Metasurfaces have garnered significant attention in recent years due to their exceptional ability to manipulate electromagnetic waves at subwavelength scales [1,2]. The electromagnetic responses of metasurfaces are primarily governed by the resonant modes of their subwavelength elements [3,4]. While considerable attention has been devoted to electric and magnetic multipoles, recent studies have highlighted toroidal multipoles as a distinct class of elementary electromagnetic sources [5,6]. The toroidal dipole (TD), characterized by poloidal currents and a circular magnetic field, typically exhibits weak responses in natural materials, making it difficult to detect. However, metasurfaces provide a versatile platform to generate strong TD responses [7–13]. The electromagnetic configuration of the TD mode can be engineered through the precise design of metallic or dielectric structures. Additionally, an intriguing anapole state can be created through destructive interference between TD and electric dipole (ED) modes [14–18]. The unique electromagnetic configuration associated with the TD mode has found applications in enhancing photoluminescence [10], nonlinear harmonic generation [11], and sensing technologies [12].

On the other hand, bound states in the continuum (BICs), as another fascinating mode, have also become a subject of significant interest due to their theoretically infinite quality factors ($Q$-factors) [19–21]. BICs are characterized as localized modes despite their presence within a continuous spectrum. Extensive research has reported the occurrence of BICs in both metallic and dielectric metasurfaces. Furthermore, the operating wavelengths of BICs can be tuned across various frequency ranges through the modification of structural parameters [22–31]. Additionally, BICs with the characteristic of TD mode have been recently reported based on dimer and quadrumer metasurfaces [32–36]. Although BICs are not directly accessible via external excitations, they can be converted into quasi-BICs through the adjustment of geometric parameters, which are available by incident electromagnetic waves. Quasi-BICs exhibit ultrahigh $Q$-factors and an enhanced electromagnetic field, showing promise for applications such as low-threshold lasing [22], high-efficiency nonlinear frequency conversion [27], and advanced sensors [24].

While substantial progress has been made in the development of TD and BIC modes in metasurfaces, most of them remain static after fabrication. In recent years, there has been growing interest in active metasurfaces that offer tunable functionalities [37,38]. A common approach involves incorporating materials with tunable properties, such as graphene [39], liquid crystals [40], semiconductors [41], magneto-optical materials [42], and phase-change materials [43–46], into metallic or dielectric metasurfaces. These materials enable dynamic tunability of optical properties through external stimuli such as voltage, heat, optical pulses, or magnetic fields. ${\mathrm{Ge}}_2{\mathrm{Sb}}_2{\mathrm{Te}}_5$ (GST), in particular, has attracted attention for its ability to undergo significant changes in refractive index during phase transition [47,48]. GST transitions from an amorphous to a crystalline state when heated above 160°C and can revert to its amorphous state through rapid annealing at approximately 640°C. Notably, the phase transition of GST is nonvolatile, allowing it to maintain stability in a certain phase at room temperature. In addition to thermal stimulus, current and optical pulse can also induce phase transition in GST [47,48]. The tunable refractive index of GST in the visible and infrared spectra has facilitated its integration into metasurfaces for the development of tunable optical devices [49–58]. Additionally, the conductivity of GST varies dramatically during the phase transition, which has recently been utilized in conjunction with metasurfaces to generate tunable terahertz devices [59–61]. While there has been significant research on GST-based tunable metasurfaces in the visible and infrared ranges, investigations on GST-based tunable terahertz devices remain relatively limited. Particularly, the dynamic control of TD and BIC modes utilizing GST-based terahertz metasurfaces has not been reported to date.

In this work, we demonstrate the realization of dynamically tunable TD and quasi-BIC modes through the integration of symmetric and asymmetric aluminum (Al) dumbbell aperture arrays with GST. The symmetric aluminum dumbbell aperture array exhibits a strong TD response in the terahertz range. By modifying the radii of dumbbell apertures to break the structural symmetry, a quasi-BIC mode can be excited in the asymmetric aluminum dumbbell aperture array, in addition to the TD mode. When GST is incorporated into both the symmetric and asymmetric aluminum dumbbell aperture arrays, the TD and quasi-BIC modes can be dynamically tuned through the phase transition of GST. As GST transitions from its amorphous to crystalline state, significant modulation of transmission occurs around the TD and quasi-BIC frequencies due to changes in conductivity. Experimental fabrications of hybrid Al/GST dumbbell aperture arrays have been conducted, and the measured transmission spectra align well with simulated results. The symmetric hybrid Al/GST dumbbell aperture array exhibits a maximum modulation efficiency of 97.8% at 0.486 THz. In contrast, the asymmetric hybrid Al/GST dumbbell aperture array achieves maximum modulation efficiencies of 94.3% and 96% at 0.352 and 0.483 THz, respectively. These results highlight the potential of hybrid Al/GST dumbbell aperture arrays for applications in terahertz modulators and filters.

Figure 1(a) presents a schematic of the hybrid Al/GST dumbbell aperture array. The metasurface consists of an aluminum film perforated by a periodic dumbbell aperture array and overlaid with a GST layer. As illustrated in Fig. 1(b), the unit cell contains two dumbbell apertures, each formed by two circular apertures with identical radii, in conjunction with a rectangular aperture. The radii of the two dumbbell apertures are denoted as $r_1$ and $r_2$, while the distance between the centers of two circular apertures is designated as $d$. The width of rectangular aperture is represented by $w$, and the array period along the $x$- and $y$-direction is defined as $P_x$ and $P_y$, respectively. The thickness of the polyimide substrate is set to 25 μm, and both the GST and aluminum layers have a thickness of 200 nm. Consequently, the aluminum dumbbell apertures are filled with GST, and the surrounding aluminum film is overlaid with a GST layer. In its amorphous phase, GST exhibits low conductivity, behaving as a low-loss dielectric material in the terahertz band. In the case of the symmetric hybrid Al/GST dumbbell aperture array ($r_1=r_2$), a strong TD response is induced under $y$-polarized incidence. This structure generates two circular currents flowing in opposite directions at the edges of the circular apertures, resulting in magnetic fields with a head-to-tail configuration that facilitates the excitation of the TD mode. Additionally, a transmission peak emerges in the terahertz spectrum, arising from the contributions of both ED and TD modes. In the case of the asymmetric hybrid Al/GST dumbbell aperture array ($r_1\ne r_2$), the transmission spectrum reveals an additional peak, corresponding to the excitation of a quasi-BIC mode. The quasi-BIC mode exhibits distinct electromagnetic field distributions in contrast to the TD mode. Furthermore, significant enhancement of the electromagnetic field occurs within the dumbbell apertures as a result of the excitation of the quasi-BIC. As GST gradually transitions to its crystalline phase, both TD and quasi-BIC modes experience a decline due to the increase in GST conductivity. This process is accompanied by a substantial decrease in transmittance, as schematically depicted in Fig. 1(c). Therefore, dynamically tunable TD and quasi-BIC modes can be achieved through the utilization of symmetric and asymmetric hybrid Al/GST dumbbell aperture arrays, offering promising opportunities for the development of terahertz modulators and filters.

We begin by numerically investigating the optical properties of the aluminum dumbbell aperture structures without the GST layer using COMSOL Multiphysics software. Figure 2(a) shows the unit cell of the aluminum dumbbell aperture array. In the simulations, periodic boundary conditions were employed in both the $x$- and $y$-directions, while a perfectly matched layer was used in the $z$-direction. The conductivity of aluminum was set to $3.56\times {10}^7\mathrm{S}/\mathrm{m}$ [62], and the complex permittivity of the polyimide substrate was set to $2.93+0.13i$ [63]. A user-controlled mesh, comprising triangular and tetrahedral elements, was employed for mesh discretization. The mesh with “extremely fine” element size was utilized to achieve a high degree of accuracy. As depicted in Fig. 2(a), the structure is excited by a normally incident, $y$-polarized plane wave. The geometric parameters of the aluminum dumbbell aperture array are $r_1=30\mathrm{\mu m}$, $d=120\mathrm{\mu m}$, $w=10\mathrm{\mu m}$, and $P_x=P_y=250\mathrm{\mu m}$. The calculated transmission spectra for varying radii $r_2$ are presented in Fig. 2(b). In the case of the symmetric aluminum dumbbell aperture array ($r_1=r_2=30\mathrm{\mu m}$), a transmission peak is observed at 0.5 THz with a transmittance of 0.81. In order to elucidate the electromagnetic origin of the resonant peak, we conducted calculations of the surface current, electric field, and magnetic field distributions at 0.5 THz, as shown in Fig. 2(e). The two dumbbell apertures exhibit identical distributions of current, electric field, and magnetic field. Notably, for $r_1=r_2$, the unit cell comprises a single dumbbell aperture, and the period along the $y$-direction is halved to $P_y/2$. Clockwise and counterclockwise currents are induced at the left and right circular apertures, respectively. These current distributions produce two magnetic dipoles oriented in opposite directions. Moreover, as displayed in Fig. 2(e), a head-to-tail magnetic field can be formed between the two circular apertures within the unit cell as well as between the two circular apertures in adjacent unit cells, which is a typical feature of the TD mode [5,6]. In addition, a significant enhancement of the electric field is observed in the central gap of the dumbbell aperture.

To further clarify the multipole origin of the resonance, we calculated the scattered powers corresponding to different multipole moments utilizing the multipole decomposition method [14,15], as presented in Fig. 2(d). Five primary multipole moments were considered: ED, TD, magnetic dipole (MD), electric quadrupole (EQ), and magnetic quadrupole (MQ). At 0.5 THz, the scattered power is dominated by the ED moment, with a smaller contribution from the TD moment. However, at 0.526 THz, the scattered power associated with the TD moment surpasses that of the ED moment, while the contributions from the MQ, EQ, and MD moments remain minimal. Due to the broad nature of the transmission peak, the scattered powers associated with the ED and TD moments exhibit competitive behavior near the peak frequency. As a result, the resonant peak arises from the contributions of both TD and ED modes.

Recent studies have extensively explored BICs in metasurfaces [19–21], with symmetry-protected BICs being prevalent in metasurfaces exhibiting structural symmetry [19–21]. While BICs cannot be directly excited by incident electromagnetic waves, they can be converted into quasi-BICs through modification of the geometric parameters. These quasi-BICs exhibit high-Q resonances in the transmission or reflection spectra. A recent investigation has demonstrated that dumbbell aperture structures are capable of supporting the BIC mode [17]. The BIC mode can be transformed into the quasi-BIC mode by modifying the length of the dumbbell aperture to break the in-plane inversion symmetry [17]. It should be noted that the in-plane inversion symmetry can also be broken by changing the radius of the dumbbell aperture. Different from the previous work [17], we vary the radius $r_2$ to investigate the excitation of the quasi-BIC in aluminum dumbbell aperture structures. When $r_1=r_2=30\mathrm{\mu m}$, the BIC mode cannot couple with the normally incident electromagnetic wave due to symmetry mismatch. However, as $r_2$ deviates from 30 μm, the in-plane inversion symmetry is broken, transforming the BIC into a quasi-BIC, which gives rise to a Fano-type resonance in the transmission spectrum, as shown in Fig. 2(b). The quasi-BIC resonance shifts to lower frequencies as $r_2$ increases, disappearing entirely when $r_2=30\mathrm{\mu m}$. For $r_2=50\mathrm{\mu m}$, the quasi-BIC resonance appears at approximately 0.398 THz. We conducted an analysis of the surface current, electric field, and magnetic field distributions at 0.398 THz, as illustrated in Fig. 2(g). The dumbbell aperture with $r_2=50\mathrm{\mu m}$ generates two circular currents flowing in opposite directions, analogous to those in the dumbbell aperture with $r_1=30\mathrm{\mu m}$, but with reversed directions. Furthermore, the magnetic fields in the dumbbell aperture with $r_2=50\mathrm{\mu m}$ are oriented oppositely to those in the dumbbell aperture with $r_1=30\mathrm{\mu m}$. Additionally, the electric fields in the central gaps of the asymmetric dumbbell apertures are significantly enhanced, exhibiting higher intensity compared to the symmetric dumbbell apertures.

We also computed the scattered powers for different multipole moments at $r_2=50\mathrm{\mu m}$, as shown in Fig. 2(f). While the scattered power associated with the ED moment remains predominant around 0.398 THz, there is a significant enhancement in the scattered power of the EQ moment. This suggests that the quasi-BIC resonance is primarily ascribed to the excitation of both ED and EQ modes. Notably, the contribution of the EQ mode to the quasi-BIC has been recently reported in dumbbell aperture structures [17]. Furthermore, the electromagnetic field distributions of the quasi-BIC are similar to those of the prior research [17]. Although the quasi-BIC in the previous study was achieved by modifying the length of the dumbbell aperture, it can also be realized through alterations in the radius of the dumbbell aperture. In addition, a broad transmission peak is observed around 0.5 THz for $r_2=50\mathrm{\mu m}$, as shown in Fig. 2(b). The scattered powers associated with the ED and TD moments are competitive in the vicinity of 0.5 THz, as displayed in Fig. 2(f). Notably, the TD moment prevails at 0.52 THz. Consequently, the asymmetric aluminum dumbbell aperture array supports the excitation of both the quasi-BIC and TD modes.

We also calculated the $Q$-factors of the quasi-BIC for varying radii $r_2$, as presented in Fig. 2(c). The $Q$-factors of the resonances are obtained by fitting the transmission spectra with the Fano formula [64,65]: $$T(\omega )=T_0+A_0\frac{{[q+2(\omega -{\omega }_0)/\gamma ]}^2}{1+{[2(\omega -{\omega }_0)/\gamma ]}^2},$$where $T_0$ is the transmittance baseline, $A_0$ is the coupling coefficient, $q$ denotes the Fano fitting parameter, and ${\omega }_0$ and $\gamma$ represent the resonance angular frequency and linewidth, respectively. Then, the $Q$-factor can be calculated by $Q={\omega }_0/\gamma$. The $Q$-factor of the quasi-BIC resonance increases as $r_2$ approaches 30 μm. Additionally, we also computed the $Q$-factors of the quasi-BIC when aluminum was substituted with a perfect electrical conductor and the loss of polyimide was ignored. As displayed in Fig. 2(c), in the absence of material absorption, the $Q$-factor approaches infinity as $r_2$ nears 30 μm, a characteristic of BICs [19–21]. The practical $Q$-factor is limited by the absorption loss in aluminum and polyimide. Therefore, the symmetric aluminum dumbbell aperture array is capable of supporting a BIC mode, which transitions to a quasi-BIC mode by modifying the radii of the dumbbell apertures to break the in-plane inversion symmetry.

In the preceding discussion, we demonstrated the effective excitation of both TD and quasi-BIC modes utilizing the aluminum dumbbell aperture array. However, these modes exhibit static characteristics and present challenges for dynamic tuning in response to external stimuli. Recently, GST has garnered significant attention due to its distinctive phase-change feature [47,48]. Notably, the conductivity of GST undergoes significant variations during its phase transition. The conductivity of GST can be tuned from 108 S/m to 321,868 S/m by changing temperature [66]. It is important to note that intermediate states exist between the amorphous and crystalline phases during the phase transition of GST, which can be controlled through adjusting the annealing temperature [50,53,59,60]. The conductivity of GST can be continuously tuned by altering the crystallization ratios [59,60]. In this study, we incorporate a GST film into the previously discussed aluminum dumbbell aperture array and investigate its optical characteristics through the phase transition of GST. Figure 3(a) shows the unit cell of the symmetric hybrid Al/GST dumbbell aperture array. To explore the influence of GST phase transition, we computed the transmission spectra of the symmetric hybrid Al/GST dumbbell aperture array for varying conductivities, while maintaining the other geometric parameters of the dumbbell apertures consistent with those in Fig. 2(a). As illustrated in Fig. 3(b), for a GST conductivity of 100 S/m, a transmission peak with a transmittance of 0.71 is observed at 0.497 THz. In comparison to the dumbbell aperture structure without GST, a slight reduction in peak transmittance is observed, which can be ascribed to the loss associated with the GST. The surface current, electric field, and magnetic field distributions at 0.497 THz for GST conductivity of 100 S/m are presented in Fig. 3(e). The surface currents encircle the dumbbell apertures and flow in opposite directions, and the electric field is notably intensified in the central gap, resembling the case without GST shown in Fig. 2(e). The multipole analysis reveals that the scattered power associated with the ED moment dominates at 0.497 THz, with a smaller contribution from the TD moment, as illustrated in Fig. 3(c). The scattered powers of the ED and TD moments are nearly equal at 0.52 THz. Therefore, when GST is added on the aluminum dumbbell aperture array, both the ED and TD responses persist, accompanied by high transmittance.

As GST progressively transitions to the crystalline phase, its conductivity increases substantially, leading to a marked reduction in peak transmittance, as shown in Fig. 3(b). Correspondingly, as depicted in Fig. 3(e), the amplitude of the surface current, electric field, and magnetic field decreases due to the increased conductivity of GST. Figure 3(d) shows the scattered powers of different multipole moments as a function of GST conductivity at 0.497 THz. As GST conductivity increases, the scattered power associated with the ED moment increases, while the scattered power of the TD moment declines. At a conductivity of 10,000 S/m, the transmittance in the considered frequency range approaches 0, as the high conductivity of both GST and aluminum inhibits the transmission of the terahertz wave. As a result, it is feasible to develop a terahertz modulator utilizing a hybrid Al/GST dumbbell aperture array. Compared with traditional electric and magnetic multipoles, the TD mode exhibits distinct electromagnetic field distribution and substantial electromagnetic field localization [5,6]. The distinctive electromagnetic configuration of TD mode has been utilized to enhance photoluminescence [10] and nonlinear harmonic generation [11]. Therefore, active control of TD mode may be employed in applications involving tunable photoluminescence and nonlinear harmonic generation.

As previously discussed, a quasi-BIC can be produced in the asymmetric aluminum dumbbell aperture array by modifying the radius $r_2$ to break the structural symmetry. In this section, we investigate the integration of GST into the asymmetric aluminum dumbbell aperture array and analyze the influence of the GST phase transition on the quasi-BIC. The unit cell of the asymmetric hybrid Al/GST dumbbell aperture array is schematically shown in Fig. 4(a). Figure 4(b) presents the calculated transmission spectra for varying radii $r_2$ in the presence of GST, with the conductivity of GST fixed at 100 S/m. When $r_2$ deviates from 30 μm, the quasi-BIC resonance is induced in the presence of GST, similar to the case in the absence of GST. Specifically, for $r_2=50\mathrm{\mu m}$, the quasi-BIC occurs at 0.396 THz. The surface current, electric field, and magnetic field distributions of the quasi-BIC resonance for $r_2=50\mathrm{\mu m}$ closely resemble those observed in the absence of GST, as shown in Fig. 4(e). Furthermore, the multipole analysis for $r_2=50\mathrm{\mu m}$ reveals that the scattered power associated with the ED moment predominates at the quasi-BIC frequency, with a smaller contribution from the EQ moment, as shown in Fig. 4(d). Additionally, the scattered power associated with the TD moment dominates at 0.519 THz. Consequently, the asymmetric hybrid Al/GST dumbbell aperture array supports the excitation of both the quasi-BIC and TD modes.

Figure 4(c) displays the calculated transmission spectra for the asymmetric Al/GST dumbbell aperture array at various conductivities of GST, with $r_2$ fixed at 50 μm. As the conductivity of GST increases, there is a gradual decline in the transmittance of the two peaks associated with the quasi-BIC and TD modes. Moreover, an increase in the conductivity of GST is accompanied by a significant reduction in the amplitude of the surface current, electric field, and magnetic field, as depicted in Fig. 4(e). As presented in Fig. 4(e), when the conductivity of GST is 100 S/m, there is a notable enhancement of the electric fields in both the top and bottom dumbbell apertures, attributed to the excitation of quasi-BIC. Notably, the electric field in the bottom dumbbell aperture is stronger than that of the top dumbbell aperture, indicating that the former is more susceptible to the ohmic losses. As the conductivity of GST increases, a decline in the electric fields of both top and bottom dumbbell apertures is observed, which can be ascribed to the increased ohmic losses. The decrease in the electric field of the bottom dumbbell aperture is particularly pronounced, as it is more sensitive to the ohmic losses. When the conductivity of GST is increased to 1000 S/m and 3000 S/m, the top dumbbell aperture exhibits a stronger electric field compared to the bottom dumbbell aperture. Therefore, tunable quasi-BIC and TD modes can be achieved based on the asymmetric Al/GST dumbbell aperture array. In comparison to other resonant modes, quasi-BIC mode exhibits a high $Q$-factor and strong field confinement ability [19–21], which has been utilized to enhance nonlinear frequency conversion [27] and unidirectional radiation [29]. Consequently, the active tuning of quasi-BICs may be advantageous for applications in tunable nonlinear frequency conversion and unidirectional radiation.

Up to now, we have numerically demonstrated the dynamically tunable TD and quasi-BIC modes based on the phase transition of GST. To validate the above analysis experimentally, we fabricated both symmetric and asymmetric hybrid Al/GST dumbbell aperture arrays and measured their transmission spectra over a range of temperatures. The fabrication process involved several steps. First, a 200-nm-thick aluminum film was deposited onto a 25-μm-thick polyimide substrate via magnetron sputtering. A layer of photoresist was then spin coated onto the aluminum film, which was subsequently patterned through photolithography. Next, the aluminum film covered without the photoresist was removed via wet etching. Following this, the aluminum dumbbell aperture array was formed after removing the residual photoresist using acetone. Finally, a 200-nm-thick GST film was deposited onto the sample using magnetron sputtering. Four hybrid Al/GST dumbbell aperture arrays were fabricated, with $r_2$ chosen as 20, 30, 40, and 50 μm, respectively. The optical microscope images of the aluminum dumbbell aperture arrays with $r_2=30$ and 50 μm, before the deposition of the GST layer, are presented in Figs. 5(a) and 5(b), respectively. To induce the GST phase transition, these samples were annealed at temperatures ranging from 25°C to 300°C for 5 min at each temperature on a hot plate. The crystalline fraction of GST was controlled by adjusting the annealing temperature, leading to a progressive alteration in its conductivity. As the phase transition of GST is nonvolatile, it can be maintained in a certain phase when the temperature is dropped to room temperature. The transmission spectra of the fabricated samples under different annealing conditions were measured at room temperature by utilizing a frequency-domain THz spectroscopy system (TeraScan 1550 from Toptica).

Figure 5(c) presents the measured transmission spectra of the aluminum dumbbell aperture arrays with varying $r_2$ in the absence of GST under $y$-polarized incidence. For the symmetric aluminum dumbbell aperture array ($r_2=30\mathrm{\mu m}$), a transmission peak is observed at 0.508 THz, exhibiting a transmittance of 0.66. The measured transmission spectrum is in close agreement with the simulated results, with minor discrepancies attributed to fabrication imperfections and material losses. Figure 5(e) presents the measured transmission spectra of the symmetric aluminum dumbbell aperture array coated with GST at various temperatures. The incorporation of GST leads to a reduction in peak transmittance, reducing to 0.59 at 25°C due to the absorption loss associated with GST, which aligns with the findings from the simulated results. As the temperature rises, the peak transmittance progressively diminishes, which can be ascribed to the increased conductivity of GST during its phase transition. These experimental results are in excellent agreement with the simulated results. Figure 5(g) depicts the relationship between transmittance at 0.486 THz and temperature. As the temperature rises from 25°C to 300°C, the transmittance at 0.486 THz diminishes from 0.59 to 0.013, demonstrating substantial modulation of terahertz transmission via temperature control. The performance of transmission modulation can be evaluated by the modulation efficiency, defined as $\zeta =1-T/T_{\max }$ [66], where $T$ represents the temperature-dependent transmittance and $T_{\max }$ is the maximum transmittance. As the temperature rises, the modulation efficiency increases, reaching 97.8% at 300°C. In the past decade, a range of terahertz modulators have been developed by combining terahertz metasurfaces with tunable materials, such as semiconductors [67], vanadium dioxide [68], graphene [69], and superconductors [70], which possess the modulation efficiency of 93%, 99%, 49.3%, and 86.8%, respectively. Recently, GST was integrated with a gold circular aperture array to realize a terahertz modulator with the modulation efficiency of 88% [66]. The modulation efficiency demonstrated in our study surpasses that of the majority of previous works. This indicates the potential for developing a terahertz modulator utilizing a symmetric hybrid Al/GST dumbbell aperture array.

The quasi-BIC resonance is also observed in the asymmetric aluminum dumbbell aperture array when $r_2$ deviates from 30 μm, as shown in Fig. 5(c). Upon incorporating GST, the quasi-BIC is still present in the asymmetric hybrid Al/GST dumbbell aperture array, as displayed in Fig. 5(d). Figure 5(f) presents the measured transmission spectra of the asymmetric hybrid Al/GST dumbbell aperture array with $r_2=50\mathrm{\mu m}$ at different temperatures. As the temperature increases, the quasi-BIC resonance progressively diminishes, a trend consistent with the simulated results. Consequently, a switchable quasi-BIC can be achieved through the phase transition of GST. Figure 5(h) shows the temperature-dependent transmittance at 0.352 and 0.483 THz. As the temperature increases, the transmittance at 0.352 THz decreases from 0.436 to 0.025. In comparison, the transmittance of the symmetric hybrid Al/GST dumbbell aperture array at 0.352 THz decreases from 0.232 to 0.012, when the temperature rises from 25°C to 300°C, as shown in Fig. 5(e). Therefore, the presence of the quasi-BIC facilitates a larger transmission modulation at 0.352 THz in comparison to the case without quasi-BIC. In addition, the transmittance at 0.483 THz drops from 0.657 to 0.026 with the increase of temperature, as presented in Fig. 5(h). At 300°C, the modulation efficiencies at 0.352 and 0.483 THz are 94.3% and 96%, respectively. As a result, dual-band THz modulators can be achieved by employing the asymmetric hybrid Al/GST dumbbell aperture array.

In summary, we have numerically and experimentally demonstrated the active manipulation of TD and quasi-BIC modes using symmetric and asymmetric hybrid Al/GST dumbbell aperture arrays. The symmetric hybrid Al/GST dumbbell aperture array exhibits a pronounced TD response in the terahertz band. By modifying the radii of dumbbell apertures to break structural symmetry, a quasi-BIC resonance is induced in the asymmetric hybrid Al/GST dumbbell aperture array. Notably, both the TD and quasi-BIC modes can be dynamically tuned through GST phase transition, resulting in significant modulation of terahertz transmission. The simulated findings are consistent with the experimental measurements. While this study primarily explores the temperature-induced phase transition of GST, it is important to note that alternative stimuli, such as current or optical pulses, can also trigger the phase transition [47,48]. These symmetric and asymmetric hybrid Al/GST dumbbell aperture arrays present promising opportunities for the development of terahertz modulators and filters.