Layered Gallium Monosulfide as Phase‐Change Material for Reconfigurable Nanophotonic Components On‐Chip

The demand for information processing at ultrahigh speed with large data transmission capacity is continuously rising. Necessary building blocks for on‐chip photonic integrated circuits (PICs) are reconfigurable integrated low‐loss high‐speed modulators and switches. Phase change materials (PCMs) provide unique opportunities for integration into PICs. Here, the investigation of layered gallium monosulfide (GaS) as a novel low‐loss PCM from infrared to optical frequencies is pioneered, with high index contrast (Δn ≈0.5) at the optical telecommunication band. The GaS bandgap switches from 1.5 ± 0.2 eV for the amorphous state to 2.1 ± 0.1 eV for the crystalline state. It is demonstrated that the reversible GaS amorphous‐to‐crystalline phase transition can be operated thermally and by picosecond green (532 nm) laser irradiation. The design of a reconfigurable integrated optical modulator on‐chip based on Mach‐Zehnder Interferometers (MZI) with the GaS PCM cell deposited on one of the arms for application is presented at the telecommunication wavelength of λ = 1310 nm, where the standard single mode optical fiber exhibits zero chromatic dispersion, and at λ = 1550 nm, where a minimum optical loss of 0.22 dB km−1 is obtained. This opens the route to applications such as reconfigurable modulators, beam steering using phase modulation, and photonic neural networks.


Introduction
The demand for information processing technology at ultrahigh speed with large data transmission capacity is continuously rising due to the fast growth of global data.Efficient and reconfigurable integrated ultra-compact low-loss high-speed photonic modulators and switches are essential components for on-chip photonic integrated circuits (PICs).The unique properties of optical phase change materials (PCMs), including fast state switching between amorphous and crystalline phases, nonvolatile change of optical properties, and a large contrast of refractive index and electrical conductivity between the different phases [1,2] make them suitable for integration into PICs. [3]Although refractive index tuning can be achieved by exploiting thermo-optical effects, liquid crystal, or Pockels cells, the tunability in these cases is relatively limited (typically on the order of 10 −2 ) and requires a continuous energy supply to maintain the desired Figure 1.a) Experimentally measured Raman spectra of various CVD a-GaS films (black lines) as well as their mean value (thick colored line) and standard deviation (shadowed area).The mean value spectrum of a-GaS with the background subtracted is also shown at the top.For comparison, the Raman spectrum of c-GaS is also shown at the bottom.The mode at 418 cm −1 indicated with * is due to the sapphire substrate. [26]b) Representative spectra of the complex refractive index (n() + ik()) of various CVD a-GaS with a thickness in the range 30-150 nm, measured in various laboratories with different instruments during a round-robin test and deposited at temperatures between 50 and 180 °C (black lines) as well as their mean value (thick colored lines) and standard deviation (shadowed areas).The characteristic SEM micrograph of a-GaS showing the grainy-like morphology of the films is in the inset.
state (volatile states). [4]In contrast, PCMs offer the advantage of enabling a refractive index variation of ≈50%. [5]Furthermore, the amorphous and crystalline states of PCMs remain stable for extended periods of time, which is essential for the development of non-volatile tunable photonic devices.Therefore, PCMs are currently employed in reconfigurable nanophotonic applications such as switches, [6][7][8] modulators [8,9] via implementation of Mach-Zehnder Interferometers (MZI) and ring resonators to provide fast, low power and high contrast. [10]Recent works in this field demonstrate further development of hybrid nanophotonic devices, incorporating nonvolatile PCMs such as the prototype Ge 2 Sb 2 Te 5 (GST).Specifically, GST has been integrated in reconfigurable optical devices for applications spanning from programmable photonics, [11,12] neuromorphic computing, [13,14] nonvolatile and rewritable data storage, [1,15] to tunable metasurfaces and flat optics with amplitude/phase control, [10] cloaking, [16] and to reflective displays. [17]Nevertheless, despite providing large index contrast, the fundamental factor limiting GST use in many optical applications is its high optical absorption inducing losses.20][21][22] In this study, we pioneer the investigation of the layered monochalcogenide gallium monosulphide, GaS, as a low-loss PCM in both crystalline and amorphous phases.We experimentally demonstrate a reversible amorphous-to-crystalline and vice versa transition of GaS with a sufficiently high index contrast Δn ≈0.5 while maintaining negligible losses in the optical telecommunication band.Since GaS is a novel optical PCM, we first establish some basic properties of amorphous GaS, a-GaS, such as Raman spectra, refractive index, and extinction coefficient spectra from the UV to the NIR spectral range, as well as the optical bandgap, analyzing statistically a large number (>100) of samples in different laboratories and with different instruments.We further demonstrate that the GaS amorphous-to-crystalline phase transition can be activated both thermally and by green laser ( = 532 nm) irradiation.The reverse crystalline-to-amorphous GaS phase transition is also demonstrated using picosecond laser pulses at  = 532 nm, identifying the energy density boundaries to achieve total amorphization with minimal ablation effects.Based on those findings, we propose the design of a reconfigurable on-chip integrated optical modulator based on MZI with a GaS PCM cell deposited on one of the arms to work at the telecommunication wavelengths of  = 1310 nm, where standard single mode optical fiber exhibits zero chromatic dispersion, and of  = 1550 nm, where minimum optical losses (0.22 dB km −1 ) are obtained.Silicon nitride (Si 3 N 4 ) is chosen as a material for integrated photonic circuit building blocks, since it provides low loss waveguides in the required wavelength range. [13,23]This GaSbased design paves the way to applications such as low-loss modulators, beam steering using phase modulation, and photonic neural networks.

Optical and Structural Properties of Amorphous GaS (a-GaS)
Figure 1a shows the Raman spectra taken over a large number (>100) of CVD deposited films of amorphous GaS (a-GaS) as well as their mean value (solid line) and standard deviation (shadowed region).The analysis of the measured Raman spectra after background subtraction and fitting of the main features to Gaussian profiles, also shown in Figure 1, established the position of the a-GaS broad Raman modes at 174 cm −1 and 362 cm −1 .These modes are related to the two out-of-plane A 1 1g (189 cm −1 ) and A 2 1g (362 cm −1 ) Raman modes of crystalline GaS (c-GaS, space group P6 3 /mmc no.194) [24] as it can be inferred by the comparison with the Raman spectrum of c-GaS also shown in Figure 1.Differently from the Raman spectra of c-GaS, those of a-GaS do not show the in-plane E 1 2g mode (296 cm −1 ).The shift to lower wavenumbers of the A 1 1g mode as well as the absence of the E 1 2g mode is due to the low crystallinity and lattice expansion of a-GaS.A typical SEM image of the morphology of the a-GaS films is reported in the inset of Figure 1b, showing that the films are continuous with a grainy-like morphology (see also Figure SI#1, Supporting Information).
The a-GaS complex refractive index (n() + ik()) was derived from a Tauc-Lorentz [25] analysis of ellipsometric spectra measured statistically for > 100 samples.Figure 1b shows some individual representative measurements of the refractive index, n, and of the extinction coefficient, k, as well as their mean value and standard deviation.The range of spectra reported represents the experimentally obtained variation of the n and k spectra of a-GaS deposited at temperatures in the range of 50 -180 °C and under various conditions of precursors flow and pressure.All these factors affect the a-GaS deposition and consequently the morphology and surface defects that may exist in the films, including the stress and strain change.As an example, although all a-GaS layers have grain-like microstructure, the increase in temperature results in an increase of the grains size (see Figure SI#1, Supporting Information) and consequently, in higher values of n and k in Figure 1b.The optical energy band gap of all a-GaS samples extracted from the Tauc-Lorentz modeling is E g = 1.5 ± 0.2 eV.The imaginary part of the dielectric function of a-GaS films is characterized by a broad maximum in the range 4.5 -5.1 eV that relates to the broadening and of all electronic transitions involving S 3p , S 3s , Ga 4p, and Ga4s as identified in ref., [27] contributing to the optical absorption of GaS in the visible (Vis) and ultra-violet (UV) energy range.Those transitions broaden differently depending on the deposition conditions that also affect defects including lattice gallium and sulphur vacancies (V Ga and V S ), antisites (Ga S and S Ga ) and interstitials (Ga i and S i ) in GaS, explaining the observed changes in the n, k spectra.
In (b) the green dashed lines show, for comparison, the optical refractive index and extinction coefficient derived for the thermally amorphized GaS (see discussion below).

Thermal Crystallization of a-GaS
Changes in the structural and optical properties upon thermal crystallization were monitored in real-time using time-resolved spectroscopic ellipsometry and Raman spectroscopy.Figure 2a shows the real-time variation of the pseudodielectric function 〈 2 〉 represented here for clarity at the photon energy of 3.9 eV upon annealing in the air with a temperature increase rate of 5 °C min −1 .The 3.9 eV photon energy has been chosen as it corresponds to the energy of the first critical point (E 1 ) in the dielectric function of single crystal GaS, which should appear during the amorphous-to-crystalline transition. [27]From the additional Raman spectra shown in Figure 2c three regimes of crystallinity could be identified in the temperature dependent 〈 2 〉 profile, i.e., (i) For 20 °C < T < 250 °C, 〈 2 〉 shows negligible variation, indicating incipient crystallization at T ≈250 °C.Consistently, the Raman spectra do not change significantly, showing mainly the two broad features at 174 cm −1 and 362 cm −1 characteristic of a-GaS.(ii) For 250 °C < T < 400 °C, the 〈 2 〉 increase is associated with crystallization, as demonstrated by the appearance of the Raman modes A 1 1g at 189 cm −1 , E 1 2g at 296 cm −1 and A 2 1g at 362 cm −1 of c-GaS (see Figure 2c) over the a-GaS background and the broad component at 174 cm −1 , indicating that the two phases coexist and several intermediate states can be achieved in this temperature range.We identified T = 250 °C as the glass transition temperature (T g ) from which the atoms become increasingly mobile and move to the energetically favorable crystalline state.More details are in Figure SI#2 (Supporting Information) showing pictures of the crystallization front advancing, emphasized by the change of the color of the film due to the change of the optical bandgap from amorphous to crystalline; the SEM pictures as well as the XRD analysis demonstrate the crystallization in the hexagonal GaS phase.Noteworthy, we statistically verified that when Raman spectra are similar to that shown in Figure 2b, with the A 1 1g at 189 cm −1 , E 1 2g at 296 cm −1 , and A 2 1g at 362 cm −1, the XRD analysis always shows dominant the (002) and ( 004 Figure 2c,d show the spectra of the refractive index, n, and of the extinction coefficient, k, for the initial a-GaS and after thermal crystallization at 350 °C and 450 °C.Clearly, in the thermally crystallized GaS the feature at 3.9 eV due to the E 1 critical point of c-GaS [27] appears. The band gap of the thermally crystallized GaS was determined to be 1.75 ± 0.15 eV, i.e., it increases from that of a-GaS, moving toward that of c-GaS.Figure 2e shows the refractive index contrast Δn and Δk between amorphous and thermally crystallized/single crystal GaS as the main metric to evaluate the performance of this materials as PCM.A refractive index contrast as high as Δn = 0.5 with negligible optical losses, i.e., Δk = 10 −2 , can be reached for photon energies lower than the bandgap of the a-GaS.Furthermore, as shown in Figure 2c, in the NIR and visible ranges, the refractive index of both the single crystal, n sc (0.75 eV) = 2.40, and the thermally crystallized, n c (0.75 eV) = 2.35, GaS is lower than that of a-GaS, n a (0.75 eV) = 2.55.This behavior differs from the typical GST PCM that has a refractive index higher in the crystalline phase than in the amorphous phase.The unusual inverse optical contrast found for GaS has also been reported for GaSb, [28] and was associated with a crystallization process that causes a transition to a less metallic and more semiconducting behavior. [29]Indeed, this behavior is consistent with the Moss rule, [30] which relates the refractive index (or optical dielectric constant) with the inverse of the energy gap in semiconducting materials.
Crystallized GaS samples with a thickness of ≈100 nm, which is of interest for device design, were subsequently rapidly (10 °C sec −1 ) annealed in air to T ≈850 °C and quickly quenched down to room temperature (≈25 °C) to amorphized them.Unfortunately, from the equipment point of view, this hightemperature heating system was not under the ellipsometric control, and we could not follow the amorphization kinetics in real time.However, the Raman spectrum and ellipsometric spectrum (displayed with dashed green lines in Figure 1b) recorded at the end of this amorphization step were similar and in the range of 〉 at the photon energy of 3.9 eV, corresponding to the E 1 interband critical point of crystalline GaS, as a function of the annealing temperature with a 5 °C min −1 temperature increase rate.b) Raman spectra of as-deposited a-GaS, and after thermal crystallization at 350 °C and 450 °C.The Raman modes of c-GaS are indicated with dashed lines.The asterisk indicates the 418 cm −1 Raman mode of the sapphire substrate. [26]c) Refractive index, n, and d) extinction coefficient, k, of the of single crystal c-GaS, of a-GaS and of thermally crystallized GaS at 350 °C and 450 °C.e) Refractive index contrast between amorphous/thermally crystallized GaS and amorphous/single crystal GaS.
those reported in Figure 1, providing an indication of the thermal amorphization.This was also supported by the AFM and lateral force microscopy analysis of the sample recorded after the thermal amorphization test reported in Supporting Information (see Figure SI#3, Supporting Information) for the amorphouscrystallized-amorphized cycle.
Because of the high temperature (T>850 °C) identified for the thermal amorphization, we have been exploring the laserinduced phase transformation as described in the section below.

Laser Induced Amorphous-to-Crystalline Phase Change of GaS
Interestingly, we found that GaS phase transformations could be activated by green (532 nm) laser irradiation.Figure 3b-f show the enhanced optical contrast micrograph and maps of the ellipsometric parameters ∆ and  measured at 3.1 eV for three a-GaS 10 μm ×10 μm regions crystallized rastering a 532 nm laser beam with a 100× (NA = 0.9) microscope.The three regions correspond to different irradiation energies of 1.3, 2.6, and 5.2 nJ nm −3 (assuming a perfect Airy disc shaped laser spot and no laser power leaks with in the instrument).
The crystallinity of the laser crystallized regions was assessed by the Raman spectra reported in Figure 3h.While the Raman spectra of the amorphous background show the two broad features, the laser crystallized regions show sharp Raman features at 189 cm −1 and at 360 cm −1 assigned to the c-GaS A 1 1g and A 2 1g Raman modes. [24,31]Noteworthy, the Raman modes frequency coincides with those in Figures 1 and 2, which are associated with the XRD pattern of the hexagonal phase, indicating that  GaS ) and sapphire substrate at 418 cm −1 (I 418 sapphire ) as a function of the laser pulse energy density. [26]A picture of a laser amorphization raster performed on a 35 nm thick GaS film is shown in the inset.b) Raman spectra of the GaS film before and after laser amorphization.c) Spectra of the refractive index, n, and extinction coefficient, k, measured in the laser amorphized regions labeled in (a) as spot #1 and #2, compared to that of the as-grown a-GaS from Figure 1 (dashed line).d) Δ and Ψ maps measured at 2.5 eV on laser amorphized spot #2. also laser crystallization results in the hexagonal GaS phase (as, an example we have evidences that a shift of the Raman modes to lower wavenumber would indicate a different crystalline phase, e.g. the 3R-phase).The color contrast between the amorphous background and crystallized regions indicates different optical properties, as also supported by spectral reflectance and ellipsometry measurements of the three crystallized regions in Figure 3c,g.Figure 3i shows the complex refractive index of amorphous and laser crystallized GaS phases, leading to the optical contrast shown in Figure 3j, keeping negligible optical losses, i.e., Δk = 10 −2 .The laser crystallization clearly results in a blue-shift of the optical bandgap and a decrease of the refractive index, consistent with data in Figures 1 and 2

Reversible Laser Induced Crystalline-to-Amorphous Phase Transition
Crystallized GaS can be reversibly amorphized by green pulsed laser (30 ps,  = 532 nm) irradiation.The parameters window for laser amorphization was explored by varying the laser power and spot size on the sample.A picture of an amorphization raster using a pulse energy of 223 μJ and spot sizes ranging from 100 to 400 μm on a 35 nm thick crystallized GaS sample is shown in Figure 4a, displaying the intensity ratio between the Raman mode at 174 cm −1 of a-GaS (I 174 GaS ) and that of the sapphire substrate (I 418 sapphire ) at 418cm −1 . [26]By maximizing this ratio, it is possible to identify the amorphization region where the laser pulse preserve a-GaS with minimal GaS ablation.This condition is matched for amorphization laser energy densities between 0.10-0.15fJ nm −3 as indicated by the green shadowed region in Figure 4a.From now on, we will refer to the spots in these switching energy densities as spot #1 (0.10 fJ nm −3 ) and #2 (0.13 fJ nm −3 ).These laser amorphization energies results in very homogeneous and smooth (see AFM image in Figure SI#5, Supporting Information) amorphization, as indicated by the Ψ,Δ maps measured on  4b shows the Raman spectra of the film before and after laser amorphization.After this process, the Raman modes of the previously crystallized film disappear leading to a spectrum dominated by the broad modes of a-GaS shown in Figure 1a.
In order to study the refractive index of the laser amorphized areas, imaging ellipsometry was performed on the spot #1 and spot#2 of Figure 4a.A strong contrast on both magnitudes can be seen between the laser amorphized GaS spot and the crystallized GaS, and the maps show a very homogeneous amorphized area.Figure 4c shows the value of the refractive index measured on spots #1 and #2 compared to that measured in the crystalline area and to that of the as-grown amorphous region.Interestingly, the AFM image of the laser amorphized GaS (see Figure SI#5d, Supporting Information) shows a quite smooth morphology with a surface roughness of 5 nm, exploitable in device applications.Therefore, we have demonstrated the proof-of-concept of a cycle of crystallization and amorphization of GaS, and measured the reproducible refractive index and optical contrast at the level of the material.

Comparison of GaS with other PCMs
Low optical losses in PCMs (i.e., low values of the extinction coefficient for amorphous and crystalline phases, k a and k c ) combined with a higher refractive index contrast Δn are a crucial requirement for PCM integration in photonic applications.This is due to the proximity of the PCM patches to the optical modes. [4]or example, in applications such as optical switches [32][11,33] or photonic memories, [15,34,35] it is preferable to have a PCM with exploitable loss in one phase while the other phase is loss-free.Conversely, for phase shifters, in which phase control is needed independently of amplitude changes of the propagating signal, low losses are required for both phases. [12]In order to evaluate the suitability of different PCMs for different applications operating at different wavelengths, several figures of merit (FOM) have been proposed.Table 1 compares the value of different FOMs of Δn, Δk, Δn/k a , and Δn/k c .calculated for emerging PCMs at various technology relevant wavelengths of 1550 nm (C telecom band), of 1310 nm (O telecom band) and of 633 (nitrogen vacancy centers in diamond quantum emitters). [4]y comparing FOMs derived from the present study for GaS, it can be inferred that it presents similar capabilities as Sb 2 S 3 , [19] which is another emerging layered chalcogenide.At 1310 nm and 1550 nm, both materials are low-loss with similar values of Δn, making them suitable for integration in phase-shifters.At 633 nm, GaS and Sb 2 S 3 are better materials than GST to be integrated in optical memories as well as optical switches considering that Δn/k a is much higher than for the crystalline phase (Δn/k c ).Therefore, both GaS and Sb 2 S 3 are suitable materials for operational wavelengths in the visible.Noteworthy, for the experiments of crystallization and amorphization performed with GaS in this work, no capping layer has been deposited on GaS.Nevertheless, it stayed stable over months even after the phase change treatments, as the Raman spectra did not shown any peak due to other phases segregation or heavy oxidation, such as Ga 2 O 3 (which presents several Raman modes in the range 350-470 cm −1 and 500-700 cm −1 not observed herein); conversely segregation of antimony, Sb, and its oxidation to Sb 2 O 3 , have been reported for Sb 2 S 3 [22] during thermal phase change.Additionally, also ellipsometry spectra showing an optical bandgap below 2 eV characteristics of GaS support a negligible oxidation, as Ga 2 O 3 would have increased the optical bandgap to more than 4 eV, which was not observed in the present case (Ga 2 O 3 is a semiconducting material with an ultra-wide bandgap of ≈4.8 eV).

Integrated Reconfigurable Nanophotonic Phase and Amplitude MZI Modulator based on GaS PCM Cell On-Chip
On the basis of the determined GaS characteristics, we have designed a reconfigurable integrated optical MZI modulator onchip with a GaS PCM cell deposited on one of the arms (see sketch in Figure 5a) for a wide application range in the infrared region, in particular at  = 1550 nm, where it is obtained minimum optical loss (0.22 dB km −1 ) and at telecommunication wavelength  = 1310 nm, where standard single mode optical fiber exhibits zero chromatic dispersion (see Figure SI#5, Supporting Information).Silicon nitride (Si 3 N 4 ) is chosen as a material for integrated photonic circuit building blocks, as it provides low loss waveguides in the required wavelength range. [13,23]A micrograph of the GaS cell nanopatterned ontop of the arm is also shown in Figure 5a, demonstrating the feasibility of the nanofabrication step of GaS-on-chip.
The amplitude (intensity) modulation at the output of the MZI at telecommunication band wavelengths can be obtained via phase modulation of the propagation mode in one of the arms of the MZI, which is achieved by modulation of the effective refractive index of the propagation mode in this arm.This can be reached by a change of the state of the PCM cell placed on top the MZI arm, which can be triggered via evanescent coupling of irradiating light at  = 532 nm to the PCM cell.Laser induced amorphization is triggered with a single optical pulse (a few mW per pulse), while crystallization can be achieved by sending a train of pulses with decreasing power.
The planar Si 3 N 4 waveguide on a buried oxide substrate is designed for single transverse electrical mode (TE0) operation in the telecommunication bandwidth.The thickness of the Si 3 N 4 layer corresponds to commercially available stoichiometric silicon nitride-on-insulator wafers, which we will utilize for the fabrication of the designed and simulated reconfigurable low-loss MZI modulator on-chip.The modeled evanescent interaction be-tween the waveguide TE0 mode at  = 1550 nm and the GaS PCM cell in the amorphous and crystalline states are shown in Figure 5b,c.
Upon the phase transition c-GaS → a-GaS, the guided mode is pulled upwards by the PCM a-GaS cell resulting in higher n eff of the guided mode in comparison with crystallize state of the cell, leading to the modification of the phase of the mode.
The linear increase of the phase shift of the guided mode with the length of the GaS PCM cell on top of the arm of the MZI at working wavelength  = 1550 nm is shown in Figure 5e.The required PCM cell length needed to obtain a relative -shift of the phase as a result of the switching between PCM states was determined via numerical 2D simulations using frequency domain (FEM) COMSOL Multiphysics software and demonstrated in Figure 5d.The GaS PCM cell with length L  placed on top of one of the arms of the MZI ensures amplitude on/off switching of the signal at  = 1310 nm and  = 1550 nm at the output of the reconfigurable MZI modulator (see also for 1310 nm Figure SI#6, Supporting Information).We have demonstrated the detailed design of the proposed MZI modulator in Ref. [22]  Importantly, in order to determine the necessary thickness of the GaS PCM cell, the switching mechanism of the cell states via evanescent coupling of irradiating light at  = 532 nm should be taken into account.An amorphous GaS cell on top of a waveguide possesses a large complex refractive index (n amorph = 3.20 + 0.357i) at  = 532 nm.This creates an overlap between the highly absorptive a-GaS cell and the optical guided mode at  = 532 nm, which induces strong absorption of light by the cell, leading to the state transition a-GaS ↔ c-GaS.This leads to attenuation of the optical signal at  = 532 nm and a decrease in its transmission through the waveguide.We utilized 2D FEM simulations in COMSOL Multiphysics to calculate the cross-sectional mode and the strength of this interaction, and thus determined the transmission of light at  = 532 nm through the waveguide with a-GaS and c-GaS PCM cells on top, demonstrated in Figure 6.Maximum absorption of irradiating light at  = 532 nm within an a-GaS PCM cell is obtained for the cell with thickness h = 30 nm (Figure 6b) which leads to minimum transmission of light at  = 532 nm through the waveguide with a GaS PCM cell (Figure 6a).For other thicknesses, h, of the GaS cell, the strength of this interaction between the propagation guided mode through the waveguide at 532 nm (inside the rib waveguide) and the GaS PCM cell decreases, leading to less absorption and higher transmission at this wavelength as demonstrated in Figure 6.
Thus, to obtain a -shift of the phase at  = 1550 nm via a-GaS ↔ c-GaS transition of the PCM cell (placed on one of the arms of MZI), triggered by evanescent irradiation of the cell at  = 532 nm, a PCM cell with thickness h = 30 nm with corresponding length L  (Figure 5a) is required.However, taking into account the high optical propagation loss of 4 dB μm −1 through the waveguide with an a-GaS PCM cell (h = 30 nm) at  = 532 nm determined from Figure 6a, to obtain effective switching of the states of a long PCM cell L  we propose to pattern GaS as micro-cells [22] and position them atop the optical waveguide.The evanescent coupling of irradiating light at  = 532 nm is addressed simultaneously via, for instance, crossed waveguides with a low inser-tion loss. [13,36,37]Notably, patterning PCM layer into micro-cells and integrating them on top of a waveguide in the maximum of the electric field improves switching contrast due to possible complete phase change of the cell, leading to higher index contrast and prevents damage of the cell during irradiation, as it is experimentally demonstrated in Ref. [33] Addressing the irradiation light through low-loss crossed waveguides provides independent control over the phase state of each patterned GaS microcell. [38,39]Further improvement, namely a decrease of L  by a factor of two can be reached by placing the PCM cells in both arms of the MZI and switching them independently. [23,40]

Conclusions
In conclusion, we have introduced GaS as a switchable phase change material with a wide bandgap suitable for photonic applications.To the best of our knowledge, the Raman spectra, refractive index, and glassy temperature of amorphous GaS have been identified.Additionally, we have demonstrated the refractive index contrast between amorphous GaS and thermally/laser crystallized GaS, approximately ∆n = 0.5, while maintaining lowloss (k = 0) operation at the telecom C-band ( = 1550 nm).We have also demonstrated the reversible amorphous-to-crystalline transition, achieving the amorphization of GaS through picosecond laser pulses.Finally, we have designed a reconfigurable integrated optical modulator on-chip based on a MZI with a GaS PCM cell deposited on one of the arms, for a wide application range in the infrared region, in particular at the telecommunication wavelength  = 1310 nm, where standard single mode optical fiber exhibits zero chromatic dispersion, and at  = 1550 nm, where minimum optical loss (0.22 dB km −1 ) is obtained.The feasibility of the integration of GaS on a silicon nitride photonic platform compatibly with the Si 3 N 4 chip fabrication process is given by the low deposition CVD temperature of 50 °C for the a-GaS which is compatible with chips fabricated and patterned with conventional photoresists.The GaS patch of the desired shape can be then obtained by conventional lift-off of the photoresist (e.g., dipping in room temperature acetone and 60 °C NMP with ultrasonic agitation) without damaging the GaS.Furthermore, this wider bandgap GaS approach also offers the possibility of a more sustainable process Te-free and Ge-free (e.g., GST-family).Additionally, GaS offers the possibility to control the state of GaSbased phase shifters on the same chip individually by optically inducing phase transition through the irradiation of visible light.This opens up the route to such applications as low-loss modulators, beam steering using phase modulation, and photonic neural networks.

Experimental Section
Amorphous GaS, a-GaS, layers with a thickness in the range of 5 -150 nm were grown by chemical vapor deposition (CVD) on Coring glass and sapphire using a Ga 2 S 3 precursor decomposed at 800 °C in H 2 flow.In order to investigate the range of deposition parameters leading to amorphous deposition, the H 2 flow was changed in the range 30 −300 sccm, the pressure changed in the range 0.3-100 Torr and the substrate temperature for the a-GaS deposition was varied between 50 and 180 °C.180 °C is considered an upper limit to have amorphous films as some incipient crystallization may occur.
X-ray photoelectron spectroscopy (XPS) measurements were carried out by a Scanning XPS Microprobe (PHI 5000 Versa Probe II, Physical Electronics) equipped with a monochromatic Al K x-ray source (1486.6 eV), with a spot size of 200 μm.Survey (0-1200 eV) and high-resolution spectra (C1s, O1s, S2p, S2s, Ga2p3, Ga3d) were recorded in FAT mode at a pass energy of 117.40 and 29.35 eV, respectively.Spectra were acquired at a take-off angle of 45°with respect to the sample surface.Surface charging was compensated using a dual beam charge neutralization system, and the hydrocarbon component of C1s spectrum was used as an internal standard for charging correction, and it was fixed at 285 eV.XPS demonstrated a GaS = 1:1 stoichiometry.
Morphology was determined by atomic force microscopy (AFM) using the Autoprobe CP (Thermomicroscope).The sample topography was recorded in a single-pass mode using a gold-coated Si tips (their frequency is ≈80 Hz) in non-contact mode.Further scanning electron microscopy (SEM) was carried out for the morphological characterization of the samples with a Zeiss Supra 219 40 FEG SEM equipped with a Gemini field emission gun.Analyses were carried out at an extraction voltage of 3 kV and 221 a 30-μm aperture.The AFM was also operated in lateral force microscope (LFM) and electrical force microscopy (EFM) modes to emphasize different phases.
Raman spectroscopy was performed with a LabRam Horiba set up using a ×100 microscope objective (NA = 0.9) and excitation wavelength of 532 nm and 1 mW to monitor and induce the laser crystallization.Under the Raman setup, the sample was mounted on a programmable moving stage.Crystallized patterns were produced by drawing 10 μm by 10 μm squares in steps of 0.5 μm with exposure times at each point of 5, 10, and 20 s to ensure the crystallization.
Optical properties, namely spectra of the complex pseudodielectric function, ⟨⟩ = ⟨ 1 ⟩ + i⟨ 2 ⟩ = (n+ik) 2 , were measured by spectroscopic ellipsometry (UVISEL Horiba) in the photon energy range 0.75 -6.5 eV with a resolution of 0.05 eV.The ellipsometric measurements were fitted to a three-media substrate/film/surface roughness/air model, where the glass substrate was experimentally measured prior to deposition.The GaS layer was parameterized using the Tauc-Lorentz oscillators model to derive the refractive index, n, and extinction coefficient, k, of the amorphous and crystallized GaS.The surface roughness was modeled using a Bruggeman effective medium approximation (BEMA) of 50% GaS and 50% voids.
A Linkam THMSEL600 heating cell with programmable temperature control was attached to spectroscopic ellipsometer and used for temperature dependent measurements to investigate the crystallization dynamics.The dynamic temperature-dependent measurements were conducted at a 70°incidence angle and with a 5 °C min −1 heating rate.The main purpose was to determine the onset of the a-GaS crystallization temperature and measure the associated change in the refractive index.The ellipsometer spot size on the sample was 2 mm x 5 mm; hence, the optical measurements refer to this area, which was checked to be homogeneous (by Raman and microscopy maps) from the amorphous and crystallization point of view.
The imaging spectroscopic ellipsometry measurements were performed with an ACCURION EP4 acquiring Psi and Delta in rotating compensator mode at 50°and 60°angles of incidence.For the UV range (200 nm to 350 nm in steps of 5 nm) an Accurion NC3 nanochromat objective with 7x magnification was used, and for the VIS range (340 nm to 1000 nm in steps of 10 nm) a Nikon CF Plan WD 20.3 with 10x magnification was used.
For the amorphization experiments, frequency doubled laser pulses from a Nd: YAG/YVO 4 laser were used.The wavelength of the laser radiation was 532 nm.The pulse duration was 30-35 ps with a measured pulse energy of 223 μJ.The sample was irradiated with a weakly focused beam with low beam divergence (total angular spread of 2.7°) and relatively large spot diameters, which were measured with the knife's edge method.The sample was placed on top of a 3-axis translation stage for rastering with the laser.The experiments were carried out in the air and at room temperature.The rastering was performed in the following way: the focus of the laser beam was above the sample surface in air.For each spot in the raster, all laser parameters were kept the same.After each laser "shot" the sample was lifted by 0.5 mm upwards toward the focus region to decrease the spot size and therefore increase the energy per area of the laser on the sample surface.The focus region itself was never reached and always remained above the sample.For each spot, the sample was at rest and one single laser pulse was applied.In the end, additional marks were created with the laser, to know the orientation of the raster for later characterization.The distance between the rows and columns in the raster is 500 μm, the FWHM of the laser spot was between 320 and 53 um and was decreased by 14 μm per 500 μm when stepping the sample upwards through a range of 9.5 mm to irradiate at 20 different energy densities.The spots in the raster were written from bottom to top and from left to right.
) reflection indicative of a preferential (0001) orientation of the hexagonal crystallized GaS.(iii) For 400 °C < T < 550 °C, 〈 2 〉 reaches a constant plateau value.The Raman spectra measured at 450 °C shows sharp A 1 1g , E 1 2g , and A 2 1g Raman modes and the disappearance of the amorphous background associated with the almost complete crystallization of the GaS film, whose optical 〈 2 〉 signal does not change further.

Figure 2 .
Figure2.a) Real time variation of 〈 2 〉 at the photon energy of 3.9 eV, corresponding to the E 1 interband critical point of crystalline GaS, as a function of the annealing temperature with a 5 °C min −1 temperature increase rate.b) Raman spectra of as-deposited a-GaS, and after thermal crystallization at 350 °C and 450 °C.The Raman modes of c-GaS are indicated with dashed lines.The asterisk indicates the 418 cm −1 Raman mode of the sapphire substrate.[26]c) Refractive index, n, and d) extinction coefficient, k, of the of single crystal c-GaS, of a-GaS and of thermally crystallized GaS at 350 °C and 450 °C.e) Refractive index contrast between amorphous/thermally crystallized GaS and amorphous/single crystal GaS.

Figure 3 .
Figure 3. a) Sketch of the 532 nm laser crystallization of a-GaS.b) Ellipsometric enhanced contrast micrograph indicating with green polygons (regions of interest, ROIs) the regions where the ellipsometric parameters were measured.Ellipsometric c) Δ and d) Ψ maps measured at 3.1 eV.e) Atomic force microscopy (AFM) topography of a whole laser crystallized area.f) Optical micrograph of the regions crystallized at different irradiation energy of 1.3 nJ nm −3 , 2.6 nJ nm −3 , and 5.2 nJ nm −3 (from left to right).g) Reflectance spectra measured at an angle of incidence of 50°in the three laser crystallized areas; h) mean (solid line) and standard deviation (shadowed area) of the Raman spectra.i) Refractive index, n, and extinction coefficient, k, of amorphous and crystallized GaS.j) Refractive index contrast between amorphous and crystallized GaS.The shadowed area represents the interval of contrast achievable by changing the laser irradiation conditions.

Figure 4 .
Figure 4. a) Intensity ratio between Raman modes of a-GaS at 174 cm −1 (I 174GaS ) and sapphire substrate at 418 cm −1 (I 418 sapphire ) as a function of the laser pulse energy density.[26]A picture of a laser amorphization raster performed on a 35 nm thick GaS film is shown in the inset.b) Raman spectra of the GaS film before and after laser amorphization.c) Spectra of the refractive index, n, and extinction coefficient, k, measured in the laser amorphized regions labeled in (a) as spot #1 and #2, compared to that of the as-grown a-GaS from Figure1(dashed line).d) Δ and Ψ maps measured at 2.5 eV on laser amorphized spot #2.
. The standard deviation on Δn and Δk is reported in Figure 3j.The shadowed regions indicate that for the laser crystallized GaS, intermediate states can be achieved depending on laser irradiation energy, pulses, and time.The AFM topography in Figure 3e taken on one of the squares also shows clearly the contrast due to the different morphology of the crystallized and amorphous regions (more AFM details are shown in Figure SI#4, Supporting Information).

Figure 5 .
Figure 5. a) Sketch of the GaS cell placed on top of one arm of the MZI modulator; a micrograph of the GaS cell nanopatterned ontop of the arm is also shown, demonstrating the nanofabrication step of GaS-on-chip.b,c) Simulated TE0 guided mode profile at  = 1550 nm through the Si 3 N 4 waveguide (width -1200 nm, thickness -334 nm) on an SiO 2 substrate with a-GaS (left panel) and c-GaS (right panel) cells integrated on the waveguide.The effective refractive index of the guided mode is labeled.GaS cell thickness is 30 nm. d) GaS PCM length L  needed to attain relative -shift of the phase of the guided TE0 mode in the MZI arm equipped with the cell on top of a Si 3 N 4 waveguide as a function of thickness of the cell at  = 1550 nm (red curve) and  = 1310 nm (blue curve).e) Phase shift as a function of the length of the GaS PCM cell at  = 1550 nm placed on one arm of MZI, the thickness of PCM cell h is labeled in the legend.

Figure 6 .
Figure 6.a) Transmission of the guided mode at  = 532 nm through a Si 3 N 4 waveguide (thickness -334 nm) on a SiO 2 substrate with an a-GaS (solid diamond points) and a c-GaS (open diamond points) cell on top of the waveguide, as a function of the length of the cell.The thickness h of c-GaS cells corresponds to similar color a-GaS cells.b) Absorption of the light at  = 532 nm within an a-GaS PCM cell as a function of the length of the cell.The thickness of PCM cell h is labeled in the legend.