Reconfigurable GeTe’s Planar RGB Resonator Filter–Absorber

Abstract

This study presents a reconfigurable planar photonic device capable of dynamically switching between optical filter and absorber functionalities by exploiting the phase transition properties of GeTe, a chalcogenide phase-change material. The device adopts a Metal–Dielectric–PCM architecture composed of silver (Ag), silicon dioxide (SiO₂), and GeTe layers, each playing a distinct role: the silver layer governs the transmission and absorption efficiency, the SiO₂ layer controls the resonance conditions, and the GeTe layer determines the device’s scattering behavior via its tunable optical losses. Numerical simulations revealed that the structure enables high RGB transmission in the amorphous state and broadband absorption in the crystalline state. By adjusting geometric parameters—especially the metallic thickness—the device exhibits finely tunable spectral responses under varying polarizations and incidence angles. These findings highlight the synergistic interplay between material functionality and layer configuration, positioning this platform as a compact and energy-efficient solution for applications in tunable photonics, optical sensing, and programmable metasurfaces.

1. Introduction

Electromagnetic absorbers and optical filters are essential components in the manipulation and control of light and electromagnetic waves, playing an important role in applications ranging from sensing and optical communications to thermal emitters and reconfigurable photonic devices. While absorbers are designed to minimize both reflectance and transmittance over specific spectral ranges (for instance, dissipating the incident energy as heat), filters operate by selectively transmitting desired wavelengths while blocking others. In this context, a growing body of recent research has explored a wide variety of device architectures capable of delivering such selective control, with particular emphasis on structures that leverage resonant phenomena to enhance performance. Among these, Fabry–Pérot resonators have attracted considerable attention due to their ability to support sharp and tunable spectral features arising from multiple-beam interference effects. These resonators, which consist of a cavity formed by two partially reflective surfaces, can be precisely engineered to exhibit narrowband transmission or absorption peaks at specific wavelengths, making them particularly suitable for use in the visible and near-infrared. Recent studies have demonstrated an extensive range of Fabry–Pérot-based designs operating within these spectral windows, exploiting material platforms, cavity geometries, high-performance filtering, or absorption [1,2,3,4,5,6,7,8,9,10].

A promising method for realizing lithography-free tunable perfect light absorbers (PLAs) involves graphene-based multilayer structures, where the surface conductivity of graphene can be adjusted via an externally applied voltage. Similar approaches have been reported for terahertz and mid-infrared frequencies in [11,12]. Recently, Fabry–Perot cavity-based metal–dielectric–metal (MDM) structures have also been explored as a means to achieve perfect absorption due to their simple fabrication process, which does not require lithography [13,14,15]. These Fabry–Perot cavity-based PLAs typically consist of a dielectric spacer layer sandwiched between a thick bottom metallic mirror and a thin, semitransparent top metallic mirror. By tuning the effective impedance of the absorber structure to match that of the incident medium, reflection can be minimized. The transmittance is further suppressed by the thick metallic mirror at the bottom. The strong light absorption within the Fabry–Perot cavity is driven by the large phase shift that occurs during multiple round trips of light inside the cavity [16]. Following the abovementioned principles, we propose a reconfigurable, lithography-free filter-absorber, underscoring their potential for low-cost, high-performance optoelectronic applications.

A particularly promising approach to enhance and dynamically reconfigure the optical response of Fabry–Pérot resonators—and resonant photonic structures more broadly—relies on the integration of phase-change materials (PCMs). These materials. such as Ge₂Sb₂Te₅ (GST) and Sb₂S₃, exhibit significant and reversible changes in their optical properties (refractive index and extinction coefficient) upon undergoing transitions between their amorphous and crystalline phases. Such transitions can be triggered thermally, optically, or electrically, enabling non-volatile and programmable control over the photonic response of a device [17,18,19,20,21,22]. A key challenge in realizing fully reconfigurable phase-change material (PCM)-based metasurfaces (MSs) and photonic integrated circuits (PICs) lies in developing a reliable and efficient method for inducing phase transitions in the PCM elements [18]. To date, most studies have relied on direct thermal conversion of plain or patterned PCM arrays using external stimuli such as bulky thermal heaters or continuous-wave lasers with broad beamwidths that illuminate the whole device surface. It is important to note that these techniques typically offer unidirectional switching—primarily enabling full or partial crystallization of initially amorphous PCMs. To fully harness the potential of PCMs, there is a strong need for reprogrammable, pixel-level PCM-based MSs and PICs that can locally modulate the amplitude and phase of scattered or guided light. Achieving such precise control requires the use of short electrical pulses or tightly focused laser pulses to selectively switch the PCM state within individual MS unit cells. This strategy is equally vital for reconfigurable PICs, where local adjustments to the PCM nanostructures determine the overall optical functionality. Laser-based switching and associated optical setups for characterizing dynamic PCM-based nanophotonic devices have been thoroughly discussed in reference [23]. While optical switching methods currently offer a more practical solution for addressing individual subwavelength PCM structures, significant effort is still needed to develop and optimize electrical alternatives. In response to the rapid progress in dynamic nanophotonics, review articles have examined and presented the benefits and limitations of various tuning approaches in a detailed way [24,25,26,27,28,29,30,31,32,33,34,35].

The integration of phase-change materials (PCMs) into optical systems has significantly expanded the functional versatility of nanophotonic devices, enabling a wide range of applications in dynamically reconfigurable photonics. These materials, which exhibit pronounced and reversible changes in their optical properties upon phase transition, allow for the active modulation of light without the need for bulky mechanical components or a continuous external power supply. For instance, GST has been employed to design broadband absorbers with thermally controlled switchable near-field imaging capabilities, demonstrating potential for applications in optical encryption and information hiding [36]. Similarly, VO₂-based metamaterials have been explored for achieving giant circular dichroism and polarization conversion in the terahertz range, providing versatile platforms for multifunctional photonic devices [37]. Beyond photonic modulation, thermochromic VO₂ has also been successfully integrated into temperature-adaptive radiative cooling devices, enabling selective activation of heat dissipation with high angular stability, which opens new avenues for energy-efficient building and vehicle applications [38].

PCMs have also been successfully employed in the development of programmable and reconfigurable perfect absorbers [39,40,41,42,43], which can selectively control spectral absorption based on the phase state of the material. They have also proven effective in the realization of tunable thermal emitters [44], where the emissivity can be dynamically adjusted for applications in thermal management and infrared camouflage. Moreover, the incorporation of PCMs in reconfigurable metasurfaces [32,45,46,47,48] has enabled the dynamic control of wavefronts, phase, and polarization, opening pathways for compact, multifunctional optical elements. In addition, advances in metalens technology [49,50] have benefited from PCM integration, allowing for tunable focusing and imaging capabilities in a single planar structure. The chosen material, GeTe, stands out due to its superior optical and functional properties compared to other widely used phase-change materials, such as GST. GeTe exhibits a more pronounced refractive index contrast between its amorphous and crystalline states, which enhances the modulation of optical resonances and allows for more effective control over transmission and absorption characteristics. Additionally, it features lower optical losses in the amorphous phase, favoring high-transmission regimes—a key requirement for the design of reconfigurable photonic devices. Another important advantage is its lower crystallization temperature and faster switching speed, making it more suitable for applications that demand rapid and energy-efficient phase transitions. These factors make GeTe a more effective alternative to GST in tunable multilayer structures aimed at dynamic light control, as proposed in this study. In the next topic, will be presented the optical constants of GeTe in both phases, highlighting the high contrast essential for efficient spectral modulation.

In this paper, we propose a reconfigurable planar photonic device based on the phase-change material (PCM) GeTe, whose reversible amorphous-to-crystalline transitions enable dynamic modulation of optical functionality. The device adopts a Metal–Dielectric–PCM architecture composed of silver (Ag), silicon dioxide (SiO₂), and GeTe layers, designed to control light transmission and absorption within the visible spectrum. In the amorphous phase, the structure operates as an RGB optical filter, selectively transmitting specific color bands. When GeTe transitions to its crystalline phase, the device suppresses transmission and exhibits narrowband absorption across the visible range, functioning as a narrowband absorber. This dual-mode behavior allows programmable switching between filtering and absorption, making the system highly suitable for dynamic photonic applications such as color display technologies, spectral imaging, and reconfigurable metasurfaces. Compared to our previous work involving In₃SbTe₂-based resonators [51] operating in the infrared range (1000–2500 nm), the present structure overcomes the spectral limitation by extending phase-change tunability into the visible domain (400–700 nm). This is achieved without the need for nanostructures or metasurface designs, using only a lithography-free, planar multilayer configuration. To the best of our knowledge, this is the first report of a Fabry–Pérot-type phase-change resonator capable of dynamically switching between RGB-selective filtering and visible-light absorption. The spectral response is tunable via geometrical parameters, specifically the metal and dielectric layer thicknesses, and the device maintains its performance under TE and TM polarizations and under oblique incidence, further reinforcing its applicability to compact, CMOS-compatible photonic systems.

2. Device’s Configuration and Methods

The proposed reconfigurable device is based on a Metal–Dielectric–PCM structure, including silicon dioxide (SiO₂), germanium telluride (GeTe), and silver (Ag). The topmost layer consists of silver, which influences the optical response, while the bottom GeTe layer enables phase-change functionality. The structure is planar in the xy-plane, with incident white light interacting along the z-axis. The thickness of each layer is denoted as h_Ag, h_SiO2, and h_GeTe, as shown in Figure 1. The selection of GeTe, a phase-change material, allows for substantial variations in its optical properties during the transition between the amorphous and crystalline phases. This tunability enables precise control over optical responses, particularly in selective absorption and filtering applications. The device operates in two distinct states: in the amorphous phase, it functions as an optical RGB filter, while in the crystalline phase, it acts as an RGB absorber. The phase transition is controlled through melt-quenching (RESET) for the amorphous state and thermal annealing (SET) for the crystalline state.

The crystallization of GeTe occurs when the material is heated above its glass transition temperature, approximately 165 °C [52], using long-duration laser pulses (ranging from a few nanoseconds to microseconds) [53]. This process enables atomic rearrangement, leading to the crystalline phase. Conversely, amorphization is induced by short-duration laser pulses (ranging from a few picoseconds to nanoseconds) [53], which heat the material above its melting temperature, about 725 °C [54]. The subsequent rapid cooling prevents atomic reorganization, preserving the material in a disordered amorphous state. Numerical analyses and simulations were performed using the frequency-domain Finite Element Method (FEM) [55] within the infrared electromagnetic spectrum range (400–700 nm), employing the licensed COMSOL 5.5 Multiphysics software, specifically employing the Wave Optics Module in the frequency-domain to analyze electromagnetic wave propagation [56]. The optical constants of silver were modeled based on the Drude–Lorentz dispersion [57], while those of GeTe followed the data presented in [58]. The refractive index of silicon was considered to be 1.46 throughout the analyzed spectrum. These curves can be seen in Figure 2. Regarding the relative permeability, all materials were assumed to be non-magnetic (μr = 1). The computational domain was designed in 2D planar geometry, with dimensions set to 150 nm in width and 2051 nm in height. Periodic boundary conditions were applied along the lateral sides to simulate an infinite periodic array. The input port is positioned 1000 nm below the top surface of the Au layer. The solver configuration used a direct solver (MUMPS) with high relative tolerance (10⁻⁶), and convergence tests were performed to validate mesh independence and result accuracy. Material dispersion was modeled by importing wavelength-dependent refractive index and extinction coefficient data for GeTe and Ag through user-defined interpolation functions. The full mesh comprises 2505 domain elements and 320 boundary elements, resulting in 5284 degrees of freedom. The initial simulation parameters include layer thicknesses of h_Ag = 24 nm, h_SiO2 = 1025 nm, and h_GeTe = 2 nm. Boundary conditions are defined along the structure’s edges, while the electromagnetic wave propagates along the z-direction, both at normal incidence and obliquely, to analyze TE and TM modes. The simulations also included parametric sweeps over wavelength, incidence angle, and layer thickness to explore spectral tunability.

Figure 2 illustrates the complex refractive index n and extinction coefficient k of GeTe in both its amorphous and crystalline phases as a function of wavelength. A notable observation is that the refractive index in the crystalline phase n-cry is significantly higher than in the amorphous phase n-amr, exhibiting a steep increase at longer wavelengths. This behavior indicates a higher optical density in the crystalline state. Furthermore, the extinction coefficient of the crystalline phase k-cry remains elevated across the entire spectral range, signifying substantial optical losses due to absorption. In contrast, the amorphous phase exhibits a relatively low extinction coefficient k-amr, which decreases progressively with increasing wavelength, suggesting enhanced optical transparency. These distinct optical properties make crystalline GeTe highly suitable for applications as an optical reflector, while the amorphous phase, characterized by its lower absorption, is well-suited for transmissive optical devices.

The relative permittivity of the materials [59] is given by the following equation:

$${\epsilon }_r(\omega )=\epsilon {\prime }_r(\omega )-i{\epsilon ″}_r(\omega )$$

(1)

where ${\epsilon \prime }_r(\omega )$ and ${\epsilon ″}_r(\omega )$ denote the real and the imaginary parts of the material’s permittivity, respectively, defined as ${\epsilon \prime }_r(\omega )=n^2-k^2$ and ${\epsilon ″}_r(\omega )=2nk.$

3. Results and Discussions

In this analysis, we consider a plane wave incident on the proposed structure, propagating along the x-z directions with periodically defined boundary conditions. Once excited at the top of the structure, this wave interacts with the material, giving rise to various optical scattering mechanisms. The magnitude of the scattered light can be expressed as follows [60]:

$$A(\omega )+R(\omega )+T(\omega )=1$$

(2)

where $A(\lambda )$, $R(\lambda )$, and $T(\lambda )$ represent the absorbed, reflected, and transmitted power fractions, respectively. According to the principle of energy conservation, the sum of these fractions must always equal unity. In this work, we focus on analyzing the absorption and transmission spectra associated with the scattering of light during the phase transitions of GeTe.

The transmission and absorption spectra are shown in Figure 3a,b. It can be observed that the high contrast between the GeTe phases switches the functionality of the structure from a transmissive filter to an absorber. In the amorphous state (a-GeTe), high transmittance is observed, whereas in the crystalline state (c-GeTe), high absorption occurs. The results reveal that in the a-GeTe state, the structure exhibits distinct transmission peaks, with transmittance values of 65.78% in the blue region, 58.92% in the green region, and 48.72% in the red region. In contrast, in the c-GeTe state, transmission is completely suppressed, while absorption is significantly enhanced, reaching 67.26% in the blue region, 73.11% in the green region, and 78.26% in the red region. However, considering that a perfect absorber should exhibit absorption higher than 90% and, similarly, an optical transmission filter should achieve a transmission above 70%, the proposed structure does not meet these criteria. Nevertheless, as this design allows for easy fabrication, the metallic layer thickness can be adjusted to achieve the desired absorption/transmission.

Thus, we numerically analyzed the optical response as a function of the metal layer thickness, with the results presented in Figure 4 through transmission and absorption spectra. We considered thickness variations of 8 nm relative to the initially analyzed value and observed that increasing the silver thickness enhances absorption while consequently reducing transmission. Conversely, decreasing the silver thickness increases transmission and reduces absorption. This trend can be observed more precisely in Figure 5, where the maximum transmission ranges from 76% to 85%, while the maximum absorption varies between 94% and 99%, considering specific RGB wavelengths.

Using empirical polynomial fitting methods, we established a relationship to control the efficiency of the structure as an optical filter, as shown in

Table 1. Polynomial relations between the silver thickness h_Ag and the absorber/filter resonant peaks for the RGB wavelengths in the amorphous and crystalline phases.

Amorphous Phase	Polinomial Expression
Blue	$\mathrm{{\rm T}}(h_{\mathrm{Ag}})=1,{57.10}^{-5}{h_{\mathrm{Ag}}}^3-1,{53.10}^{-3}{h_{\mathrm{Ag}}}^2+2,{92.10}^{-2}{h}_{\mathrm{Ag}}+6,{22.10}^{-1}$	(3)
Green	$\mathrm{{\rm T}}(h_{\mathrm{Ag}})=2,{57.10}^{-5}{h_{\mathrm{Ag}}}^3-2,{08.10}^{-3}{h_{\mathrm{Ag}}}^2+3,{14.10}^{-2}{h}_{\mathrm{Ag}}+6,{81.10}^{-1}$	(4)
Red	$\mathrm{{\rm T}}(h_{\mathrm{Ag}})=2,{55.10}^{-5}{h_{\mathrm{Ag}}}^3-1,{77.10}^{-3}{h_{\mathrm{Ag}}}^2+1,{35.10}^{-2}{h}_{\mathrm{Ag}}+8,{43.10}^{-1}$	(5)
Crystalline Phase	Polinomial Expression
Blue	$\mathrm{A}(h_{\mathrm{Ag}})=-2,{7.10}^{-4}{h_{\mathrm{Ag}}}^2+3,{44.10}^{-2}{h}_{\mathrm{Ag}}-1,{44.10}^{-3}$	(6)
Green	$\mathrm{A}(h_{\mathrm{Ag}})=-4,{2.10}^{-4}{h_{\mathrm{Ag}}}^2+4,{1810}^{-2}{h}_{\mathrm{Ag}}-2,{714.10}^{-2}$	(7)
Red	$\mathrm{A}(h_{\mathrm{Ag}})=-4,{95.10}^{-4}{h_{\mathrm{Ag}}}^2+4,{48.10}^{-2}{h}_{\mathrm{Ag}}-4,{63.10}^{-3}$	(8)

. For optical filters, third-degree polynomial equations were employed, whereas for the absorber, second-degree polynomial equations were used. These mathematical models provide a predictive approach to optimizing the optical response by fine-tuning the silver layer thickness. Thus, both absorption and transmission can be controlled as a function of different silver thicknesses within the range of the analyzed values. By adjusting the metallic layer thickness, it is possible to tailor the device’s optical properties to achieve the desired performance for specific applications. Therefore, for this configuration, a thickness of 40 nm can be considered optimal for a perfect absorber, while a thickness of 8 nm is suitable for an optical filter.

Therefore, for this configuration, a thickness of 40 nm has been considered optimal for a perfect absorber, while a thickness of 8 nm is suitable for an optical filter, and for that, the concept of impedance matching was employed. This analysis involves computing the effective surface impedance Z(ω), which can be determined using the following expression [61,62,63]:

$$Z(\omega )=\sqrt{{\displaystyle \frac{{(1+S_{11})}^2+{S_{21}}^2}{{(1-S_{11})}^2+{S_{21}}^2}}}$$

(9)

where S₁₁ and S₂₁ correspond to the reflection and transmission scattering parameters, respectively.

Figure 6a,b present the real and imaginary parts of the effective surface impedance of the proposed model for the different phases of GeTe. It is also worth highlighting the transmission efficiency (up to 75%) and absorption efficiency (up to 95%) observed in the three resonance regions. In Figure 6a the transmission spectrum is also included (blue line, right axis), enabling a more detailed analysis of the optical response of the structure. It can be observed that the transmission peaks coincide with regions where the real part of the effective impedance approaches unity and the imaginary part remains relatively small. These are conditions under which the structure is impedance-matched to free space, minimizing reflection and allowing efficient wave transmission. On the other hand, in Figure 6b, sharp resonances where Re(Z) ≈ 1 and Im(Z) ≈ 0 are associated with perfect absorption due to critical coupling. The blue line, right axis (Figure 6b), highlights the wavelengths at which these conditions are met, maximizing absorption within the cavity.

Figure 7 illustrates the physical coupling mechanism in our proposed architecture. When the geometric and optical parameters are satisfied, the total phase shift accumulated at the dielectric/metal and dielectric/PCM interfaces (represented by the reflection phase terms ϕ₁ and ϕ₂) is compensated by the optical path within the dielectric layer. This leads to a condition for constructive interference, which enhances the field confinement within the cavity. The condition is given by [64,65,66]:

$${\varphi }_1+{\varphi }_2+{\displaystyle \frac{4\pi }{\lambda }}{h}_{\mathit{SiO}2}\cos \theta =2m\pi$$

(10)

As seen previously in Figure 2, the use of GeTe as the bottom layer is advantageous due to its phase-change nature, allowing dynamic modulation of optical losses. In the amorphous state (n_a-GeTe), GeTe exhibits low absorption, supporting multiple internal reflections within the SiO₂ layer. In contrast, the crystalline state (n_c-GeTe) introduces higher optical loss, enabling strong absorption and field dissipation. This transition effectively tunes the interference and absorption behavior of the structure.

The simplified resonance condition for normal incidence can be expressed as follows:

$$2n_{\mathit{SiO}2}{h}_{\mathit{SiO}2}\cos \theta =m\lambda$$

(11)

where λ is the wavelength of the incident light, h_SiO2 is the thickness of the dielectric film, θ is the angle of incidence, and m is an integer indicating the resonance order.

As shown in Figure 7, it is evident that the SiO₂ layer plays a key role in controlling the resonance behavior of the structure. Since all other parameters are kept constant, varying the thickness of the dielectric spacer h_SiO2 enables the tuning of the optical cavity to support different resonant modes.

By adjusting the thickness of the SiO₂ layer to specific values (namely 460 nm, 700 nm, 1025 nm, 1305 nm, and 1565 nm), the system satisfies the resonance condition for successive higher-order modes (m = 1 to m = 5), respectively. As illustrated in Figure 8, each increment of h_SiO2 results in the appearance of an additional standing wave node within the dielectric layer, confirming the Fabry–Pérot-type behavior of the cavity. The left panels display the electric field distributions for each mode, clearly showing the increasing number of intensity maxima. The right panels present the corresponding transmission and absorption spectra, in which a growing number of sharp resonant peaks appear with increasing m. This demonstrates that the resonance order is directly proportional to the optical thickness of the dielectric layer. The presence of GeTe as the substrate enables active modulation of the device functionality, from high transmission in the amorphous state to high absorption in the crystalline state.

In addition to the spectral performance at normal incidence, we also analyzed how the optical response of the proposed model is affected by variations in the angle of incidence. This angular analysis provides further insight into the robustness and behavior of the structure under more realistic illumination conditions. Figure 9 presents the transmission spectra of the proposed structure as a function of wavelength and incidence angle for both TE (a) and TM (b) polarizations. The color maps indicate the transmission efficiency, where red regions correspond to high transmission and blue regions represent low transmission. Distinct resonant bands are observed in both cases, shifting towards longer wavelengths as the incidence angle increases.

For TE polarization (Figure 9a), the structure maintains high transmission efficiency for incidence angles up to approximately 60°, demonstrating angular robustness. However, beyond this range, a gradual decrease in transmission is observed, becoming more pronounced as the angle approaches 90°. In contrast, for TM polarization (Figure 9b), the transmission remains relatively high over a broader angular range, exhibiting stronger resilience to oblique incidence.

The differences in angular dependence between TE and TM modes suggest that the transmission characteristics are influenced by distinct resonance mechanisms, likely involving guided modes or plasmonic interactions.

Figure 10 illustrates the angular dependence of absorption for both TE (a) and TM (b) polarizations as a function of wavelength and incidence angle. The color maps represent the absorption efficiency, where red and yellow regions indicate high absorption, while blue regions correspond to minimal absorption.

For TE polarization (Figure 10a), absorption remains low across most of the spectral range, with distinct resonant features emerging at specific wavelength–angle combinations. The absorption bands exhibit a shift towards longer wavelengths as the incidence angle increases, following characteristic dispersion behavior. Moreover, the resonances weaken significantly beyond 60°, indicating reduced absorption efficiency at oblique angles.

In contrast, for TM polarization (Figure 10b), a similar trend is observed, albeit with slightly more pronounced resonant absorption at near-normal incidence. However, as the angle increases, the absorption remains low, demonstrating the structure’s limited capacity to sustain high absorption across a broad angular range.

These results suggest that the proposed structure exhibits angle-sensitive absorption, which is more pronounced at lower incidence angles. The observed resonant features may be attributed to guided mode resonances or surface plasmon interactions, depending on the specific design and material properties.

The angular and polarization-dependent behavior of the proposed structure has significant implications for practical applications in photonics. For instance, the high transmission efficiency observed under TM polarization and oblique incidence suggests suitability for optical sensing, spectral filtering, and display systems, where incident light may arrive from diverse directions [2,4,5]. Such angular robustness is particularly beneficial in wearable or conformal optical devices, where alignment with a fixed angle is impractical [41,42]. Conversely, the angle-sensitive absorption behavior exhibited in the crystalline phase enables potential use in directional thermal emitters [44], infrared camouflage, and angular-selective photodetectors [38,43]. The polarization-selective response also opens avenues for polarimetric imaging [37] and optical communication systems, particularly those employing polarization encoding or filtering [32].

An analysis of the TM polarization absorption in this proposed structure demonstrates that high absorption levels are preserved even at oblique incidence angles. This phenomenon suggests the excitation of surface plasmon polariton (SPP) modes at the Ag/SiO₂ interfaces. The underlying dispersion relation governing the SPP modes in this system is given by [4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,26,27,28,29,30,31,32,33,34,35,36,37,38,39,40,41,42,43,44,45,46,47,48,49,50,51,52,53,54,55,56,57,58,59,60,61,62,63,64,65,66,67]:

$${\mathrm{k}}_{\mathrm{SPP}}={\mathrm{k}}_0\sqrt{{\displaystyle \frac{{\epsilon }_m·{\epsilon }_d}{{\epsilon }_m+{\epsilon }_d}}}$$

(12)

where ε_m and ε_d correspond to the complex permittivities of the metal (Ag) and dielectric (SiO₂), respectively.

As k_SPP increases and approaches infinity, the associated mode tends asymptotically to the surface plasmon frequency ω_SPP. At this point, the real part of the metal’s permittivity satisfies the following condition for SPP excitation:

$$\left\{\mathrm{Re}\right\}{\epsilon }_m<-{\epsilon }_d$$

(13)

This condition is fulfilled throughout the visible spectrum, as evidenced in Figure 11, where Re(ε_Ag) is negative and has a greater magnitude than the nearly constant, positive ε_SiO2. This strong contrast in optical permittivity allows for efficient coupling of the incident TM-polarized light to SPP modes at the Ag/SiO₂ interface.

Figure 12a–c presents the spatial electric field distribution component for different incidence angles and wavelengths, highlighting the field enhancement and distribution within the resonant structure. As the wavelength increases from λ₁ = 441 nm to λ₃ = 627 nm, a clear increase in the spatial extent of the electric field lobes is observed. At shorter wavelengths (441 nm), the field distributions show tightly confined and closely spaced modes, while at longer wavelengths (577 nm and especially 627 nm), the modes appear more spread out. This behavior reflects the wavelength-dependent scaling of the effective resonant modes, which naturally extend over larger spatial regions at higher wavelengths. These variations in field profile across different angles and wavelengths demonstrate the tunable and angle-dependent response of the proposed structure.

4. Conclusions

In summary, this work introduces a reconfigurable optical structure based on a Metal–Dielectric–PCM architecture, whose performance can be dynamically switched between RGB filtering and absorption by harnessing the phase-transition properties of GeTe. The functionality of the device is defined by a synergistic contribution of its constituent layers: the silver layer enables control over transmission and absorption efficiency, the SiO₂ layer governs resonant field confinement, and the GeTe layer modulates scattering behavior through its phase-dependent optical losses. The proposed structure demonstrates robust spectral selectivity, angle sensitivity, and polarization dependence. Furthermore, polynomial models were developed to predict and optimize device performance through thickness tuning. Beyond its versatile optical behavior, the device stands out for its structural simplicity and fabrication feasibility, showing strong potential for integration in dynamic photonic systems such as tunable filters, absorbers, and optoelectronic sensors.