Esrom Kifle

Pavel Loiko

Javier Rodríguez Vázquez De Aldana

Carolina Romero

Víctor Llamas

Josep Maria Serres

Magdalena Aguiló

Francesc Díaz

Lizhen Zhang

Zhoubin Lin

Javier Rodríguez Vázquez

D E Aldana

Haifeng Lin

G E Zhang

Viktor Zakharov

Andrey Veniaminov

Valentin Petrov

Uwe Griebner

Xavier Mateos
email: xavier.mateos@urv.cat

L I Wang

And Weidong Chen

Low-loss fs-laser-written surface waveguide lasers at >2 µm in monoclinic Tm 3+ :MgWO 4

teaching and research institutions in France or abroad, or from public or private research centers.

Magnesium monotungstate (MgWO4) has been recognized recently as an excellent host material for doping with trivalent thulium ions (Tm 3+ ) [1,2]. It belongs to the monoclinic crystal class (sp. gr. C 4 2h -P2/c, wolframite, [Fe,Mn]WO4, type structure) [3] and shows a single crystallographic site for Tm 3+ ions (the Mg 2+ one, symmetry: C2). Charge compensation for the heterovalent doping is provided by univalent alkali-metal cations (e.g., Na + ) in the flux [4]. MgWO4 exhibits good thermo-mechanical properties, such as high thermal conductivity <κ> = 8.7 Wm -1 K -1 [5] and low anisotropy of the thermal expansion, αa = 11.22, αb = 8.09 and αc = 8.77 [10 -6 K -1 ] [4], leading to promising power scaling capabilities.

For Tm 3+ ions (electronic configuration: [Xe]4f 12 ), MgWO4 shows attractive spectroscopic properties, i.e., intense and broad emission bands for the 3 F4 → 3 H6 transition with polarized light, and large Stark splitting of the ground-state, ΔE( 3 H6) = 633 cm -1 [6]. Thus, the broad polarized emission bands of Tm 3+ :MgWO4 extend well above 2 μm avoiding the structured water vapor atmospheric absorption. The latter is a key issue for the generation of femtosecond pulses in mode-locked (ML) lasers in the 2 μm spectral range [7]. It was recently confirmed by a graphene ML Tm 3+ :MgWO4 laser which generated 86 fs pulses at a wavelength of 2017 nm (bandwidth: 53 nm) at a repetition rate of 87 MHz [2]. Furthermore, continuous wavelength tuning was achieved from 1897 to 2062 nm [2]. As MgWO4 is an efficient Raman-active material, laser emission at even longer wavelengths (up to 2093 nm) was realized due to the electron-phonon coupling [6]. A continuous-wave (CW) Tm 3+ :MgWO4 laser generated 3.09 W at 2022-2034 nm with a slope efficiency as high as 50% [1].

Due to its attractive thermal and spectroscopic properties, Tm 3+ :MgWO4 is also promising for ML waveguide (WG) lasers above 2 μm. While Tm 3+ -doped monoclinic double tungstate (MDT) bulk crystals have been widely explored to this aim [8], WG laser operation has never been demonstrated with monoclinic divalent-metal monotungstates. In the present work, we demonstrate the first Tm 3+ :MgWO4 WG laser featuring low passive losses and high laser efficiency, as a first step towards highrepetition-rate (GHz-range) ML WG lasers [9].

As a fabrication method, we selected femtosecond Direct Laser Writing (fs-DLW). This method is based on focusing a fs laser beam inside a small volume (few μm 3 ) within a transparent material resulting in partial amorphization of the damaged area leading to a permanent refractive index variation (Δn > 0 or Δn < 0) [10]. The advantages of fs-DLW are: short interaction time and high precision, single step procedure, suitability for various materials incl. anisotropic crystals, variable WG geometries, moderate to low propagation losses. Efficient fs-DLW thulium WG lasers have been demonstrated [11,12]. Lancaster et al. reported on a buried channel WG laser inscribed in a Tm 3+ :ZBLAN glass delivering 205 mW at ~1.89 μm with a slope efficiency η of 67% and low passive losses δloss of 0.4±0.2 dB/cm [11]. Kifle et al. demonstrated a surface WG laser in a Tm 3+ :KLu(WO4)2 crystal generating 171 mW at ~1.85 μm with η = 38% and δloss = 0.7±0.3 dB/cm [12].

The Tm 3+ :MgWO4 crystal was grown by the Top-Seeded Solution Growth (TSSG) method using Na2WO4 as a solvent [13].

The Tm 3+ doping level was 0.89 at.% (NTm = 1.41×10 20 cm -3 ). A rectangular sample with dimensions 1.86(Ng)×3.96(Nm)×3.05(Np) mm 3 was cut and polished along the optical indicatrix frame of this biaxial crystal [6]. The DLW was performed using 120-fs pulses with a central wavelength of 795 nm from a Ti:Sapphire regenerative amplifier at a repetition rate of 1 kHz. A small portion of the pulse energy (86 nJ, measured after the focusing optics) was employed with polarization E || Nm. The fs radiation was focused into the crystal along the Ng-axis through the top surface using a 40× microscope objective (N.A. = 0.65). The sample was translated at a speed of 400 μm/s along the Np-axis producing damage tracks. Surface channel WGs with a half-ring depressed-index cladding (Δn > 0, classified as type III microstructures [10]) were produced.

Their geometry was studied using a confocal scanning laser microscope LSM 710 (Carl Zeiss) equipped with a polarizer (P), an analyzer (A), a blue GaN laser (405 nm), and a 20× objective (N.A. = 0.75), Fig. 1(a). The spatial resolution was 0.24 μm. The diameter of the cladding was 40, 50 or 60 μm. The inner part of the guides appeared to be darker than the surrounding bulk area due to light scattering at the curved cladding (the waveguiding conditions are not satisfied at high N.A. of the objective). The axis of the WG was located at 20-30 μm beneath the top surface. Except of the damage tracks no cracks in the writing areas nor in the bulk volume were observed. The damage tracks passed continuously through the whole length of the 3.05 mm long sample (along the Np-axis), Raman spectroscopy is a sensitive tool to study the structure alteration at the μ-scale [14]. The polarized Raman spectrum of the Tm 3+ :MgWO4 crystal for the p(mm)𝑝 ̅ geometry (Porto's notations) is shown in Fig. 2(a). The most intense Raman band denoted as ν1( A1g) is observed at ~916 cm -1 and assigned to symmetric stretching W -O vibrations in the [WO6] octahedra [13]. The peak intensity, width (FWHM) and position were monitored over the sample end-facet resulting in μ-Raman maps, Fig. 2(c-d). In the damaged regions, a drop of intensity, broadening of the band and its shift to longer frequencies are observed indicating a reduction of crystallinity (a partial amorphization). The crystalline quality of the core is well preserved as manifested by the almost unchanged Raman response with respect to the bulk regions.

In Tm 3+ :MgWO4, the maximum absorption cross-section, σabs, for the 3 H6 → 3 H4 pump transition is 1.43×10 -20 cm 2 at 800.3 nm, Fig. 3(a). The peak stimulated-emission (SE) cross-section, σSE, at wavelengths >2 μm where the laser operation is expected, is 0.85×10 -20 cm 2 at 2015 nm, Fig. 3(b). Both values are specified for E || Nm. The upper laser level ( 3 F4) lifetime is 1.93 ms [13].

The laser cavity comprised a flat pump mirror coated for high transmission, HT (T = 96%) at 0.80 μm and for high reflection, HR at 1.80-2.06 μm, and a set of flat output couplers (OCs) having a transmission TOC = 1.5-30% at the laser wavelength. We also used a special bandpass OC coated for HT at <2 μm and providing TOC = 1.6% at >2.05 μm. All the OCs provided HR at ~0.80 μm, so the WG was pumped in a double-pass. The crystal was mounted on a passively cooled BK7 glass block and placed between the cavity mirrors without index-matching among them. The mirrors were gently pressed towards the WG endfaces resulting in a geometrical cavity length of ~3.1 mm. The pump power from a CW Ti:Sapphire laser was up to ~2 W at 800.3 nm (M 2 ≈ 1). The pump, polarized with E || Nm (in the crystal), was focused using a 10× microscope objective (N.A. = 0.28, f = 20 mm, T = 75% at 0.80 μm) to a spot with a diameter of 25±5 μm. The pump coupling efficiency, ηcoupl, was estimated to be 88.4±1% from pump-transmission measurements at 830 nm. The pump is confined in the slightly D-shaped cladding profile; no leakage into the bulk was detected, Fig. 4(a). The single-pass pump absorption under non-lasing conditions, ηabs, NL, determined at 800.3 nm by the end-fire method, gradually decreased with the incident pump power Pinc due to the ground-state bleaching. This effect was enhanced for smaller cladding diameters. The absorbed pump power Pabs was determined from ηabs, NL at the threshold power for each OC and accounting for double-pass pumping.

The input-output characteristics for the 60 μm WG are shown in Fig. 5(a). Exceptionally for the best-performing TOC = 3%, we removed the optical elements placed to attenuate Pinc resulting in higher pump power. The laser generated 320 mW at ~2015 and 2036 nm with a slope efficiency η of 64.4% (vs. Pabs). The laser threshold was at Pabs = 126 mW and the optical-to-optical efficiency at the maximum pump level was 24.7% (vs. Pinc). The total pump absorption (in two passes) was 48.1%. The laser slope efficiency slightly increased with the output coupling, reaching η = 69.3% for TOC = 20%. The laser threshold decreased for smaller TOC, from 186 mW (30% OC) down to 117 mW (1.5% OC). No thermal roll-over in the output dependences was detected at least until Pabs = 0.6 W; the power scaling was limited by the available pump. The laser operated for tens of minutes without a significant drop in the output power.

The polarization of the laser emission was linear (E || Nm), naturally selected by the strong anisotropy of the gain in Tm 3+ :MgWO4. The laser spectra are shown in Fig. 5(b). For small TOC < 9%, the emission occurred at ~2.02 and 2.04 μm. For higher output coupling (9-30%), only the short-wavelength emission was supported due to the quasi-three-level nature of the 3 F4 → 3 H6 Tm 3+ laser scheme, i.e., influenced by reabsorption. The passive losses in the 60 μm WG were estimated using the Caird analysis [15], Fig. 5(c). The resulting internal slope efficiency was η0 = 70.6±0.4% and δloss = 0.04+0.1 dB/cm. This value is well below of those reported previously for fs-DLW WGs in Tm 3+ :ZBLAN glass (0.4±0.2 dB/cm) [11]. Moreover, the passive loss is comparable with the best reports on surface channel (ridge) WGs in Tm 3+ :MDT single-crystalline films fabricated by Liquid Phase Epitaxy (LPE) and Ar + ion beam milling (<0.11 dB/cm) [16].

We intentionally studied the laser performance employing the bandpass OC, Fig. 5(a,b, dark green). The laser exhibited a low laser threshold of 120 mW and generated 132 mW at ~2080 nm with η = 38.9%. This wavelength is well beyond the limit for the purely electronic 3 F4 → 3 H6 Tm 3+ transition in MgWO4 (2017 nm, between the Stark sub-levels with the energies 5591 cm -1 ( 3 F4) and 633 cm -1 ( 3 H6) [6]), i.e., the first demonstration of a vibronic WG laser. It is assigned to the electron-phonon coupling between the electronic transitions of Tm 3+ and the low-frequency Raman modes of the host [17], Fig. 2(a), such as 121 (Ag), 134 or 154 (both Bg) cm -1 .

The performance of the surface WG lasers with different size of the cladding (40-60 μm) is compared in Fig. 5(d). The slope efficiency was similar for all the studied guides while the laser threshold clearly decreased for smaller cladding diameter: it amounted to 108 mW (50 μm WG) and 82 mW (40 μm WG), both for TOC = 3%. The laser polarization for all the WGs was E || Nm.

All the studied WG lasers operated in the fundamental transverse mode. The latter might benefit from additional mode "filtering" provided by the reabsorption (gain-guiding) [18]. An example of the mode for the 40 μm WG is shown in Fig. 6 To conclude, the monoclinic crystal Tm 3+ :MgWO4 is promising for highly-efficient low-loss (<0.1 dB/cm) fs-DLW waveguide lasers at wavelength above 2 µm, due to its attractive spectroscopic, vibronic and thermal properties. In the present work, we achieved the highest slope efficiency (69.3%) and output power (320 mW) for any thulium waveguide laser fabricated by femtosecond direct laser writing. Furthermore, the first "vibronic" Tm waveguide laser with the longest emission wavelength of 2080 nm for Tm-doped WGs is also demonstrated. The designed photonic micro-structures with a size in the few millimeter range are highly attractive for the generation of ultrashort pulses in compact ML lasers operating at high repetition rates (GHz-range), as well as for surface activation, e.g., leading to pulsed operation via evanescent-field coupling.

Funding. Spanish Government (MAT2016-75716-C2-1-R (AEI/FEDER,UE), FIS2017-87970-R); Generalitat de Catalunya (2017SGR755); Junta de Castilla y León (SA287P18); National Natural Science Foundation of China (61975208, 51761135115, 61850410533, 61575199, 61875199); Ministry of Science and Higher Education of the Russian Federation (Goszadanie 2019-1080); Deutsche Forschungsgemeinschaft (PE 607/14-1).

Fig. 1 .

 1 Fig. 1. Confocal microscopy study of fs-DLW surface channel WGs in Tm 3+ :MgWO4: (a) end-facet view, transmission mode, polarized light (P || Ng); (b,c) top surface view, transmission mode: (b) a general view in polarized light (P || Np); (c) a close look on the 60 μm WG in crossed polarizers (P || Np, A || Nm); (d) the same area as in (c) in reflection mode. λ = 405 nm. P -polarizer, A -analyzer.

Fig. 1 (

 1 b). The study of the sample in crossed polarizers (P and A) revealed bright areas localized at the damage tracks indicating a local alteration of the size and orientation of the optical indicatrix, Fig. 1(c). The damage tracks reached the top surface, Fig. 1(d), producing a shallow ablation groove at the external tracks.

Fig. 2 .

 2 Fig. 2. Micro-Raman mapping of the fs-DLW surface channel WG (size: 50 μm) in Tm 3+ :MgWO4: (a) polarized Raman spectrum for the p(mm)𝑝̅ geometry, numbers denote the Raman frequencies in cm -1 , λexc = 514 nm; (b-d) maps of the crystal end-facet representing the variation of (b) peak Raman intensity, (c) peak frequency and (d) peak width (FWHM) for the ~916 cm -1 Raman band.

Fig. 3 .

 3 Fig. 3. Spectroscopy of Tm 3+ ions in monoclinic MgWO4: (a) absorption cross-section, σabs, for the 3 H6 → 3 H4 transition, and (b) stimulatedemission (SE) cross-section, σSE, for the 3 F4 → 3 H6 transition. The light polarization is E || Nm. The vertical arrows in (a),(b) indicate the pump and emission wavelengths for the waveguide laser, respectively.

Fig. 4 .

 4 Fig. 4. (a-c) Pump modes at the output facet of the fs-DLW WGs in Tm 3+ :MgWO4: the WG cladding diameters are (a) 40 μm, (b) 50 μm and (c) 60 μm. White dashed lines indicate the WG cladding and the air/crystal interfaces. The pump polarization (E || Nm) is horizontal; (d) measured single-pass pump absorption in the WGs under non-lasing conditions, ηabs, NL: symbols -experimental data, curve -simulation with a rate-equation model. The pump wavelength is 800.3 nm.

Fig. 5 .

 5 Fig. 5. (a-c) Fs-DLW Tm 3+ :MgWO4 surface channel WG (60 μm) laser: (a) input-output dependences, η -slope efficiency; (b) laser emission spectra measured at the maximum Pabs; (c) Caird analysis: the plot of inverse of the slope efficiency, 1/η, vs. inverse of the output-coupling loss, 1/γOC, γOC = -ln(1 -TOC); (d) Comparison of the output performance of WGs with cladding diameters of 40 -60 μm: η -slope efficiency, TOC = 3%. The laser polarization is E || Nm.

  (a). It is well confined within the WG cladding. The 1D intensity profiles, Fig. 6(b), are well fitted to a Gaussian distribution (goodness of the fit: R 2 > 0.99) yielding mode diameters 2wL (|| Ng) = 27.2 μm and 2wL (|| Nm) = 24.5 μm. The mode ellipticity is weak (e = 1.11).The numerical aperture (N.A.) of the guides was estimated by measuring the divergence of the laser beam θ (half-angle) as N.A. = sinθ. It was similar along the vertical and horizontal directions, N.A. = 0.082 and 0.087±0.005, respectively. Within the approximation of a set-index guide, N.A. 2 ≈ 2ncoreΔn, one obtains Δn = ncladding -ncore of about -1.7×10 -3 (here, ncore ~2.03 for E || Nm[6]). A similar value was reported for Tm 3+ :ZBLAN glass (Δn ~ -1.5×10 -3 )[11].

Fig. 6 .

 6 Fig. 6. (a) Spatial near-field laser mode profile of the 40 μm WG fabricated by fs-DLW in Tm 3+ :MgWO4. Pabs = 0.4 W, TOC = 9%. The laser polarization is horizontal (E || Nm). White lines indicate the WG cladding and the air / crystal interfaces; they are drawn as guides for the eye; (b) 1D intensity plots of the intensity profile: symbols -experimental points, curves -their Gaussian fits.

Acknowledgments.

Víctor Llamas is a fellow of Eurecat's "Vicente López" PhD grant program.

Disclosures. The authors declare no conflicts of interest.


Gurvan Brasse

Pavel Loiko

Clara Grygiel

Abdelmjid Benayad

Franck Lemarie

Viktor Zakharov

Andrey Veniaminov

Jean-Louis Doualan

Alain Braud

Patrice Camy
email: patrice.camy@ensicaen.fr

Liquid Phase Epitaxy growth, structure and spectroscopy of highlydoped 20 at.% Yb 3+ :LiYF 4 thin films

Keywords: Liquid phase epitaxy, lithium yttrium fluoride, thin films, ytterbium ions, luminescence

Highly doped (20 at.%) Yb 3+ :LiYF4 single-crystalline thin films are grown on (001)-oriented bulk undoped LiYF4 substrates by Liquid Phase Epitaxy using lithium fluoride (LiF) as a solvent. The growth temperature lies around 741 °C (0.5-1 °C supercooling) and the growth rate is 1.8-2.6 μm/min. The singlecrystalline nature of the films is confirmed by X-ray diffraction and polarized Raman spectroscopy. The film morphology is studied and discussed. The polarized spectroscopic properties of Yb 3+ ions are reported, indicating a stimulated-emission cross-section of 0.88×10 -20 cm 2 at 993.9 nm in π-polarization and a radiation trapping free lifetime of the 2 F5/2 state of 2.00 ms indicating weak concentrationquenching. The crystal-field splitting of Yb 3+ multiplets is resolved at 12 K. Highlydoped Yb 3+ :LiYF4 / LiYF4 homoepitaxies are promising for waveguide and thindisk lasers at ~1 μm.

Introduction

Lithium (rare earth) fluoride crystals with a general chemical formula of LiLnF4 (where Ln = Gd, Y or Lu is the host-forming cation) doped with trivalent laser-active rare-earth ions (RE 3+ ) are known for efficient laser emission in the near-infrared [START_REF] Loiko | Comparative spectroscopic and thermo-optic study of Tm:LiLnF4 (Ln = Y, Gd, and Lu) crystals for highly-efficient microchip lasers at ~2 μm[END_REF]. The LiLnF4 crystals belong to the tetragonal crystal class (sp. gr. I41/a) and they have a scheelite-type ordered structure [START_REF] Garcia | Structure of the laser host material LiYF4[END_REF].

As host matrices, these crystals possess several advantages. First, they provide good thermo-mechanical properties such as high thermal conductivity (e.g., for Yb:LiYF4, κa = 4.1 W/mK and κc = 5.2 W/mK [START_REF] Aggarwal | Measurement of thermo-optic properties of Y3Al5O12, Lu3Al5O12, YAIO3, LiYF4, LiLuF4, BaY2F8[END_REF]), weak anisotropy of thermal expansion and negative thermooptic coefficients dn/dT leading to weak negative thermal lensing [START_REF] Hardman | Energy-transfer upconversion and thermal lensing in high-power end-pumped Nd:YLF laser crystals[END_REF]. Second, they have attractive optical properties such as broad transparency range, low refractive indices and natural birefringence eliminating the depolarization losses (e.g., for LiYF4, no = 1.449 and ne = 1.471 at ~1 μm [START_REF] Barnes | Temperature variation of the refractive indices of yttrium lithium fluoride[END_REF]). Third, the low maximum phonon energies (hνph = 446 cm -1 for LiYF4) [START_REF] Miller | Lattice vibrations of LiYF4[END_REF] reduce the probability of non-radiative (NR) relaxation for the excited-states of the RE 3+ ions [START_REF] Walsh | Spectroscopy and modeling of solid state lanthanide lasers: Application to trivalent Tm 3+ and Ho 3+ in YLiF4 and LuLiF4[END_REF]. Finally, for the RE 3+ doping, the LiLnF4 crystals offer a single substitutional rare-earth site and high available doping levels [START_REF] Loiko | Comparative spectroscopic and thermo-optic study of Tm:LiLnF4 (Ln = Y, Gd, and Lu) crystals for highly-efficient microchip lasers at ~2 μm[END_REF][START_REF] Vannini | High efficiency room temperature laser emission in heavily doped Yb:YLF[END_REF]. The Czochralski growth of bulk LiLnF4 crystals is also well developed [START_REF] Cockayne | The czochralski growth and laser characteristics of Li(Y,Er,Tm,Ho)F4 and Li(Lu,Er,Tm,Ho)F4 scheelite single crystals[END_REF].

The early studies focused mainly on Nd 3+ doping of LiYF4 crystals [START_REF] Clarkson | High-power diode-bar end-pumped Nd:YLF laser at 1.053 µm[END_REF]. Ytterbium (Yb 3+ ) ions, as compared to the Nd 3+ ones, have greater potential for laser emission at ~1 μm. Yb 3+ (electronic configuration: [Xe]4f 13 ) has a simple energy-level scheme eliminating unwanted energy-loss processes such as excited-state absorption or energy-transfer upconversion. The inband pumping scheme results in high pump Stokes efficiency and reduced heat loading. The Yb 3+ ions can be easily pumped by commercial InGaAs laser diodes emitting at ~0.96 μm [START_REF] Coluccelli | Room-temperature diode-pumped Yb 3+ -doped LiYF4 and KYF4 lasers[END_REF].

Diode-pumped bulk Yb:LiYF4 [START_REF] Coluccelli | Room-temperature diode-pumped Yb 3+ -doped LiYF4 and KYF4 lasers[END_REF][START_REF] Kawanaka | Tunable continuous-wave Yb:YLF laser operation with a diode-pumped chirped-pulse amplification system[END_REF] and Yb:LiLuF4 [START_REF] Yasukevich | Continuous wave diode pumped Yb:LLF and Yb:NYF lasers[END_REF] lasers were reported. The possibility of laser operation with heavily Yb 3+ doped crystals was demonstrated: a roomtemperature 30 at.% Yb 3+ -doped LiYF4 laser generated a maximum peak power of 4.0 W at 1059 nm with a slope efficiency of 62.8% [START_REF] Vannini | High efficiency room temperature laser emission in heavily doped Yb:YLF[END_REF]. Mode-locking of Yb:LiLnF4 oscillators was also realized [START_REF] Coluccelli | Diode-pumped passively mode-locked Yb:YLF laser[END_REF][START_REF] Lin | Crystal growth, spectroscopic characterization, and sub-100 femtosecond mode-locked operation of a Yb:LiLuF 4 laser[END_REF][START_REF] Pirzio | 87 fs pulse generation in a diode-pumped semiconductor saturable absorber mirror mode-locked Yb:YLF laser[END_REF]. Pirzio et al. reported on a SESAM mode-locked Yb:LiYF4 laser delivering 87 fs pulses at ~1052 nm at a repetition rate of 100 MHz [START_REF] Pirzio | 87 fs pulse generation in a diode-pumped semiconductor saturable absorber mirror mode-locked Yb:YLF laser[END_REF]. A great improvement of the diodepumped laser performance was achieved under cryogenic temperatures [START_REF] Zapata | Power scaling of cryogenic Yb:LiYF 4 lasers[END_REF]. Zapata et al. demonstrated a cryogenically cooled (80 K) diode-pumped Yb:LiYF4 laser delivering an output power of 224 W at 995 nm with a slope efficiency of 68% and extremely low quantum defect [START_REF] Zapata | Power scaling of cryogenic Yb:LiYF 4 lasers[END_REF]. These crystals were also considered for radiation cooling [START_REF] Seletskiy | Local laser cooling of Yb:YLF to 110 K[END_REF].

Besides bulk laser applications, the LiYF4 crystals are well suited for fabrication of RE 3+doped single-crystalline thin films by Liquid Phase Epitaxy (LPE) [START_REF] Rogin | Liquid phase epitaxy of LiYF 4[END_REF]. Such active structures are of interest for waveguide lasers [START_REF] Starecki | Rare earth doped LiYF 4 single crystalline films grown by liquid phase epitaxy for the fabrication of planar waveguide lasers[END_REF]. The growth, spectroscopy and efficient waveguide laser operation of LiYF4-based epitaxies doped with Pr 3+ [START_REF] Starecki | Red and orange Pr 3+ :LiYF 4 planar waveguide laser[END_REF], Yb 3+ [START_REF] Bolaños | 2.8 W end-pumped Yb 3+ :LiYF 4 waveguide laser[END_REF], Tm 3+ [START_REF] Loiko | Watt-level Tm:LiYF 4 channel waveguide laser produced by diamond saw dicing[END_REF], Ho 3+ [START_REF] Loiko | Ho:LiYF 4 planar waveguide laser at 2.05 μm[END_REF] and Nd 3+ [START_REF] Douysset-Bloch | Growth by liquid phase epitaxy and characterization of Nd:YLiF 4 layers[END_REF] ions were reported. In particular, a 4 at.% Yb, 5 at.% Gd:LiYF4 / LiYF4 LPEbased waveguide laser generated 2.8 W at 1020 nm with a slope efficiency of 43% [START_REF] Bolaños | 2.8 W end-pumped Yb 3+ :LiYF 4 waveguide laser[END_REF]. Note that the Gd 3+ ions were added for the enhancement of the refractive index contrast between the substrate and the film. RE 3+ -doped LiYF4 thin films were also fabricated by Pulsed Laser Deposition (PLD) [START_REF] Garapon | Nd 3+ -doped LiYF 4 thin films prepared by pulsed laser deposition[END_REF][START_REF] Barsanti | Nd 3+ -doped fluoride film grown on LiYF 4 substrate by pulsed laser deposition[END_REF] and Matrix-Assisted Pulsed Laser Evaporation (MAPLE) [START_REF] Barsanti | Optical response and surface morphology of crystalline Nd 3+ -doped fluoride films grown on monocrystalline LiYF 4 substrates by pulsed laser deposition[END_REF].

Another possible application of LPE-grown crystalline thin films is their use in thin-disk lasers instead of the traditional technology which requires handling with thin (few hundreds of μm) laser elements. This would require either high transition cross-sections for the RE 3+ ions or high RE 3+ doping levels. The latter condition can be easily satisfied in Yb 3+ -doped LiYF4. So far, efficient thin-disk lasers based on epitaxies doped with Yb 3+ [START_REF] Rivier | Thin-disk Yb:KLu(WO 4 ) 2 laser with single-pass pumping[END_REF], Tm 3+ [START_REF] Vatnik | Efficient thin-disk Tm-laser operation based on Tm:KLu(WO 4 ) 2 /KLu(WO 4 ) 2 epitaxies[END_REF] and Ho 3+ [START_REF] Mateos | Holmium thin-disk laser based on Ho:KY(WO 4 ) 2 /KY(WO 4 ) 2 epitaxy with 60% slope efficiency and simplified pump geometry[END_REF] ions were reported. Very recently, we achieved laser operation in highly Tm 3+ -doped (20 at.%) LiYF4 thin films [START_REF] Soulard | Laser operation of highly-doped Tm:LiYF 4 epitaxies: towards thin-disk lasers[END_REF].

In the present work, we report for the first time on the LPE growth, structural and morphological characterization and detailed optical spectroscopy of highly-doped (20 at.%) single-crystalline LiY1-xYbxF4 thin films on oriented bulk LiYF4 substrates intended for the potential use in waveguide and thin-disk lasers.

Elaboration of single-crystalline films

LPE is very suitable for the growth of thick single-crystalline films onto bulk substrates of the same nature (this process is called homoepitaxy). It allows the growth of high optical grade epitaxial layers with a thickness lying between few μm and few hundreds of μm. In the present work, the experimental process to get such layers consisted of the following steps: (i) the growth of undoped LiYF4 crystals by the Czochralski (Cz) method, (ii) the orientation of the crystals and fabrication of the required oriented substrates for the epitaxial growth, (iii) the epitaxial growth of Yb 3+ -doped LiYF4 films onto the oriented substrates and (iv) preparation / shaping / polishing of the epitaxy to achieve the required dimensions and optical quality regarding the potential laser application.

Cz growth of undoped LiYF4 single-crystals

High optical quality undoped LiYF4 single crystals were grown by the Cz method. As raw materials, we used LiF (Alfa-Aesar, optical grade 99.5%) and Y2O3 (Alfa Aesar, 4N-REO) following the molar ratio of 52% LiF -48% YF3, according to the related phase diagram of the LiF-YF3 binary system [START_REF] Thoma | Phase equilibria in the system LiF-YF 3[END_REF], Fig. 1. The oxide precursor Y2O3 was fluorinated using a large excess of ammonium hydrogen bifluoride (NH4HF2) solution under heating at 250 °C, following the chemical equation:

Y2O3 + 6 NH4HF2 → 2 YF3 + 6 NH4F + 3 H2O (T = 250 °C).
The dry cake obtained in this way needs to be annealed at 650 °C under a controlled argon atmosphere to remove the residual NH4F and moisture. After synthesizing the required fluoride precursor YF3, the Cz growth of bulk LiYF4 was performed under an oxygen-free Ar atmosphere using a [001]-oriented seed of undoped LiYF4. The pulling rate was fixed at 1 mm/h. An example photograph of a LiYF4 boule is shown in Fig. 2(a).

The substrates were oriented along the (001) crystallographic plane because of the following reasons: (i) the growth direction of undoped LiYF4 crystal was along the [001] axis and, consequently, it was easier to prepare the substrates with better uniformity and optical quality in the orthogonal plane; (ii) due to isotropic thermo-mechanical properties of LiYF4 in the a-b plane, it is easier to perform LPE.

Typically, substrates with transverse dimensions of 30×11 mm 2 and a thickness of 3.0 mm were used for the LPE process, so that bulk crystals grown by the Cz technique were around 60 mm in height and 30-40 mm in diameter. The substrates were finely polished on both faces and a special attention was paid concerning their flatness (measure to be λ/8) and roughness of the polished surfaces (measured to be in the range of few nm).

Epitaxial growth of Yb:LiYF4 single-crystalline layers

The schematic of the LPE setup is described in detail in [START_REF] Starecki | Rare earth doped LiYF 4 single crystalline films grown by liquid phase epitaxy for the fabrication of planar waveguide lasers[END_REF][START_REF] Brasse | Liquid Phase Epitaxy growth of Tm 3+ -doped CaF 2 thin-films based on LiF solvent[END_REF]. It can be divided into two main areas: the upper and bottom zones which are isolated from each other by an electrovalve. The upper area is an exchange airlock that allows for changing the samples, substrates and stirrer, while keeping the furnace in the proper temperature regime without contaminating the melt in the crucible located in the growth chamber. The glassy carbon crucible containing raw materials is installed in one-end-sealed silica tube which constitutes the growth chamber and is located in the heating zone of a tubular furnace where the thermal gradient is the lowest. The furnace has a dual heating zone that ensures a homogeneous temperature field through the whole height of the melt in the crucible and the temperature is controlled with a precision of 0.1 °C. The growth of fluoride crystalline materials requires oxygen-free atmosphere of high purity.

In the case of Yb:LiYF4 growth, the reagents are LiF (Alfa-Aesar, 99.5%), Y2O3 and Yb2O3 (both Alfa Aesar, 4N-REO). The rare-earth oxides Ln2O3 (Ln = Y, Yb) are fluorinated according to the procedure described in Section 2.1 yielding LnF3 (Ln = Y, Yb) fluorides. The raw materials that compose the molten bath used for the LPE growth are taken according to the molar ratio 27% (YF3 + YbF3) -73% LiF, in the manner to work at lower temperatures and to anticipate the vaporization of the LiF solvent, which will move the equilibrium on the phase diagram. The Yb 3+ doping level is set to 20 at.% with respect to Y 3+ , according to the formula LiY0.80Yb0.20F4.

The fluorinated raw materials are introduced in the LPE growth chamber to be degassed under heating and then they are superheated to ensure good homogenization of the molten bath. An additional mechanical stirring of the LiF -YF3 -YbF3 mixture is also done.

A part of the phase diagram of the LiF-YF3 binary system relevant for the LPE growth experiment is shown in Fig. 3. Our own studies confirm incongruent melting of LiYF4 with a peritectic molar composition about 49% YF3 -51% LiF, in agreement with [START_REF] Rogin | Liquid phase epitaxy of LiYF 4[END_REF][START_REF] Thoma | Phase equilibria in the system LiF-YF 3[END_REF]. For the selected bath composition, 27% (YF3 + YbF3) -73% LiF, the liquidus temperature was determined to be ~744 °C with an uncertainty of about 3-4 °C. This value agrees better with the phase diagram constructed by Thoma et al. [START_REF] Thoma | Phase equilibria in the system LiF-YF 3[END_REF]; the liquidus temperatures determined by Rogin et al. [START_REF] Rogin | Liquid phase epitaxy of LiYF 4[END_REF] seem overestimated. The apparent onset of crystallization (by visual observation) was approximately at 741.5 °C. The transition from dissolution to growth upon cooling was not very sharp (within a temperature range of about 1-1.5 °C).

After homogenizing of the bath, the temperature was slowly decreased just over the apparent onset of crystallization in a supersaturation regime and stabilized. After installing the substrate in the exchange airlock, the epitaxial growth process began by dipping the substrate into the molten bath. The growth temperature was set in the range of 741-740.5 °C (i.e., the apparent supercooling was 0.5-1.0 °C). The substrate was rotated at a speed of 10 rev/min in the molten bath. The dipping duration varied between 20 min and 3 hours depending on the desired layer thickness, as well as on the particular temperature and viscosity of the bath. The mean growth rate estimated at a fixed dipping duration of 30 min amounted to 1.8±0.5 µm/min and 2.6±0.5 µm/min for 0.5 °C and 1 °C supercooling, respectively. This value accounts only for the uniform thickness of the Yb:LiYF4 layer, i.e., when excluding the features of its bumpy surface and polycrystalline solvent residuals (see Section 4.1).

The obtained growth rates at a given supersaturation reasonably agree with the previous work of Rogin et al. where the bath composition of 40% (YF3 + REF3) -60% LiF was studied (i.e., located closer to the peritectic one). Thus, we can conclude that the growth mechanism of the layer is relying on attachment of new building units anywhere at the layer edge (in contrast to the regime of small supercoolings when this can happen only at kink positions). This growth mechanism provides higher growth rates which change almost linearly with increasing the apparent supercooling.

In our work, for obtaining high optical quality Yb:LiYF4 layers, a relatively narrow range of growth temperatures (and, accordingly, apparent supercoolings) was used. Further increase of the supercooling to 3 -5 °C resulted in higher growth rates albeit with lower control of the layer uniformity, thickness and formation of macroscopic defects (cracks) and appearance of polycrystalline material (a mixture of Yb:LiYF4 and LiF).

After removing the substrate from the molten bath, a smooth cooling ramp is applied to avoid crack formation in the grown layer [START_REF] Rogin | LiYF 4 liquid phase epitaxy using an inverted slider geometry[END_REF]. A photograph of the as-grown Yb:LiYF4 / LiYF4 epitaxy is shown in Fig. 2(b). Approximately 2/3 of the substrate contain the grown film which is separated from the non-dipped part of the substrate by the growth meniscus. The film is transparent, but its surface is covered by a white-colored residual crystallized solvent (LiF). It is easily removed by the top polishing of the layer.

Experimental

The surface morphology of the as-grown films was studied employing a confocal and interferometric optical microscope (Sensofar, S-neox) equipped with a blue light-emitting diode (LED, λ = 405 nm) in the reflection mode. The surface roughness was studied in the confocal mode.

The polished end-facet of the epitaxy was studied using a confocal laser microscope (LSM 710, Carl Zeiss) equipped with a polarizer (P), an analyzer (A), and a blue GaN laser (λ = 405 nm) in transmission mode. Two microscope objectives were used (one: 20×, N.A. = 0.75 and another: 50×, N.A. = 0.95).

The structure of the films was studied by single-crystal X-ray diffraction (XRD) using a Bruker AXS D8 Discover diffractometer with a four-circle goniometer, a line focus Cu X-ray source equipped with a Goebel mirror and a Ge (220) double-bounce monochromator providing a parallel and monochromatic (Cu Kα1, λ = 0.15406 nm) incident X-ray beam. A Ge (220) crystal analyzer was used to further limit the scintillator detector acceptance. Symmetric 2θ-θ scans and reciprocal space maps (RSM) obtained by taking a series of 2θ-ω patterns (±0.5° around the reference) at successive ω values (ω = θ ± Δ; ±0.5° around the reference) were recorded. For the RSM representation, the DxTools viewer [START_REF] Boulle | DxTools: processing large data files recorded with the Bruker D8 diffractometer[END_REF] was used.

The composition of the films was studied by Energy Dispersive X-ray (EDX) spectroscopy using a scanning electron microscope (SEM, Jeol 6400). It was equipped with a tungsten filament thermoelectronic gun and a 10 mm² X-microanalysis diode (Oxfordinstrument).

The polarized Raman spectra were measured with a Renishaw inVia confocal Raman microscope equipped with a half-wave (λ/2) plate and a polarizer (P) using a Leica x50 objective (N.A. = 0.75). The excitation wavelength λexc was 488 nm (Ar + laser line).

First, the spectroscopic studies were performed at room temperature (RT, 293 K). The absorption spectra were measured using a spectrophotometer (Lambda 1050, Perkin Elmer) and a Glan-Taylor polarizer. The luminescence spectra were measured using an optical spectrum analyzer (OSA, Ando AQ6315-E) and a Glan-Taylor polarizer. As an excitation source, we used a CW Ti:Sapphire laser tuned to ~0.93 μm. The spectral resolution was 0.5 nm. The luminescence decay was studied using a 1/4 m monochromator (Oriel 77200) and an InGaAs detector. The luminescence was excited by the output of a ns optical parametric oscillator (Horizon, Continuum) tuned to ~0.96 μm. The luminescence decay curve was fitted with a single-exponential law, Ilum(t) = I0exp(-t/τlum).

For low-temperature (LT, 12 K) absorption and luminescence studies, the sample was mounted on an APD DE-202 closed-cycle cryo-cooler equipped with an APD HC 2 Helium vacuum cryo-compressor and a Laceshore 330 temperature controller. A silver paint was used for better thermal contact between the sample and the cryo-cooler. The LT absorption spectrum was measured using a 20 W halogen lamp with a spectrally-calibrated emission and the OSA. The LT luminescence spectrum was measured using the same OSA. The spectra resolution was 0.2 nm.

Results and discussion

Surface morphology

The morphology of the top surface of the as-grown Yb:LiYF4 film was first studied using a confocal optical microscope in the reflection-mode, Fig. 4. This study revealed the presence of two contributions to the surface topography. First, these are "hills" with a transverse size of few tens of μm and a height of few μm separated by "valleys" both belonging to the Yb:LiYF4 film. On top of the "hills", dendritic polycrystalline crystalline structures due to the solvent residuals (LiF) can be observed while the film "valleys" are clean from the solvent. No surface cracks are observed in the Yb:LiYF4 film. Such a morphology is typical for LPE-grown fluoride thin-films [START_REF] Soulard | Laser operation of highly-doped Tm:LiYF 4 epitaxies: towards thin-disk lasers[END_REF][START_REF] Brasse | Liquid Phase Epitaxy growth of Tm 3+ -doped CaF 2 thin-films based on LiF solvent[END_REF]. The bumpy surface of the layer is attributed to two main reasons. First, upon extraction from the solution, a thin liquid film (about few µm) cannot be completely removed. Upon cooling, this film crystallizes leading to an additional increase of the layer thickness and a precipitation of the excess of LiF at the surface forming the dendritic structures. Second, a tendency to follow such a morphology can be assigned to higher viscosity of the flux at lower growth temperatures determined by the selected composition of the solution. Moving the bath composition closer to the peritectic point is expected to reduce the flux viscosity potentially improving the surface smoothness of the epitaxial layer.

It is known that the excess of the melt can be removed from the layer surface by fast spinning in the horizontal dipping geometry of LPE. It was used for the fabrication of fluoride (Yb:CaF2) epitaxial layers [START_REF] Peña | Yb:CaF 2 grown by liquid phase epitaxy[END_REF]. However, vertical dipping keeping a small part of the substrate clean is useful for further polishing of the epitaxy when this part is used as a plane reference. It also prevents the Pt holder from being covered by crystallized solvent (otherwise, it may be difficult to remove the epitaxy without fracturing it).

The surface roughness of the film was further examined, Fig. 5. This plot clearly reveals the presence of "hills" and "valleys", as well as sharp solvent residuals. The root mean square (r.m.s.) surface roughness is about 0.7 μm. For comparison, the thickness of the films used in this study lies in the range of 40-100 μm. Note that the surface roughness of the laser-gradepolished LiYF4 substrates is in the nanometer range.

The substrate / layer interface was further studied by observing the polished end-facet of the epitaxy using a confocal laser microscope in transmission mode (such mode allowed us to avoid the effect of the surface imperfections), Fig. 6. A clean and straight interface is observed. No cracks in the film propagating along its growth direction are visible, see Fig. 6(a). The light intensity for the layer is slightly different owing to the refractive index variation due to the Yb 3+ doping. For the substrate / layer interface observed with crossed polarizers (P || [001], A || [100]), Fig. 6(b), only a weak stress field in the substrate is visible, which means a weak lattice mismatch for the films.

The "hilly" surface morphology of the as-grown epitaxial films does not present a serious limitation for their applications. Typically, the as-grown epitaxies are subjected to top surface polishing to ensure a uniform layer thickness, as well as to control the latter in a precise way. The final (post-polishing) thickness of the layer, as well as the refractive index contrast with respect to the substrate determine the mode guiding properties of the epitaxy (e.g., singletransverse-mode operation). Moreover, the top polishing eliminates the unwanted surface roughness and reduces the propagation losses in the layer. Another approach to produce a relatively smooth surface of the RE 3+ -doped (active) layer is its overgrowth by an undoped cladding layer: during the thermal equilibrium step before the overgrowth, the rough layer surface will be liquified leading to a smooth layer / cladding interface [START_REF] Rogin | Liquid phase epitaxy of LiYF 4[END_REF]. In this case, the surface state of the cladding is not very critical. For waveguide applications, two opposite side facets of the epitaxy should be polished as well.

The processing of epitaxies intended for the use in thin-disk elements is similar, i.e., it involves laser-grade polishing of the layer surface and the opposite substrate face [START_REF] Mateos | Holmium thin-disk laser based on Ho:KY(WO 4 ) 2 /KY(WO 4 ) 2 epitaxy with 60% slope efficiency and simplified pump geometry[END_REF]. Later on, both surfaces are subjected to dielectric coatings and the substrate is soldered to a heat sink. Note that for thin-disk laser applications, the epitaxies do not need to exhibit a refractive index contrast. The fabrication of thin-disk elements based on the LPE technology can be referred as a "bottom-up" approach. The more widespread one (a "top-down" approach) is the fabrication of thin crystal plates from bulk crystals by mechanical processing. It enables fabrication of plates with a thickness ranging between few tens and few hundreds of µm (depending on the mechanical properties of the material). The "top-down" approach is simpler from the technological point of view; however, such thin crystal plates suffer from internal stresses and surface bulging both affecting their laser performance. However, LPE-based thin-disk laser elements have not been commercialized so far.

The fabrication of planar waveguides by direct optical bonding of thinned crystal plates prepared from bulk crystals is also known [START_REF] Shepherd | High-power planar dielectric waveguide lasers[END_REF]. Mackenzie et al. developed single-and doublecladding planar waveguide structures produced by direct bonding of undoped and RE 3+ -doped Y3Al5O12 and sapphire (α-Al2O3) crystal plates [START_REF] Mackenzie | 15 W diode-sidepumped Tm:YAG waveguide laser at 2 µm[END_REF]. Such systems appear promising for power scaling at the expense of tolerated spatial beam quality. However, a direct application of this approach to fluorides will face a problem of lower hardness of the latter (as compared to oxides mentioned above). Handling with LiYF4 crystal plates with a thickness below 100 µm is very complicated and from this point of view, LPE is more reliable and reproducible.

Structural study

The phase purity and the structure of the grown films were confirmed by X-ray diffraction (XRD), Fig. 7(a). In the measured XRD pattern of the film, only the reflections assigned to the tetragonal LiYF4 phase (cf. Fig. 7(b) showing the diffraction peaks according to the Inorganic Crystal Structure Database (ICSD) card No. 81-2254 [START_REF] Garcia | Structure of the laser host material LiYF 4[END_REF]) are observed. The 20 at.% Yb:LiYF4 film has a tetragonal structure (sp. gr. C 6 4h -I41/a, No. 88, centrosymmetric point group 4/m). The films are isostructural to undoped LiYF4 which presents a scheelite (CaWO4) type structure. The orientation of the film well follows that of the bulk undoped LiYF4 substrate, namely, along the [001] axis. Indeed, in the XRD pattern, there are only two intense reflections both corresponding to the (00l) Miller's indices, namely (004) at 2θ ≈ 33.4° and (008) at 2θ ≈ 70.2°. By plotting the measured XRD pattern in a semi-log scale, we make it possible to observe weak parasitic reflections coming from slight mosaicity of the film among which, the peak at 2θ ≈ 38.3° can be unambiguously assigned as (202). However, their intensity is by 3 orders of magnitude weaker than that for the (00l) reflections.

Prior to further discussion, we describe shortly the structure of Yb:LiYF4. For undoped LiYF4, the lattice constants are a = 5.164(1) Å and c = 10.741(2) Å (the number of the formula units in the unit-cell: Z = 4), so that the volume of the unit-cell V is 286.5(1) Å 3 and the theoretical density ρcalc = 3.99 g/cm 3 [START_REF] Garcia | Structure of the laser host material LiYF4[END_REF]. The Li + ion has four and the Y 3+ ion has eight nearestneighbor F -ions. The two Y-F distances are 2.244 Å and 2.297 Å [START_REF] Garcia | Structure of the laser host material LiYF4[END_REF]. The symmetry of the Y 3+ site in the slightly distorted dodecahedron [YF8] is S4 (a slight distortion of D2d for scheelite crystals) [START_REF] Garcia | Structure of the laser host material LiYF4[END_REF][START_REF] Garcia | Structure of the laser host material LiYF 4[END_REF]. The [LiF4] tetrahedra are corner-sharing with the edge-sharing [YF8] dodecahedrons.

When doped with Yb 3+ ions, the latter are expected to replace the Y 3+ ones in the S4 symmetry sites. This process is favored by the closeness of ionic radii of these cations (RYb = 1.12 Å and RY = 1.16 Å for the VIII-fold fluorine coordination [START_REF] Kaminskii | Laser crystals: their physics and properties[END_REF]). Note that the isostructural stoichiometric compound, LiYbF4, exists [START_REF] Kaminskii | Detection of stimulated Raman scattering in the LiYbF 4 crystal[END_REF]. Its lattice constants are a = 5.133 Å and c = 10.587 Å [START_REF] Kaminskii | Detection of stimulated Raman scattering in the LiYbF 4 crystal[END_REF].

A close look on the (008) reflection, Fig. 7(c), reveals a split peak structure, with the narrower component at smaller 2θ angles (related to the substrate) and the broader one at higher 2θ (corresponding to the film). This reflects the decrease of the lattice constants of the film due to the difference in ionic radii of Y 3+ and Yb 3+ .

To determine the lattice constants of the film, we used the following equation for tetragonal crystals:

, 4 sin 2 2 2 2 2 2 2         + + = c l a k h   ( 1
)
where θ is the diffraction angle, (hkl) are the Miller's indices of the reflection, and λ = 0.15406 nm is the wavelength of X-rays. We analyzed the (134), ( 202), ( 004) and (008) reflections which were unambiguously assigned. The resulted values are a = 5.157 Å and c = 10.714 Å. They are smaller than those for the undoped crystal, as expected. The lattice mismatch Δa/asubst = -0.14% and Δc/csubst = -0.25%. The compositions LiYF4 -LiYbF4 form an isostructural series of solid solutions LiY1-xYbxF4, so that a linear evolution of the lattice constants is expected yielding to the following calculated values: acalc = 5.158 Å and ccalc = 10.710 Å, which are in good agreement with the experimental observations. The reciprocal space mapping (RSM) of the (103) node for the Yb:LiYF4 layer is shown in Fig. 8(a) and it leads to the corresponding rocking curve, Fig. 8(b). The (103) direction is not perpendicular to the surface of the substrate. By this way, it takes into account the strain induced by the substrate on the lattice parameter a in the plane of the epitaxial layer. The full width at half maximum (FWHM) of the rocking curve is 0.113°, which reflects very low mosaicity of the crystalline film. In other words, there is a weak local disorientation of crystalline planes regarding the normal to the surface.

A typical EDX spectrum of the raw surface of the Yb:LiYF4 film is shown in Fig. 9. The EDX clearly confirms the presence of Yb 3+ ions in the film, together with the host-forming elements (Y, F). Note that light Li atoms cannot be detected by EDX. A weak signal from O originate from the post-growth hydroxyl and oxygen absorption on the residual solvent, while the Si traces can be explained either by possible weak contamination coming from the LPE silica tube constituting the growth chamber or by a Si pollution of the SEM chamber which is also used for analyzing Si wafers. The traces of C are referred to organic contamination of the sample, especially by the use of acetone for cleaning the sample.

The actual concentration of Yb 3+ ions in the layer with respect to Y 3+ was determined by EDX to be 19.6±1 at.% (NYb = 27.0×10 20 cm -3 ), corresponding to a segregation coefficient KYb = Nlayer/Nsolution = 0.98±0.05, in agreement with our previous studies [START_REF] Bolaños | 2.8 W end-pumped Yb 3+ :LiYF 4 waveguide laser[END_REF][START_REF] Loiko | Watt-level Tm:LiYF 4 channel waveguide laser produced by diamond saw dicing[END_REF]. Close to unity value of KYb is determined by the small difference of ionic radii of Y 3+ and Yb 3+ cations. The layers exhibited a relatively uniform distribution of Yb 3+ ions, with a variation of ~2 at.% along the dipping (vertical) direction.

Raman spectra

The polarized Raman spectra were measured using a polished end-facet of the epitaxy in the a(ij)a configuration (where both i, j = π or σ), according to the Porto's notations [START_REF] Damen | Raman effect in zinc oxide[END_REF]. The studies were performed for both the bulk LiYF4 substrate and the Yb:LiYF4 thin-film. The results are shown in Fig. 10.

LiYF4 crystallizes in the scheelite structure. The scheelite primitive cell containing two LiYF4 formula units has 36 degrees of freedom. At the center of the Brillouin zone Г (k = 0), these vibrational modes are distributed over the irreducible representations of C4h as follows: 3Ag + 5Bg + 5Eg + 5Au + 3Bu + 5Eu [START_REF] Miller | Lattice vibrations of LiYF4[END_REF]. One Au and one Eu modes correspond to rigid translations of the whole crystal, and other Au and Eu modes are IR-active. The gerade (g) modes (5Bg + 5Eg) are Raman-active and the remaining modes (Bu) are silent.

In the Raman spectra of the substrate, Fig. 10(a), the most intense mode at 262 cm -1 is assigned as Ag. The highest energy vibrations are observed at 424 (Bg) and 443 (Eg) cm -1 . These two are related to the Li motions. In LiYF4, the internal binding of the [LiF4] -tetrahedra does not differ too much from other ionic forces in the crystal, so it is difficult to assign the internal and external vibrations [START_REF] Miller | Lattice vibrations of LiYF4[END_REF][START_REF] Salaün | Lattice dynamics of fluoride scheelites: I. Raman and infrared study of LiYF 4 and LiLnF 4 (Ln = Ho, Er, Tm and Yb)[END_REF].

For the layer, the polarized Raman properties are well preserved, Fig. 10(b). There are only minor shifts in the peak positions of the bands (within 1 cm -1 ). This is in agreement with the Raman observations for stoichiometric LiYbF4 [START_REF] Kaminskii | Detection of stimulated Raman scattering in the LiYbF 4 crystal[END_REF][START_REF] Salaün | Lattice dynamics of fluoride scheelites: I. Raman and infrared study of LiYF 4 and LiLnF 4 (Ln = Ho, Er, Tm and Yb)[END_REF]. The differences are revealed by examining the intense bands at 262 and 424 cm -1 . For both bands, the peak Raman intensity for the film decreases and the bands are broadened. Indeed, the FWHM of the 424 cm -1 band increases from 11.3 cm -1 (substrate) to 13.0 cm -1 (film). Thus, we conclude that the anisotropic crystal structure of LiYF4 is well preserved for the film, however, a certain reduction of its crystallinity with respect to the substrate is expected.

Optical spectroscopy

Tetragonal LiYF4 crystals are optically uniaxial (the optical axis is parallel to the crystallographic c-axis) [START_REF] Barnes | Temperature variation of the refractive indices of yttrium lithium fluoride[END_REF]. Thus, there exist two principal light polarizations, denoted as π (E || c) and σ (E ⊥ c) with the corresponding refractive indices ne and no. As ne > no, this crystal is positive uniaxial.

For comparison, we studied a 10 at.% Yb:LiYF4 bulk crystal. The stimulated-emission (SE) cross-sections, σSE, were determined from the measured RT luminescence spectra calibrated for the spectral response of the set-up W'i(λ) using the Füchtbauer-Ladenburg (F-L) equation [START_REF] Aull | Vibronic interactions in Nd:YAG resulting in nonreciprocity of absorption and stimulated emission cross sections[END_REF]:

, d ) ( ' 3 / 1 ) ( ' 8 ) ( , 2 rad 2 5 SE   =   =            j j i i W W c n (2)
where, λ is the light wavelength, <n> is the mean refractive index at the emission wavelength <λlum>, c is the speed of light, τrad is the radiative lifetime of the emitting state 2 F5/2 and i = π or σ is the light polarization. The polarization-averaging is performed as 1/3(2σ + π).

The results are shown in Fig. 11(a). For the LPE-grown film, the maximum σSE is 0.88×10 -20 cm 2 at 993.9 nm in π-polarization. Another intense emission peak is observed at 1018.8 nm. These two peaks are assigned to the 0' → 1, 2 and 0' → 3 transitions, respectively (here, s = 0...3 and p' = 0'...2' indices number the Stark sub-levels of the ground-state and the excited-state, respectively, as each 2S+1 LJ multiplet is split into a total of 2J + 1 sub-levels [START_REF] Bensalah | Growth of Yb 3+ -doped YLiF 4 laser crystal by the Czochralski method, Attempt of Yb 3+ energy level assignment and estimation of the laser potentiality[END_REF]). The total emission bandwidth (FWHM) Δλlum is 41.2 nm (if analyzing the peaks separately, it is ~18 nm for the most intense one). For σ-polarization, the SE cross-sections are lower, namely σSE = 0.42×10 -20 cm 2 at 1018.1 nm.

The SE cross-sections for the film slightly differ from those for the highly-doped bulk crystal, Fig. 11(a). In particular, the emission band for the film is broadened, as expressed by larger Δλlum value (compare with Δλlum = 37.8 nm for π-polarized light for the bulk crystal); the peak σSE value for the film is also slightly reduced. The broadening of the emission band is confirmed by calculation of the polarization-averaged integrated σSE value which is almost the same for both the bulk crystal and the layer. The reduction in intensity and broadening of the emission spectra for the films is in line with the results of Raman spectroscopy.

Previously for heavily doped (30 at.%) Yb:LiYF4 single-crystals, Vannini et al. reported σSE = 0.83 (π) and 0.34(σ) [10 -20 cm 2 ] at ~1018 nm as calculated by the F-L method [START_REF] Vannini | High efficiency room temperature laser emission in heavily doped Yb:YLF[END_REF] which agrees with our observations.

The RT absorption spectrum for the epitaxy was measured for light propagating orthogonal to the polished top-surface (i.e., along the c-axis). Thus, only the σ-polarization was available. The absorption cross-sections were calculated from the measured absorption coefficient, σabs = αabs/NYb, where NYb = 2.75×10 21 cm -3 is the Yb 3+ ion density. For another polarization (π), the absorption cross-sections were calculated using the corresponding σSE spectrum with the reciprocity method (RM) [START_REF] Payne | Infrared cross-section measurements for crystals doped with Er 3+ , Tm 3+ and Ho 3+[END_REF]:

      - - = kT E hc Z Z ZPL 2 1 abs SE / exp ) ( ) (      , ( 3a
) ) / exp( kT E g Z m k k m k m - =  . ( 3b
)
Here, h is the Planck constant, hc/λ is the photon energy (in cm -1 ), k is the Boltzmann constant, T is the crystal temperature (RT), EZPL is the energy difference between the lowest lying Stark sub-levels of the two multiplets (the so-called zero-phonon line (ZPL), 0 → 0'), Zm are the partition functions of the lower (m = 1) and upper (m =2) Yb 3+ multiplets, g m k is the degeneracy of the sub-level with the number k and the energy E m k measured from the lowest sub-level of each multiplet. Such a calculation could suffer from reabsorption losses affecting the measured luminescence spectra. However, this effect is greatly reduced in the studied films with a thickness of few tens of μm.

The σabs spectra are shown in Fig. 11(b). For the Yb:LiYF4 layers, the maximum σabs = 0.67×10 -20 cm 2 at 959.7 nm in π-polarization and the FWHM of the absorption peak is 19.3 nm. For σ-polarization, the absorption cross-section at the same wavelength is lower (σabs = 0.48×10 - 20 cm 2 ). This absorption line is due to the 0 → 1' transition. As compared to those for the singlecrystal, the absorption spectra experience a broadening which is especially evident for πpolarization.

The crystal-field splitting for Yb 3+ multiplets in Yb:LiYF4 epitaxial films was determined from LT absorption and emission spectra, Fig. 12. The spectra were interpreted accounting for the Raman spectra in order to exclude the vibronic sidebands (marked in Fig. 12 by "hv"). The assignment of electronic transitions (between the Stark sub-levels) is according to [START_REF] Bensalah | Growth of Yb 3+ -doped YLiF 4 laser crystal by the Czochralski method, Attempt of Yb 3+ energy level assignment and estimation of the laser potentiality[END_REF][START_REF] Lupei | Electron-phonon coupling effects on Yb 3+ spectra in several laser crystals[END_REF]. The following set of Stark sub-levels was obtained: 2 F7/2 = (0, 216, 245, 482) cm -1 and 2 F5/2 = (10295, 10420, 10554) cm -1 , cf. Fig. 13, yielding the partition functions Z1 = 1.820 and Z2 = 1.739 and the ratio Z1/Z2 = 0.955 (at room temperature). The determined Stark splitting is similar to that reported previously for bulk low-doped 0.5 at.% Yb:LiYF4 crystal, namely 2 F7/2 = (0, 218, 248, 485) cm -1 and 2 F5/2 = (10293, 10416, 10554) cm -1 [START_REF] Bensalah | Growth of Yb 3+ -doped YLiF 4 laser crystal by the Czochralski method, Attempt of Yb 3+ energy level assignment and estimation of the laser potentiality[END_REF].

For Yb 3+ ions, the luminescence at ~1 μm spectrally overlaps with the absorption due to the 2 F7/2 → 2 F5/2 transition. Thus, with increasing the Yb 3+ doping level, the luminescence lifetime τlum measured for bulk samples can be elongated due to reabsorption (also called the radiation trapping effect). To avoid it, several approaches were proposed, such as the pinhole method [START_REF] Kühn | Model for the calculation of radiation trapping and description of the pinhole method[END_REF] and the powder suspension method [START_REF] Kisel | Spectroscopy and continuous-wave diode-pumped laser action of Yb 3+ :YVO 4[END_REF]. The first one is non-destructive and thus it was selected in the present work.

Other possible effects which may influence τlum are the concentration-quenching and the energy-migration to impurities both leading to the shortening of the lifetime. Note that for Yb 3+ ions in LiYF4, the non-radiative relaxation from the emitting state ( 2 F5/2) is negligible.

The RT luminescence decay curve for the 20 at.% Yb:LiYF4 epitaxy is shown in Fig. 14(a), as measured using the smallest pinhole diameter d. It is clearly single-exponential in agreement with a single site for Yb 3+ ions (symmetry: S4, C.N. = VIII). The extrapolation of the linear fit of the dependence of τlum on d, Fig. 14(b), to the limit of d → 0 gives an estimation for the radiation-trapping free lifetime τ0 = 2.00±0.05 ms.

For comparison, we also studied two bulk Yb:LiYF4 crystals doped with 2 at.% and 10 at.% Yb. The corresponding decay curves are shown in Fig. 14(a). For the crystal with 2 at.% Yb, τ0 = 2.26 ms. Its only slightly decreased for the crystal with 10 at.% Yb (2.24 ms). Thus, the concentration-quenching owing to the energy migration for this range of doping concentrations is relatively week. From this considerations, one can assume that τ0 ≈ τrad. The slightly lower τ0 for the film than that for the single-crystal may originate from the varied crystal-field both due to the form of the material (thin crystalline film) and high Yb 3+ doping.

Previously, De Loach et al. determined the luminescence lifetime for Yb:LiYF4 crystals with a low doping concentration (the so-called intrinsic one) as τlum,0 = 2.16 ms [START_REF] Deloach | Evaluation of absorption and emission properties of Yb 3+ -doped crystals for laser applications[END_REF]; the authors also estimated the radiative lifetime by means of the RM as τrad = 2.27 ms. Yasukevich et al. evaluated τlum,0 for the isostructural Yb:LiLuF4 crystal by the powder suspension method to be 1.94 ms [START_REF] Yasyukevich | Spectral kinetic properties of Yb 3+ :Na4Y6F22 and Yb 3+ :LiLuF4 crystals[END_REF]. Our work agrees well with these results.

Conclusions

Liquid Phase Epitaxy is a suitable method for the growth of highly-doped (20 at.% Yb, in the present work) Yb 3+ :LiYF4 single-crystalline thin-films on undoped LiYF4 substrates. In the present work, the maximum film thickness of 215 μm was reached; it can be varied by changing the dipping duration and the growth temperature. The grown films show a proper orientation, high crystallinity and low mosaicity, a uniform thickness over the substrate with the lack of cracks and well-preserved anisotropic emission properties of Yb 3+ ions. A slight broadening of the emission spectrum of the Yb 3+ :LiYF4 thin films (more evident for the πpolarization) is detected as compared to the corresponding single-crystals which may be favorable for laser applications, especially for mode-locked lasers. A weak concentrationquenching is observed as confirmed by the radiation trapping free upper laser level lifetime, ~2.0 ms, indicating weak unwanted energy-transfer to impurities.

The developed epitaxies are attractive for thin-disk laser applications. For a 100 μm-thick film, the estimated small-signal single-bounce (double-pass) pump absorption at 960 nm is ~23%. This parameter can be further increased by growing films with even higher doping levels (30-50 at.% Yb); however, this may be accompanied by stronger effect of energy-migration on the upper laser level lifetime. Still, the lattice mismatch for such Yb 3+ doping levels is expected to be low. Another alternative is the use of (100) oriented LiYF4 substrates. This will allow one to access another light polarization (π) corresponding to higher transition cross-sections both in absorption and emission. However, a possible challenge here is the anisotropy of the thermal conductivity and thermal expansion in the film plane. [START_REF] Thoma | Phase equilibria in the system LiF-YF 3[END_REF] and Rogin et al. [START_REF] Rogin | Liquid phase epitaxy of LiYF 4[END_REF]. Red squares liquidus temperatures, red circlesapparent onset of crystallization (both after [START_REF] Rogin | Liquid phase epitaxy of LiYF 4[END_REF]), blue squareliquidus temperature, blue asteriskgrowth temperature (both -this work). Blue dashed linethe selected bath composition.

List of figure captions

Figure 1 .

 1 Figure 1.Phase diagram of the LiF -YF3 binary system (after[START_REF] Thoma | Phase equilibria in the system LiF-YF 3[END_REF]), red solid line marks LiYF4 solid phase, blue dashed line indicates the composition selected for the LPE growth.

Figure 2 .

 2 Figure 2. (a) Photograph of an as-grown undoped LiYF4 boule used for preparing the substrates for LPE. The growth direction is along the [001] axis; (b) Photograph of the Yb:LiYF4 / LiYF4 epitaxy directly after the LPE growth process. The substrate is oriented along the (001) plane.

Figure 3 .

 3 Figure 3.A fragment of the phase diagram of the LiF -YF3 binary system relevant for the LPE growth experiments, after Thoma et al.[START_REF] Thoma | Phase equilibria in the system LiF-YF 3[END_REF] and Rogin et al.[START_REF] Rogin | Liquid phase epitaxy of LiYF 4[END_REF]. Red squares liquidus temperatures, red circlesapparent onset of crystallization (both after[START_REF] Rogin | Liquid phase epitaxy of LiYF 4[END_REF]), blue squareliquidus temperature, blue asteriskgrowth temperature (both -this work). Blue dashed linethe selected bath composition.

Figure 4 .

 4 Figure 4. Observation of the raw surface of a LPE-grown Yb:LiYF4 film (top view) using a confocal optical microscope. Reflection mode, bright field, the objective magnification is 20×, λ = 405 nm.

Figure 5 .

 5 Figure 5. Surface roughness plot of the raw surface of a LPE-grown Yb:LiYF4 film, obtained using a confocal optical microscope. Black lines indicate the range of root mean square (r.m.s.) deviation, inset -the corresponding topography map, while linethe analyzed direction. Reflection mode, λ = 405 nm.

Figure 6 .

 6 Figure 6. Confocal microscope images of the polished end-facet of the Yb:LiYF4 / LiYF4 epitaxy: (a) polarized light, P || [001], (b) a close look on the substrate / layer interface in crossed polarizers (P || [001], A || [100]). Transmission mode, the objective is (a) 20×, N.A. = 0.75, (b) 50×, N.A. = 0.95, λ = 405 nm.

Figure 7 .

 7 Figure 7. Single-crystal X-ray diffraction (XRD) study of the 20 at.% Yb:LiYF4 / LiYF4 epitaxy: (a) single-crystal XRD pattern of the film; (b) theoretical XRD pattern of LiYF4 (ICSD card No. 81-2254); (c) a close look on the (008) reflection. Numbers indicate the Miller's indices, (hkl).

Figure 8 .

 8 Figure 8. (a) Reciprocal space mapping of the (103) node for the Yb:LiYF4 layer; (b) the rocking curve of this node.

Figure 9 .

 9 Figure 9. Typical EDX spectrum of the 20 at.% Yb:LiYF4 film grown by the LPE method (top raw surface).

Figure 10 .

 10 Figure 10. Polarized Raman spectra of the undoped LiYF4 substrate and the LPEgrown 20 at.% Yb:LiYF4 layer in the a(ij)a, i, j = π, σ, configuration (Porto's notations), numbers indicate Raman frequencies in cm -1 , FWHMfull width at half maximum, λexc = 488 nm.

Figure 11 .

 11 Figure11. (a) Stimulated-emission (SE), σSE, and (b) absorption, σabs, cross-section spectra for the 2 F5/2 ↔ 2 F7/2 Yb 3+ transition in 20 at.% Yb:LiYF4 LPE-grown layer in π and σ polarizations; the spectra for the bulk 10 at.% Yb:LiYF4 crystal are given for comparison. σSE: calculation using Eq. (2), the F-L formula.

Figure 12 .

 12 Figure12. Low-temperature (LT, 12 K) absorption and luminescence spectra for the 20 at.% Yb:LiYF4 LPE-grown layer in σ polarization, λexc = 933 nm. "+" mark the electronic transitions, "hv"the vibronic sidebands. The spectral resolution is 0.2 nm.

Figure 13 .

 13 Figure 13. Crystal-field splitting of Yb 3+ multiplets in highly-doped (20 at.%) Yb:LiYF4 epitaxial films, the arrows indicate the electronic transitions in absorption and emission at 12 K.

Figure 14 .

 14 Figure 14. (a) Luminescence decay curve for the 20 at.% Yb:LiYF4 LPE-grown film (a semi-log plot): black curveexperimental data, green linesingle-exponential fit, the data for 2 at.% Yb and 10 at.% Yb:LiYF4 bulk crystals are given for comparison, all the curves are measured using the smallest pinhole; (b) evaluation of the radiation trapping-free lifetime τ0 for the epitaxy by the pinhole method. λexc = 960 nm, λlum = 1030 nm.

Acknowledgements

This work was supported by French Agence Nationale de la Recherche (ANR) through the projects LabEx EMC3 (ANR-10-LABX-09-01), SPLENDID2 (ANR-19-CE08-0028), and the European project "NOVAMAT" co-funded by the European Community funds FEDER and the Normandie region.


Lauren Guillemot

Pavel Loiko

Zhongben Pan

Jean-Louis Doualan

Alain Braud

Patrice Camy

Mid-Infrared Laser Emissions of Tm 3+ -doped Garnets: The Case Study of Disordered Tm:CNGG Crystal

Mid-Infrared Laser Emissions of Tm 3+ -doped Garnets: The Case Study of Disordered Tm:CNGG Crystal

Lauren Guillemot Recently, disordered garnets such as CNGG (calcium niobium gallium garnet) doped with Tm 3+ and codoped with Tm 3+ and Ho 3+ ions have attracted a lot of attention for generation of ultrashort pulses at ~2 µm [START_REF] Pan | Generation of 84-fs pulses from a mode-locked Tm:CNNGG disordered garnet crystal laser[END_REF]. Pan et al. reported on a Tm oscillator operating on the 3 F4 → 3 H6 transition and delivering 84 fs pulses at 2018 nm (emission bandwidth: >50 nm) and demonstrated continuous wavelength tuning between 1879-2086 nm [START_REF] Pan | Generation of 84-fs pulses from a mode-locked Tm:CNNGG disordered garnet crystal laser[END_REF]. It is interesting to extend the emission range of such crystals further into mid-IR. It can be done by exploiting either (i) vibronic emissions relying on electron-phonon-coupling or (ii) less common electronic 3 H4 → 3 H5 transition [START_REF] Loiko | Highly efficient 2.3 µm thulium lasers based on a high-phonon-energy crystal: evidence of vibronic-assisted emissions[END_REF].

A 3.2 at.% Tm:CNGG crystal was grown by the Czochralski method. The stimulated-emission (SE) crosssection, σSE, spectra for its near-and mid-IR emissions were determined using the Füchtbauer-Ladenburg equation with the radiative lifetimes τrad and branching ratios βJJ' calculated using the Judd-Ofelt formalism, Fig. 1(a). For the 3 F4 → 3 H6 transition, σSE = 0.44×10 -20 cm 2 at 1865 nm and the emission extends until 2.2 µm owing to a phonon sideband. For the 3 H4 → 3 H5 transition, σSE = 0.28×10 -20 cm 2 at 2331 nm (emission bandwidth: 132 nm). The gap between these electronic transitions is bridged by vibronic processes. The reabsorption-free luminescence lifetimes τlum( 3 F4) = 3.26 ms and <τlum( 3 H4)> = 0.181 ms, Fig. 1(b). The laser experiments were performed using an [111]-cut uncoated Tm:CNGG sample (thickness: 6.0 mm). A hemispherical cavity was formed by a flat pump mirror coated for HT at ~0.79 µm and for HR at 1.90-2.35 µm and several concave output couplers (OCs) (RoC = -100 mm). We used one "conventional" non-selective OC (#1) and two special "shortpass" OCs (#2 and #3) designed to force the laser oscillations above 2 µm, Fig. 2(a). The pump source was a Ti:Sapphire laser tuned to 785.8 nm. Two types of emission were detected: (i) a purely electronic 3 F4 → 3 H6 one using OC #1 (maximum power: 500 mW at 1.99 µm with a slope efficiency η of 26.1%), and (ii) a mixture of vibronic emission at 2.08 or 2.13 µm and a purely electronic 3 H4 → 3 H5 one at 2.33 or 2.37 µm using OCs #2 and #3 (maximum power: 548 mW with a total η = 58.2%), Fig. 2(b,c). The observed laser lines agreed with the crystalfield splitting of the corresponding multiplets determined at 12 K, Fig. 1(c). The weaker contribution of the 3 H4 → 3 H5 emission is assigned to strong self-quenching of the 3 H4 lifetime. Tm:CNGG crystals with optimized doping levels (1 -3 at.%) are very promising for ultra-broadband mid-IR laser sources.

Fig. 1

 1 Fig. 1 Spectroscopy of 3.2 at.% Tm:CNGG crystal: (a) Stimulated-emission (SE) cross-sections, σSE, for (from left to right) the 3 H4 → 3 F4, 3 F4 → 3 H6 and 3 H4 → 3 H5 Tm 3+ transitions; (b) RT luminescence decay curves from the 3 H4 and 3 F4 states; (c) crystal-field splitting of the 3 F4 and 3 H4 multiplets (revealed at 12 K).

Fig. 2

 2 Fig. 2 Mid-infrared Tm:CNGG laser: (a) transmission spectra of the output couplers (OCs), values specify TOC at 2 µm, 2.13 µm and 2.3 µm; (b) input-output dependences, η -slope efficiency; (c) typical spectra of laser emission.

Pavel Loiko 1 , Zhongben Pan 2 , Jean-Louis Doualan 1 , Alain Braud

  1,* , 1 , and Patrice Camy 1 1. Centre de Recherche sur les Ions, les Matériaux et la Photonique (CIMAP), UMR 6252 CEA-CNRS-ENSICAEN, Université de Caen Normandie, 6 Boulevard du Maréchal Juin, 14050 Caen Cedex 4, France. E-mail: lauren.guillemot@ensicaen.fr 2. Institute of Chemical Materials, China Academy of Engineering Physics, Mianyang, 621900, China