Surface plasmons (SPs) are unique two-dimensional electromagnetic waves existing at metal-dielectric interfaces. Over the past two decades, SPs have shown great potential in various applications, such as extraordinary light transmission [1,2], high resolution imaging [3,4], biochemical sensing [5,6], and integrated plasmonic circuitries [7]. Metasurfaces are artificial two-dimensional functional surfaces composed of planar structures, which have exhibited unprecedented capabilities in manipulating free-space light, including phase [8–11], amplitude [12], and polarization [13–15], enabling the design of complex light propagation behaviors and associated devices [16–18]. Recent advances have shown that such a high controlling degree of freedom can also be introduced into the SP regime [19].

Thus far, various metasurface-based SP devices have been demonstrated, including directional and asymmetric SP couplers [20], SP vortex generators and interferometers [21–23], SP holograms [24], etc. However, most of them are working in a passive manner, hindering their applications in fulfilling the requirements of modern tunable on-chip systems. One of the most attractive approaches is to integrate functional materials into the metasurface design, like those reported in the free-space regime, for example, VO2 [25–28], graphene [29,30], high electron mobility transistor [31], liquid crystals [32], and MEMS [33], which modulate the resonance responses of the structures through changes in conductivity, refractive index, and geometry deformation. However, these materials require continuous stimuli to keep their properties. This is good for fast modulation, but poses limitations on power consumption, fabrication cost, and applying environments.

Recently, a novel phase-change material, Ge2Sb2Te5 (GST), exhibiting distinct variable and nonvolatile optical properties, has drawn increasing attention in achieving active metasurface devices for both linear and nonlinear outputs [34,35]. It mainly possesses two material states, an amorphous state as a dielectric and a crystalline state as a metal [36]. The states can be controlled by multiple routes, such as heating [37], laser illumination [38], and bias voltage [39], making it applicable in various scenarios. Diverse GST-based free-space metasurface devices have been demonstrated, including modulators [40,41], color displays [42], and photonic memories [43,44]. However, related studies are seldom carried out in the SP regime. In 2023, Zhang et al. demonstrated a GST-based nonvolatile reconfigurable terahertz varifocal SP metalens [45], confirming such a possibility.

In this work, inspired by the focusing ability of a Fresnel zone plate (FZP), we experimentally demonstrate a dual-function switchable SP device using a GST metasurface in the terahertz regime. By introducing GST into the dark zones of a well-designed column-shape spin-dependent directional SP coupler, the device excites plane-wave SPs at the amorphous state while focusing SPs at the crystalline state. Upon spin direction changes, the direction of the excited SPs reverses accordingly. Here, the amorphous to crystalline transform is realized through an annealing process with gradually rising temperatures, while the back transform is realized through nanosecond laser illumination. As the state of GST is nonvolatile after each processing procedure, the response of the SP device is retained. The measured results agree well with the design. Our method paves the way towards nonvolatile and tunable on-chip devices.

The basic building block of the proposed metasurface is a subwavelength metallic slit resonator (SR) with a width of a and a length of b, which is a commonly applied structure for coupling free-space light to SPs; see the lower right inset in Fig. 1(a). Only the polarization component perpendicular to the SR is useful and the SPs are excited towards its two sides like in-plane dipole radiations [46]. According to the Huygens’s principle, when many SRs of the same orientation are arranged in a column with a distance d less than the SP wavelength, plane-wave SPs will be excited with propagation directions perpendicular to the column; when they are arranged in a column following an FZP distribution, focusing SPs will be excited. Here, to increase the functionality, we adopt a two-column design; see Fig. 1. The SRs in the two columns are set to be perpendicular to each other. Under an arbitrarily polarized incidence, the SPs excited towards the right side Er and left side El can be expressed as $$\begin{gathered} Er=A(E1cosθeiϕ+E2eiδsinθ), \\ El=A(E1cosθ+E2eiδsinθeiϕ), \end{gathered}$$(1)where E1 and E2 are two orthogonally polarized components of the incidence with their directions respectively perpendicular to the SRs in the two columns, A is the coupling coefficient of the SR, θ is the angle between E1 and x axis, δ is the phase difference between E1 and E2, ϕ=kSPg is the propagation phase of the SPs over the distance g between two columns, and kSP=2π/λSP is the SP wave number with λSP being the SP wavelength. By analyzing Eq. (1), it can be found that when θ=45°, g=λSP/4, and the incident light is circularly polarized, we have |Er|=0 and |El|=21/2AE1 under right-handed circularly polarized (RCP, δ=−π/2) incidence, while |Er|=21/2AE1 and |El|=0 under left-handed circularly polarized (LCP, δ=π/2) incidence. Our design is just based on such a configuration, which corresponds to a spin-dependent directional SP coupler [47]. Its device unit cell is schematically shown in the upper right inset in Fig. 1(a).

Figures 1(a)–1(c) illustrate the schematics of our dual-function switchable SP device. The two columns of SRs are set along the y axis. Several parts of the SRs are covered by GST patches. These parts are determined by the even zones of FZP; see Fig. 1(d). The radius of the Fresnel zones rm can be calculated by rm=mλSPf+m2λSP24,(2)where m is the integral indicator of each zone, and f is the focal length. Here, the even zones are classified as dark zones, since the GST patches can shut down the SP excitation responses of the SRs below them. In contrast, the odd zones are classified as bright zones. To ensure that the SRs in the even zones are fully covered, the GST patches exceed the SRs below. When GST is in the amorphous state, it functions as a dielectric and all the SRs work. The device excites plane-wave SPs towards the left under RCP incidence and the right under LCP incidence, which is denoted as a directional plane-wave SP coupler; see Fig. 1(a). By annealing the GST above its transition temperature, it is transformed to the crystalline state and functions as a metal. Thus, the SRs in the dark zones are shortened; only the SRs in the bright zones work. The device becomes an SP FZP, which excites focusing SPs directionally towards the left under RCP incidence and the right under LCP incidence; see Fig. 1(b). By illuminating the crystalline GST with a nanosecond laser, it is transformed back to the amorphous state. The device then recovers the functionality of a directional plane-wave SP coupler; see Fig. 1(c). Notice that the function switching here arises from the modulation of GST to the SR resonance. The excess GST patches out of the SRs neither affect the interaction region of this modulation, nor bring observable influence on the propagation path of SPs due to their ultrathin feature.

To validate the feasibility of the above strategy, we employ two passive samples to mimic the conditions of GST in its two different states. Both samples are designed and characterized in the terahertz regime, as shown in Fig. 2(a). The samples are made from 200-nm-thick aluminum film (σ=3.56×107S/m) patterned on a 650-μm-thick silicon substrate (εSi=11.9). The SR has a dimension of a=12μm and b=66μm, which resonates at 0.75 THz where the SP excitation efficiency is the strongest. Since the metal nearly performs like a perfect conductor at terahertz frequencies, the SP dispersion is nearly equal to that in free space. Thus, g is set to 100 μm to make the device work at 0.75 THz (λSP=400μm). For the first sample (sample 1), both columns contain 65 SRs with d=100μm, corresponding to GST in the amorphous state, i.e., a directional plane-wave SP coupler. For the second sample (sample 2), only the SRs in the bright zones of sample 1 are retained, corresponding to GST in the crystalline state, i.e., a directional SP FZP with a focal length of f=3mm. Both samples were fabricated using a conventional photolithography method. First, the silicon substrate was sequentially cleaned with acetone, isopropanol, and deionized water. Then, a 200-nm-thick aluminum film was deposited on the substrate using thermal evaporation. After that, photoresist was spun-coated onto the aluminum surface, followed by UV exposure using a predesigned mask. After development, corresponding photoresist patterns were obtained. Next, the samples were placed in an acidic solution for wet etching, where the unprotected metal was etched away. Finally, the samples were obtained by immersing them in acetone to remove the remaining photoresist. The microscope images of sample 1 and sample 2 are illustrated in Figs. 2(b) and 2(e), respectively.

To measure the SP excitation responses, we utilized an all-fiber near-field scanning terahertz microscopy (NSTM) system based on the time-domain method [48,49]. The drive laser was a femtosecond fiber laser with a central wavelength of 1560 nm, a pulse duration of ∼100fs, and a repetition frequency of 100 MHz. It was first divided into two beams by a fiber splitter. One was for pumping a commercial photoconductive emitter to generate broadband linearly polarized terahertz radiation (y-polarized), while the other was for pumping a commercial near-field z-polarized terahertz probe to detect the terahertz SP fields [50]. Figure 1(a) illustrates the measuring scheme of the essential terahertz part. The generated terahertz radiation was first collimated by a lens, and then passed through a linear polarizer to initialize the output polarization angle to be either +45° or −45°. After illuminating on the sample from the substrate side, desired terahertz SPs were excited in the structure side and detected by the probe. By rater scanning the probe along the sample surface with a certain distance of 50 μm, the SP distributions were mapped. Figures 2(c) and 2(f) show the measured normalized SP intensity (|Ez|2) distributions of sample 1 and sample 2 in the x−y plane under RCP and LCP incidences, respectively. The results were synthesized using those under linearly +45°- and −45°-polarized incidences with Ez,RCP=Ez,+45°−iEz,−45° and Ez,LCP=Ez,+45°+iEz,−45°, respectively. The scanning areas were all 12mm×6.6mm. For sample 1, plane-wave SPs are clearly directionally excited towards left under RCP incidence and right under LCP incidence. While for sample 2, corresponding plane-wave SPs become focusing SPs (see the focal points inside the white dashed circles), showing the effectiveness of the FZP concept in SP control. The focal length is measured to be about 3 mm, which is consistent with the design. Besides, we have also carried out numerical simulations on the two samples using CST Microwave Studio. Open boundaries are applied in all the directions. The Ez field distributions of SPs are recorded using a field monitor. The results are shown in Figs. 2(d) and 2(g), which agree well with the measurements. All these demonstrate that our strategy in exciting spin-dependent directional plane-wave SPs and focusing SPs is effective. If GST can be well tuned from a dielectric phase to a metal phase, a dual-functional switchable function is possible.

Based on the above results, the phase-transition performance of GST is the key in achieving the dual-function switching application. In order to study the modulation effect of GST on terahertz waves, a 100-nm-thick GST film was deposited on a silicon substrate using magnetron sputtering and measured using a conventional all-fiber terahertz time-domain spectroscopy (THz-TDS) system. Figure 3(a) illustrates the measured amplitude transmission spectra of the GST film. The green dashed-dotted line shows the result of the initially fabricated GST film (unheated), which is almost transparent to the terahertz wave. The color solid lines show the results under increasing annealing temperature from 150°C to 340°C. The annealing process was performed simply by putting the sample onto a hot plate set to the target temperature for 1 h and then naturally cooling it down to room temperature for measurement. Owing to the nonvolatile feature of GST, the material property of GST remains, which no longer needs a continuous heating process. It is seen that as the annealing temperature increases, the transmission amplitude gradually decreases from around 0.97 to 0.41. We also plot the corresponding transmission amplitudes at 0.75 THz as a function of the annealing temperature in Fig. 3(d), which change dramatically around 280°C and show a saturated effect when further increasing the annealing temperature above 340°C. After that, we illuminated the GST film with a nanosecond laser with a central wavelength of 1064 nm, a pulse width of 10 ns, a spot size of 9 mm, and an energy density of 120mJ/cm2. The corresponding transmission amplitude increases abruptly to around unity, which is nearly the same as that of the unheated one [see the purple dashed line in Fig. 3(a)], meaning that the GST was changed back to its initial state.

To check the conductivity change of the GST film, a thin-film approximation theory is applied [51], where its complex conductivity σGST(ω)=σr(ω)+iσi(ω) can be expressed as σGST(ω)=1+nsi−(1+nsi)tGST(ω)Z0dGSTtGST(ω),(3)where nsi=3.42 is the refractive index of the silicon substrate, Z0=377Ω is the impedance of vacuum, dGST=100nm is the thickness of the GST film, and tGST(ω) is the complex transmission of the GST film. Figure 3(b) illustrates the calculated real-part conductivity of the GST film σr(ω), and the specific values at 0.75 THz are extracted in Fig. 3(d), which show opposite varying trends to that of transmission amplitudes, as a larger conductivity corresponds to a larger screening effect on the terahertz wave. This indicates that GST is gradually transformed to the crystalline state. The conductivity can reach 1.68×105S/m at an annealing temperature of 340°C, which is large enough to shorten the SRs [45]. After being illuminated by the nanosecond laser, the conductivity changes back to the unheated case, which abruptly decreases to only 9.30×102S/m. This indicates that GST is switched back to the amorphous state. As for the imaginary-part conductivity of the GST film σi(ω), it is two orders smaller than that of σr(ω).

To show the effectiveness of the measured conductivity, we further approximately calculate the relative permittivity of the GST film εGST(ω) by [52] εGST(ω)=−σi(ω)ωε0+iσr(ω)ωε0(4)and import it into CST Microwave Studio. Here, ε0 is the permittivity of vacuum. Figure 3(c) illustrates the corresponding simulated amplitude transmission spectra, while the extracted transmission amplitudes at 0.75 THz are illustrated in Fig. 3(d). They all agree well with the measured results. This also indicates that we can directly apply the measured permittivity of GST to numerically study its modulation effect.

As the final step, we fabricated and characterized the proposed dual-function switchable SP device in Fig. 1. The fabrication process began with the same procedures as those for sample 1 in Fig. 2(b), followed by GST deposition and patterning on top of it with careful alignment. Firstly, photoresist was spun-coated on (another fabricated) sample 1. After UV exposure using another pre-designed mask, a new photoresist pattern was obtained through development. Then, a 100-nm-thick GST film was deposited on top of it by magnetron sputtering. Finally, the device was obtained by immersing the sample in acetone to remove the undesired GST parts. The microscope images of the device, including an overall image and a partially zoomed-in one, are illustrated in Fig. 1(e). The annealing processes and SP measurements were also identical to the above relevant content.

The corresponding measured normalized results under RCP and LCP incidences are shown in Figs. 4(a) and 4(c), respectively. Clear spin-dependent directional SP excitation behaviors are observed in all cases. The GST is initially in the amorphous state after fabrication; its dielectric property makes the device a directional SP coupler, which excites plane-wave SPs. Upon increasing the annealing temperature to 200°C, the SP excitation response is nearly unchanged, since the temperature is not high enough to make obvious phase transition in GST and the conductivity is still very low. As the annealing temperature continues increasing to 260°C, phase transition happens in GST; the conductivity of GST dramatically increases. The shortening effect on the SRs in the dark zones becomes obvious but not enough. Thus, both plane-wave and focusing SPs are excited with the plane-wave SPs becoming weaker while focusing SPs become stronger. Focal points are clearly observed at the two sides of the device under LCP and RCP incidences, respectively; see the SP intensities inside the white dotted circles. Increasing the annealing temperature to 280°C, GST is almost transitioned to the crystalline state. The high conductivity greatly shortens the SRs below, making the device function switch to an SP FZP. Thus, SP focusing behavior dominates. This becomes even clearer at 300°C annealing temperature, where GST is in the crystalline state. After being illuminated by a nanosecond laser, the device is switched back to a directional plane-wave SP coupler. The SP excitation results in the initial unheated case and laser illuminated case are very consistent with those of sample 1 in Fig. 2(c), while the corresponding results at 300°C annealing temperature are also very consistent with those of sample 2 in Fig. 2(f). The above results demonstrate the dual-function switchable and reconfigurable ability of our device very well. Notice that there is no comparability among measured absolute intensities at different rows in Fig. 4 owing to some experimental limitations, such as the system state and alignment differences in the relative position between the sample and the probe. This does not affect the internal physics of our design, as the switching behavior is clearly observed.

To make the above process clearer, we extract the SP intensity profiles along the white dashed lines in Figs. 4(a) and 4(c), which just locate at the focal lines of the FZP, as shown in Figs. 5(a) and 5(c). The results are normalized by their own maxima. It is seen that the SP intensities are around a similar level below 200°C annealing temperature, and an SP intensity peak at the focal point gradually emerges and becomes dominated as the annealing temperature increases to 300°C. After laser illumination, the SP intensity profile is changed back, which is nearly the same as the initial unheated case. We also carry our numerical simulations on the device under RCP and LCP incidences at different annealing temperatures and after laser illumination by importing the measured GST permittivity into the simulation model. The corresponding simulated normalized SP intensity distributions are shown in Figs. 4(b) and 4(d), while normalized SP intensity profiles at the focal lines are shown in Figs. 5(b) and 5(d), respectively. The results agree well with the measured results.

Here, we show that 100-nm-thick GST patches are enough to achieve the dual-function switching, even if it is thinner than the metallic slits. Theoretically, one can use thicker GST, since thicker GST corresponds to higher surface conductivity in its crystalline state, and thus a stronger shortening effect on the underlying SRs. In this case, the switching can occur at an even lower annealing temperature. However, it should be noted that thicker GST may require more intense nanosecond laser illumination for reamorphization.

The thermal control here is to obtain forward phase transition of GST from amorphous to crystalline. Meanwhile, to achieve sufficient modulation in the terahertz regime, the crystalline state of GST must be further pushed to the hexagonally closest packed phase [53]. Though being slow and not that precise, thermal control provides a simple, robust, large-scale, and cost-effective manner in tuning the phase transition of GST. Apart from thermal control, other ways like electrical and optical controls can also accomplish similar modulations, while finding advantages in speed and precision. Electrical control relies on Joule heating to realize the forward phase transition [54], while optical control relies on the photothermal effect [55]. The modulation times of them in the terahertz regime are reported to be in second and minute scales, respectively. Both offer more precise control by adjusting corresponding stimuli strengths. Besides, one can also consider GST as a semiconductor and apply an optical pump to tune the instant conductivity by photocarriers, where the modulation time can achieve picosecond scale [54].

It should also be mentioned that though the thermal control for forward phase transition here is slow, the backward phase transition by nanosecond laser illumination can be very fast, which is on the nanosecond timescale [45,56]. Meanwhile, by adjusting the power of the nanosecond laser, multi-level modulation of SP excitation is also possible and can be more precise [45]. To achieve fast back-and-forth modulation using phase transition, one may synthetically apply different methods, such as utilizing electrical control for forward phase transition while nanosecond laser illumination for backward phase transition. Further increasing the modulation speed requires exploration of both new stimuli mechanisms and GST growth technology.

In summary, we have experimentally demonstrated a generic method for achieving dual-function switchable SP devices in the terahertz regime using a GST metasurface composed of two-column well-defined SRs and GST patches. The SRs are arranged in a way to realize spin-dependent directional SP excitation at 0.75 THz, while the GST patches are designed to only cover the SRs in the even Fresnel zones based on the FZP concept. The phase transition property of the GST can turn “on” and “off” the SRs at the amorphous state and crystalline state through conductivity modulation from near 9.30×102 to 1.68×105S/m, making the device function as a spin-dependent directional plane-wave SP coupler and a spin-dependent directional SP FZP, respectively. Such a controlling concept is first verified passively using two samples without GST patches by either remaining or removing the SRs in the even Fresnel zones. Then, its active switching ability is demonstrated with the formal design of the GST metasurface, where the function switching from an SP plane-wave coupler to FZP is achieved by annealing the device from 150°C to 300°C, and the function back switching is reconfigured by illuminating the device with a nanosecond laser. Benefitting from the non-volatile property of GST, all the measurements are carried out at room temperature after each process. The measured results agree well with expectation, which are further confirmed by numerical simulations with measured GST parameters, proving the feasibility of our method very well. Besides the proposed one, our method can in principle be extended to achieve switching between more versatile SP excitation functionalities, which can find broad applications in low power consumption and low cost on-chip communications and sensing.