Aurély Bagghi

Rania Ghena

Nicolas Gautier

Nicolas Stephant

Philippe Deniard

Rémi

Rémi Dessapt

New alkali lanthanide-free

come

INTRODUCTION

Polyoxometalates 1,2 (POMs) are metal-oxide anionic clusters of early transition metals (usually Mo, W, V) whose redox and optical properties can be finely tuned according to their chemical composition, charge, shape and size. They are currently undergoing a remarkable development these last decades, due to their high relevance in many domains connected to materials science including single molecule magnets (SMMs), 3,4 photochromism, 5,6 catalysis, 7 photocatalysis [8][9][10] and photoluminescence (PL) sensing. 11,12 In this very active field, polyoxometalates incorporating visible-emitting lanthanide Ln 3+ ions (Ln-POMs) [13][14][15][16][17] have been intensively investigated to design stimuli-responsive luminescent materials and sensors for luminescence thermometry, 18,14 anticounterfeiting and luminescence security printing, 19 and luminescence bio-sensing. 20,21 In most of cases, the POM ligands act as sensitizers to enhance the emission of the Ln 3+ ions via an antenna effect, 22,23 in which the photoexcitation into the high energy OM ligand-to-metal charge transfer (LMCT) bands is followed by intramolecular POM-to-Ln 3+ energy transfers and emission processes due to f-f transitions within the Ln 3+ ions. The Weakley-type [Eu(W 5 O 18 ) 2 ] 9-polyoxotungstate 24 is a very efficient emitter at room temperature which has been exploited to elaborate a wide range of chemical probes for the detection of analytes in water i.e., heavy metal cations, 25 H 2 O 2 , 26 amines, 27 human papillomavirus, 29 and enzymes. 29 In contrast, its integration into PL sensors able to detect trace levels of water in liquids or gas has been much more scarcely reported. 30 However, the design of new Ln-POMs exhibiting high PL properties at room temperature is still highly challenging, and very few candidates fulfill all required structural and electronic criteria. 31 In particular, only the Ln 3+ ions with an emitting level lying in the OM LMCT bands are able to undergo efficient POM-to-Ln 3+ energy transfers, 31,32 which dramatically restrains the choice of suitable Ln 3+ /POM combinations. Moreover, considering rising costs associated with the difficulties of extracting or recycling rare earths, water-responsive Ln-free POM luminescent materials could be a promising lower-cost alternative.

Very recently, two Ln-free Keggin-type POMs incorporating Bi 3+ and Pb 2+ ions as emitting centers were reported as crystallographically uncharacterized tetrabutylammonium salts. 33 They demonstrated reversible solid-state vapoluminescent properties in the presence of water vapor, involving an emission color change between wet and dry samples. Many other PL Ln-free POMs have already been elaborated since the 1990s, [34][35][36] essentially as hydrated alkali salts. Their solid-state emission originates directly from the OM LMCT triplet levels, or after intramolecular energy transfers, from discrete energy levels lower in energy than the LMCT states, for POMs containing additional transition metal ions (Cr 3+ , Mn 4+ …). 34 However, these compounds have been very scarcely exploited in materials science, 37 essentially because most of them are luminescent only at low temperatures (below 100 K). Indeed, the crystallized water molecules form an extended hydrogen-bonding network connecting the oxygen POM surface, and O-H vibrators are responsible for strong PL quenching at room temperature by promoting non-radiative deactivation processes from the emitting levels. 34,36 The Anderson-type [SbW 6 O 24 ] 7-unit is one of the most performant PL Ln-free POMs, 35 and its potassium salt K 5.5 H 1.5 [SbW 6 O 24 ]6H 2 O 38 exhibits a green luminescence which is even distinguishable with naked eyes below room temperature. Two other hydrates have been also reported, 39,40 but their PL properties remain still uninvestigated.

Herein, we postulate that the solid-state emission of alkali [SbW 6 O 24 ] 7--containing materials could be reversibly tuned by controlling their hydration degree. As a proof of concept, we report a complete study of the precise impact of thermal dehydration/rehydration processes on the structure, the particles size and morphology, and the photophysical properties of Na 7 [SbW 6 O 24 ]16H 2 O, 39 K 5 Na 2 [SbW 6 O 24 ]12H 2 O, 40 , and their crystal structures have been solved from ab initio powder X-ray diffraction determination. Then, the room-temperature solid-state absorption and PL properties of both hydrated and anhydrous series have been thoroughly investigated, and structure-property relationships have been evidenced. As envisaged, the thermal dehydration processes cause a tremendous emission enhancement, and PL quantum efficiencies depend on the nature of the alkali ions. Moreover, it is demonstrated here that the luminescence properties of the anhydrous phases at room temperature are highly tunable with the presence of humidity in the atmosphere. Exposure to moisture-laden air induces strong PL quenching effects which are due to the complete regeneration of the hydrated parent salts, the rehydration rates varying with the structures of the hydrates. The anhydrous phases are then easily regenerated by soft thermal treatments, making them highly efficient sustainable and reusable water-responsive luminescent materials. O has been elaborated in water from a one-step synthesis procedure inspired from that previously reported, 40 O (Fig. S3, ESI †). In addition, the K/Sb, K/W and W/Sb elemental ratios in K 7 [SbW 6 O 24 ]•6H 2 O were determined by Scanning Transmission Electron Microscopy (S/TEM) and Energy Dispersive X-ray Spectroscopy (EDS) analyses (Fig. S4, ESI †) and are equal to 6.9, 1.2 and 6.0, respectively, in very good agreement with the expected ones. Finally, the hydration degree i.e., the quantity of crystallized water molecules has been determined from thermogravimetric analysis (see section 1.3). 1b). The sodium clusters are formed by six edge-shared {NaO 6 } octahedra involving oxo ligands of adjacent POMs and water molecules. These latter link the POM surface via eighteen H-bonds (with OO distances ranging from 2.66 to 2.95 Å) with six oxo ligands of the central {SbO 6 } octahedron, and eight terminal and bridging  2 -oxo ligands of the {W 6 O 18 } ring (Fig. 1b and Fig. S5, ESI †). The supramolecular layers are stacked along the c-axis and are connected by {NaO 4 (H 2 O) 2 } octahedra involving four terminal oxo ligands of two adjacent POMs (Fig. 1c). Each POM anion also links two sodium clusters of neighboring layers via two H-bonds (OO distance of 2.80 Å) (Fig. S5, ESI †). Finally, additional water molecules not directly interacting with Na + ions are localized between the layers, and link the POM surface via two other H-bonds (OO distance of 2.72 Å).

RESULTS AND DISCUSSION

Structural descriptions of

K 5 Na 2 [SbW 6 O 24 ]•12H 2 O crystallizes in the 𝑅3 𝑚 space group. Its crystal lattice is described herein in the hexagonal metric to facilitate the comparison with that of its anhydrous homologue (see section 2.1.). The [SbW 6 O 24 ] 7-units form a hexagonal arrangement and are assembled with Na + cations and water molecules into supramolecular layers in the (ab) plane (Fig. 1d). These latter are stacked along the c-axis, and separated by the larger K + ions. Each POM anion is surrounded by six [Na(H 2 O) 6 ] + octahedra forming a honeycomb-like arrangement. The [Na(H 2 O) 6 ] + entities connect the POM surface via eighteen H-bonds (OO distances of 2.68 and 2.95 Å) which involve all terminal and bridging  2 -oxo ligands of the {W 6 O 18 } ring. Along the c-axis, two [Na(H 2 O) 6 ] + cations of neighboring layers also connect the POM surface via six other H-bonds (OO distance of 2.90 Å), involving only the µ 2 -oxo ligands (Fig. 1e).

Thermal dehydration study

The thermal behaviors of the three hydrated materials have been investigated by a combination of thermal and HT-PXRD analyses. The thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) curves of Na 7 [SbW 6 O 24 ]•16H 2 O (Fig. 2a) show a weight loss of 13.61 % upon heating between room temperature and 120 °C, which is well consistent with expected removal of the sixteen crystallized water molecules (theoretical weight loss of 14.01 %). This proceeds in three distinct steps characterized by sharp endothermic peaks in the DSC curve. A first weight loss (4.70 %) is observed in the temperature range of 46-74 °C, and is attributed to the removal of six water molecules (theoretical weight loss of 5.24 %). A second weight loss (5.31 %) , well characterized by a sharp endothermic peak on the DSC curve. Noticeably, a broad exothermic peak is observed in the 400-650 °C range which has been attributed to the progressive crystallization process of the anhydrous phase.

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Crystal structures

The structures of Na 7 [SbW 6 O 24 ], K 5 Na 2 [SbW 6 O 24 ] and K 7 [SbW 6 O 24 ] have been solved from ab initio powder X-ray diffraction determination and refined by the Rietveld method. 42 The determination of the structural model of the three compounds was first started with the pattern of Na 7 [SbW 6 O 24 ] at 650 °C. As the structure factor of these compounds is largely dominated by the [SbW 6 O 24 ] 7-cluster, it was decided to use this information as a rigid body to build the structural model. The cluster model was taken from 39 At this point, it was assumed that this cluster is robust enough to not vary too much from one structure to another. In addition, it is supposed to be not very sensitive to temperature variation to fit with the above mentioned patterns recorded at high temperature. A hexagonal unit cell for Na 7 [SbW 6 O 24 ] was found using Expo2014 software 43 thanks to an embedded version of Dicvol06. 44 The difficulty to find unambiguously a space group from systematic extinctions let us consider to use, in the P1 space group, the hexagonal cell parameters found previously (the symmetry being later determined in the direct space). At this point, a measured density of the compound (5.67 g/cm 3 ) in regard of the cell volume led to 3 formula units per hexagonal unit cell. The above mentioned cluster was introduced in Fox software 45 ] is displayed in Fig. 4, and it has been arbitrary chosen to describe the crystal packing in the series of isostructural anhydrous phases. The [SbW 6 O 24 ] 7-units are arranged into hexagonal supramolecular layered networks in the (ab) plane (Fig. 4a), and are in closed proximity. The shortest intermolecular OO distance is of 3.094(18) Å, indicating significant electrostatic anion-anion interactions. The POM layers are stacked along the caxis and separated by alkali ions (Fig. 4b) which occupy two distinct Wyckoff positions (Table 1). In the first one (3b), the Na + ions are surrounded by six O-atoms in a regular octahedral environment (Fig. 4c). In the second one (18f), they are surrounded by seven O-atoms and occupy a distorted monocapped trigonal prismatic site (Fig. 4d). The same structural model has been used for K 5 Na 2 [SbW 6 O 24 ] and K 7 [SbW 6 O 24 ] and the results of the Rietveld refinements are reported in Table S1, ESI † and Table S2, ESI †, and displayed in Fig. S9, ESI † and Fig. S10, ESI †, respectively. Let us notice that, in the crystal structure of K 5 Na 2 [SbW 6 O 24 ], the 3b Wyckoff position is solely occupied by K + ions while the 18f Wyckoff position is randomly occupied by both K + and Na + ions with a site occupation fraction of about 0.72 and 0.28, respectively. Please do not adjust margins Please do not adjust margins

FTIR-ATR and FT-Raman characterizations

Based on results of the thermal analysis, powder samples of 2). Thus, the thermal treatment suppresses the H-bonding network resulting in an increase of the  as (W=O) frequencies. 46,47 The FT-Raman spectroscopy has been also used as a powerful complementary tool to confirm that the integrity of the POM is maintained after thermal dehydration. The Raman spectrum of Na 7 [SbW 6 O 24 ]16H 2 O (Fig. 5b) shows several peaks in the 50-800 cm - 1 range, corresponding to the W-O and Sb-O bonds involving  2 -oxo and  3 -oxo bridges of the [SbW 6 O 24 ] 7-unit. 48 Among them, the bands at 590 and 676 cm -1 can be assigned to both stretching W-O and Sb-O modes.

Rehydration and deuteration processes

The rehydration capacity of the three anhydrous materials in moisture-laden air has been investigated. The freshly thermally dehydrated powder samples were kept for three days at room temperature, in a closed chamber with a high relative humidity (RH) level of 100%. Then, the samples were characterized by PXRD and thermal analyses. As shown in Fig. 6, the PXRD pattern of

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To complete this study, single-crystals of Na 7 [SbW 6 O 24 ]16H 2 O were grind manually in a mortar, and the evolution of the particles size and morphology during successive dehydration/rehydration cycles was investigated in detail by SEM (Fig. 7). For each cycle, the hydrated phase was heated at 200 °C for 10 minutes and then, rehydrated upon exposure to moisture-laden air (100% RH) in a closed chamber for three days at room temperature. The evolution of the specific surface area of particles was also determined using the BET method. The starting particles of Na 7 [SbW 6 O 24 ]16H 2 O have a fairly high size dispersion ranging from 150 nm to a few micrometers (Fig. 7a), associated to a low specific surface area of 2.53 m 2 /g. Then, the first thermal treatment induces a fragmentation of the particles leading to sizes of about 100-200 nm small particles of Na 7 [SbW 6 O 24 ] (Fig. 7b). However, these particles are highly aggregated giving rise to only a weak increase in the specific surface area (3.00 m 2 /g). After complete rehydration, the particles are converted to very small faceted crystals of Na 7 [SbW 6 O 24 ]16H 2 O, quite homogeneous in shape and with a mean size in the 100-400 nm range (Fig. 7c). Interestingly, the associated specific surface area (10.00 m 2 /g) is higher than that of the starting particles of the hydrated compound. The second thermal treatment leads to small particles of Na 7 [SbW 6 O 24 ] with a mean size comparable to that of their hydrated mother particles (Fig. 7d). Nevertheless, the particles of Na 7 [SbW 6 O 24 ] are much less aggregated than those obtained directly after the first thermal treatment (Fig. 7b), resulting in a significant increase of the specific surface area which reaches 13.09 m 2 /g. Finally, no significant changes in the particles size and morphology of both hydrated and anhydrous phases are observed during other successive rehydration/dehydration processes (Fig. 7e-7i). Let us notice that, after one similar dehydration/rehydration cycle, K 5 Na 2 [SbW 6 O 24 ]•12H 2 O exhibits a comparable behaviour, and a significant decrease in its particles size from few micrometers to about 100-200 nm (Fig. S16, ESI †). Finally, by adapting the rehydration protocol described above, the deuterated analogue of K 5 Na 2 [SbW 6 O 24 ]12H 2 O has been prepared in order to better evidence the impact of the crystallized water molecules on the PL properties of the hydrated compound (see section 3). Thus, a freshly thermally dehydrated K 5 Na 2 [SbW 6 O 24 ] powder sample was kept under N 2 atmosphere in a closed chamber containing heavy water, for five days at room temperature. The PXRD analysis (Fig. S17,ESI †) confirms that the obtained compound is well isostructural to K 5 Na 2 [SbW 6 O 24 ]12H 2 O. In addition, the FTIR-ATR spectrum of the deuterated material clearly evidences the vibrational signatures of D 2 O and, to a lesser extent, HDO molecules (Fig. S18, ESI †).

Solid-state photophysical properties

Absorption and emission properties

The solid-state photophysical properties of the hydrated and anhydrous alkali Ln-free POM materials have been thoroughly investigated and compared (Table 2). The absorption properties have been studied at room temperature by diffuse reflectance spectroscopy of microcrystalline powder. The absorption spectra of all materials (Fig. S19, ESI †) show a broad absorption band in the range 250-320 nm which is assigned to mixed intense OW and OSb ligand-to-metal charge-transfer (LMCT) transitions. The optical band gap is of 296 nm (4.19 eV), 301 nm (4.12 eV) and 318 nm (3.90 ] i.e., a significant increase of 77%. In addition, the removal of water molecules directly bonded to the alkali ions increases their polarizing power and strengthens electrostatic interactions with POMs. Both effects induce a broadening and a redshift of the lowest energy absorption band toward higher wavelength. 49 Upon irradiation at 254 nm with a standard UV lamp, S20, ESI †. The PLE spectra monitored at 513 nm show a broad band peaking at 254 nm with a shoulder at about 290 nm. These bands are in well agreement with the LMCT transitions observed in the absorption spectra, and are mainly dominated by the 1 A 1g  1 T 1u transition of the OW charge transfer photoexcitation. 33 The PL spectra monitored at 254 nm show a broad and structureless band centered at 513 nm. This emission is associated to a very large stoke shift of about 20174 cm - S21,ESI †). It can be assigned to the 3 T 1u  1 A 1g transitions originating from the two OW LMCT emitting triplet states of the [SbW 6 O 24 ] 7-unit. 34 In the case of 2), the strong PL quenching can be attributed to consequent nonradiative de-excitation processes due to O-H vibrators connecting the {WO 6 } octahedra. Indeed, the hydrogen bonding network between Ln-free POMs and the neighboring water molecules creates dipole-dipole interactions which expand the luminescent {WO 6 } octahedral sphere and increases the rate of radiationless deactivation of the excited states. 34 To confirm this, the emission properties of K 5 Na 2 [SbW 6 O 24 ]•12H 2 O have been compared to those of its deuterated isotype (see section 2.3), because D 2 O is often reported as a much poorer quencher that H 2 O. 50 Upon similar photoexcitation at λ ex = 254 nm, the PL profiles of both compounds are roughly comparable (Fig. S23,ESI †), but the deuterated compound exhibits a little bit more intense emission characterized by a slightly longer luminescence lifetime of τ = 5.79 ± 0.26 µs. These results seem to confirm that nonradiative deactivation of the emitting triplet states in show tremendous luminescence enhancement which strongly contrasts with the weak PL responses of the hydrates series. In particular, Na 7 [SbW 6 O 24 ] exhibits a highly bright greenish-white emission (Fig. 8a).

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Please do not adjust margins For both hydrated and anhydrous compounds, the excited state radiative (k r ) and nonradiative (k nr ) decay rates, extracted from equations ( 1) and ( 2), are gathered in Table 2 and depicted in Fig. 9.

Considering that the hydrated compounds exhibit very low quantum yields (<1%) at room temperature, their associated k r values have been considered as negligible compared to k nr ones. As expected, the nonradiative decay rates of the anhydrous compounds are extremely lower than those of the hydrated ones (Fig. 9a). For instance, k nr is of 129.7×10 respectively. This well confirms that the thermal dehydration processes at 200 °C remove all nonradiative de-excitation pathways caused by O-H vibrators, strongly improving the PL performances of the alkali Ln-free POM materials at room temperature. Concurrently, the radiative decay rates of the anhydrous materials are much greater than those of their hydrated analogues (Fig. 9b) which is also in direct line with their brighter luminescence. Focusing now specifically on the series of anhydrous materials, k r is almost two times higher for Na 7 [SbW 6 O 24 ] (4.1×10 3 s -1 ) than for K 5 Na 2 [SbW 6 O 24 ] (1.7×10 3 s -1 ) and K 7 [SbW 6 O 24 ] (2.3×10 3 s -1 ), whereas at the same time, k nr is more than two-fold lower (Fig. 9a). This well evidences that radiative decay is more favored when [SbW 6 O 24 ] 7-is associated with only sodium ions in the crystal lattice. Thus, replacing K + ions by smaller Na + ones may increase the rigidity of the ionic structure of such materials, and could significantly reduce nonradiative deexcitation pathways, as already observed in other luminescent alkali tungstates and molybdates. , the rehydration processes are associated with strong and complete PL quenching effects at room temperature, due to the highly contrasted PL responses of both the anhydrous and hydrated compounds (see section 3.1.). These results evidence that the three anhydrous materials are highly reactive towards water molecules in the atmosphere that they are able to optically detect according to a PL turn-off mechanism. This differs from the vapoluminescence mechanism proposed for compounds (TBA) 4 [PW 11 O 39 Bi] and (TBA) 5 [PW 11 O 39 Pb] for which exposure to humidity induces significant modifications of the luminescence profiles resulting in emission color change. 33 O were grind manually in a mortar, and then heated at 200°C for 10 minutes. The obtained anhydrous materials were kept at room temperature in a closed chamber at 100% RH. Samples (60 mg) were collected at different exposure time, and their PL spectra were monitored (Fig. S27, ESI †). The temporal evolution of the percentages of PL intensity loss are displayed in Fig. 10b. The rehydration rates of the samples were compared by considering their PL quenching half-life time (t 1/2 ), namely the time required for the PL intensity loss to reach half of its final value. The t 1/2 values are short for K 5 Na 2 [SbW 6 O 24 ] (42 s) and K 7 [SbW 6 O 24 ] (7.8 min), and the PL intensity loss is complete for about 60 min. In contrast, the t 1/2 value is much longer for Na 7 [SbW 6 O 24 ] (49.5 min), and the PL intensity loss reaches 99 % for 8 hours. As an example, the PXRD pattern of

Φ F = k r /(k r + k nr ) (1)
k nr = 1/τ -k r (2)

Physical measurements. Fourier transform infrared (FT-IR) and

Fourier transform infrared with attenuated total reflectance (FTIR-ATR) spectra were recorded in the 4000-400 cm -1 range on a BRUKER Vertex equipped with a computer control using the OPUS software. Fourier transform Raman (FT-Raman) spectra were collected at room temperature under an excitation wavelength of 1064 nm (Nd:YAG laser) using a FT-Raman Bruker RFS 100 spectrophotometer. The nominal power was 300 mW. Solid-state FT-Raman spectra were recorded at 4 cm -1 resolution over the wavenumber range 3-4000 cm -1 , with 100 scan accumulation. Differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA) were performed by flowing air with a heating and cooling rate of 5°C/min on a SETARAM TG-DSC 111 between room temperature and 800 °C. Powder X-ray Diffraction (PXRD) patterns were collected at room temperature on a Bruker D8 Advance instrument in the Bragg-Brentano geometry, equipped with a front germanium monochromator, a copper anode (CuK-L3 radiation λ=1.540598 Å) and a LynxEye PSD detector. Scanning Transmission Electron Microscopy (S/TEM) characterization was performed on a Cs-probe corrected Themis Z G3 (Thermo Fisher Scientific) operating at 80 kV accelerating voltage. High Angle Annular Dark Field (HAADF) images were acquired with a 30.5 mrad convergence angle and 50 -200 mrad collection angles and Energy Dispersive X-ray Spectroscopy (EDS) maps were acquired with 4 windowless silicon drift detectors (Super-X system). HT-PXRD experiments were carried out on a Bruker D8 Advance instrument using Cu K-L2-L3 (λ = 1.540598, 1.54433 Å) radiation and a Våntec PSD detector. The morphology of powder was investigated with a field-emission gun scanning electron microscope JEOL JSM7600F operating at 7 kV. Density measurements were collected at room temperature on a AccuPyc II 1340 V1.09 with helium as analysis gas. Nitrogen volumetric measurements treated by the Brunauer-Emmett-Teller (BET) approaches were collected on a TriStar II 3020 device. Diffuse reflectance spectra were collected at room temperature on a finely ground sample with a Perkin-Elmer Lambda 1050 spectrometer equipped with a 150 mm diameter integrating sphere coated with Spectralon®, a highly reflecting fluoropolymer. Diffuse reflectance was measured from 250 to 1000 nm with a 2 nm step using Spectralon® as reference (100% reflectance). The reflectance data were treated by a Kubelka-Munk transformation 41 to better determine the absorption thresholds. Room-temperature photoluminescence spectra were recorded on a Jobin-Yvon Fluorolog 3 fluorometer equipped with a photomultiplier (excitation source: 400 W Xe arc lamp and 150W pulsed map for decay times). The emission spectra were corrected for detection and optical spectral response of the spectrofluorimeter and the excitation spectra were weighed for the spectral distribution of the lamp intensity using a photodiode reference detector. Quantum yields were recorded on a Jasco FP-8500 spectrofluorometer equipped with an integrating sphere ILF-835 (100 mm). The three hydrates are weakly or nonluminescent at room temperature (Φ F < 1 %), because crystallized water molecules connecting the oxygen POM surface through complex Hbonding networks promote non-radiative deactivation processes from the OW charge-transfer emitting triplet states by multiphonon relaxation. However, after complete thermal dehydration, the anhydrous phases exhibit a tremendous solidstate photoluminescence enhancement, in direct line with a significant decrease in the excited state nonradiative decay rates due to the removal of all O-H vibrators. In this series, the PL quantum yield is greater for Na 7 [SbW 6 O 24 ] (Φ F = 36 %) than for K 7 [SbW 6 O 24 ] (Φ F = 13 %) and K 5 Na 2 [SbW 6 O 24 ] (Φ F = 9 %), underlining that the substitution of K + by Na + ions strengthens the rigidity of the ionic structure and directs favorably the

Conclusions

  using K 2 WO 4 and KSb(OH) 6 as W and Sb sources, respectively. Single crystals of Na 7 [SbW 6 O 24 ]•16H 2 O 39 have been obtained from a new synthetic route using Na 2 WO 4 2H 2 O, and SbCl 5 instead of KSb(OH) 6 to avoid the formation of K 5 Na 2 [SbW 6 O 24 ]•12H 2 O as impurity. The powder X-ray diffraction (PXRD) patterns of both hydrated materials match well their structural models (Fig. S1, ESI †). The new K 7 [SbW 6 O 24 ]•6H 2 O compound has been elaborated from a synthesis protocol adapted from that of K 5 Na 2 [SbW 6 O 24 ]•12H 2 O, without adding NaCl in the aqueous medium. The obtained crystals were efflorescent, and attempts to solve their structure by single-crystal X-ray diffraction analysis were unsuccessful even at low temperature. Let us notice that their PXRD pattern (Fig. S2, ESI †) differs from that of K 5.5 H 1.5 [SbW 6 O 24

Fig 1 .

 1 Fig 1. (a) Polyhedral representation of the [SbW6O24] 7-POM. (b) and (c) Crystal packing in Na7[SbW6O24]•16H2O. (d) and (e) Crystal packing in K5Na2[SbW6O24]•12H2O (grey octahedra: WO6, pink octahedra: SbO6, green octahedra: NaO6, orange sphere: oxygen atoms of the POM unit, blue sphere: oxygen atoms of water molecules, purple sphere: potassium. H-bonding interactions are displayed as blue dotted lines. H atoms are not displayed).

Fig. 2 2 -

 22 Fig. 2 (a) TGA (black) and DSC (blue) curves of Na7[SbW6O24]16H2O. (b) PXRD patterns of Na7[SbW6O24] upon heating between 110 °C and 650 °C

  journal is © The Royal Society of Chemistry 20xx Please do not adjust margins Please do not adjust margins the crystal structure of Na 7 [SbW 6 O 24 ]16H 2 O.

Fig. 3

 3 Fig. 3 Observed, refined and difference patterns of Na7[SbW6O24] after Rietveld refinement (RBragg = 3.60, GOF = 1.31, Rwp = 10.74).

Fig. 4

 4 Fig. 4 Polyhedral representations of the crystal packing in Na7[SbW6O24]. (a) Supramolecular layers of (a) the [SbW6O24] 7-units and (b) the {NaO6} and {NaO7} polyhedra in the (ab) plane. (c) Stacking of the POM layers and the Na + ions in (c) the 3b and (d) the 18f Wyckoff positions (grey octahedra: WO6, pink octahedra: SbO6, blue octahedra: NaO6, green octahedra: NaO7, orange sphere: oxygen).

Na 7 [

 7 SbW 6 O 24 ], K 5 Na 2 [SbW 6 O 24 ] and K 7 [SbW 6 O 24 ] have been elaborated by thermally dehydrating their hydrated homologues at 200 °C for 10 minutes. Then, the anhydrous phases have been characterized by infrared and Raman spectroscopies. As an example, the FTIR-ATR and Raman spectra of Na 7 [SbW 6 O 24 ] are displayed in Fig. 5, in comparison with those of Na 7 [SbW 6 O 24 ]16H 2 O. As expected, the vibration bands of water molecules around 3500 cm -1 (OH-stretching modes) and 1600 cm -1 (HOH-bending modes) in the FTIR-ATR spectrum of Na 7 [SbW 6 O 24 ]16H 2 O are not observed in that of Na 7 [SbW 6 O 24 ] (Fig. 5a). Both spectra show the vibrational signature of [SbW 6 O 24 ] 7-in the range 1000-400 cm -1 . The bands corresponding to the pure stretching  as (W=O) modes involving terminal oxo ligands are observed at 866 cm -1 and 932 cm -1 in Na 7 [SbW 6 O 24 ]16H 2 O and are slightly shifted at higher frequencies (872 cm -1 and 953 cm -1 ) in Na 7 [SbW 6 O 24 ]. This can be explained considering that the W=O bonds at the surface of [SbW 6 O 24 ] 7-are involved in many H-bonding interactions with the water molecules in Na 7 [SbW 6 O 24 ]16H 2 O (see section 1.

Fig. 5 1 )

 51 Fig. 5 (a) FTIR-ATR spectra of Na7[SbW6O24] (red line) and Na7[SbW6O24]16H2O (black line). (b) FT-Raman spectra of Na7[SbW6O24] (red line) and Na7[SbW6O24]16H2O (black line).Four more intense bands are observed at 881, 898, 939 and 954 cm - 1 which correspond to the stretching modes of the shorter W=O

Na 7 [

 7 SbW 6 O 24 ] after rehydration is strictly similar to that of Na 7 [SbW 6 O 24 ]16H 2 O, and no remaining peaks of the anhydrous phase are observed. The TGA curve of the rehydrated Na 7 [SbW 6 O 24 ] also shows that the weight loss observed in the 25-200 °C range is consistent with the removal of sixteen water molecules (Fig. S13, ESI †), confirming that the rehydration process is complete. Similar results have been obtained for K 5 Na 2 [SbW 6 O 24 ] and K 7 [SbW 6 O 24 ] (Fig. S14, ESI †) which well evidence the full regeneration of K 5 Na 2 [SbW 6 O 24 ]•12H 2 O and K 7 [SbW 6 O 24 ]•6H 2 O. Noticeably, the rehydration processes are perfectly reversible, and the initial anhydrous phases can be then easily reset again by a simple thermal treatment at 200 °C for 10 minutes. In addition, these materials exhibit a good cyclability over successive dehydration/hydration processes (Fig. S15, ESI †).

Fig. 6

 6 Fig. 6 PXRD patterns of Na7[SbW6O24] before (black line) and after (red line) rehydration at a relative humidity (RH) level of 100% for three days at room temperature, compared with that of the Na7[SbW6O24]16H2O reference (blue line).

Fig. 7

 7 Fig. 7 SEM images of Na7[SbW6O24]16H2O and Na7[SbW6O24] particles during 4 successive cycles of thermal dehydration (200 °C, 10 minutes)/rehydration (100% RH, 3 days) processes.

  τ = 7.71 ± 0.09 µs (Fig. S22, ESI †) and a very low quantum yield (Φ F < 1%). Comparable results have been obtained for K 5 Na 2 [SbW 6 O 24 ]•12H 2 O (τ = 4.82 ± 0.49 µs, Φ F < 1%) and K 7 [SbW 6 O 24 ]•6H 2 O (τ = 7.17 ± 0.14 µs, Φ F < 1%) (Fig. S22, ESI †). As expected, the three hydrated materials exhibit very limited PL performances at room temperature. Considering the large amount of water molecules involved in H-bonding interactions with the POM surface in the crystal structures of Na 7 [SbW 6 O 24 ]•16H 2 O and K 5 Na 2 [SbW 6 O 24 ]•12H 2 O (see section 1.

K 5 Na 2 [

 2 SbW 6 O 24 ]•12H 2 O mainly occurs by multiphonon relaxation induced by O-H vibration modes. Noticeably, upon similar photoexcitation (λ ex = 254 nm) at room temperature, Na 7 [SbW 6 O 24 ], K 5 Na 2 [SbW 6 O 24 ] and K 7 [SbW 6 O 24 ]

The PLE spectra of Na 7 [

 7 SbW 6 O 24 ] (Fig. 8b), and K 5 Na 2 [SbW 6 O 24 ] and K 7 [SbW 6 O 24 ] (Fig. S24, ESI †) monitored at 513 nm are well comparable with those of their corresponding hydrates (Fig. S20, ESI †), indicating that the photoexcitation processes remain unchanged. Moreover, the PL spectrum of Na 7 [SbW 6 O 24 ] (Fig. 8b) and those of K 5 Na 2 [SbW 6 O 24 ] and K 7 [SbW 6 O 24 ] (Fig. S24, ESI †) monitored at 254 nm show that both the position in energy and the shape of the PL band does not hardly vary after dehydration, resulting in a very slight variation of the CIE chromaticity coordinates of (0.2800, 0.3804) for Na 7 [SbW 6 O 24 ], (0.2819, 0.3814) for K 5 Na 2 [SbW 6 O 24 ] and (0.2690, 0.3705) for K 7 [SbW 6 O 24 ] (Fig. S25, ESI †). However, the photoluminescence lifetime of Na 7 [SbW 6 O 24 ] is increased tenfold (τ = 88.00 ± 0.60 µs) in relation with that of Na 7 [SbW 6 O 24 ]•16H 2 O (τ = 7.71 ± 0.09 µs), and its luminescence quantum yield reaches 36 %. Although less pronounced, significant enhancement of the PL intensity is also observed for K 5 Na 2 [SbW 6 O 24 ] (τ = 54.01 ± 0.35 µs, Φ F = 9 %) and K 7 [SbW 6 O 24 ] (τ = 55.73 ± 0.04 µs, Φ F = 13 %) in relation with their hydrated analogues (Fig. S24, ESI †).

Fig. 8

 8 Fig. 8 (a) Photographs of Na7[SbW6O24]16H2O and Na7[SbW6O24] upon UV excitation (λex = 254 nm) at room temperature. (b) Room-temperature excitation spectrum (red line) monitored at 513 nm and emission spectrum (black line) monitored at 254 nm of Na7[SbW6O24].

Fig. 9

 9 Fig. 9 Comparison of the excited state (a) radiative (kr) and (b) nonradiative (knr) decay rates in the series of hydrated and anhydrous alkali Ln-free POM materials.

  3 s -1 for Na 7 [SbW 6 O 24 ]•16H 2 O and is divided by a factor almost 18 for Na 7 [SbW 6 O 24 ] (7.3×10 3 s -1 ).

9

 9 This journal is © The Royal Society of Chemistry 20xx J . N a m e ., 2013, 00, 1-3 | Please do not adjust margins Please do not adjust margins Similarly, the k nr values of K 7 [SbW 6 O 24 ]6H 2 O (139.5×10 3 s -1 ) and of K 5 Na 2 [SbW 6 O 24 ]12H 2 O (207.5×10 3 s -1 ) significantly drop to 15.6×10 3 s -1 and 16.8×10 3 s -1 for K 7 [SbW 6 O 24 ] and K 5 Na 2 [SbW 6 O 24 ],

K 5 Na 2 [

 52 SbW 6 O 24 ] exposed upon 100% RH for 60 min well confirms the complete regeneration of K 5 Na 2 [SbW 6 O 24 ]•12H 2 O (Fig. S28, ESI †). This kinetics study shows that although the three anhydrous materials are isostructural, their rehydration rates strongly vary with the nature of the alkali ions. The fast PL quenching effects of K 5 Na 2 [SbW 6 O 24 ] and K 7 [SbW 6 O 24 ] indicates that the regeneration of K 5 Na 2 [SbW 6 O 24 ]•12H 2 O and K 7 [SbW 6 O 24 ]•6H 2 O occurs very quickly, while upon similar conditions, the conversion from Na 7 [SbW 6 O 24 ] to Na 7 [SbW 6 O 24 ]•16H 2 O is much slower. If further investigations are still needed to fully rationalize the dehydration/rehydration mechanisms of these alkali POM salts, structural filiations between hydrated and anhydrous phases could be one of the key parameters that control their rehydration kinetics. Indeed, as discussed in section 2.1., the structures of K 5 Na 2 [SbW 6 O 24 ]•12H 2 O and K 5 Na 2 [SbW 6 O 24 ] show strong similarities in the arrangement of POMs and alkali ions into alternated hexagonal supramolecular layers stacked along the c-axis. Thus, the incorporation of water molecules into the crystal lattice of K 5 Na 2 [SbW 6 O 24 ] could involve very limited structural reorganization resulting in fast rehydration processes. In strong contrast, the phase transformation from Na 7 [SbW 6 O 24 ] to Na 7 [SbW 6 O 24 ]•16H 2 O implies breaking the layered structures and significant rearrangements of both anionic and cationic species which could be in direct line with the slower rehydration of the anhydrous sodium compound.

Fig. 10 (

 10 Fig. 10 (a) Room-temperature PL spectra monitored at 254 nm of Na7[SbW6O24] before (black line) and after (blue line) rehydration for three days at 100% RH, compared with that of pristine Na7[SbW6O24]•16H2O (red line). (b) Evolution of the percentage of PL intensity loss of Na7[SbW6O24] (red square), K5Na2[SbW6O24] (black square) and K7[SbW6O24] (blue square). Solid lines are guides for the eyes. Inset: PL quenching half-life times of the different anhydrous materials.

Na 7 [

 7 In summary, three new water-responsive luminescent alkali Lnfree POM materials Na 7 [SbW 6 O 24 ], K 5 Na 2 [SbW 6 O 24 ] and K 7 [SbW 6 O 24 ] have been successfully elaborated by thermally dehydrating at 200°C SbW 6 O 24 ]16H 2 O, K 5 Na 2 [SbW 6 O 24 ]•12H 2 O and K 7 [SbW 6 O 24 ]•6H 2 O. The structures of the isostructural anhydrous compounds have been solved from ab initio powder X-ray diffraction determination. FTIR-ATR and FT-Raman spectroscopy analyses have also confirmed the integrity of the [SbW 6 O 24 ] 7-unit after the thermal treatment.

7 [SbW 6 O 24 ]6H 2 O.

  and the new compound K First, the thermal behaviors of the three hydrates have been fully investigated by a combination of

	Nantes Université, CNRS, Institut des Matériaux de Nantes Jean Rouxel, IMN, F-
	44000 Nantes, France. E-mail : remi.dessapt@cnrs-imn.fr, philippe.deniard@cnrs-
	imn.fr
	Electronic Supplementary Information (ESI) available: Additional PXRD patterns, FT-

IR and FT-Raman spectra, TGA/DSC measurements, S/TEM-EDS spectrum, SEM images, crystallographic data and refinements parameters, room-temperature solid-state UV-vis absorption, PL and PLE spectra, CIE chromaticity diagrams, and luminescence decay curves of the hydrated and anhydrous materials. CIF files of the three new anhydrous materials. See DOI: 10.1039/x0xx00000x 2 | J. Name., 2012, 00, 1-3 This journal is © The Royal Society of Chemistry 20xx Please do not adjust margins Please do not adjust margins thermogravimetric measurements, High Temperature Powder X-Ray Diffraction (HT-PXRD) analysis, scanning electron microscopy (SEM), FT-IR and FT-Raman spectroscopies. Their complete thermal dehydration has led to three new isostructural anhydrous phases namely, Na

7 [SbW 6 O 24 ], K 5 Na 2 [SbW 6 O 24 ], and K 7 [SbW 6 O 24

1. Hydrated alkali Ln-free POM materials: Na 7 [SbW 6 O 24 ]•16H 2 O, K 5 Na 2 [SbW 6 O 24 ]•12H 2 O and K 7 [SbW 6 O 24 ]•6H 2 O

1.1. Synthesis

The microcrystalline powder of K

5 Na 2 [SbW 6 O 24 ]•12H

  2

7 [SbW 6 O 24 ]•6H 2 O has

  ]•6H 2 O.38 The presence of the [SbW 6 O 24 ] 7-unit in K

			been evidenced by comparing its FT-IR
	spectrum	with	those	of	Na 7

[SbW 6 O 24 ]•16H 2 O and K 5 Na 2 [SbW 6 O 24 ]•12H

  2

Na 7 [SbW 6 O 24 ]•16H 2 O and K 5 Na 2 [SbW 6 O 24 ]•12H 2 O The crystal structures of Na 7 [SbW 6 O 24 ]•16H 2 O and K 5 Na 2 [SbW 6 O 24 ]•12H 2 O are

  depicted in Fig.1, and briefly described as followed. Both compounds contain the Anderson-Evans-type [SbW 6 O 24 ] 7-polyanion which is built upon a Sb 5+ ion in a central cavity of the crown formed by six edge-shared {WO 6 } octahedra (Fig.1a). In each compound, the [SbW 6 O 24 ] 7-entities are assembled with alkali ions and water molecules in numerous hydrogen bonding interactions.

Na 7 [SbW 6 O 24 ]•16H 2 O crystallizes

  in the 𝑃1 space group. The [SbW 6 O 24 ] 7-anions connect {Na 6 O 8 •14H 2 O} formal clusters into supramolecular layers in the (ab) plane (Fig.

anhydrous Na 7 [SbW 6 O 24 ] material

  . These results clearly evidenced that, when progressively increased the temperature, the complete thermal dehydration process of

	occurs between 74 and 110 °C which
	corresponds to the removal of six other ones. The third weight loss
	(3.60 %) in the temperature range of 110-120 °C is attributed to the
	removal of the last four water molecules (theoretical weight loss of
	3.50 %), leading to a new

Na 7 [SbW 6 O 24 ]•16H 2 O involves

	the gradual formation of two new
	compounds with intermediary hydration degrees, namely
	Na 7 [SbW 6 O 24 ]•10H 2 O and Na 7 [SbW 6 O 24 ]•4H 2 O. Their structural and
	photophysical properties are beyond the scope of our present study
	and will be subject to further investigations in our future work. No
	weight loss occurs from 120 °C and 700 °C which corresponds to the
	melting point of

Na 7 [SbW 6 O 24

7 [SbW 6 O 24 ] and K 7 [SbW 6 O 24

  thanks to a Fenske-Hall matrix transformation, in the P1 space group. The asobtained partial structural model introduced in JANA2020 as a rigid body was completed by difference Fourier maps prior to use individual atom positions. At the end of the procedure, the symmetry was found to correspond to the 𝑅3 space group. Although the structure determination was first obtained from high temperature patterns, the following structure description is given for Na

] after cooling down to 30 °C during HT-PXRD experiment. All the procedure was done under dry nitrogen atmosphere to avoid any rehydration. However, despite these precautions and may be because of its highly fast reactivity towards humidity (see below), this could not be achieved for K

5 Na 2 [SbW 6 O 24 ].

  Thus, its structure reported in ESI † is that of a data collection at 500 °C. Due to the structure factor largely dominated by the tungsten and antimony atoms, sodium Atomic Displacement Parameters (ADP) were constrained as well as all of those of the oxygen atoms. Fig.3shows the observed, calculated and difference patterns for Na 7 [SbW 6 O 24 ] and Table1shows the complete description of its structural model.The structure of Na

7 [SbW 6 O 24

Table 1 .

 1 Atom positions, site occupation fraction and atomic displacement parameters of Na7[SbW6O24] at 30 °C.

	S.G. 𝑅3 , a(Å) = 10.53600(18), c(Å) = 16.1942(5), R Bragg = 3.60, GOF = 1.31, R wp = 10.74
	a t o m	x		y		z	S O F W y c k o f f	U iso (Å 2 )
	Na1	0		0		0.5	1		3b	0.021(3)
	Na2	0.1110(12)		0.2729(10)	0.1862(6)	1		18f	0.021(3)
	O1	0.520(2)		0.1774(17)	0.4010(9)	1		18f	0.016(3)
	O2	-0.048(2)		-0.363(2)	0.3964(11)	1		18f	0.016(3)
	O3	0.1889(15)		0.1389(17)	0.2454(12)	1		18f	0.016(3)
	O4	0.200(2)		-0.003(2)	0.4125(10)	1		18f	0.016(3)
	Sb1	0		0		0	1		3a	0.0025(18)
	W1	-0.17427(16) -0.31023(14) 0.33367(11)	1		18f	0.0139(13)
	*U aniso (W1): 0.0055(16); 0.018(2); 0.0152(9); 0.0041(10); 0.008(2); 0.0052(16)
		6x10 3
					observed
					calculated
					difference
		4x10 3			Bragg peak
	intensity	2x10 3
		0
		10	15	20	25 30	35 40	45	50	55 60	65
						2 (degrees)

7 [SbW 6 O 24 ]•16H 2 O, K 5 Na 2 [SbW 6 O 24 ]•12H 2 O and K 7 [SbW 6 O 24 ]•6H 2 O, respectively

  eV) for Na . Strikingly, the absorption threshold is systematically red shifted for their anhydrous analogues, and reach 346 nm (3.58 eV), 321 nm (3.86 eV) and 334 nm (3.71 eV) for Na

7 [SbW 6 O 24 ], K 5 Na 2 [SbW 6 O 24 ] and K 7 [SbW 6 O 24 ], respectively

  . This has been interpreted considering that the thermal treatment increases the concentration of absorbing [SbW 6 O 24 ] 7-units in the crystal lattices, strengthening electrostatic anion-anion interactions. For example, the [SbW 6 O 24 ] 7-volume concentration shifts from 1.81 ×10 -3 mol cm -3 in Na

7 [SbW 6 O 24 ]•16H 2 O to

  3.20 ×10 -3 mol cm -3 in Na

7 [SbW 6 O 24

Na 7 [SbW 6 O 24 ]16H 2 O, K 5 Na 2 [SbW 6 O 24 ]•12H 2 O

	a n d
	K 7 [

SbW 6 O 24 ]•6H 2 O exhibit

  a weakly intense greenish-white emission at room temperature, barely observable with naked eyes. Their steady-state photoluminescence excitation (PLE) and emission spectra are displayed in Fig.

SbW 6 O 24 ]•16H 2 O,

  (0.3123, 0.4092) for K

					1 , and is
	characterized by the Commission Internationale de l'Eclairage (CIE)
	chromaticity	coordinates	of	(0.3019,	0.4002)	for
	Na 7 [

5 Na 2 [SbW 6 O 24 ]•12H 2 O and

  (0.2958, 0.3950) for K

7 [SbW 6 O 24 ]•6H 2

O (Fig.

Table 2 .

 2 Photophysical data of the considered anhydrous and hydrated alkali Ln-free POM materials.

E

g [eV] a λ em [nm] b τ [s] c F [%] d k r [×10 3 s -1 ] e k nr [×10 3 s -1 ] f Na 7 [SbW 6 O 24 ]16H 2 O

  Emission maximum at ex = 254 nm. c Photoluminescence lifetime monitored at ex = 254 nm. d Photoluminescence quantum yield. e Radiative decay rate (kr = F/t).

		4.19	513	7.71 ± 0.09	< 1	-	129.7
	Na 7 [SbW 6 O 24 ]	3.58	513	88.00 ± 0.60	36	4.1	7.3
	K 5 Na 2 [SbW 6 O 24 ]12H 2 O	4.12	513	4.82 ± 0.49	< 1	-	207.5
	K 5 Na 2 [SbW 6 O 24 ]	3.86	513	54.01 ± 0.35	9	1.7	16.8
	K 7 [SbW 6 O 24 ]6H 2 O	3.90	513	7.17 ± 0.10	< 1	-	139.5
	K 7 [SbW 6 O 24 ]	3.71	513	55.73 ± 0.04	13	2.3	15.6

a Optical band gap. b f Nonradiative decay rate (knr = 1/τ -kr).

Na 7 [SbW 6 O 24 ], K 5 Na 2 [SbW 6 O 24 ] and K 7 [SbW 6 O 24 ] are fully converted back into Na 7 [SbW 6 O 24 ]•16H 2 O, K 5 Na 2 [SbW 6 O 24 ]•12H 2 O and K 7 [SbW 6 O 24 ]•6H 2 O, respectively

  51,52 3.2. Reversible vapoluminescent propertiesAs described in section 2.3., when exposed to moisture-laden air (100% RH) for three days at room temperature. Noticeably, as shown in Fig.10afor Na

7 [SbW 6 O 24 ] and

  in Fig.S26, ESI † for K

5 Na 2 [SbW 6 O 24 ] and K 7 [SbW 6 O 24

Na 7 [SbW 6 O 24 ], K 5 Na 2 [SbW 6 O 24 ] and K 7 [SbW 6 O 24 ] have

  Interestingly, on the considered time scales, also a high chemical inertness towards other gaseous molecules present in the air (O 2 , CO 2 …) likely to induce side PL quenching processes. Let us also notice that the complete regeneration of the PL-active anhydrous phases can be achieved by a simple thermal treatment at 200°C for 10 minutes.The kinetics of PL quenching upon rehydration of Na

7 [SbW 6 O 24 ], K 5 Na 2 [SbW 6 O 24 ] and K 7 [SbW 6 O 24 ] have

  been investigated at room temperature. Single crystals of Na

7 [SbW 6 O 24 ]•16H 2 O and K 7 [SbW 6 O 24 ]•6H 2 O, and the microcrystalline powder of K 5 Na 2 [SbW 6 O 24 ]•12H 2

SbW 6 O 24 ]16H 2 O.

  Na 2 WO 4 •2H 2 O (2 g, 6 mmol) was dissolved in 40 ml of water. After addition of SbCl 5 (190 µL, 1.5 mmol), the pH of the colorless solution was adjusted to 7 by adding hydrochloric acid (4M) dropwise. The solution was heated to 85 °C until three quarters of the volume remained. The solution was cooled down to room temperature and then filtrated. The filtrate was left at room temperature, and colourless plate-like crystals were obtained after one week. Yield in W: 46%. FT-IR (KBr pellet, cm -1 ) H

	Please do not adjust margins
	EXPERIMENTAL SECTION
	Synthesis. All reagents and solvents used were commercially
	available.
	Na 7 [2 O 3465 (m),
	1630 (m); [SbW 6 O 24 ] 7-955 (m), 935 (s), 891 (s), 872 (s), 675 (s), 602
	(m), 451 (s).
	K 5
	Please do not adjust margins

Na 2 [SbW 6 O 24 ]•12H 2 O

  . K 2 WO 4 (988 mg, 3 mmol) was dissolved in 20 mL of water. After addition of KSb(OH) 6 (132 mg, 0.5 mmol) and NaCl (116.8 mg, 2 mmol), the pH of the colorless solution was adjusted to 7 by adding hydrochloric acid (4M) dropwise. The solution was heated at 85 °C until three quarters of the volume remained. A white precipitate is obtained during the heating. The solution was cooled down to room temperature and then filtrated. A white microcrystalline powder is obtained and washed with cold water. Yield in W: 77%. FT-IR (KBr pellet, cm -1 ) H 2 O 3516 (m), 1630 (m) ; [SbW 6 O 24 ] 7-932 (s), 887 (s), 702 (m), 604 (s), 571 (s), 451 (s).

	K 7 [

SbW 6 O 24 ]•6H 2 O

  . K 2 WO 4 (988 mg, 3 mmol) was dissolved in 20 mL of water. After addition of KSb(OH) 6 (132 mg, 0.5 mmol), the pH of the colorless solution was adjusted to 7 by adding hydrochloric acid (4M) dropwise. The solution was heated to 85 °C until three quarters of the volume remained. The solution was cooled down to room temperature and then filtrated. The filtrate was left at room temperature and highly efflorescent colourless plate-like crystals were obtained after two days. Yield in W: 63%. FT-IR (KBr pellet cm -

	1 ) H 2 O 3454 (m), 1651 (m) ; [SbW 6 O 24 ] 7-935 (s), 885 (s), 683 (s), 571
	(s), 449 (m).

Na 7 [SbW 6 O 24 ], K 5 Na 2 [SbW 6 O 24 ] and K 7 [SbW 6 O 24 ]. Single-crystals of Na 7 [SbW 6 O 24 ]16H 2 O and K 7 [SbW 6 O 24 ]•6H 2 O or microcrystalline powder of K 5 Na 2 [SbW 6 O 24 ]•12H 2 O were

  grind manually in a mortar, and the obtained white powders were heated at 200 °C in an oven for 10 minutes. FTIR-ATR

Na 7 [SbW 6 O 24 ]: 953

	(s), 872 (s), 652 (s), 615
	(m), 578 (m), 493 (s), 450 (s).

K 5 Na 2 [SbW 6 O 24 ]: 940

	(s), 866 (s), 648
	(m), 600 (s), 567 (s), 487 (s), 446 (s).

K 7 [SbW 6 O 24 ]: 940

	(s), 866 (m),
	652 (m), 600 (m), 569 (s), 487 (s), 450 (s).

This journal is © The Royal Society of Chemistry 20xx J . N a m e ., 2013, 00, 1-3 | 3

Acknowledgements

This work was supported by the CNRS, Nantes Université, the Région des Pays de la Loire and the Ministère de l'Enseignement Supérieur et de la Recherche. The CIMEN Electron Microscopy Center in Nantes is also greatly acknowledged. The authors would like to thank Pr. Pierre Frère and Nagham Ibrahim (Moltech-Anjou, University of Angers, France) for their expert assistances in the characterization of luminescence quantum yield measurements.

ARTICLE

Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

Conflicts of interest

There are no conflicts to declare.


Jérémy Potet

Mathilde Gay

Laurent Bramerie

Monique Thual

Ivan N Cano

Giuseppe Caruso

Ricardo Rosales

Derek Nesset

Fabienne Saliou

Gaël Simon

Philippe Chanclou

Real-Time DSP-Free 100Gbit/s/λ PAM-4 Fiber Access Link using EML and Direct Detection

Keywords: Access networks, FTTH, PON, mobile network, PAM-4, EML, Fronthaul

I. INTRODUCTION

HERE is much interest in increasing the capacity of fiber access links to meet the growing bandwidth demands for both fixed and mobile broadband services. To this end, both the ITU-T and IEEE have initiated standards for bi-directional point-to-point fiber links at 50 Gbit/s line rate [START_REF]Bidirectional 10 Gb/s, 25 Gb/s, and 50 Gb/s Optical Access PHY[END_REF][START_REF]Higher speed bidirectional, single fibre, point-to-point optical access system[END_REF]. Furthermore, the ITU-T is developing the Higher-Speed PON (HSP) recommendations targeting 50 Gbit/s line rate in passive optical network (PON) systems [START_REF]Higher Speed Passive Optical Networks[END_REF]. Naturally, with the completion of those standards, the research interest moves to the next capacity step for fiber access networks (FANs) e.g., 100 Gbit/s. While 100 Gbit/s is a reality for short range applications as datacenter (DC) [START_REF] Verbist | Real-Time and DSP-Free 128 Gb/s PAM-4 Link Using a Binary Driven Silicon Photonic Transmitter[END_REF], the specificity of the access network leads to different constraints, especially in terms of optical budget and reach. An overview of the topologies of the access network is depicted in Fig. 1. Two main architectures exist: point to point (PtP) and point to multipoint (PtMP). The PtP topology is dedicated to mobile, business or specific networks (education, banks). PtMP is well-known for Fiber To The Home (FTTH) using PON. The required link budgets (e.g. 20 dB for PtP (Class A) [START_REF]Bidirectional 10 Gb/s, 25 Gb/s, and 50 Gb/s Optical Access PHY[END_REF][START_REF]Higher speed bidirectional, single fibre, point-to-point optical access system[END_REF] and 29 dB for PtMP (Class N1) [START_REF]Higher Speed Passive Optical Networks[END_REF]) together with transmission distances covering typically 20 km, are more challenging in FAN than other short reach scenarios. To meet the link budget, the optical receiver (Rx) sensitivity is a critical issue. For this reason, coherent approaches have been studied for 100 Gbit/s line rate FAN systems [START_REF] Zhang | Single Wavelength 100-Gb/s PAM-4 TDM-PON Achieving Over 32-dB Power Budget Using Simplified and Phase Insensitive Coherent Detection[END_REF][START_REF] Zhang | Demonstration of 100-Gb/s/λ PAM4 TDM-PON supporting 29-dB power budget with 50-km reach using 10G-class O-band DML transmitters[END_REF][START_REF] Matsuda | Hardware-Efficient Signal Processing Technologies for Coherent PON Systems[END_REF][START_REF] Birk | Real-Time Single-Carrier Coherent 100 Gb/s PM-QPSK Field Trial[END_REF]. However, few reports [START_REF] Birk | Real-Time Single-Carrier Coherent 100 Gb/s PM-QPSK Field Trial[END_REF] real time demonstrations due to the development cost of customized digital signal processing (DSP) chips at this bitrate. On the other hand, 100 Gbit/s with 4-level Pulse Amplitude Modulation (PAM-4) has been demonstrated with direct detection (DD) and offline DSP [START_REF] Xue | 100G PAM-4 PON with 34 dB Power Budget Using Joint Nonlinear Tomlinson-Harashima Precoding and Volterra Equalization[END_REF][START_REF] Che | Direct modulation of a 54-GHz distributed Bragg reflector laser with 100-GBaud PAM-4 and 80-GBaud PAM-8[END_REF][START_REF] Torres-Ferrera | 100+ Gbps/λ 50 km C-Band Downstream PON Using CD Digital Pre-Compensation and Direct-Detection ONU Receiver[END_REF][START_REF] Borkowski | World's First Field Trial of 100 Gbit/s Flexible PON (FLCS-PON)[END_REF]. In [START_REF] Potet | Real Time 100 Gbit/s/λ PAM-4 Experiments for Future Access Networks over 20 km with 29 dB Optical Budget[END_REF], we showed that using a Mach-Zehnder Modulator (MZM) and DD, PAM-4 could be detected without DSP. Still, it is of further interest to assess if a DSP-free Rx could achieve high link budgets and properly detect a PAM-4 optical signal with a simpler transmitter (Tx) like an externally modulated laser (EML) composed of a monolithically integrated distributed feedback laser and an electro-absorption modulator. The use of such Tx would avoid higher complexity and cost compared to legacy technology such as G-PON and XG(S)-PON. In this paper we demonstrate for the first time to our knowledge, a single wavelength real-time PAM-4 100 Gbit/s/λ fiber access link in O-band generated with an EML Tx and without DSP in the Rx. Fiber transmission over 20 km is realized with an optical budget (OB) of 30 dB using analog pre-equalization, optical amplification, and DD. We study the impact of the optical amplifier location in three configurations: booster, preamplifier, and both booster and pre-amplifier targeting PtP and PtMP scenarios. The results are also compared with those obtained previously in [START_REF] Potet | Real Time 100 Gbit/s/λ PAM-4 Experiments for Future Access Networks over 20 km with 29 dB Optical Budget[END_REF] with a MZM Tx. The analog electrical finite impulse response (FIR) filter and its impact on the electrical modulation signal at the Tx-side are also presented. The experimental setup is presented in Fig. 2 showing the Optical Line Terminal (OLT), the optical network unit (ONU) and the link. In this work we study different configurations depending on the target optical budget. For the back to back reference (part III.A), no transmission link and no amplifiers are introduced. For the PtP configuration, we have tested the case with a semiconductor optical amplifier (SOA) as a booster (Part III. B), and with a SOA as a preamplifier (Part III.C). Finally, for the PtMP configuration (Part III. D), both booster and pre-amplifier are required in order to reach the optical budget. In this case the booster was a Praseodymium doped fiber amplifier (PDFA) as detailed later. The detail of the setup is as follows: at the OLT Tx side, we used a Pulse Pattern Generator (PPG) to generate a pair of pseudo random bit sequences of length 2 15 -1 (PRBS 15) 50 Gbit/s Non-Return to Zero signals. The length of the PRBS sequence was limited by the size of the random access memory of the BER tester programmed for PAM-4 decoding. The PAM-4 100 Gbit/s signal is encoded inside the PPG thanks to a 3-bit digital-to-analog converter (DAC) on which we use 2 inputs. The electrical signal passes through an analog 6-taps FIR filter with 55 GHz -3 dB bandwidth (BW) and time spacing of 7.5 ps. The FIR optimizes the channel response. The equalized electrical signal is amplified and applied to the electro absorption modulator (EAM) section of an EML chip. The laser and the EAM sections of the EML are biased at 92 mA and -2 V respectively, leading to a PAM-4 extinction ratio (ER) of 5.6 dB (defined as the ratio of the outer levels) and an optical modulation amplitude (OMA) equal to 4 dBm. The EML temperature is set and maintained via a temperature controller at 20°C. The EML output optical signal is coupled into a micro-lensed fiber. The optical power coupled into the fiber is +3.5 dBm. This EML emits at 1309 nm and presents a -3 dB electro-optical (EO) BW of 46 GHz. In [START_REF] Potet | Real Time 100 Gbit/s/λ PAM-4 Experiments for Future Access Networks over 20 km with 29 dB Optical Budget[END_REF], we used a MZM with 40 GHz BW and 8 dB ER (OMA = 8.1 dBm). After possible amplification (depending on the case study), the optical signal is sent through 20 km of standard single mode fiber (SSMF), leading to a cumulated chromatic dispersion equal to -2 ps/nm (at 1309 nm), and a Variable Optical Attenuator (VOA) is inserted to emulate the losses of an optical distribution network (ODN). The ONU Rx detects the optical signal using a 42 GHz PIN photodiode (PD) coupled with an integrated transimpedance amplifier (TIA). To increase the Rx sensitivity, a SOA is added as a pre-amplifier on the second PtP configuration (Part III. C) and in the PtMP configuration (Part III. D) without an optical filter in between the pre-amplifier SOA and PD. A polarization controller is inserted in front of this SOA to compensate for its 1-dB polarization dependent gain and a VOA in front of the PD protects it from any damage. Finally, the detected signal is sent to BERT for Bit Error Rate (BER) measurement. The BER is measured on each of the three eyes consecutively. We fix the decision time and scan the amplitude decision threshold to determine the minimum BER for each level by programming the BERT with the pattern corresponding to each level (Fig. 3). The corresponding PAM-4 BER is calculated by combining the BER of each eye using the following equation [START_REF] Szczerba | 30 Gbps 4-PAM transmission over 200 m of MMF using an 850 nm VCSEL[END_REF]:

BER = 1 2 minBER 1 + minBER 2 + 1 2 minBER 3
Fig. 3. PAM-4 BER measurement method. The temporal threshold is fixed, the amplitude threshold sweeps on the amplitude of the PAM-4 signal and the BER is measured for each of the three eyes of the signal.

III. RESULTS AND DISCUSSION

A. Back to back Fig. 5 presents the reference BER measurement in back-to-back (BtB) without optical amplification. We measure a reference Rx To reach an OB suitable for PtP Class A (i.e., 20 dB), we add a SOA as a booster amplifier (SOA1) i.e. at the OLT Tx side (Fig. 7 a)). Fig. 6 presents the booster (SOA1) output power (full blue square curve) and the gain (full red circle curve) as a function of the SOA input power. SOA1 has a small signal gain of 28 dB, a saturation input power of -16 dBm at 1 dB gain compression and a noise figure (NF) of 7 dB at 1309 nm wavelength, for bias current of 444 mA at 20°C. In our previous work [START_REF] Borkowski | World's First Field Trial of 100 Gbit/s Flexible PON (FLCS-PON)[END_REF], to avoid nonlinear effects during fiber propagation, the launched power had to be limited to 12 dBm [START_REF] Xue | 100G PAM-4 PON with 34 dB Power Budget Using Joint Nonlinear Tomlinson-Harashima Precoding and Volterra Equalization[END_REF]. We thus add an attenuator before the SOA to reduce the input power to -17 dBm and obtain a Tx output power of 12 dBm (Fig. 6). In this way, SOA1 works very close to its linear regime which limits both the gain compression in the amplifier to 0.5 dB and the fiber nonlinearities. As observed on the BER measurements on Fig. 7 b), the ASE noise introduced by the SOA leads to a ~1 dB penalty on the Rx sensitivity compared to the reference measurement (no SOA). In Fig. 7 b), the reference measurement is only relevant to the received optical power and not to the OB. We measure an Rx sensitivity of -9.5 dBm and a corresponding OB of 21.5 dB both in BtB and after 20 km of SSMF. The chromatic dispersion has a negligible effect after 20 km transmission at the operating wavelength (1309 nm) as seen on the eye diagrams of Fig. 7 c) captured in BtB (up) and after 20 km (down) for a received power of -9 dBm. An alternative to the amplified Tx for PtP links is a pre-amplified Rx as depicted in Fig. 8 a). The pre-amplifier (SOA2) presents a small signal gain of 19 dB, an input saturation power of -20 dBm as shown on Fig. 6 (dot curve), a NF of 5.9 dB and 1 dB polarization dependent gain. Fig. 8 b) plots the measured BER and Fig. 8 c) presents the detected electrical eye diagrams at BER = 10 -2 in BtB and after 20 km. In this setup, we obtain a Rx sensitivity of -18 dBm at BER = 10 -2 , corresponding to an OB of 21.5 dB (Tx power of +3.5 dBm). We also show on Fig. 8 b), results from [START_REF] Borkowski | World's First Field Trial of 100 Gbit/s Flexible PON (FLCS-PON)[END_REF] with a MZM at the Tx. The MZM has a higher ER (8 dB) and is chirp free, however it has a higher cost than an EML. We note from Fig. 8 b) the sensitivity measured with the MZM based Tx is almost the same as with EML. Hence, at this bit rate and operating wavelength, the Rx noise is the main limiting factor.

C. Pre-amplifier SOA (PtP)

D. Booster and preamplifier (PtMP)

Finally, we study a future single wavelength 100 Gbit/s PtMP residential scenario. To meet the required OB, we insert in this case both a booster and a pre-amplifier (Fig. 9 a)). SOAs exhibit a larger noise figure than fiber amplifiers; hence, to limit the optical noise we use a PDFA as booster. The PDFA has a noise figure of 5 dB, and its output power is set to 12 dBm to avoid nonlinearities in fiber propagation [START_REF] Borkowski | World's First Field Trial of 100 Gbit/s Flexible PON (FLCS-PON)[END_REF]. The detected electrical eye diagrams are open (Fig. 9 c)) and we measure a Rx sensitivity of -18 dBm in BtB and after 20 km (Fig. 9 b)). Despite the presence of a second amplifier which introduces more noise in the link compared to the case without booster, the Rx sensitivity remains the same as the case with only pre amplified Rx. We reach an OB of 30 dB, which is compatible with N1 Class (14-29 dB) OB for PONs [START_REF]Higher Speed Passive Optical Networks[END_REF]. The black curve gives the performances obtained previously with the MZM. Despite the lower OMA from the EML, the performance is comparable. IV. DISCUSSION AND CONCLUSIONS While the association of two non-optically filtered amplifiers as booster and pre-amplifier provides the best results, having both simultaneously remains questionable for residential broadband applications due to the cost constraints. The experiments show that with a SOA either as a booster or pre-amplifier, the OB for PtP can be achieved. However, since monolithic integration on a single chip of a SOA with an EML is easier than with a PIN diode, the booster SOA seems more technically feasible. Moreover, it is more reasonable to add an optical amplifier at the OLT because of the controlled environment operation and energy consumption. We demonstrate a real time single wavelength 100 Gbit/s/λ PAM-4 transmission using only analog pre-equalization and optical amplification. The equalization scheme used in this work is relatively simple with a 6-tap analog FIR filter, showing that a 100 Gbit/s/λ IM/DD link is achievable without complex DSP. We test three different non-filtered optical amplifier configurations -booster, preamplifier, and both-, and thus achieve OB of 20.5 dB, 21.5 dB and 30 dB respectively (higher than ITU-T budget class N1) with an integrated O-band EML in BtB and through 20 km of SSMF. The use of fiber amplifier in access network has not been deployed, mainly due to cost and integration issues, recent works on Bismuth doped fiber amplifier [START_REF] Mikhailov | Amplified Transmission Beyond C-and L-Bands: Bismuth Doped Fiber Amplifier for O-Band Transmission[END_REF] further open this possibility. These results demonstrate a way forward for bidirectional PtP standards along with illustrating what could be possible at 100 Gbit/s in PON applications.

Fig. 1 .

 1 Fig. 1. Access networks topologies overview

Fabienne

  Fig. 2. Experimental setup

Fig. 4 a

 4 Fig. 4 a) shows the modulating electrical eye diagram without and with the FIR analog compensation. The S21 measurements shown on Fig. 4 b) are done with a 70 GHz vector network analyzer (VNA). The red dot curve is the Tx (RF chain and EML) response without compensation (3 dB EO BW of 38 GHz), and the black dash curve is the FIR filter response with the parameters used in this work. Finally, the blue full curve is the compensated transmitter response, the FIR filter enhances spectral components between 10 GHz and 30 GHz.

Fig. 4 .

 4 Fig. 4. (a) Electrical eye diagrams of modulating signal without and with compensation. (b) Analog FIR filter S21 response (black), transmitter response before (red) and after compensation (blue). The tap coefficients are optimized through the observation of the eye diagram and measured BER. The effect of equalization is clearly observed in the quality of the eye diagram in Fig. 4 a). After compensation, the trajectories are cleaner, and the eyes are more balanced and symmetric.

Fig. 6 .

 6 Fig. 5. BER versus average received optical power and optical budget (top axis). The insert is the optical eye diagram at the PD output at a received power of -8 dBm. B. Booster SOA (PtP)

Fig. 7 .

 7 Fig. 7. (a) Experimental setup with boosted transmitter. (b) BER versus average received optical power and optical budget (top axis), with 20 km SSMF propagation and BtB. (c) Optical eye diagrams at received optical power of -9 dBm.

Fig. 8 .

 8 Fig. 8. (a) Experimental setup with pre amplified receiver (b) BER versus average received optical power and optical budget (top axis), with 20 km SSMF propagation and BtB. (c) Optical eye diagrams at a received power of -18 dBm.

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Fig. 9 .

 9 Fig. 9. (a) Experimental setup with boosted transmitter and pre amplified receiver. (b) BER versus average received optical power and optical budget in back-to-back and with 20 km fiber propagation. (c) Optical eye diagram at received power of -18 dBm.


Jérémy Potet
email: jeremy.potet@orange.com

Gaël Simon

Fabienne Saliou

Philippe Chanclou

Mathilde Gay

Laurent Bramerie

Monique Thual

Hélène Debregeas

Elena Duran

Natalia Dubrovina

Cross Gain Modulation Mitigation with Automatic Gain Control of Bidirectional SOA for DSP-free 50G-PON

We demonstrate how to mitigate cross gain modulation impairments induced when using a bidirectional SOA shared at OLT side in 50G-PON. Automatic gain control of the SOA reduces by 4 dB the XGM impairments induced by the upstream burst on the downstream signal. ©2022 The Author(s)

Introduction

Telecom operators currently exploit gigabit capable-passive optical network (G-PON) generation and start to deploy 10 Gbit/s PON (as XGS-PON). The ever-growing bandwidth demands pushed the standardization bodies to standardize the next generation PON systems. The latest International Telecommunication Union (ITU-T) standard concerns the 50G-PON [START_REF] Dubrovina | Higher Speed Passive Optical Networks: Requirements[END_REF] (50 Gbit/s PON). Some points concerning the practical realization of such system remain undetermined for the industry. One of the difficulties is to reach a high link optical budget (up to 29 dB for N1 class) at such a bitrate. For that purpose, in [START_REF] Saliou | DSP-free and Shared SOA for HS-PON Transmissions with up to 30dB Optical Budget and 15dB dynamic range[END_REF], we proposed to use a Semiconductor Optical Amplifier (SOA) to increase the link budget and meet the requirements of the standard based on the legacy PON infrastructure. Locating the SOA at the OLT side reduces cost and complexity, as a single SOA is shared by all the users of the Optical Distribution Network (ODN) and operates in a controlled environment, the operator's central office. In terms of cost, the best solution seems to use an SOA for amplifying both the downstream (DS) and the upstream (US). It is known that operating an SOA in bidirectional amplification regime can bring some issues related mainly to the Cross Gain Modulation (XGM) associated to the upstream burst mode transmission: when a US reaches the OLT-side SOA (receiver side), the SOA gain available for the DS signal falls since the upstream burst consumes part of its gain [START_REF] Simon | 50Gb/s Real-Time Transmissions with Upstream Burst-Mode for 50G-PON using a Common SOA Pre-amplifier/Booster at the OLT[END_REF]. Since the lifetime of the SOA's carriers is short (typically a few tens of ps or less) [START_REF] Zilkie | Carrier Dynamics of Quantum-Dot, Quantum-Dash, and Quantum-Well Semiconductor Optical Amplifiers Operating at 1.55 µm[END_REF], the carrier's density and so the gain of the SOA imprints the gain fast fluctuations on the DS signal (the US burst envelope) and brings intensity noise on the US. XGM can then reduce the quality of the signal in both directions. In this paper, we propose a solution based on controlling the bias current of the SOA and synchronize it with the upcoming envelope of the burst, to compensate the fluctuations of the gain. Some other solutions were proposed to reduce XGM impairments such in [START_REF] Kim | Reduction of cross-gain modulation in the semiconductor optical amplifier by using wavelength modulated signal[END_REF][START_REF] Lee | Bidirectional amplification with linear optical amplifier in WDM-PON[END_REF][START_REF] Francis | A single-chip linear optical amplifier[END_REF][START_REF] Chan | Cross gain modulation suppression in SOA using polarization diversified loop[END_REF] but these solutions require additional components or specific optical amplifiers. Fig. 3 a) presents the behavior of the SOA in bidirectional amplification in the context of PON (i.e. with an US link in burst mode), when the driving current is continuous and constant. When an US burst reaches the amplifier, the burst consumes some SOA's gain and the gain available for the signal on the DS direction falls temporarily. At the output of the SOA, the DS signal presents depletions. When such DS signal is detected, some errors are directly linked to cross gain modulation. To avoid this phenomenon, we propose to manage the SOA's gain by driving its bias current, to provide more gain to the system when an US burst crosses the SOA and decrease it when the SOA is only crossed by the DS signal, as shown in Fig. 3 b). Two conditions must be satisfied: the SOA gain variations and incoming bursts must be synchronized, and the SOA gain must be managed according to the burst magnitude, as a powerful incoming burst will consume more gain and imprints more on the DS signal than a less powerful burst. When it comes to the field exploitability of such solutions, it is worth noting that commercially available PON systems already manage the calendar (timing) of the US bursts to avoid collisions. The commercially available OLT also possesses information related to the optical power of the US bursts of the different ONUs through the Received Signal Strength Indication (RSSI). For our lab experiment, we employed a current gate signal synchronized with the burst envelope, which comes from the gate generator used to drive the SOA (Fig. 3 b)). By adjusting the amplitude and the time delay of the gate, we can compensate the depletion of the DS signal due to the burst in US direction. We first assess each link individually before testing the bi-directional case. Fig. 4 shows the performances of the 50 Gbit/s DS and of the US link alternately switched on. We reached 29.4 dB OB with a sensitivity of -15 dBm on the DS link, matching the N1 class (14-29 dB) budget requirement. In US (burst mode), a 13 dB OB and a sensitivity of -15 dBm are measured. The high error floor on US results (BER = 4.10 -3 ), is due to the very low conversion gain (19 V/W) of the available DC-30GHz photodiode (PD) which provides a low output amplitude. The use of a a) b) receiver with higher sensitivity or with burst mode TIA [START_REF] Yin | Linear Burst-Mode Receivers for DSP-Enabled Passive Optical Networks[END_REF][START_REF] Borkowski | Real-time burst-mode operation of an integrated SOA-PIN/TIA receiver for 25 Gbit/s/λ and faster T(W)DM-PON[END_REF] should solve the problem and enable 50 Gbit/s burst mode transmission reaching 29 dB OB in US [START_REF] Simon | Real-Time 58.2Gb/s Equalization-Free NRZ Mode Burst Transmission for Upstream HS-PON and beyond with Monolithically Integrated SOA-UTC Receiver[END_REF] as the eye diagram presents a good opening as seen on the insert on Fig. 4. We now assess the bi-directional case and the effect of XGM. In this experiment, we drive the SOA with a constant current. Fig. 5 presents the DS BER measurement at a fixed power of -7 dBm on the receiver when the US power Pus at point B varies. Cases with a DS power Pds at point A of -5.6dBm (blue circles) and -2.6dBm (orange squares) are considered. First, with very low US signal (Pus < -15dBm), the error floor is slightly degraded for higher DS power (BER = 5.1.10 -7 for Pds= -2.6dBm and 1.4.10 -7 for Pds= -5.6dBm); this is mainly due to the signal that slightly saturates the SOA gain leading to a lower extinction ratio in this case (the so-called self-gain modulation SGM). Secondly, in each case we observe a BER degradation when the US increases (for Pus > -10 dBm when Pds= -2.6 dBm and for Pus > -15dBm when Pds= -5.6dBm) due to the XGM owing from the contra-propagating signal. Fig. 5 presents the optical eye diagram of the DS signal at the input of the PD (point C on Fig. 1), with (Fig. 5 b)) and (Fig. 5 c)) US signal (for Pus = -4.6 dBm). This highlights the presence of XGM affecting the quality of the eye diagram as a second smaller eye seems to appear behind the first one on Fig. 5 c). We then studied the effect of SOA AGC on the DS signal BER. On Fig. 6, same experiments as in Fig. 5 are performed and results with or without the SOA AGC are compared. The beneficial effect of dynamically controlling the gain of the SOA is clear on this result. The US power starting to degrade the DS signal is pushed to higher values (from Pds > -15dBm to Pus > -10dBm when Pds= -5.6dBm and from Pus > -10dBm to a value higher than -4.9 dBm when Pds= -2.6dBm). Moreover, at a given US power of -4.6 dBm, it leads to a BER enhancement of up to 2 decades (for Pds= -5.6 dBm). In terms of US power, the supported US power on the SOA at a same DS BER can be 4 dB higher as seen on Fig. 6. When Pds is low, the US power at which a degradation happens is also lower and the XGM cannot be completely mitigated by the SOA AGC. Even if we can drive the SOA with higher current, the SOA will saturate and will not be able to compensate the XGM indefinitely. The study of the BER degradation owing from XGM on the US signal was not possible given the low performance of our burst photodiode. However, we can observe its impact on Fig. 7, which presents the burst and the eye diagram from the US signal with and without DS signal in the SOA for Pds=-5.6 dBm. We can observe less power on the burst and eye diagram closure with DS signal.

Experimental setup

Results and discussion

Conclusion

We demonstrate a technic for the mitigation of the effect of cross gain modulation in a 50G-PON system based on bidirectional semiconductor optical amplifier. By synchronizing the amplifier driving current gate with the upcoming burst, the cross gain modulation reduction leads to a BER improvement of up to 2 decades for the downstream signal and the upstream signal can be 4 dB higher on the SOA, to reach the same BER on the downstream signal.

Fig. 1 :

 1 Fig. 1: Experimental setup The experimental setup is presented on Fig. 1. In DS direction, we use an externally modulated laser (EML) composed of a distributed feedback laser and an electro-absorption modulator (DFB-EAM) integrated on a single chip from Almae Technologies. The EAM is modulated with a 50 Gbit/s Non-Return to Zero On-Off Keying Pseudo Random Bit Sequence signal of length 2 31 -1 (NRZ-OOK PRBS31) generated by a pulse pattern generator (PPG1). The DFB biased at 100 mA emits light at 1310 nm, the EAM was biased at -1.1V with a Vpp of 800 mV leading to an extinction ratio (ER) of 6 dB. A Thermoelectric Cooler (TEC) is used to maintain the temperature of the EML at 20°C. The optical signal at the output of the chip is coupled into a micro-lensed fiber providing 4 dBm output power (with signal modulation). The optical signal passes through an optical isolator (ISO1) and a Coarse Wavelength Division Multiplexing filter (CWDM1) is employed to separate US and DS. The DS signal is amplified by a booster SOA and is controlled at a room temperature of 20°C and at 280 mA when the automatic gain control (AGC) is turned OFF. As shown on Fig 2., the SOA presents a 17.7 dB small signal gain at 1310 nm (19.1 dB at 1270 nm), a saturation input power of -20 dBm and a low polarization dependent gain (PDG) of 0.5 dB. To ensure

Fig. 2 :

 2 Fig. 2: Gain versus SOA input power at 280 mA and 20°C After amplification, the DS signal crosses another CWDM filter (CWDM2) and reaches a variable optical attenuator (VOA1). The direct detection is realized by a photodiode (PD) PIN-TIA with 42 GHz 3 dB electro-optical bandwidth (EO-BW). The electrical signal is sent to the error detector (ED1) for bit error rate (BER) measurement. Concerning the US link, a gate generator sends the burst envelope (20 µs width and 125 µs periodicity) to both the second pulse pattern generator (PPG2) to trigger the data emission and an external cavity laser (ECL) which operates at 1270 nm. PPG2 generates the NRZ PRBS31 electrical signal for the 50 Gbit/s burst US link. The electrical signal modulates a lithium niobate Mach-Zehnder modulator (MZM) of 40 GHz EO-BW biased at 4 V leading to an ER of 7 dB. The optical signal is sent to ISO2, VOA2 and then reaches CWDM2. For the upstream link, the SOA works as a preamplifier in front of a PD PIN DC-30GHz. The detected signal finally reaches the ED2.

Fig. 3 :

 3 Fig. 3: Behavior of SOA in bidirectional amplification without (a)) and with (b)) AGC.

Fig. 4 :

 4 Fig. 4: BER versus average received optical power for DS (blue circle) and US (orange circle).

Fig. 5 :

 5 Fig. 5: BER measured on the DS for different US power injected in the SOA a), eye diagram without (b)) and with (c)) US signal in the SOA.

Fig. 6 :

 6 Fig. 6: BER versus US optical power for a DS power of -5.6 dBm (circle) and -2.6 dBm (square) at the input of the SOA with and without AGC.

Fig. 7 :

 7 Fig. 7: US burst and eye diagram without and with DS signal.

Acknowledgements

The authors would like to thank Anritsu for the loan of the PPG 64G MP1900A. a) b) c)


Jérémy Potet
email: jeremy.potet@orange.com

Mathilde Gay

Laurent Bramerie

Monique Thual

Fabienne Saliou

Gaël Simon

Philippe Chanclou

Real-Time 100 Gbit/s/lambda PAM-4 Fiber Link supporting 4λ Operation with a Common Fiber Amplifier for Future Mobile X-haul and Point to Point Access Networks

de niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Introduction

After the commercial deployment of the 5 th generation of mobile network (5G), the work on the 6 th generation (6G) has begun. To handle new usages unlocked by future 6G networks, the future optical access network needs to be capable to transport the great amount of data that 6G will lever. Mobile Back/Mid/Fronthaul optical links capable to transport 100 Gbit/s to up to 1 Tbit/s are required [START_REF] Rajatheva | White paper on broadband connectivity in 6G[END_REF] on a point-to-point topology (PtP). New usages such as industry 4.0 and metaverse should also benefit from these future high bitrate fiber links. The current PtP standards [START_REF]Bidirectional 10 Gb/s, 25 Gb/s, and 50 Gb/s Optical Access PHY[END_REF] used for mobile X-haul use intensity modulation and direct detection (IM/DD) with the Non-Return to Zero On-Off Keying (NRZ-OOK) modulation format and provide up to 25 Gbit/s per wavelength. Coherent technologies intend to meet the requirements for such transmission systems [START_REF] Suzuki | Simplified digital coherent-based beyond-100G optical access systems for B5G/6G [Invited[END_REF] but the added complexity and cost compared to an IM/DD system remains a drawback. Standardisation bodies like the Institute of Electrical and Electronics Engineers (IEEE) have already edited specification on 100 Gbit/s links [4]. For these links, the targeted channel insertion losses must be above 15 dB and the optical budget (OB) must reach to 21.5 dB. These requirements are defined for a reach of 30 km of the 100GBASE-ER4-30 [4] specification.

In this paper, we experimentally assess the use of a common optical fiber amplifier on a 4-levels pulse amplitude modulation (PAM-4) 100 Gbit/s/. This link supports 4λ operation with the wavelength plan of the multi-source agreement (MSA) specifying optical link at 4λx100

Gbit/s for 30 km reach (400G-ER4-30) [START_REF]IEEE Standard for Information technology--Local and metropolitan area networks--Specific requirements--Part 3: CSMA/CD Access Method and Physical Layer Specifications Amendment 4: Media Access Control Parameters, Physical Layers, and Management Parameters for 40 Gb/s and 100 Gb/s Operation[END_REF]. We used a Praseodymium Doped Fiber Amplifier (PDFA) which is a fiber amplifier fabricated in fluoride glass and amplifying in the O band thanks to the stimulated emission of Pr 3+ ions pumped at 1000 nm. We assess the wavelength amplification range of our PDFA and its capacity to amplify linearly 4 channels based on the Local Area Network-WDM (LWDM) wavelength plan used for the MSA 400G-ER4-30.

Finally, we assess the performances of our system to transmit 100Gbit/s/ PAM-4 signal in back-to-back (BtB) and through 30 km propagation in standard single mode fiber (SSMF). The detected electrical signal is sent to a bit error rate (BER) detector (BERT) for symbol error rate (SER) evaluation. The BER is measured on each of the three eyes corresponding to 𝐵𝐸𝑅 1 , 𝐵𝐸𝑅 2 and 𝐵𝐸𝑅 3 of the PAM-4 signal by adjusting the amplitude decision threshold [START_REF] Potet | Real Time 100 Gbit/s/λ PAM-4 Experiments for Future Access Networks over 20 km with 29 dB Optical Budget[END_REF]. The SER is calculated by the following equation [START_REF] Che | Direct modulation of a 54-GHz distributed Bragg reflector laser with 100-GBaud PAM-4 and 80-GBaud PAM-8[END_REF]: To assess the capability of the PDFA to amplify multi-wavelength signals and reach the performance standard's requirements, we first assess our amplified link in a single wavelength tunable configuration. Fig. 3 presents the sensitivity and the optical budget versus the wavelength of the Tx in BtB. For this experiment, the optical budget (OB) is defined as 𝑃 𝑇𝑋 (𝑎𝑡 𝑝𝑜𝑖𝑛𝑡 𝐶) -𝑃 𝑅𝑋 (𝑎𝑡 𝑝𝑜𝑖𝑛𝑡 𝐸) (between the PDFA output and the OBPF output). To obtain these results, only Ch 2 is turned on. The emitting wavelength of the ECL is tuned from 1270 nm to 1340 nm with a 10 nm step. The sensitivity of the receiver at the forward error correction (FEC) threshold is defined at SER=10 -2 based on the FEC threshold of SD-FEC [START_REF]3: 50-Gigabit-capable passive optical networks (50G-PON): Physical media dependent (PMD) layer specification[END_REF]. The sensitivity of the receiver stays in a range of 2.2 dB for all the wavelength range. It is not the case for the OB which depends on the launch power at the output of the PDFA (Plaunch) and so depends on the gain of the amplifier at the Tx wavelength. With only one signal in the PDFA, the wavelength range to obtain 21.5 dB or more OB is from 1282 nm to 1325 nm giving a wavelength tunability of 43 nm. Based on the MSA 400G-ER4-30 in the context of transport networks, the WDM wavelength plan is with channel centered at: 1304.6 nm, 1306.9 nm, 1309.1 nm and 1311 nm with 400 GHz channel frequency spacing. Fig. 4 presents the optical spectrum of the signal at the input (Fig. The sensitivity of the receiver is assessed through the SER measurements on the four channels. Fig. 5 presents the SER versus average received optical power at point E, in BtB and with 30 km of propagation in SSMF. As depicted on the output power versus input power curve (Fig. 7) of our PDFA at 1308 nm (middle of the comb), increasing the input power by 4.5 dB would increase the output by 3 dB. In these conditions, the channel insertion losses would reach up to 19.15 dB after 30 km fiber propagation.

Experimental setup

𝑆𝐸𝑅 = 1 2 ⁄ 𝐵𝐸𝑅 1 + 𝐵𝐸𝑅 2 + 1 2 ⁄ 𝐵𝐸𝑅 3 .

Results and discussion

Fig. 1 :

 1 Fig. 1: Experimental setup. The experimental setup is depicted on Fig.1. The 100Gbit/s (50 GBaud) real time PAM-4 signal is generated by a PPG which generates two pseudo random bit sequence of length 2 15 -1 (PRBS15) and PAM-4 encoded thanks to a 2-bit DAC. The electrical modulation signal passes through an analog 6-taps finite impulse response filter (FIR) with a time spacing of 7.5 ps and an electrical bandwidth of 64 GHz for analog pre-equalization [6]. The electrical modulation signal is then sent to two optical transmitters (Tx). The Tx called "Channel 2" (Ch 2) is composed of an external cavity laser (ECL) with a wavelength which can be tuned between 1260 nm and 1360 nm, and a Mach-Zehnder modulator (MZM) with a -3-dB electro-optical bandwidth (EO-BW) of 40 GHz. The MZM is driven at 4.0 Vpp leading to an extinction ratio (ER) of 8 dB. The second Tx called "Ch 3" is a distributed feedback laser (DFB) and an electro-absorption modulator (EAM) integrated on the same chip which forms an externally modulated laser (EML) emitting at

Fig. 2 :

 2 Fig. 2: PDFA gain versus signal wavelength for a total mean power of 0 dBm input power.

Fig. 2

 2 Fig.2presents the gain of the PDFA at emitter's side (booster) with a signal input power of 0 dBm versus the single input wavelength. The amplification bandwidth at -3 dB is equal to

Fig. 3 :

 3 Fig. 3: Optical budget (blue square curve) and sensitivity at FEC threshold (red circle curve) of the Rx versus the wavelength of Ch 2.

Fig. 4 :

 4 Fig. 4: Optical spectrum of the WDM comb signal at the input of the PDFA (a)) and at the output (b)) with a flatness better than 0.5 dB.

  4 a)) and at the output (Fig. 4 b)) of the PDFA. The input signals are set to present a flatness better than 0.5 dB. On the signal at the output of the PDFA, the signal is not distorted and the flatness of the signal stays better than 0.5 dB. Between the 4 wavelengths transmitted in our experimental setup, 2 are modulated and 2 are continuous wave (CW) signals. The 1 st modulated signal is Ch 3 coming from the DFB-EAM and emitting at 1309.1 nm and the 2 nd modulated signal is alternatively each other three values of the wavelength plan for analysis of Ch 1, Ch 2 and Ch 4.

Fig. 5 :

 5 Fig. 5: SER versus average received optical power measured on point E in BtB (dash curve) and after 30 km of SSMF (full curve) for Ch 1, Ch 2 and Ch 4 (MZM) and Ch 3 (EML).

Fig. 6 :

 6 Fig. 6: Channel insertion losses (blue squares) and channel optical budgets (red circle) for the four channels in BtB.

Fig. 6

 6 Fig. 6 presents the channel insertion losses (blue square) and the OB (red circle) for the four wavelengths of the comb for each channel. Channel insertion losses are defined as 𝑃 𝑇𝑋 (𝑎𝑡 𝑝𝑜𝑖𝑛𝑡 𝐶) -𝑃 𝑅𝑋 (𝑎𝑡 𝑝𝑜𝑖𝑛𝑡 𝐸) -𝛼 𝑂𝐵𝑃𝐹 for the four wavelengths of the comb. The output power of the PDFA (𝑃 𝑇𝑋 ) is equal to 6.6 dBm on the three CW and 6.4 dBm for Ch 3. Channel

Fig. 7 :

 7 Fig. 7: PDFA output power versus input power.

Conclusions

We assess the behaviour of a common praseodymium doped fiber amplifier in a multi-wavelengths real time and digital signal processing free 100Gbit/s/λ transmission with two modulated wavelengths over 30 km of standard single mode fiber with up to 16.15 dB of experimentally measured channel insertion losses. PDFA is still complex to be commercialized massively due to its cost and integration difficulties. However the PDFA can be replaced for example, by a bismuth doped fiber amplifier [10] which can be fabricated in silica fiber allowing an easier fabrication and integration than the fluoride glass used for PDFA. On the contrary, the use of a semiconductor optical amplifier instead of a fiber amplifier can be complicated in links supporting multiwavelengths transmission mainly due to crosstalk issues and non-linear gain compression. This work sketches a possible IM/DD system working at 400 Gbit/s operation on 4λ with a single common O-band fiber amplifier.