There are mainly two colors display categories according to their generation principles which are chemical and structural color. Structural coloration offers significant advantages in high purity, high resolution, and environmental protection^{[1, 2]}. Almost all the structural colors were generated by the light−matter interaction. Several structures and resonant effects have been demonstrated by previous works, such as multilayer films based on Fabry−Perot resonance, metasurfaces based on Mie resonance, plasmonic resonances, and guided mode resonance^[3−5]. Metasurfaces, composed of subwavelength structures of two-dimensional planar antennas, have been demonstrated strong control over incident wavefronts, enabling various important optical functionalities like metalens, holographic display^[6], thermal emission control^[7] and color filtering^{[8, 9]}. All-dielectric subwavelength structures provide strong control of light through the excitation of both electric and magnetic multipolar Mie-type resonances^{[10, 11]}. This not only retains the advantages of metal plasmonic (i.e. good color saturation and high resolution), but also overcomes limitations such as increased peak width due to high metal losses, offering a broader color gamut.

Active and reconfigurable control of light in all-dielectric metasurfaces could broaden its applications, such as dynamic display, optical sensing, and anti-counterfeiting. Several methods have been proposed to achieve this goal, including chemical, mechanical, graphene^[12] and environmental modulation. Electrochemical methods^[13] have been applied to manipulate the growth and dissolution of polyaniline (PANI) on gold nanorods, enabling dynamic modulation of the refractive index and extinction coefficient. Mechanical tuning^[14] has also been explored, where a robotic arm is employed to stretch a metalens on a flexible substrate, resulting in adjustments to the angle and periodicity of the metallic array. Another approach involves environmental modulation, achieved by immersing the metasurface in various fluids. For instance, a dimethyl sulfoxide (DMSO) solution has been used as a refractive index matching layer^[15], leading to enhanced color saturation. However, these methods are in terms of switching speed, integration complexity, and tuning capabilities.

The application of chalcogenide phase change materials (PCMs) in nanostructures could effectively address these issues^[16]. PCMs conduct rapid and non-volatile switching between crystalline and amorphous states under external stimuli such as heat, electricity^[17], and laser^[18], bringing significant changes in material physical properties (resistivity, refractive index, and so on)^{[19, 20]}. Moreover, multi-level control can be achieved through intermediate states of PCMs^{[21, 22]}, which provides the possibility of grayscale manipulating of light. The pioneering research that integrates phase change materials with structural color is multilayer films. In this study^[8], the dielectric material within the resonant cavity is the phase-change material Ge₂Sb₂Te₅. After the phase-change process, the peak position could achieve a shift of approximately 50 nm. Subsequently, Carrillo et al.^[23] integrated phase-change materials with the substrate of metasurfaces, achieving dynamic tuning based on plasmonic resonances. This device could realize cyan, magenta and yellow (CMY) color display in the crystalline state, which in the amorphous state, resonance would disappear, resulting in white light reflection. In the domain of all-dielectric metasurfaces, Galarreta et al.^[24] proposed a hybrid structure combining Ge₂Sb₂Te₅ and Sb₂S₃ with metasurfaces based on Mie resonances. This configuration allowed independent control of resonance peaks with the phase transition, realizing a significant switching ratio. While metasurfaces combining PCMs have been reported^[25−32], they are limited to using phase change as the tuning method and can only achieve color changes. However, in many application situations, the modified methods not only require a variation in reflective color but also need a switching function. Moreover, traditional structural colors with significant losses are incapable of simultaneously achieving high transmittance and reflectance.

In this paper, we propose an all-dielectric phase change metasurface and numerically analyze the optical response in the visible spectrum for dynamic color generation. Low-loss phase change material antimony sulfide is selected as the active material, which enables low optical absorptance. Besides, phase change materials have been shaped as elliptical cylinders, introducing polarization tuning as an additional tuning mechanism. By altering structural parameters, the optical spectrum of the metasurface could be modified across the whole visible range. Color shift and on/off switching have been achieved due to the combination of PCMs and polarization as tuning mechanisms. Additionally, strategically selecting dielectric materials with low loss in the visible spectrum enables high reflectance in specific bands while maintaining high transmittance, achieving transflective coloration.

The schematic of the proposed dynamic full-color metasurface is shown in Fig. 1(a). CF₂ is chosen as the substrate material due to its relatively low refractive index and near-zero absorption in the visible region, assumed to be lossless and with non-dispersive refractive index n = 1.4. Then, low-loss phase change material Sb₂S₃ is chosen as functional material due to its lower optical loss in the visible spectrum and high refractive index contrast between amorphous and crystalline states. Sb₂S₃ was used in the field of batteries in the past due to its photovoltaic conversion capabilities and its ability to embed/de-embed lithium ions^[33−35], recent research has explored its use in tunable photonic devices using PCMs. The complex permittivity of Sb₂S₃ is displayed in Fig. 1(c). Herein, the numerical simulations were performed assuming vertical incidence of light with both TE and TM polarization modes, where the polarization direction of the electric field is along the y-axis for TM mode and x-axis for TE mode. For setting of boundary conditions: periodic boundary conditions in the x and y axis directions, and open boundary conditions in the z axis direction. Ports are set up on the upper and lower ends of the device, and the floquet mode is set to TE and TM. Frequency solver is chosen as the solution method. Additionally, complete and uniform amorphization or crystallization were assumed. The colors are evaluated by calculating the x−y coordinates and plotting them on the International Commission on Illumination (CIE) 1931 Chromaticity Chart.

The Mie scattering theory is mainly employed to investigate the scattering of light by particles when the size is comparable to the light wavelength. It is a rigorous solution to the scattering of elastic waves by particles based on the solution of Maxwell's equations for plane electromagnetic waves. The theory defines Mie resonance, which is a resonant scattering phenomenon and mainly classified as magnetic resonance and electric resonance. Magnetic resonance refers to the resonance mode excited in dielectric materials with a high dielectric constant under the influence of an electric field. This can induce resonance modes like metallic magnetic resonance structures, causing the dielectric to generate displacement currents equivalent to the conduction current in metallic structures. Displacement currents can, in turn, produce a magnetic field, enabling dielectric particles with initially non-apparent magnetism and a high dielectric constant to exhibit magnetic resonance. Electric resonance occurs when the electric field is predominantly concentrated inside dielectric particles during the propagation of electromagnetic waves. If the dielectric constant of dielectric particles is high, a significant displacement current can be generated inside the dielectric particles, thereby exciting strong electric resonance^{[36, 37]}. The mth-order Mie coefficient^[38] is given by the following formulas：

1am=ncψm(ncx)ψm'(nex)−neψm(nex)ψm'(ncx)ncψm(ncx)ξm'(nex)−neξm(nex)ψm'(ncx),

2bm=neψm(ncx)ψm'(nex)−ncψm(nex)ψm'(ncx)neψm(ncx)ξm'(nex)−ncξm(nex)ψm'(ncx),

where x=2πr0λ, λ is the wavelength of the incident wave in a vacuum, r0 is the radius of the particle, nc stands for the complex refractive index of the particle, and ne represents the refractive index of the ambient medium. ψm(x) and ξm(x) represent the Riccati−Bessel function.

In the subwavelength regime, the contribution of the first-order dipole is significantly higher than that of higher-order dipoles, dominating the scattering behavior. According to the simplified approach introduced by Lewin^[39], the scattering coefficients can be reduced to:

3a1≈−i23(nex)3ϵcF(ncx)−ϵeϵcF(ncx)+ϵe,

4b1≈−i23(nex)3F(ncx)−1F(ncx)+2,

where ϵc is the dielectric constant of the particle, ϵe is the dielectric constant of the ambient medium, and

5F(θ)=2(sinθ−θcosθ)(θ2−1)sinθ+θcosθ,

with θ=ncx. When a1 tends to infinity, F(θ)=−2ϵeϵc, and the particle can exhibit strong electric dipole resonance, assuming ϵc≫ϵe, F(θ)≈0. Taking the first positive solution, we obtain

6λ=2πr0nc4.49,

similarly, when b1 tends to infinity, F(θ)=−2, the particle can induce intense magnetic dipole resonance. Taking the first positive solution in this case yields λ=2r0nc. While the extinction coefficient of the particle was not considered in the derivation process, the changing trend of the resonance wavelength still holds significant guiding implications.

To investigate the structure-function relationship between the proposed metasurface and the effects of phase change and polarization, we numerically simulate the optical responses in the visible light range (400−700 nm) by varying the major and minor axes of the ellipsoidal rod. As well as this the unit cell periodicity in the two directions has been modified while keeping the thickness of the phase change material (t_pcm) fixed at 120 nm. Additionally, we demonstrate the electromagnetic mechanisms of the two modulation methods to provide a more comprehensive explanation of the optical response. The unit cell design of the device is illustrated in Fig. 1(b).

Firstly, we demonstrate the influence of different major and minor axes on the optical response of the metasurface, with the periodicity fixed at T_x = T_y = 350 nm. In the TM polarization mode, the metasurface exhibits reflection peaks, and a redshift occurs with increasing R_y (Figs. 2(a) and 2(b)). With an increase in R_x, the peak position, peak intensity, and full width at half maximum (FWHM) of the reflection peaks all slightly increase (Figs. 3(a) and 3(b)). The increase in FWHM is due to the expanded range of resonant radius. When the PCM transitions from the amorphous to the crystalline state, a significant redshift in peak position and a substantial decrease in peak intensity occurs due to the increased refractive index and extinction coefficient. Moreover, the tuning capability of the metasurface increases with the volume of the phase change material (Fig. 2(c) and Fig. 3(c)). Simulation results indicate that the trend of resonance wavelength with increasing radius and particle refractive index aligns well with Mie resonance theory. In the TE polarization mode, reflection peaks are only observed when R_y exceeds 50 nm, and the reflectance rapidly increases with an increase in radius (Figs. 2(d) and 2(e)). However, when R_y is fixed at 30 nm (Figs. 3(d) and 3(e)), the metasurface exhibits almost no reflection in both amorphous and crystalline states because the minor axe of the elliptical cylinder is too short, resulting in resonances outside the visible light range.

Next, we explored the influence of different periodicity on the metasurface, with major and minor axes fixed at R_x = 130 nm and R_y = 30 nm, respectively. The variation in transmittance and reflectance of the metasurface as the periodicity T_x and T_y range from 300 to 400 nm is depicted in Fig. 4. As T_x increases (Fig. 4(d)), the metasurface's reflection peak experiences a slight blueshift, while the reflectance and FWHM undergo a slight increase. The transmission spectrum's trough also becomes narrower. This trend is attributed to the fact that, in the TM mode, an increase in T_x is equivalent to a decrease in R_x, contrary to the trend observed in Fig. 3(a). When T_y increases, the metasurface's reflection peak undergoes a significant redshift, transitioning from cyan to yellow and finally to red. Moreover, the reflectance and saturation increase noticeably, and the transmission spectrum color becomes lighter, approaching white. Since R_y is much smaller than T_y, it cannot be assumed that increasing T_y is equivalent to decreasing R_y. On the contrary, increasing T_y corresponds to a substantial increase in R_x, leading to an increase in resonance wavelength. Overall, T_y has a significant impact on the optical response of the metasurface, affecting the peak position, peak intensity, and FWHM of the reflection peaks. In contrast, T_x has a minor impact on the metasurface's optical response, primarily influencing the FWHM of the reflection peaks.

Then, we conducted a simulation of the electric dipole (ED), magnetic dipole (MD), electric quadrupole (EQ), magnetic quadrupole (MQ) distribution at different structure geometry in TM mode, shown as Fig. 5, to discuss the underlying mechanism. According to these results, we can explain the phenomenon of splitting of the reflection peaks in Fig. 2(a). At R_y = 30 nm, the MD density is so weak, but ED has big amplitude at ~561.5 nm, which causes the occurrence of reflection peak (Fig. 5(a)). And the contribution to particle scatter of MD is enlarging with the increase of R_y, resulting in the split of reflection peak (Fig. 5(b)). ED is severely affected by R_x according to Figs. 5(a) and 5(c)，while MD is relatively stable, so, the peak positions of the spectrum shift are along with R_x. This phenomenon aligns well with Fig. 3. In summary, R_x has a significant impact on MD, while R_y has a greater influence on ED. They have an independent control on reflection spectra, providing guidance for device parameters design. Furthermore, to investigate the effect of phase change on switching function, the multipole distribution of the crystalline state device was also simulated (Fig. 5(d)). Because of the increased extinction coefficient, the resonance intensity of the crystalline state is significantly lower than that of the amorphous state, resulting in lower reflectance. The wavelength range of resonance is also widened, leading to an increase in FWHM.

We compared the performance of this work with several published articles on structural color, shown as Table 1. This work uses metasurface to generate vivid colors and achieves dynamic tuning in two dimensions. Phase change materials enables color variation, while switching can be realized by adjusting the polarization state. Innovations are made in both functionality and tuning mechanisms compared to published works on dynamic structural colors. From the data presented in the Table 1, static structural colors have the better color gamut value, because dynamic tuning needs to balance tuning capability and color performance, which may introduce extra losses, thereby affecting color gamut value. This work prominently highlights the novel structural functionalities. The color gamut values in the Table 1 represent all colors demonstrated in this paper, indicating exploration space remains regarding the relationship between structural parameters and colors.

Finally, we simulated the electromagnetic response of a single unit at a wavelength of 561.5 nm, with the structural parameters shown in Fig. 6(a). In the TM polarization mode, the major diameter of the elliptical cylinders played an important role, inducing strong electric and magnetic dipole resonances at the ends and interior of the rod when the PCM was in its amorphous state (Fig. 6(c)). Consequently, sharp reflection peaks were observed (Fig. 6(b)). Upon the phase transition of the material to the crystalline state, the electric and magnetic dipole resonances vanished, leading to a shift in the reflection peak. In the TE polarization mode, the primary influence stemmed from the short diameter of the ellipsoidal rod. However, there were no corresponding electric dipole resonances at the ends of the short diameter in both amorphous and crystalline states (Fig. 6(d)), resulting in the absence of reflection peaks. While the PCM metasurface technology proposed in this article enables polarization-dependent dynamic color generation, there are still certain limitations associated with this technique, such as the requirement for high-precision lithography and etching processes. We believe that these challenges can be addressed through advancements in semiconductor process technology.

We demonstrated an all-dielectric metasurface based on elliptical phase change cylinders for full-color transflective generation, which can be actively tuned by PCMs and polarization. Firstly, we designed the metasurface as nanoscale elliptical cylinders composed of low-loss phase change materials. Due to different lengths of diameters, an incident wave of different polarization directions induces different optical responses. The optical characteristics of the metasurface are influenced by changes in refractive index and extinction coefficient when the phase of PCMs transitions. Additionally, we exclusively choose low-loss materials to accommodate both transmissive and reflective generation. Through numerical simulations, we investigated the structure-activity relationship of the metasurface and analyzed its electromagnetic mechanisms. In the TM polarization mode, the amorphous metasurface exhibits vibrant colors due to Mie resonance, offering the potential for full-color generation by adjusting structural parameters. As the physical properties of the material change, the crystalline metasurface undergoes color changes, which the tuning capabilities increase with the volume of the phase-change material. In the TE polarization mode, reflective colors are only observed when the short radius (R_y) exceeds 50 nm. To sum up, we believe that the structure discussed in this paper can provide novel insights into the design of optical metasurfaces.