Metasurfaces are ultrathin, optically planar structures that exhibit exotic abilities to freely tailor electromagnetic (EM) waves [1], [2], [3], [4]. Initially, these structures comprised metallic microstructures and nanostructures, but their efficiency and performance were hindered by substantial nonradiative losses in the metallic materials [5], [6], [7], [8], [9], [10]. To overcome this limitation, researchers have shifted their focus to low-loss dielectric materials with high refractive indices, resulting in increased research on all-dielectric metasurfaces [11], [12]. All-dielectric metasurfaces are particularly attractive for color displays as they can exhibit impressive structural colors by leveraging the Mie resonance phenomena related to the geometrical properties of the device in the visible range. Common dielectric materials used for generating structural colors with Mie resonance effects are silicon and titanium dioxide [13], [14]. A wide range of colors can be obtained within the visible spectrum by carefully designing the shape and size of dielectric nanodisks or nanopillars. Despite notable progress in studying structural colors, most nanostructures have static optical properties, limiting their applications. Hence, researchers have developed various techniques to enhance the tunability of these structures. Phase-change materials (PCMs), microfluidics, and active materials have shown promise in the construction of metasurfaces with dynamic and reversible optical responses, which is particularly attractive and practical for display applications [15], [16], [17], [18], [19], [20]. Both phase-change and flexible materials allow for dynamic, rapid, and reversible control of optical device properties [21], [22], [23], [24], [25], [26].

One extensively studied PCM is Ge2Sb2Te5 (GST). GST exhibits a change in refractive index during structural transitions. It can switch back and forth between different states billions of times within subnanosecond time scales. Once switched, it does not require additional energy to maintain its new state [27], [28], [29], [30]. Combining the PCM GST with thin-film Fabry-Pérot device-based resonant cavities, plasmonic nanograting devices, and plasmonic metasurfaces has successfully produced tunable color generation [31], [32], [33]. However, using GST to adjust the color gamut is not considered ideal owing to its high absorption of above-bandgap photon energies and the relatively small variation in the real component of the refractive index, known as ΔRe(n), at visible frequencies [33].

In contrast, a relatively unexplored PCM called Sb2S3 shows great promise in tunable visible photonics [34]. Sb2S3 possesses an adjustable band gap in the visible spectrum, ranging from 2.0 to 1.7 eV. When coupled with an optical resonator, Sb2S3 demonstrates a remarkable response to its reversible structural phase transition in the visible spectrum, making it highly attractive for tunable visible photonics applications. Sb2S3 demonstrates a larger refractive index difference in the visible spectrum than GST during its structural phase transition. This characteristic makes Sb2S3 highly suitable for developing electrically and optically adjustable active photon devices. Additionally, both the amorphous and crystalline phases of Sb2S3 remain stable at room temperature, which is crucial for ensuring the stability of optical devices [35], [36]. Notably, recent studies indicate that the switching between the crystalline and amorphous states of Sb2S3 can be triggered optically and electrically. This multifaceted switching capability further enhances the versatility of Sb2S3 in practical applications. Researchers have successfully incorporated Sb2S3 into all-dielectric/PCM hybrid metasurfaces and demonstrated its potential for tunable color generation [37], [38].

Our research employs polydimethylsiloxane (PDMS) as a stretchable substrate to achieve tunable PCM metasurfaces in the visible wavelength region. PDMS is widely recognized for its flexibility, inertness, nontoxicity, and nonflammability. Recently, there has been considerable interest in exploring tunable nanophotonics on PDMS substrates owing to the ability to mechanically deform of photonic nanostructures fabricated on flexible substrates, enabling easy, rapid, and repeatable tuning of photonic devices [40], [41], [42], [43], [44], [45]. These artificial materials comprising solid metamolecules exhibit semirigidity and semiflexibility when placed on a soft PDMS substrate. Referred to as flexible metasurfaces, they possess tunable and reconfigurable material properties. This approach has been widely adopted in various applications of stretchable, flexible, and tunable photonic devices such as solar cells [46], iLEDs [47], tunable filters [48], and hemispherical electronic eye cameras. In 2017, Tseng et al. expanded the study of stretchability to aluminum metasurfaces, achieving full-spectral control of structural color [16]. However, despite notable advancements, these metasurfaces are still limited by high metal losses. Zhang et al. demonstrated a mechanically tunable all-dielectric metasurface in 2020 [17]. To date, only few studies have been conducted on the use of PCM metasurface for stretchable structural color.

This study presents a novel approach combining PCM with flexible substrates to introduce tunable PCM metasurfaces. These metasurfaces exhibit an active switchable optical response, allowing for the excitation of electric dipole (ED) resonances in the visible wavelength range. By incorporating PDMS, an elastic dielectric material, we can fine-tune the near-field interactions between resonant dielectric elements through mechanical strain. This strain-induced tuning enables the shifting of the resonance peaks of the structure across the entire visible spectrum, resulting in a broad color gamut that can be actively adjusted. Furthermore, we demonstrate that the reflected light color gamut can be dynamically switched through the amorphous-to-crystalline transition of the PCM Sb2S3. This multidimensional approach addresses a long-standing limitation in solid-medium patterning, where the structure cannot be altered once fabricated. We offer a versatile and dynamic platform for designing and fabricating tunable optical devices by integrating PCM and flexible substrates. This study opens new possibilities for developing advanced photonic technologies with unprecedented control over light-matter interactions.