All‐Optical Switching of Structural Color with a Fabry–Pérot Cavity

Fine tuning the optical responses of thin‐film devices is highly attractive for emerging applications, such as optical memories, solar cells, nanophotonics, and photodetectors. Even though thin‐film technology is well established, dynamically switching the optical responses of thin films after fabrication remains challenging because of passive materials and device structures. This work demonstrates an approach for all‐optical switching of structural colors excited by a Fabry–Pérot (FP) cavity inside a metal–dielectric–metal (MDM) thin‐film stack. A low‐loss phase‐change material (PCM), that is, antimony trisulfide (Sb2S3), which is embedded in the stack, enables efficient FP cavity resonance in the visible spectrum. 1) Color reflectivity of >60%; 2) multistructural colors using a single MDM cavity; 3) a wide dynamic range of colors of up to ≈220 nm; and 4) a gamut coverage of more than 80% of standard RGB (sRGB) are achieved. The all‐optical switching is realized via the crystallization and reamorphization of Sb2S3 using continuous‐wave and pulsed lasers, respectively. The findings provide a framework for the cost‐effective realization of dynamically responsive thin‐film‐based nanophotonic devices.


Introduction
Thin-film structures have been commonly utilized in many applications, such as structural color windows, [1] optical memory, [2] and photovoltaic cells. [3]Although their performance can be altered with different designs and materials, they are intrinsically static once the devices are fabricated.Dynamically tuning the optical response of thin film structures can enable active multifunctional devices to address the demand for tunable optical thin-film technology.Fabry-Pérot (FP) cavity is a basic thin-film structure, consisting of a dielectric layer sandwiched by two metallic layers to form a metal-dielectric-metal (MDM) cavity. [4]The light interference of the MDM cavity is dominated by the thickness and optical properties of the dielectric material.Fundamentally, light interference in the MDM cavity follows the formula of λ/4n, where λ is the reflection wavelength and n is the real refractive index (RI) of the dielectric material.[7] So far, quite a few works have demonstrated tunable responses in the MDM structures via thickness modulation.For example, an electrical bias was applied to migrate a silver film based on the ionic transport through a layer of amorphous iron oxide, which led to the thickness variation of the thin-film stack.As a consequence, dynamic structural colors were realized via the reversible time-resolved optical response. [1]In addition, by varying the intensity of the ultraviolet light incident onto a conductive polymer, the polymer's thickness can be altered accordingly. [8]oreover, the thickness of poly(N-isopropyl acrylamide) (PNIPAm) brush polymer sandwiched between the silver and chromium metal layers can be thermally changed, leading to tunable thin-film structural colors. [9]Indeed, electrical and thermal tuning stimuli are limited by their speed.
Besides thickness modulation, the alternative way to change the interference of the MDM cavity is via RI modulation of the dielectric layer using phase-change materials (PCMs).Due to the rapid development of nonvolatile photonics in recent years, PCMs offer a unique combination of properties, making them a suitable candidate for tailoring the response of the MDM cavity.][16][17][18][19][20][21][22][23][24][25][26][27][28][29] As a typical PCM, germanium-antimony-tellurium (GST) was sandwiched between two indium tin oxide (ITO) layers in an MDM cavity to demonstrate stable color changes, with a back metal reflector to enhance color purity and light confinement.The reflective color varied with the thickness of the GST layer across the visible spectrum. [30]However, the color changes were limited between the amorphous and crystalline states only, and the large optical losses in GST degraded color reflection efficiency and purity.On the other hand, a transmissive color filter in an MDM cavity was demonstrated using a thin film of GST, [31] but its tuning range was small due to the weak field confinement inside the GST film.Increasing the thickness of the GST layer would induce remarkable optical losses and reduce color purity.So far, high-performance tunable structural color assisted by refractive index modulation of the MDM cavity is yet to be realized.A low-loss PCM is key for sustaining high-field confinement inside the MDM cavity at the visible spectrum.
[37][38][39][40][41] Moreover, their optical properties can be switched via external stimuli, such as electrical, optical, or thermal stimuli.Among them, optical tuning offers the fastest switching speed.Upon exposure to a continuous wave (CW) laser, these materials absorb the laser energy transforming their phase from amorphous to crystalline.The intermediate state, a mixture of amorphous and crystalline phases with different ratios between amorphous and crystalline, can be obtained by precisely manipulating the CW laser energy absorbed by PCMs.During the reamorphization, PCMs absorb a large amount of energy in an ultrashort time to randomize molecules again.This process refers as melt quenching and is usually implemented using a pulsed laser. [42]Previous research has demonstrated the optical reamorphization of Sb 2 S 3 using a few femtosecond laser pulses.[45] In contrast, fast reamorphization is achievable using picosecond (ps) lasers with relatively higher-energy per pulse and optimizing the total pulse energy absorbed by the Sb 2 S 3 film.In addition, stable changes between multiple intermediate states of Sb 2 S 3 during crystallization are still elusive.The realization of alloptical switching of the multiple states of Sb 2 S 3 will advance the development of ultrafast, tunable nanophotonics and optoelectronic devices with a multilevel and dynamic response in the visible spectrum. [46,47]n this work, we incorporated a single layer of amorphous Sb 2 S 3 in the MDM cavity consisting of silver at the bottom and titanium at the top.It acts as a FB cavity to enhance light confinement inside the Sb 2 S 3 layer.Upon the phase change of Sb 2 S 3 , the reflection resonance is modulated across a broad visible spectrum.Reversible switching, that is, from crystallization (from the amorphous state followed by multiple intermediate states to the crystalline state) to reamorphization and vice versa was realized using a CW-laser and a ps laser.

Results and Discussion
Figure 1a shows the design concept for tunable structural colors generated from an MDM cavity of multilayer thin-film structures.To fabricate the proposed cavity, a 5 nm-thick titanium (Ti) layer was initially deposited as an adhesion promoter, followed by a 100 nm silver (Ag) layer to serve as a low-loss back reflector.After that, a 25 nm-thick silicon nitride (Si 3 N 4 ) layer was deposited to isolate the silver layer from the 100 nm Sb 2 S 3 film atop, which was then deposited using radio frequency (RF) sputtering.The following layers were subsequently deposited, Si 3 N 4 (25 nm), Ti (10 nm), and aluminum oxide (Al 2 O 3 , 10 nm) to protect the titanium from oxidation.Details on the fabrication steps can be found in the Experimental Section and Figure S2 (Supporting Information).The thickness of each layer was optimized for the maximal reflectivity and tuning range.The tuning mechanism of the reflective colors is based on the RI changes of Sb 2 S 3 when it changes from an amorphous, intermediate state to fully crystalline.The MDM cavity exhibits several advantages, such as angle-independent reflectivity, simple fabrication techniques, and precisely controllable peak resonance wavelength. [48]e quantitatively measured the optical properties of a thin film of Sb 2 S 3 at the amorphous and crystalline states by ellipsometry.The results of the RI (n) and the extinction coefficient (k) are plotted in Figure 1b.To obtain a full crystalline phase of Sb 2 S 3 , we annealed the film on a hotplate at 300 °C for 5 min.It is important to note that without a top protection layer, the Sb 2 S 3 film will be damaged during annealing due to sulfur evaporation.The maximum contrast of the measured RI (Δn) between the amorphous and crystalline Sb 2 S 3 states is around 1.2 at the wavelength of 600 nm, and the corresponding difference of extinction coefficient (Δk) is 0.4.The figure of merit (FOM) of Sb 2 S 3 (Δn/Δk) is about 3. In comparison, the FOM of GST at the same wavelength is only %0.04. [49]Such giant FOM enables an extensive tuning range of the resonance wavelength of the MDM cavity in the visible regime.Intermediate states of Sb 2 S 3 can be obtained by mixing amorphous and crystalline phases in different proportions. [43]The percentage of crystalline phase in the total amorphous volume is defined to differentiate the intermediate states, such as 20% c-Sb 2 S 3 and 40% c-Sb 2 S 3 .The permittivity of Sb 2 S 3 at intermediate states was calculated using the following equation. [50] where ε amor , ε inter , and ε crys are the permittivity of Sb 2 S 3 in the amorphous, intermediate, and crystalline phases, respectively.The crystallinity factor, m, varies from zero (a-Sb 2 S 3 ) to the unity of one (c-Sb 2 S 3 ).As depicted in Figure 1b, the calculated RI increases with the phase change from amorphous, intermediate states, to crystalline states.Meanwhile, it shows the same trend of the extinction coefficient.Reversible switching of the phase of the Sb 2 S 3 film between the crystalline, intermediate, and amorphous states can be realized using a CW laser and a ps laser, respectively.To study the all-optical reconfigurability of Sb 2 S 3 , we deposited the following stack layers on a silicon substrate: titanium (5 nm), aluminum (100 nm), silicon nitride (50 nm), Sb 2 S 3 (45 nm), and silicon nitride (50 nm).Then, we measured the Raman spectrum at each state of Sb 2 S 3 , as shown in Figure 1c.The optical images after reversible switching are illustrated in Figure S3 (Supporting Information).Raman spectroscopy has been used as an indirect technique for examining the change in crystallinity of PCMs. [44,45,51]The crystallization was realized by a CW laser with a wavelength of 532 nm and average power of 6 mW.We observed a gradual shift of the Raman peaks from 90 to 275 cm À1 upon the transformation from the amorphous (as RF sputtered) to intermediate to a fully crystalline phase (similar to the Raman spectrum in the case of hotplate annealing) after increasing the laser exposure time to the sample, as shown in the left panel of Figure 1c. [45]The phase transformation across different phases was obtained by precisely controlling the laser exposure time while keeping the laser power density at a constant of 200 kW cm À2 .The total exposure time to complete the crystallization process was %17 s, which can be several orders of magnitude lower after increasing the CW laser power density absorbed by the Sb 2 S 3 film.
To study the phase transition during the reamorphization, we initially annealed the thin-film structures using a hotplate at 300 °C for 5 min to ensure that the whole substrate surface was in the fully crystalline phase.After that, we used a singlepulse ps laser with a width of %10 ps and a wavelength of 1064 nm.By increasing the pulse energy density from 0 to 56 mJ cm À2 , it reversibly switched the phase from crystalline, multiple intermediate states, and back to amorphous, as shown in the right column of Figure 1c. [45]The diameter of the ps laser spot is around 30 μm, which is sufficient to cover the entire MDM cavity pixel area.Large-scale phase change can be realized by scanning the surface area via a motorized stage.
The MDM cavity design was optimized via simulation to validate various metallic materials as the top and back reflectors to achieve the maximal reflectivity and large tuning range.The simulated reflections of the MDM cavity using aluminum (Al), silver (Ag), gold (Au), palladium (Pd), titanium (Ti), and chromium (Cr) as the top reflector are plotted in Figure 2a.It shows that the Al top reflector resulted in the narrowest Fano-shape resonance that reflected a wide range of visible light, which reduces the color saturation.Al quickly tends to be oxidized in the ambient environment, which degrades the peak resonance of reflection. [52]We observed that the reflection peaks of Ag and Au were around 510 and 525 nm, respectively.However, they exhibited high reflection at longer wavelengths (λ > 600 nm) and reduced color saturation.The reflection at the long wavelength using Cr was reduced with the resonance strength, defined as the difference between the peak and the bottom reflections.Even though the resonance strength of Pd increased, the reflection at the long wavelength was even higher than that with Cr.Among all the candidates, Ti excels at the overall performance as the top metal with a main reflection peak near 540 nm, a high resonance strength of around 70%, and the lowest reflection at the long wavelength.The top reflector plays a significant role in enhancing the reflective color and confining the incident light inside the Sb 2 S 3 film.
After choosing titanium with a thickness of 10 nm as the top reflector, we investigated the effect of the back reflector using different metals on the MDM cavity response while keeping the thickness constant at 100 nm.The simulated reflections are plotted in Figure 2b.The reflection strength of Ti was around 51%, which is the lowest, followed by Cr (54%).Pd increased the reflection strength to 60%, while Al had a higher reflection strength of 69%.For Au, the reflection strength further increased to 80%.Ag exhibited the highest reflection strength of %83% due to its smallest RI and extinction coefficient among all the candidates.Based on the analysis, different types of back metals showed a similar response with varying reflection strength.Ti and Ag were selected as the top and back reflectors in the optimized MDM cavity.
Subsequently, we optimized the thickness of Ti as the top reflector to achieve the highest color saturation.The simulated reflections of different thicknesses from 5 to 25 nm are plotted in Figure 2c.As expected, the thinnest film (5 nm) exhibited the highest reflection due to the lowest absorption losses within the MDM cavity, hence increasing the Q-factor.However, the full width half maximum (FWHM) of its reflection curve was broader than other curves.It can be attributed to the weak coupling of incident light within the MDM cavity.When the thickness increased from 10 to 25 nm, the FWHM decreased from 84 to 52 nm, respectively, and the reflection strength decreased simultaneously from 85% to 45%.Based on the analysis, it indicates that the thickness of the top metal plays a crucial role in improving the reflection strength.Beyond that, Figure 2d plots the simulated reflections of the back reflector (Ag) with different thicknesses.A negligible difference in the FWHM and reflection strength of those curves was observed.It is attributed to the thickness of the back reflector being larger than the penetration depth of the silver film.
Based on the validation of the thin film performance of each layer in the MDM cavity, we carried out the simulation study and the experiments to investigate the efficient dynamic switching of structural colors of the MDM cavity.The simulated reflections of the MDM cavity with multistate Sb 2 S 3 are plotted in Figure 3a.For more details on simulation settings and parameters, see the Experimental Section and Figure S1 (Supporting Information).At the amorphous state (a-Sb 2 S 3 ), the reflection peak exhibits an FWHM of approximately 80 nm at a wavelength of 540 nm, which results in green color.At the intermediate states, as the crystallinity ratio increases from 20% to 60%, the reflection peak gradually shifts from wavelength 547 to 577 nm, and the FWHM increases to 118 nm.The increase in the FWHM of the reflection peak is due to the increased crystallinity ratio in the Sb 2 S 3 film, leading to the increased optical loss of the MDM cavity.Meanwhile, the increase in the RI of Sb 2 S 3 accounts for the redshift of the reflection peak.When the crystallinity ratio reaches 80%, the resonance curve is dramatically changed and becomes broad at the longer visible-wavelength range, which results in red color.The giant change in the reflection spectrum occurs due to two reasons, the RI change of Sb 2 S 3 and the dominance of the crystalline Sb 2 S 3 phase, which shrinks the Sb 2 S 3 film thickness at the high crystallinity ratio due to the large density of c-Sb 2 S 3 . [53,54]Upon reaching the full crystalline phase, the reflection peak resonance vanishes due to the strong  absorption of c-Sb 2 S 3 , leading to broadband reflection across the visible spectrum, evidenced by the white color shown in the inset.The simulated reflection of Sb 2 S 3 with different thicknesses is shown in Figure S4 (Supporting Information).The inset of Figure 3a shows the calculated reflective colors of each state of Sb 2 S 3 .The maximum color tuning with Sb 2 S 3 phase change over RI change ( Δλ Δn ) is 133 nm/RI and is improved due to the strong MDM cavity resonance. [31,55]o verify these simulation results, we fabricated the MDM cavity and measured its reflection spectrum for various states of Sb 2 S 3 (Figure 3b).At the a-Sb 2 S 3 state, the measured reflection spectrum shows a reflection peak near a wavelength of 480 nm, which is broader than the theoretical prediction in Figure 3a due to light scattering near rough interfaces of amorphous layers stacked in the MDM cavity.To obtain intermediate states of Sb 2 S 3 , we used a photoscanning microscope with a Â 50 objective and a CW laser of a wavelength of 532 nm to perform area scanning.By gradually increasing the CW laser energy, we achieved distinct intermediate states of Sb 2 S 3 accordingly.After that, a full crystallization state was achieved by increasing the laser power density from 0 to 200 kW cm À2 at a constant laser scan speed of 20 μm s À1 and a line step width of 333 nm.Using a laser power density of 40 kW cm À2 , the resonance peak of the measured reflection of 20% c-Sb 2 S 3 shows a redshift to 500 nm.For intermediate states of Sb 2 S 3 ranging from 40% c-Sb 2 S 3 to 60% c-Sb 2 S 3 , the corresponding reflection peaks are redshifted to wavelengths of 530 to 550 nm.At 80% c-Sb 2 S 3 , the measured reflection resonance became broad at the longer wavelength of the visible spectrum.For a fully crystalline Sb 2 S 3 film, the reflection resonance vanished and resulted in reflecting the incident colors.In general, the measured reflection resonance matches the simulated reflection after considering the RI change and thickness shrinkage during the phase change of the Sb 2 S 3 film.The inset of Figure 3b shows the corresponding measured optical images of structural colors at different states.The experimental value of the color tuning ( Δλ Δn ) is 183 nm/RI.These results demonstrate the versatility of intermediate states in low-loss phase-change materials for achieving multiple structural colors using a single film of Sb 2 S 3 within the MDM cavity.
We simulated the field profile of reflections to analyze the confinement of the electric field intensity within the MDM cavity, and the results are plotted in Figure 3c.In the a-Sb 2 S 3 state, the intensity distribution of the electric field intensity was primarily confined within the Sb 2 S 3 layer at a wavelength of 540 nm.In the case of 20% c-Sb 2 S 3 , 40% c-Sb 2 S 3 , and 60% c-Sb 2 S 3 , similar field profiles were observed at shifted wavelengths of 547, 556, and 577 nm, respectively.For 80% c-Sb 2 S 3 , the crystalline phase of Sb 2 S 3 dominated and resulted in thickness shrinkage by %22% and the incident field confined inside the Sb 2 S 3 film at a longer wavelength of 700 nm.In the case of c-Sb 2 S 3 , the thickness shrinkage by %38%, and the large optical losses of the c-Sb 2 S 3 film weaken the light confinement of the incident field and result in broad reflection response across the visible spectrum.Apparently, the proposed MDM cavity was able to confine the incident field inside Sb 2 S 3 film while tuning the reflected light peak wavelength across a broad range of the visible spectrum.
To investigate the effect of Sb 2 S 3 thickness on the reflected color, we simulated the reflected colors of different Sb 2 S 3 phases as a function of the thickness of the Sb 2 S 3 in the proposed MDM cavity.Figure 4a shows all the calculated colors generated from the cavity.For each state of Sb 2 S 3 , its thickness variation from 1 to 200 nm offers a wide gamut including blue, orange, amber, purple, green, and red colors.On the other hand, changing the Sb 2 S 3 phase can switch between these colors.Sb 2 S 3 with a thickness of around 100 nm shows the largest color tuning range up to 220 nm and resulting in colors from green to red.
Figure 4b shows the chart of the Commission on Illumination (CIE) of colors achieved in Figure 4a.The CIE color plot characterizes the colors of the reflection spectrum versus wavelength by decomposing the reflection spectrum into two color coordinates, that is, x and y, which are specified as a point on the chromaticity diagram.Chromaticity coordinate calculations were performed by multiplying the reflection spectrum and the color-matching functions for the three primary colors: red, green, and blue.The tristimulus values were then obtained by integrating each spectrum and normalization.The three primary colors are marked on the CIE color plot as the black triangle and labeled with characters red (R), green (G), and blue (B).The center point on the CIE color plot represents a white color.The maximum calculated color gamut achieved using the MDM cavity with different Sb 2 S 3 thicknesses and phases is %80% of the sRGB spectrum.In addition, we obtained saturated green and purple colors outside the sRGB limit.The color gamut using single thickness and different phases of Sb 2 S 3 is 28% of the sRGB spectrum.
Structural colors should maintain their color purity over a wide field of view, greater than the central vision of the binocular eye around 60°. [56] Subwavelength nanoengineered structural color devices usually are sensitive to viewing angle, [44,57,58] which is not favorable for specific applications, such as color filters. [31]e optimized the metal and dielectric thicknesses to achieve angle-independent color for a large range of the incident angles.Then, we simulated the color angle dependency of the proposed MDM cavity up to a field of view of 60°and plotted the results in Figure 4c.At the a-Sb 2 S 3 state, the reflection peak maintained its strength and FWHM uniformly across the simulated incident angle spectrum.We also examined the angle dependency for the 60% c-Sb 2 S 3 state and found it angle insensitive.As a result, the proposed MDM cavity is angle insensitive, which is independent after the phasing modulation of Sb 2 S 3 as well.In addition, we studied the feasibility of using the proposed MDM cavity for curved surfaces and wearable devices.More details can be found in the Experimental Section and Figure S5 (Supporting Information).

Conclusion
In this study, we have demonstrated MDM cavity consisting of a multilayer of metallic and low-loss phase-change dielectric layers to realize tunable structural colors.All-optical reversible switching of structural color was realized using CW and pulsed lasers to change the phases of Sb 2 S 3 via the crystallization and reamorphization processes.A hybrid effect, that is, RI modulation and thickness modulation of the Sb 2 S 3 layer, is dominant in the tuning mechanism.This approach offers a reflection peak resonance with an FWHM of 80 nm at a wavelength of 540 nm and a resonance strength of more than 60% at the amorphous state of Sb 2 S 3 , owing to the large FOM of %3.In addition, the resonance wavelength of the MDM cavity shifts up to 220 nm, corresponding to 80% gamut of sRGB.The optimal thickness of Sb 2 S 3 for maximum color tuning was found to be 100 nm.We believe that there is room for further improvement in the spectral bandwidth and color purity of the reflected colors, both from the design and materials perspectives.One potential enhancement is to increase the electric field confinement by incorporating a 1D grating or photonic crystal nanocavity with Sb 2 S 3 .This would help to optimize the light-matter interaction inside the nanocavities.Additionally, the use of novel low-loss and high-bandgap PCMs has the potential to further boost the performance of the nanocavities.The approach is suitable for scaling up via cost-effective large-area thin-film fabrication.It is a promising approach for developing ultracompact and configurable structural colors in nanophotonic devices, anticounterfeiting, color displays, wearable devices, and high-capacity data storage devices.

Experimental Section
Reflection and Field Profile Simulations: The finite-difference time-domain (FDTD) method, specifically using the Lumerical FDTD Solution software, [59] was used to simulate the reflection of light from an MDM cavity.To ensure the accuracy of the simulation, we incorporated measured RIs of the materials used in the cavity.We utilized a linearly polarized source with a wavelength range of 400-700 nm and implemented a perfectly matched layer in the vertical direction (z-axis) and periodic boundary conditions in the horizontal plane (x-y plane).The mesh size employed was less than 1 nm.To visualize the electric field intensity profile within the MDM cavity at the reflection peak wavelength, we utilized a vertical 2D monitor.Additionally, we employed a horizontal 2D monitor above the source to measure the reflection.
Fabrication of the MDM Cavity: The process for fabricating tunable thinfilm structural colors based on Sb 2 S 3 is depicted in Figure S2 (Supporting Information).The substrate utilized was crystalline silicon (c-Si) with a native oxide layer of SiO 2 %2.3 nm thick.The samples were initially diced, followed by cleaning with acetone and isopropyl alcohol (IPA), and subsequently subjected to ultrasonic cleaning in acetone for 10 min at room temperature.Subsequent rinsing with acetone and IPA was performed.A layer of Ti %5 nm thick was then deposited via electron beam evaporation (EBE) using a Denton Explorer system, serving as an adhesion promoter.The Ti deposition occurred at a chamber pressure of less than 5 Â 10 À7 Torr and a rate of 1 Å s À1 .Without breaking the vacuum, a layer of Ag %100 nm thick was deposited as a back metal reflector using the same EBE system at a rate of 1 Å s À1 .A Si 3 N 4 film %25 nm thick was subsequently deposited via inductively coupled plasma-chemical vapor deposition (ICP-CVD) using an Oxford Plasmalab System 380 at a substrate temperature of 50 °C.The gases employed during CVD were SiH 4 , N 2 , and Ar, with flow rates of 20.6, 23.5, and 30 sccm, respectively.Plasma ignition occurred at an RF power of 50 W and ICP power of 1000 W, with a chamber pressure of 17 mTorr.The CVD deposition was performed using ICP power of 800 W only, without RF power, at a chamber pressure of 15 mTorr in order to prevent etching of the Sb 2 S 3 using Ar plasma.An Sb 2 S 3 film %100 nm thick was then deposited via RF sputtering using an AJA sputtering system at room temperature, utilizing Ar plasma at a flow rate of 21.6 sccm and RF power of 20 W, with a chamber pressure of 10 mTorr.An additional Si 3 N 4 film %25 nm thick was deposited via ICP-CVD using (Oxford Plasmalab System 380) with the same deposition settings as the bottom Si 3 N 4 layer to protect the Sb 2 S 3 film and boosted the melt-quenching process during reamorphization.It was followed by depositing a 10 nm-thick Ti layer via EBE as a top metal mirror resonator at a rate of 1 Å s À1 and a chamber pressure of 5 Â 10 À7 Torr.The final step in the fabrication process involved the protection of the top Ti layer with a 10 nm-thick Al 2 O 3 layer deposited via atomic layer deposition (ALD) using a (Beneq ALD TFS 200) system at a deposition temperature of 85 °C in order to prevent phase change to the crystalline form of the Sb 2 S 3 .
Reflection Measurement: The reflection spectra were measured using a microspectrophotometer (CRAIC QDI-2010) equipped with a Â 36 objective and a 6 Â 6 μm 2 aperture size.The integration time was set to 350 msec, using number of spectrum scans then average equal to 30 and a sampling time of 12 msec.The light source utilized was a mercury lamp.
Material Characterization: The optical RI of materials utilized in the MDM cavity was measured using variable-angle spectroscopic ellipsometry (JA Wollam VB400).The optical RI of Sb 2 S 3 was measured while coating Sb 2 S 3 film with a top protective layer of Si 3 N 4 %50 nm.The RI was fit using the Tauc-Lorentz model, and the overall mean square error (MSE) was less than 20 for all measured angles.We used a Cauchy model for fitting the aluminum oxide and silicon nitride RI.In addition, we used a Lorentz model to fit the silver RI.The optical RI of materials used in the MDM cavity is plotted in Figure 1a,b, and S1 (Supporting Information).The measured microscopic Raman was done using confocal Raman imaging microscopes (WiTec alpha300 R).We used an objective Â50 and 532 nm CW laser with a focused laser spot diameter of less than 1 μm.The grating used was 1800 groves mm À1 with a center wavelength of around 532 nm.

Figure 1 .
Figure 1.The proposed MDM cavity for all-optical tunable thin-film structural color.a) The schematic of the conceptual design of the MDM cavity shows the tunability of reflective colors upon the phase change of the low-loss Sb 2 S 3 thin film.b) The measured RI (n) and corresponding extinction coefficient (k) of Sb 2 S 3 at the amorphous and crystalline states.Values of n and k at intermediate states are based on calculation.c) Optically reversible switching of Sb 2 S 3 thin film between amorphous, intermediate, and crystalline states.(Left panel) Crystallization of Sb 2 S 3 using a 532 nm CW laser with different exposure times.(Right panel) Reamorphization of Sb 2 S 3 using a 1064 nm ps laser with different pulse energies.

Figure 2 .
Figure 2. The analysis of the type and thickness effect of the top and back reflectors on the reflection of the MDM cavity.a) Simulated reflections of the top reflector with different metals using 100 nm amorphous Sb 2 S 3 film in the MDM cavity.b) Simulated reflections of the back reflectors using different metals of thickness 100 nm.c) Simulated reflections of the Ti top reflector with thickness varying from 5 to 25 nm.d) Simulated reflections of the Ag back reflector with thickness varying from 50 to 150 nm.

Figure 4 .
Figure 4. Color coverage and angle-independent characterization of the MDM cavity.a) The simulated thickness and phase change variation on the structural colors.b) The simulated CIE map plot upon the thickness variation and multiple states of Sb 2 S 3 .c) Reflection versus incident angle in MDM cavity for a-Sb 2 S 3 and 60% c-Sb 2 S 3 phases.