 10 17 ions/cm 2 (D).

Figure 5 :

 5 Figure 5: BF-TEM images and corresponding SAED pattern (inset) of ErPO4 sample obtained

Figure 6 :

 6 Figure 6: BF-TEM images and corresponding SAED pattern (inset) of ErPO4 sample obtained

Figure 7 :

 7 Figure 7: BF-TEM images and corresponding SAED pattern (inset) of ErPO4 sample obtained

Figure 8 :

 8 Figure 8: BF-TEM images and corresponding SAED pattern (inset) of ErPO4 sample obtained

  The images shown in (C), (E), (G), and (I) were obtained upon magnification of regions represented by white boxes in (B), (D), (F), and (H), respectively. The mottled diffraction contrasts due to ion-irradiation were clearly observed in (C), (E), (G), and (I).

Figure 9 :Figure 12

 912 Figure 9: Electron-beam induced recrystallization of amorphous ErPO4. BF-TEM images and corresponding SAED patterns obtained before (A) and after (B) Au 2+ ion-irradiation. BF-TEM

Table 2 :

 2 Electronic and Nuclear Stopping Powers for ErPO4The electronic and nuclear stopping factors of dual ion-beam (Au 2+ + He + ) were obtained through summation of electronic and nuclear stopping powers of their respective single ion -beams Sequential ion-irradiation of ErPO4

	Ion-beam	Electronic Stopping	Nuclear Stopping	ENSP
		Power (dE/dx)e (eV/nm)	Power (dE/dx)n (eV/nm)
	Au 2+ (1.5 MeV)	1725	4034	0.4276
	He + (160 keV)	279	2.442	114.2
	(Au 2+ + He + ) *	2004	4036	0.4965

*

Table 3 :

 3 Simultaneous ion-irradiation of ErPO4

	Steps	Flux	Temperature	Au 2+ fluence	He + fluence	Eelectronic	Enuclear	Eelectronic/
		(ions/cm 2 /s)	of TEM foil	(ions/cm 2 )	(ions/cm 2 )	(keV/cm 3 )	(keV/cm 3 )	Enuclear
	1	Au 2+ : 2.0 × 10 10	28°C	1.0 × 10 13	1.40 × 10 15 2.3 × 10 20 (Au)	5.3 × 10 20 (Au)	~ 9
		He + : 3.0 × 10 12				4.8 × 10 21 (He)	4.2 × 10 19 (He)
	2			5.0 × 10 13	7.37 × 10 15 1.0 × 10 21 (Au)	2.3 × 10 21 (Au)	~10
						2.5 × 10 22 (He)	2.2 × 10 20 (He)
	3			1.0 × 10 14	1.50 × 10 16	2.1 × 10 21 (Au)	4.7 × 10 21 (Au)	~10
						5.1 × 10 22 (He)	4.5 × 10 20 (He)
	1	Au 2+ : 2.0 × 10 10	43°C	1.0 × 10 13	2.83 × 10 15 2.3 × 10 20 (Au)	5.3 × 10 20 (Au)	~16
		He + : 5.0 × 10 12				9.7 × 10 21 (He)	8.4 × 10 19 (He)
	2	Au 2+ : 2.0 × 10 10		5.0 × 10 13	1.35 × 10 16 1.0 × 10 21 (Au)	2.3 × 10 21 (Au)	~17
		He + : 5.4 × 10 12				4.6 × 10 22 (He)	4.0 × 10 20 (He)
	3	Au 2+ : 2.0 × 10 10		1.0 × 10 14	2.78 × 10 16	2.1 × 10 21 (Au)	4.7 × 10 21 (Au)	~18
		He + : 5.6 × 10 12				9.5 × 10 22 (He)	8.3 × 10 20 (He)

Table 4 :

 4 Simultaneous ion-irradiation of ErPO4

	Steps	Flux	Temperature	Au 2+ fluence	He + fluence	Eelectronic	Enuclear	Eelectronic/
		(ions/cm 2 /s)	of TEM foil	(ions/cm 2 )	(ions/cm 2 )	(keV/cm 3 )	(keV/cm 3 )	Enuclear
	1	Au 2+ : 2.0 × 10 10	56°C	1.0 × 10 13	7.57 × 10 15 2.3 × 10 20 (Au)	5.3 × 10 20 (Au)	37
		He + : 1.4 × 10 13				2.6 × 10 22 (He)	2.3 × 10 20 (He)
	2			5.0 × 10 13	3.69 × 10 16 1.0 × 10 21 (Au)	2.3 × 10 21 (Au)	39
						1.3 × 10 23 (He)	1.1 × 10 21 (He)
	3			1.0 × 10 14	7.12 × 10 16	2.1 × 10 21 (Au)	4.7 × 10 21 (Au)	36
						2.4 × 10 23 (He)	2.1 × 10 21 (He)
	4			1.5 × 10 14	1.09 × 10 17	3.1 × 10 21 (Au)	7.0 × 10 21 (Au)	37
						3.7 × 10 23 (He)	3.2 × 10 21 (He)
	5			2.0 × 10 14	1.42 × 10 17	4.1 × 10 21 (Au)	9.3 × 10 21 (Au)	36
						4.8 × 10 23 (He)	4.2 × 10 21 (He)
	1	Au 2+ : 2.0 × 10 10	100°C	1.0 × 10 13	1.52 × 10 16 2.3 × 10 20 (Au)	5.3 × 10 20 (Au)	57
		He + : 2.9 × 10 13				5.2 × 10 22 (He)	4.5 × 10 20 (He)
	2			5.0 × 10 13	7.12 × 10 16 1.0 × 10 21 (Au)	2.3 × 10 21 (Au)	55
						2.4 × 10 23 (He)	2.1 × 10 21 (He)
	3			1.0 × 10 14	1.41 × 10 17	2.1 × 10 21 (Au)	4.7 × 10 21 (Au)	54
						4.8 × 10 23 (He)	4.2 × 10 21 (He)
	4			2.0 × 10 14	2.87 × 10 17	4.1 × 10 21 (Au)	9.3 × 10 21 (Au)	55
						9.8 × 10 23 (He)	8.5 × 10 21 (He)

Table 5 :

 5 Electron beam irradiation of amorphous ErPO4

	Steps	Au 2+ ion	Temperature	Electron flux	Electron	TEM observation
		fluence	of TEM foil	(e -/nm 2 /s)	exposure	conditions
		(ions/cm 2 )	(°C)		time (s)
	1	1.0 × 10 14	26	170	-	-
	2			270	t ≤ 90 min	Cond. 3, Obj. 2, α
						= -41, β = 22,5,
						Spot size 3, Mag.
						SA 25500x -
						29000x
	3			270	90 ≤ t ≤ 165	Cond. 3, Obj. 2, α
						= -41, β = 22,5,
						Spot size 1, Mag.
						SA 25500x -
						29000x
	3			1130	167 ≤ t ≤ 195 Cond. 3, Obj. 2, α
						= -41, β = 22,5,
						Spot size 1, Mag.
						SA 25500x -
						29000x
	4			5040	200 ≤ t ≤ 260 Cond. 3, Obj. 2, α
						= -41, β = 22,5,
						Spot size 1, Mag.
						SA 55000x -
						62000x

Table 6 :

 6 Lattice constants of xenotime-type ErPO4

	Sample	Lattice constants
	ErPO4	a = 6.8613 (1) Å
		c = 6.0097 (1) Å
		Volume = 282.92 Å 3

Table 7 :

 7 Interplanar distance dhkl values of electron beam induced recrystallized samples Crystal structure of xenotime-type YPO4 (Space group: I41/amd). The crystal structures were generated using the VESTA software.[START_REF] Momma | VESTA: a three-dimensional visualization system for electronic and structural analysis[END_REF]

	Diffraction spots	(h k l)	dhkl -Experimental (Å) dhkl -Theoretical (Å)
	1	(1 0 1)	4.5523	4.5379
	2	(2 0 0)	3.4504	3.4474
	3	(1 1 2)	2.6044	2.5635

Acknowledgement

The authors would like to thank the French National Research Agency (ANR JCJC-X-MAS; Project # ANR-17-CE06-0004) for the financial support. The ion-irradiation experiments performed at the CSNSM facility: JANNuS-Orsay (CSNSM Univ Paris-Sud/ CNRS, Orsay, France) was supported by the EMIR French accelerator network. The authors would also like to thank Dr. Nicolas Clavier for discussions about the sintering procedure for ErPO4 samples.


MD - Loïc Le Guennec
email: loic.leguennec@aphp.fr

Matthieu Schmidt

Frédéric Clarençon

Ahmed Mohamed Elhfnawy

Flore Baronnet

Michel Kalamarides

Guillaume Lebreton

Charles-Edouard Luyt

Mechanical Thrombectomy in Acute Ischemic Stroke Patients under Venoarterial Extracorporeal Membrane Oxygenation

Use of venoarterial extracorporeal membrane oxygenation (VA-ECMO) in adult to treat refractory cardiogenic shock has increased in recent years, and ischemic stroke is the most frequent VA-ECMO-induced cerebrovascular complication. No adult case of mechanical thrombectomy (MT) has been reported. We describe herein two adult patients on VA-ECMO with acute ischemic stroke, successfully treated with MT, and discuss feasibility and safety of such procedure.

BACKGROUND

Venoarterial extracorporeal membrane oxygenation (VA-ECMO) is used to provide circulatory support in patients with refractory cardiogenic shock 1 2 . Ischemic stroke is among the most frequent complications occurring in VA-ECMO-treated patients [3][4][5] . Due to the increased risk of secondary hemorrhagic transformation, the need for curative anticoagulation, and the ECMO-related hemostasis disorders, thrombolysis for acute ischemic stroke in patients on ECMO is theoretically contra-indicated, and has never been reported so far in an adult patient. We describe herein two adult patients from our tertiary care center, with acute ischemic stroke occurring while on VA-ECMO support, successfully treated by mechanical thrombectomy (MT).

CASES PRESENTATION

The first case was a 51-year-old man with a history of ischemic dilated cardiomyopathy admitted to the intensive care unit (ICU) for cardiogenic shock. Despite optimal medical treatment, his condition worsened and a femoro-femoral VA-ECMO was implanted in the left groin under local anesthesia. Five days after VA-ECMO implantation, he presented a sudden right hemiplegia with aphasia with an initial national institute of health stroke scale (NIHSS) of 18. CT-angiography (CTA) was performed, since ECMO is an MRI-incompatible device, and showed left middle cerebral artery (MCA) M1 segment occlusion. MT was urgently performed via a right femoral artery access, using a Solitaire stent retriever (Medtronic, Irvine, CA). Time to groin puncture was 4 hours, with a successful result. Modified Thrombolysis In Cerebral Infarction (mTICI) score was at 3 at the end of the procedure. Brain CT scan performed 24h after recanalization showed no sign of hemorrhagic transformation. Patient's cardiac condition progressively improved, and VA-ECMO was removed on day 9. Ten day after the ischemic stroke, he was discharged from the ICU and transferred in the cardiology department. At this time, he only presented a slight aphasia, with a Rankin score of 1. One year later he had normal neurological examination with a Rankin score of 0.

The second case was a 35 year-old man, who was admitted for cardiogenic shock complicating acute myocardial infarction. Coronary angiography showed proximal left anterior descending artery occlusion, that was successfully stented. At the end of the procedure, he developed refractory cardiogenic shock requiring implantation of a peripheral, left femoro-femoral ECMO, and a sudden left hemiplegia with an initial NIHSS of 24. CTA showed distal right MCA occlusion. After multidisciplinary team discussion, MT was performed via the right femoral artery, using a Solitaire stent retriever. Time to groin puncture was 6.5 hours, with a successful result (Figure 1; A, B). mTICI score was at 3 at the end of the procedure. Brain CT performed 24h after recanalization showed no sign of hemorrhagic transformation, but a right hemispheric swelling with midline shift and obstruction of the right lateral ventricle (Figure 1; C). A right decompressive craniectomy was performed 72 hours after MT because of persistent coma and radiological worsened (Figure 1; D). Patient's cardiac condition progressively improved, and VA-ECMO was removed on day 5. Due to delayed awakening, a tracheostomy was performed on day 15. Patient's neurological condition progressively improved, and cranioplasty was performed 3 months after craniectomy. The patient was finally weaned from his tracheostomy and transferred in a rehabilitation department. The patient was discharged home 2 years later; at that time his was walking without help and had a Rankin score of 2.

DISCUSSION

These cases illustrate the feasibility of performing MT on ECMO in adult patients 6 . ECMO circuit is either in a VA or in a veno-venous (VV) configuration. It consists of a centrifugal pump, which provides a continuous flow in the circuit, and a membrane oxygenator that allow oxygenation and decarboxylation of the blood. In VA-ECMO, the circuit includes an inflow cannula which drains blood from the venous system, and an outflow cannula which delivers the warmed oxygenated blood back into the arterial system (Figure 2,A) in order to restore a circulatory flow in refractory cardiogenic shock and cardiac arrest. Intra-aortic balloon pump (IABP) is often added within the femoral artery in these patients (Figure 2, B) to protects against hydrostatic pulmonary edema 7 , or to improve coronary bypass graft flow and cardiac function in refractory postoperative cardiogenic shock 8 9 . It reduces left ventricular afterload, improves coronary perfusion, and provide pulsatile blood flow. In VV-ECMO, the outflow cannula delivers oxygenated and decarboxylated blood to the vena cava and the right atrium (Figure 2,C), to insure a normal blood gas exchange during acute respiratory distress syndrome (ARDS) refractory to conventional therapy [START_REF] Combes | Extracorporeal Membrane Oxygenation for Severe Acute Respiratory Distress Syndrome[END_REF] . Stroke occurs mainly during VA-ECMO 3 11 , whereas cerebral hemorrhage is mostly observed during VV-ECMO [START_REF] Luyt | Brain injury during venovenous extracorporeal membrane oxygenation[END_REF] . These complications have an important impact in terms of outcome.

In order to perform MT, the choice of the arterial vascular access to perform MT should be discussed with the bedside ECMO specialist in charge of the patient, depending on ECMO configuration. In our two cases, the femoral access opposite to the arterial inflow cannula was chosen, but transradial artery access is another option. As the patient receive continuous unfractioned heparin, it is mandatory to use vascular ultrasound during puncture to minimize complications. In both cases, no femoral IABP were implanted, but its presence should not contra-indicate MT, as the operator can perform the MT via transradial arteries. A particular situation, but rare in adult, is a VA-ECMO circuit configuration with outflow cannula delivering blood back into the axillary artery (Figure 2,D). In this case, the operator should not access transradial artery ipsilateral to the outflow cannula. In VV-ECMO configuration, all arterial access can be approached without constraint.

Concerning the timing of MT, our cases mainly differ, regarding the onset of the VA-ECMO, in relation to the neurological event.

In the first case, the neurological symptoms occurred suddenly after 5 days of VA-ECMO support in an alert patient, allowing us to determine precisely the stroke onset time. Unfortunately, patients under VA-ECMO are too often on invasive mechanical ventilation and heavily sedated, thus compromising neurological examination and stroke onset time determination. Although some patients need sedations many VA-ECMO patients remain sedated for the sole reason that they are under VA-ECMO, mainly for safety reasons. Fortunately, the recent literature goes against this dogma 13 14 , and patients are lightly sedated, or extubated when possible, giving the opportunity to have proper neurological evaluation. For the second case, the timing was different: immediately after a coronary angiography, he presented sudden neurological symptoms suggestive of cerebral infarction, probably of cardioembolic origin, related to atherosclerotic material release out of vascular plaques in the setting of coronary angiography [START_REF] Werner | Stroke in patients undergoing coronary angiography and percutaneous coronary intervention: incidence, predictors, outcome and therapeutic options[END_REF] . Due to the initial presentation with refractory cardiogenic shock, the first step was to restore respiratory and hemodynamic compromises using oro-tracheal intubation and mechanical ventilation, catecholamine infusion and VA-ECMO insertion. Even if it delayed the management of the stroke (6.5 hours were needed to perform this sequence, although all human and technical resources were available on site), the patient was finally transferred to the radiology department, where CT scan, CTA and MT were performed. For this latter case, the evolution differed from that of our first case mainly because the former had too late MT, complicated with brain infarction and brain swelling requiring hemicraniectomy. However, significant neurologic recovery was finally observed in this patient.

In both cases, the main point that was debated was about performing or not an MT: indeed, VA-ECMO usually requires continuous infusion of unfractioned heparin to avoid circuit thrombosis and/or thrombo-embolic events, which in our cases had to be counterbalanced with the risk of reperfusion syndrome occurrence with secondary hemorrhagic transformation after MT. Indeed, it is not known whether VA-ECMO increases the risk of MT-induced cerebral hemorrhage, and if we refer to the literature, the only published case of IAMT on ECMO has been reported by Sun et al [START_REF] Sun | Mechanical Thrombectomy in an Infant With Acute Embolic Stroke[END_REF] , in a 9-month-old infant under VA-ECMO, inserted for cardiovascular collapse after parainfluenza pulmonary infection associated with acute cor pulmonale. In this case report, the infant developed both left MCA and basilar artery occlusions. MT could only have been performed within the posterior circulation, with a successful result, and without secondary hemorrhagic transformation. Finally, poor clinical outcome was observed, due to a large intracerebral hemorrhage in the left MCA territory. Together, these three cases suggest MT safety under ECMO support, although further studies are needed to determine the precise risk of secondary hemorrhagic transformation after IAMT in this specific population.

LEARNING POINTS/TAKE HOME MESSAGES

-MT feasibility on ECMO -Light sedation/extubation when possible in order to have proper neurological evaluation and stroke onset time determination.

FIGURESFigure 1 :

 1 FIGURES

Figure 2 :

 2 Figure 2: ECMO circuits (A) Femoro-femoral VA-ECMO circuit (B) Femoro-femoral VA-ECMO circuit with controlateral IAPB (C) Femoro-jugular VV-ECMO circuit (D) Femoro-axillary VA-ECMO circuit ECMO; extracorporeal membrane oxygenation VA; venoarterial VV; venovenous IAPB; Intra-aortic balloon pump


Stephane Calvez
email: scalvez@laas.fr

Antoine Monmayrant

Olivier Gauthier- Lafaye

Thermally-tunable Cavity Resonator-Integrated Guided-mode Resonance Filters

We report the fabrication of Cavity Resonator-Integrated Guided-mode Resonance Filters (CRIGFs) using a hybrid lithium-niobate/silicon oxynitride technological platform that allows the exploitation of lithium niobate advantageous material properties while maintaining standard nanoprocessing techniques. The beneficial use of this approach is illustrated with the first demonstration of thermal tuning of a CRIGF.

Introduction

Cavity Resonator-Integrated Guided-mode Resonance Filters (CRIGFs) are waveguide Fabry-Perot micro-cavities with an intra-cavity out-of-plane input/output grating coupler whose design corresponds to a Guided-Mode Resonance Filter (GMRF) of finite size [START_REF] Kintaka | Cavity-resonator-integrated guided-mode resonance filter for aperture miniaturization[END_REF][START_REF] Buet | High angular tolerance and reflectivity with narrow bandwidth cavity-resonator-integrated guided-mode resonance filter[END_REF][START_REF] Laberdesque | Highorder modes in cavity-resonator-integrated guided-mode resonance filters (CRIGFs)[END_REF][START_REF] Rassem | Waveguide mode in the box with an extraordinary flat dispersion curve[END_REF] (see Fig. 1). As such, the overall device operating principle is ruled by the excitation of and emission from supported localized Fabry-Perot modes [START_REF] Laberdesque | Highorder modes in cavity-resonator-integrated guided-mode resonance filters (CRIGFs)[END_REF]. The response is thus a fixed-wavelength(s) passband filter in reflection (and a notch filter in transmission) whose peak reflectivity and angular dependence are governed by the overlap between the excitation signal and the localized modes spatial and spectral properties. The resulting reflective characteristics and tolerances [START_REF] Buet | High angular tolerance and reflectivity with narrow bandwidth cavity-resonator-integrated guided-mode resonance filter[END_REF] make them attractive for use as wavelength-selective mirrors in extended-cavity diode lasers (ECDLs) [START_REF] Buet | Wavelength-stabilised external-cavity laser diode using cavity resonator integrated guided mode filter[END_REF][START_REF] Monmayrant | Performances of a Butterfly-packaged External cavity laser diode with CRIGF mirror[END_REF][START_REF] Monmayrant | Wavelength stabilized External Cavity Quantum Cascade Lasers using Cavity Resonator Integrated Grating Filters[END_REF]. However, when it comes to introducing tunability into CRIGFs (and associated ECDLs), the above-mentioned operating principle renders ineffective the use of angular tuning as commonly implemented with other (diffractive, volume Bragg or GMRFs) grating-based approaches. The only viable solutions therefore consist in altering the CRIGF micro-cavity physiccal properties either via its geometry or its constitutive set of refractive indices. To that extent, CRIGFs with spatially-graded grating structures have been successfully demonstrated [START_REF] Augé | Tunable graded cavity resonator integrated grating filters[END_REF] and used to introduce broadband tuning of ECDLs [START_REF] Gauthier-Lafaye | Graded CRIGF filters for tunable external cavity lasers[END_REF]. Nevertheless, to gain in laser stability and packaged module compactness, it is desirable to devise, demonstrate and implement systems exploiting displacement-free tunable filters. In that context, it is worth noting that GMRFs with suitable tuning mechanisms including electro-refractive [START_REF] Sharon | Light modulation with resonant grating-waveguide structures[END_REF][START_REF] Chang | Tunable Liquid Crystal-Resonant Grating Filter Fabricated by Nanoimprint Lithography[END_REF][START_REF] Katchalski | Light modulation with electro-optic polymer-based resonant grating waveguide structures[END_REF], optofluidic [START_REF] Xiao | A tunable submicro-optofluidic polymer filter based on guided-mode resonance[END_REF] and thermal [START_REF] Uddin | Guided-Mode Resonant Thermo-Optic Tunable Filters[END_REF] actuation schemes have already been reported and could readily be transferred to CRIGFs with the preference being given to all-solid approaches.

In this paper, we report the demonstration of thermally-tunable, and therefore movementfree, Cavity Resonator-Integrated Guided-mode Resonance Filters based on a hybrid lithiumniobate (LN)/silicon oxynitride (SiON) platform, the latter being selected as it combines convenient nano-processing afforded by the SiON technology with the ability to exploit the favorable properties of a hard-to-process material : lithium niobate.

Design

To demonstrate movement-free tuning of a CRIGF resonance, we selected the lithium niobate on insulator technology as this material platform is compatible with the fabrication of low-loss integrated optics [START_REF] Poberaj | Lithium niobate on insulator (LNOI) for micro-photonic devices[END_REF][START_REF] Boes | Status and Potential of Lithium Niobate on Insulator (LNOI) for Photonic Integrated Circuits[END_REF] and provides various electronic means to modify the material refractive index, namely using either thermal, electro-optic, piezo-electric or even acousto-optic effects [START_REF] Poberaj | Lithium niobate on insulator (LNOI) for micro-photonic devices[END_REF][START_REF] Boes | Status and Potential of Lithium Niobate on Insulator (LNOI) for Photonic Integrated Circuits[END_REF]. More specifically, as summarized in Table 1, LiNbO3 is an attractive material to use to make thermally-active filters as both its refractive index thermal coefficient and its expansion coefficient are greater than the ones of the materials typically used so far (Si3N4 for the waveguide core and of AF32 glass (α=3.2⋅10 -6 K -1 ) for the transparent substrate).

Although the CRIGFs could be made by direct corrugation of the waveguide LiNbO3 core, we opted for the hybrid SiON/LN embodiment represented on Fig. 1 since, as detailed below, this approach not only offers an easier processing route [START_REF] Boes | Status and Potential of Lithium Niobate on Insulator (LNOI) for Photonic Integrated Circuits[END_REF][START_REF] Chang | Heterogeneous integration of lithium niobate and silicon nitride waveguides for wafer-scale photonic integrated circuits on silicon[END_REF] but also a greater design flexibility thanks to a higher number of optimization parameters.

The CRIGFs were designed for operation at a wavelength of 1550 nm using a combination of rigorous coupled-wave analysis [START_REF] Chaumet | Electromagnetic modeling of large subwavelength-patterned highly resonant structures[END_REF] and coupled-mode theory [START_REF] Laberdesque | Highorder modes in cavity-resonator-integrated guided-mode resonance filters (CRIGFs)[END_REF]. More specifically, the vertical planar waveguide is constituted of a 297-nm-thick X-cut LiNbO3 core surrounded by a 2 µm SiO2 under-cladding layer and an upper-cladding bilayer made of a 72 nm Si3N4 layer surmounted by 323 nm of silica. The upperclad bilayer allows to optimize the grating effective index difference (i.e. the grating coupling strength) and the overall stack reflectivity. These thicknesses were chosen first and foremost to ensure low-loss TE-polarized (s-polarized) singlemode propagation on an X-cut LiNbO3 substrate with an effective index of 1.821. They also constitute an anti-reflection coating as shown on Fig. 2 right and as calculated using the scattering matrix method [START_REF] Coldren | Diode Lasers and Photonic Integrated Circuits[END_REF] with refractive indices taken from references [START_REF] Moretti | Temperature dependence of the thermo-optic coefficient of lithium niobate, from 300 to 515 K in the visible and infrared regions[END_REF][START_REF] Arbabi | Measurements of the refractive indices and thermo-optic coefficients of Si_3N_4 and SiO_x using microring resonances[END_REF] and summarized in Table 1. Assuming that the Si3N4 layer is fully etched in the Distributed Bragg Reflectors (DBRs) that establish the CRIGF cavity, the effective index difference between the etched (without the Si3N4 layer, of effective index n1) and unetched (with the Si3N4 layer, of effective index n2) sections of the DBR and GC was evaluated to be 0.0451 (see Fig. 2. left). Using the belowmentioned (thin-film approximation) formulae [START_REF]Vertical-Cavity Surface-Emitting Laser Devices[END_REF], DBRs with Np=400-pair and a fillingfactor of 0.5 and a period of 432.5 nm exhibit a modal reflectivity, RDBR, at a wavelength of =1550 nm greater than 99.9% with a full-width-at-half-maximum (FWHM) stopband  of ~25.4 nm and thereby support highly-confined localized modes.

𝑅 = (1)
=  asin | | (2)
Finally, we chose a 21-period grating coupler with a grating pitch of ΛGC=865 nm and a fill-factor of 0.45 as well as phase-adjustment sections of LPS=1.125 ΛGC. With these parameters, the passband reflective filter response sitting an anti-reflective background is predicted to have a FWHM bandwidth of ~0.85 nm with, at resonance, a maximal reflectivity of 85.3% and a minimal transmission of 14.4% as shown in Fig. 3. We note that the remaining 0.3% loss correspond to waveguide losses at the end of the DBRs. Additionally, the spectral range of ripple-free transmission around the resonant wavelength corresponds to the abovementioned ~25-nm-wide DBR stopband.

Finally, using the thermo-optical coefficients of references [START_REF] Moretti | Temperature dependence of the thermo-optic coefficient of lithium niobate, from 300 to 515 K in the visible and infrared regions[END_REF][START_REF] Arbabi | Measurements of the refractive indices and thermo-optic coefficients of Si_3N_4 and SiO_x using microring resonances[END_REF] and assuming that the grating structure expansion is imposed by the substrate, RCWA calculations suggest that the resonant peak will linearly shift at a rate of 0.0476 nm/K with temperature, 43 % of which being due to the substrate thermal expansion. Similarly, the wavelength of the antireflection coating minimum (~1535 nm) is found to increase linearly at a rate of 0.0452 nm/K. Fig. 3. CRIGF spectral characteristics (reflection in red and transmission in green) as calculated by RCWA (using 901 orders and a mode-matched Gaussian beam [START_REF] Rassem | Numerical modeling of long sub-wavelength patterned structures[END_REF]). The grey area highlights the DBR stopband.

Fabrication

The device fabrication started from a Lithium-Niobate On Insulator (LNOI) planar underclad core waveguide on its substrate obtained from NanoLN. The 72-nm-thick Si3N4 layer constituting the first layer of the waveguide upper-cladding was deposited using an Inductive-Coupled-Plasma Plasma-Enhanced Chemical Vapor Deposition (ICP-PECVD) system with single-wavelength (532 nm) in-situ reflectivity control. The designed grating structures were then defined into the latter layer using nano-imprint lithography and dry etching with the 50µm-long grating lines oriented along the LiNbO3 crystallographic Z-axis (extraordinary axis).

As in [START_REF] Pelloquin | Soft mold NanoImprint Lithography: a versatile tool for sub-wavelength grating applications[END_REF], a master was patterned on a silicon substrate using electron-beam lithography, then replicated into a soft mold by thermal nano-imprint lithography and finally transferred by UV nano-imprint lithography from the soft mold into the resist spun on the LNOI sample. In this last step, the soft mold was peeled off along the grating lines. The grating patterns were then transferred into the Si3N4 layer using a CHF3/O2-etch with the LiNbO3 top surface serving as an etch-stop interface given this material chemical inertness and hardness. As a consequence and as shown in Fig. 4, it became easier to control the grating height (and coupling strength) despite the potential presence of a residual resist layer [START_REF] Sotomayor Torres | Nanoimprint lithography: an alternative nanofabrication approach[END_REF]. It also helped obtain smooth surfaces inside the grating trenches (with a measured roughness of 0.6 nm as compared to 3.2 nm at the top of the SiN layer) and therefore high-quality-factor devices by minimizing the roughness-induced limit of the DBR effective reflectivity and of the waveguide propagation loss. The device fabrication was completed by ICP-PECVD deposition of a 323-nm-thick SiO2 layer on top of the above-described grating structure and of a 266-nm-thick SiO2 single-layer anti-reflective coating on the rear-side of the substrate to avoid back-reflection from the latter interface.

Characterization

The device spectral characteristics around a wavelength of 1550 nm were measured using the setup shown in Fig. 5. The data were acquired with a 10-pm resolution using a fiber-coupled tunable laser whose output was relayed using free-space optics to form a beam whose waist was measured to be 5.3-µm. The camera and 850 nm LED illumination were used to image the sample and the 980 nm laser beam served to transversally locate the sample with respect to the probe beam, making use of the collinearity between the 980 and 1550 nm beams. This arrangement is essentially a substitute to using an InGaAs camera to directly monitor the 1550 nm beam. The sample was then longitudinally positioned at the waist by maximizing the reflected signal from an adjacent grating-free area. The transmission and reflectivity responses were respectively calibrated against sample-free transmission and the reflection from a silver mirror. The sample was mounted using silver paste on an aluminum mount presenting a drilled 8mm-diameter via-hole to permit the transmission measurements and whose temperature was measured using a calibrated NTC thermistor and controlled by a thermo-electric Peltier cooler. As it can be observed on top part of Fig. 6, for a substrate temperature of 20°C, the responses are composed of an antireflective background centered at 1535 nm with a resonant feature observed at ~1554.4 nm with a 0.96-nm linewidth (or equivalently a Q-factor of ~1620) and an on-resonance transmission factor and peak reflectivity of 32±2%. The latter values differ from the calculated ones. These discrepancies are tentatively attributed to residual material and roughness-induced loss contributions (which are not included in the calculations) and to the mode mismatch between the fiber beam profile and the excited CRIGF mode profile.

Furthermore, the observable out-of-resonance ripples are in part due to the DBR structure (as shown in Fig. 3), to the coated substrate Fabry-Perot fringes (that were not included in the calculations) and to spurious transmission fringes resulting from the sensitivity of the fiber part of the measurement setup to the environmental conditions (which primarily affect the device transmissivity).

To gain further insight into the characteristics of the device, the reflectivity was mapped as a function of the position along the grating lines with a slightly offset injection (with respect to the center of the CRIGF (y=0)). As in [START_REF] Laberdesque | Highorder modes in cavity-resonator-integrated guided-mode resonance filters (CRIGFs)[END_REF], Fig. 6.bottom reveals, on top of the substrate Fabry-Perot fringe pattern (with ~1-nm free spectral range), a multimode response i.e. a response with several reflectivity peaks. However, contrary to [START_REF] Laberdesque | Highorder modes in cavity-resonator-integrated guided-mode resonance filters (CRIGFs)[END_REF], this spectro-spatial map is asymmetric (see Fig. 6 bottom), a feature whose origin is attributed to the lateral unmolding steps performed during the nano-imprint lithography stage. The observed ~0.014-nm/µm resonance spatial gradient leads, here, to an 0.7-nm spectral shift over the entire grating width. It also causes a negligible spectral broadening of the measured response over the sampling beam profile.

Indeed, the measured bandwidth is close to the calculated one (Q~1830 -see Fig. 3). Should the targeted Q-factor exceed 16000 [START_REF] Augé | Tunable graded cavity resonator integrated grating filters[END_REF], an improvement in spatial inhomogeneity would be required. to 80°C, the resonance is shown to shift over a 2.31-nm span, a range ~2.4-time the filter bandwidth (see Fig. 7). The resonance-shifting rate is evaluated to be 0.0382±0.0008 nm/K.

Parabolic fits of the anti-reflection coating part of the spectra also reveals a linear shift of the AR minimum at a similar rate (0.0367±0.0057nm/K) in keeping with the predicted theoretical behavior. The discrepancy between the experimentally measured and calculated values is assigned to the difference between the actual sample surface temperature and the measured mount temperature.

Conclusion

We reported a thermally-tunable Cavity Resonator-Integrated Guided-mode Resonance Filter exploiting a hybrid lithium-niobate/silicon oxynitride technological platform. The device operating around the central wavelength of 1555 nm was shown to exhibit a Q-factor of ~1620

and to be tunable at a rate of 0.0382 nm/K. Spans larger than the filter bandwidth and an order of magnitude larger than the typical mode spacing of practical extended-cavity diode lasers [START_REF] Buet | Wavelength-stabilised external-cavity laser diode using cavity resonator integrated guided mode filter[END_REF][START_REF] Monmayrant | Performances of a Butterfly-packaged External cavity laser diode with CRIGF mirror[END_REF][START_REF] Monmayrant | Wavelength stabilized External Cavity Quantum Cascade Lasers using Cavity Resonator Integrated Grating Filters[END_REF][START_REF] Gauthier-Lafaye | Graded CRIGF filters for tunable external cavity lasers[END_REF] were also shown to be achievable upon application of a reasonable temperature rise (>25°C). Future work will include investigations of such electronically-tunable CRIGF-based ECDLs for fine tuning of the laser emission wavelength. Additionally, similarly to the work of reference [START_REF] Chang | Heterogeneous integration of lithium niobate and silicon nitride waveguides for wafer-scale photonic integrated circuits on silicon[END_REF], the demonstrated technological approach of combining the established nanostructuration of silicon oxynitride layers with materials which possess interesting properties but are hard-to-process is thought to be an attractive route to confer advanced functionalities or optimum performance to devices. The specific implementation with LNOI was performed as a step towards the exploitation of its nonlinear properties as it will be reported elsewhere.

Fig. 1 .Fig. 2 .

 12 Fig. 1. Diagram of the considered CRIGF structure. The arrows give a schematic representation of the device operating principle where an incident beam (coming from the top) is coupled to localized resonating mode which is out-coupled to form the reflection/transmission response.

Fig. 4 .

 4 Fig. 4. Atomic-Force Microscope measurement of the profile of the etched grating into the SiN layer.

Fig. 5 .

 5 Fig. 5. Characterization setup. The free-space relay is made of the f1 and f2-focal-length lens telescope. WDM are wavelength-division multiplexer (spectrally-selective beam splitters). DAQ is a A/N data acquisition card.

Fig. 6 .

 6 Fig. 6. Top: Typical spectral characteristics at 20°C of the fabricated LNOI CRIGF. Bottom: Spatial dependence of the reflectivity (at 20°C) along the CRIGF grating lines (Z-axis). The white dashed lines show the limit of the CRIGF

Fig. 7 .

 7 Fig. 7. Temperature dependence of the normalized reflectivity of the fabricated LNOI CRIGF.

Table 1 .

 1 Simulation parameters[START_REF] Moretti | Temperature dependence of the thermo-optic coefficient of lithium niobate, from 300 to 515 K in the visible and infrared regions[END_REF][START_REF] Arbabi | Measurements of the refractive indices and thermo-optic coefficients of Si_3N_4 and SiO_x using microring resonances[END_REF]

	Material	n	dn/dT	α
			(10 -5 K -1 )	(10 -6 K -1 )
	SiO2	1.444	0.95	0.55
	Si3N4	1.979	2.45	2.0
	LiNbO3	2.138	3.84	15.4

Acknowledgments

The authors would like to thank NanoLN for providing the LNOI sample onto which the device was fabricated and acknowledge technical support from the LAAS-CNRS micro and nanotechnologies platform, member of the French RENATECH network of cleanroom facilities.

Funding

ANR Reson

Disclosures

The authors declare no conflicts of interest.


Stéphane Calvez
email: scalvez@laas.fr

Antoine Monmayrant

Oliver Gauthier-Lafaye

Towards high-speed tuning Cavity Resonator-Integrated Guidedmode Resonance Filters

We report the experimental demonstration of tunable Cavity Resonator-Integrated Guided-mode Resonance Filters made on lithium niobate on insulator. Temperatureinduced tuning over a wavelength span greater than the full-width half-maximum of a filter with a Q factor of ~1600 is achieved.

Introduction

Cavity Resonator-Integrated Guided-mode Resonance Filters (CRIGFs) [1] form a family of grating-based filters whose characteristics in terms of spectral selectivity and peak reflectivity and angular tolerance make then attractive for use as wavelengthselective mirrors for extended-cavity diode lasers (ECDLs) [2,3]. Since their behavior is ruled by the excitation of a localized mode, their implementation in conventional SiN technological platforms typically leads to fixed wavelength operation. To overcome this limitation, CRIGFs with spatially-graded structures have been successfully demonstrated [4] and used to introduce broadband tuning of ECDLs [START_REF] Gauthier-Lafaye | Conference on Lasers and Electro-Optics[END_REF]. In this article, we report progress towards tunable CRIGFs with no moving parts exploiting the lithium niobate on insulator platform.

Device design and fabrication

To demonstrate resonance electronic tuning in CRIGFs, we selected the lithium niobate on insulator technology as this material platform is compatible with the fabrication of low-loss integrated optics [START_REF] Boes | [END_REF] and provides various means to electronically modify the material refractive index, namely using either thermal, electrooptic, piezo-electric or acousto-optic effects. The CRIGFs were designed for operation at a wavelength of 1550nm using a combination of rigorous coupled-wave analysis [7] and coupled-mode theory [8]. The devices under study are made on an X-cut LiNbO3 substrate and rely on a SiO2/72 nm Si3N4/297nm LiNbO3/2µm SiO2 planar waveguide. 50-µm-wide structures consisting of an 11-period 865-nm-pitch central grating coupler embedded between two 400-period 432.5-nm-pitch Distributed Bragg Reflectors were fabricated using a combination of nano-imprint lithography and dry-etching. The rear surface of the sample is covered by a 266-nm-thick SiO2 single-layer anti-reflective coating to avoid back-reflection from this interface.

Device characterization

The device spectral characteristics are measured using a fibre-coupled tunable laser around 1550 nm whose output is relayed using free-space optics to form a 5.3-µmwaist beam and on the sample. The transmission and reflectivity responses are respectively calibrated against sample-free transmission and the reflection from a silver mirror. For a substrate temperature of 20°C, a resonant feature is observed at ~1554.4 nm with a 0.96-nm linewidth and an on-resonance transmission factor and peak reflectivity of 32±2%. As shown in Fig. 1 left, when varying the sample substrate temperature, a resonance shift of 0.0382±0.0008nm/K was observed. This value is in good agreement with the rate of 0.0364nm/K derived numerically (RCWA) using the experimentally-inferred material coefficients of references [9,10]. Furthermore, as shown in Fig. 1. right, should 70-µm-separated electrodes be added to the current set of devices, an electro-optic tuning rate of 0.002nm/V is predicted.

Conclusions

We presented the design, fabrication and characterization of CRIGFs made using lithium niobate on insulator technology. Filters exhibiting Q-factors of ~1600 at a wavelength of ~1550nm and a resonance shift of ~2.6 nm upon a rise in temperature of 60°C are reported. Investigations of electro-optically-tuned devices is under way and the associated results will be shown at the meeting.

Fig. 1 :

 1 Fig. 1: left: Temperature-dependent evolution of the CRIGF normalized reflectivity right: Predicted tuning for electro-optically actuated CRIGFs based on the current design.