Metasurfaces, as 2D metamaterials consisting of an array of function-driven artificial meta-atoms, have attracted tremendous attention due to their unprecedented abilities in manipulating the phase, polarization, and amplitude of electromagnetic (EM) waves [1]. In the terahertz (THz) frequency band, metasurfaces have resulted in numerous applications, including absorbers [2], metalenses [3], beam steering [4], holography [5,6], vector beams [7], topological waveguides [8], bound states in the continuum [9,10], etc. Despite metasurfaces having exhibited clear advantages over classical optical components, their performances are fixed upon fabrication, which limits their further applications. So actively tuning the properties of EM waves is highly desirable for integrated photonic systems [11,12]. Currently, various active modulation methods have been proposed, such as liquid crystals (LCs) [13–15], semiconductors [16–18], microelectromechanical system (MEMS) [19–21], graphene [22–24], and phase-change material (PCM) [25–29]. These mechanisms of active modulation have led to a diverse range of tunable THz metasurfaces [13,16,17,20,22–24,26–29]. However, a constant stimulus is needed to preserve the working states of devices, and there has yet to be developed an energy-efficient solution for maintaining the desired function with zero static power consumption in the THz regime. Moreover, most of these approaches are limited to a single functionality, which hinders their further applications.

${\mathrm{Ge}}_2{\mathrm{Se}}_2{\mathrm{Te}}_5$ (GST), as one of the most popular chalcogenide PCMs, has been widely used in the field of data storage due to its unique phase-change characteristics [30,31]. It exhibits dramatic differences in optical properties between its amorphous and crystalline states and can be repeatedly switched between these states through thermal, electrical, and optical stimuli. Crystallization of GST can be induced by applying long and relatively moderate-intensity pulses (optical or electrical) to locally heat amorphous GST (A-GST) to its crystallization temperature or by heating the whole sample up (i.e., thermal) and holding for a short time. By contrast, amorphization can be achieved by applying high-intensity pulses with a short duration to heat crystalline GST (C-GST) above the melting point and then rapidly quenching it to prevent any recrystallization [32]. Once the crystallization or reamorphization has been realized, the state of GST is stable for years with zero hold power at room temperature (compared with another PCM, ${\mathrm{VO}}_2$, which is volatile) [31,33]. In view of these excellent properties, GST has been used in the development of integrated all-optical memory devices [34–36], photonic neuromorphic devices [37–39], color displays [40], thermal emitters [41], and plasmonic and dielectric metasurfaces [42,43] in the visible and infrared domains. In addition, Janus metasurfaces, inspired by the two-faced god Janus in Roman mythology, have shown tremendous potential in controlling EM waves by integrating different functionalities in a single structural layer [44–47]. However, in the THz regime, researches on the reversible phase transition in large-scale areas and applications of GST are still under exploration [27–29], and nonvolatile dynamic Janus metasurfaces that can realize switchable functions in both states of GST have not been reported.

In this work, we demonstrate a novel class of nonvolatile, reconfigurable, and dynamic Janus metasurfaces by incorporating PCM GST in the THz regime, which can offer a wealth of switchable functionalities. The metasurfaces comprise three sets of functional structures arranged using multiplex methods: two active sets composed of GST integrated C-shaped slit resonators (G-CRs) and a passive set composed of CRs. In the amorphous state of GST, the function of the device depends on the contribution of three sets of structures; however, the function is governed by the passive set in the crystalline state. Here, we experimentally show the reversible switching characteristic of GST on large areas by applying a single ns-laser pulse with a diameter of 1 cm to achieve amorphization of GST and heating the entire sample on a hot plate to induce crystallization. Based on this scheme, several important bifunctional metasurfaces are designed, fabricated, and experimentally characterized, including dynamic beam splitter, bifocal metalens, dual-mode focusing optical vortex (FOV) generators, and switchable metalens/FOV generators. Experimental results show that the proposed metasurfaces can realize two independent functionalities in both states and can be switched reversibly and repeatedly. Moreover, the function in a certain phase is stable with zero hold power at room temperature. The proposed nonvolatile, reconfigurable, and dynamic Janus metasurfaces combined with GST provide a novel way to realize multifunctional and compact THz devices and may have essential applications in THz integrated devices, imaging, and communications.

A schematic diagram of the proposed nonvolatile, reconfigurable Janus metasurface is shown in Fig. 1(a), which demonstrates the reversible switching between the $m$th-order FOV and metalens. To realize the modulation of phase and amplitude of transmitted THz waves, we select the Babinet-inverted, complementary C-shaped split-ring resonators–CRs as basic meta-atoms due to their strong response to THz radiation [25]. The geometrical parameters of CRs are schematically illustrated in Fig. 1(b). $P_x=P_y=80\mathrm{\mu m}$ is the period, $r$ is the outer ring radius, $w$ is the line width, $\alpha$ is the opening angle, and $\beta$ is the orientation angle. By changing the geometrical parameters of CRs, the amplitude and phase of cross-polarization can be simultaneously controlled. To enable an abrupt phase shift covering a $2\pi$ range and a nearly constant transmission amplitude, numerical simulations are carried out to select resonators by tuning $w$, $r$, and $\alpha$ of CRs using commercial software computer simulation technology (CST) Microwave Studio. The CRs are made of a 200-nm-thick aluminum (Al) film on a 2-mm-thick silicon (Si) substrate, and the $\beta$ are fixed at $\pm 45°$. A normally incident plane wave with $y$ polarization is used to excite the structure, and scattered cross-polarization is detected. Eight CRs with nearly the same transmission amplitude and a $\pi /4$ phase-change interval are selected at 0.8 THz, as shown in Fig. 1(c). To realize the proposed nonvolatile, reconfigurable Janus metasurface, we incorporate GST, which exhibits a drastic contrast in THz transmission between amorphous and crystalline states and good repeatability (see Appendix A for more details).

The working principle is presented in Fig. 1(a). The metasurface consists of three sets of functional structures arranged using multiplex methods, as shown in the black rectangular box: two active sets (${\mathrm{U}}_1$ and ${\mathrm{U}}_2$) composed of G-CRs, which can be reversibly switched between amorphous and crystalline states by optical and thermal stimuli, and a passive set (${\mathrm{U}}_3$) composed of CRs, which exhibits a fixed response. The inset shows enlarged views of ${\mathrm{U}}_1$, ${\mathrm{U}}_2$, and ${\mathrm{U}}_3$. In active structures ${\mathrm{U}}_1$ and ${\mathrm{U}}_2$, there is an additional layer of GST between the Al CRs and the Si substrate. In the amorphous state of GST, we enable the far-field diffraction patterns of ${\mathrm{U}}_2$ and ${\mathrm{U}}_3$ to exhibit nearly constant amplitude and phase delay of $\pi$ in the detection plane. The effective structures are ${\mathrm{U}}_1$ due to destructive interference. However, in the crystalline state of GST, the transmission amplitudes of G-CRs nearly approach zero (see Appendix B for more details); thus, the effective structures are ${\mathrm{U}}_3$. In other words, in the amorphous state of GST, the metasurface functionality is determined by the effective set ${\mathrm{U}}_1$; after crystallization, the amorphous G-CRs transition into crystalline G-CRs, and the function of the multiplex structures is governed by the passive set ${\mathrm{U}}_3$. By reamorphization, the function can be switched back. Here, we apply 1064-nm wavelength nanosecond laser pulses to reamorphize GST and heat the entire sample on a hot plate to induce crystallization. The following explains why the heating procedure is chosen over the short laser pulse irradiation process. When A-GST is heated to about 300°C, it transforms into a hexagonal-centered cubic phase (HCP), and when the sample is irradiated with short laser pulses, it transforms into a face-centered cubic phase (FCC) [48,49]; the THz wave transmission change through the sample during the amorphous-to-HCP process is more dramatic than during the amorphous-to-FCC process [28]. It should be noted that our device is nonvolatile without a constant stimulus to maintain (compared with the alternative PCM ${\mathrm{VO}}_2$, which is volatile). Therefore, by designing and arranging three sets of structures properly, any two functionalities can be realized in both states of GST and can be reversibly switched, without changing any optical elements in the optical path during the entire process.

First, a dynamic beam splitter is demonstrated using our Janus metasurface, as shown in Fig. 2(a). The anomalous refraction of cross-polarization THz waves propagates along two opposite directions in crystalline and amorphous states of GST at normal incidence of $y$-polarization THz waves. To experimentally prove the reliability of the designed dynamic beam splitter, we fabricated a metasurface on a 2-mm-thick Si substrate by conventional multistep lithography. The microscope optical image of part of the fabricated sample and enlarged views of basic units comprising three sets of functional structures are shown in Fig. 2(b). The metasurfaces consist of two active sets (${\mathrm{U}}_1$ and ${\mathrm{U}}_2$) composed of G-CRs and a passive set (${\mathrm{U}}_3$), which are designed and arranged to obtain the desired phase profiles. The phase distribution of structure set ${\mathrm{U}}_1$ is designed to steer the transmitted waves to the right, whereas ${\mathrm{U}}_3$ is designed to steer beams to the left; ${\mathrm{U}}_2$ is arranged to exhibit a phase delay of $\pi$ compared to ${\mathrm{U}}_3$, which results in destructive interference. Fiber-based angle-resolved THz time-domain spectroscopy (FAT-TDS) [4,25] is used to characterize the device; $y$-polarized THz waves are normally incident on the sample from the substrate side, and cross-polarized radiations are detected at different angles from $-80°$ to 80° with a 1° step. Figure 2(c) shows the measured normalized far-field intensity distributions as a function of the deflection angle and frequency. Nearly all output waves are steered to the left around the broadband frequency range from 0.5 to 0.9 THz in the crystalline state of GST, whereas, after applying a single $120\mathrm{mJ}/{\mathrm{cm}}^2$ laser pulse with 1064 nm wavelength, 7 ns pulse width, and 1 cm spot diameter, the deflected beam is steered to the right and also exhibits a broadband feature. Figure 2(d) shows the corresponding normalized intensity profiles at 0.8 THz extracted from Fig. 2(c). The deflection angles are $-36°$ and 37°, respectively, in both states, which agree well with the theoretically calculated values by the generalized Snell’s law $\theta =\pm \arcsin [\lambda /(8P)]=\pm 35.87°$, where $\lambda$ is the wavelength, and $P$ is the period. To further demonstrate the repeatability of the designed Janus device, another switching cycle is performed, and the normalized intensity distributions and corresponding profiles at 0.8 THz are shown in Figs. 2(e) and 2(f), respectively. Here, recrystallization of GST is realized by thermally annealing the sample at 300°C for 2 min on a hot plate. In both states, the deflection angles are $-35°$ and 36°, respectively. The slight deviation in results between the two cycles is primarily due to system measurement. Figure 2(g) shows the calculated and measured dependence of the deflection angle on frequency. The results reveal that the measured dispersive components deflected at different angles obey the generalized Snell’s law, which also validates the measured results. Except for a slight deviation at lower frequencies, due primarily to the large deflection angle and low signal strength, the experimental results are in good agreement with the theoretical calculations. The beam splitting ratios are calculated, which show a broadband feature in both states of GST and are switched from 6.91 to $-6.3\mathrm{dB}$ at 0.7 THz for the crystalline and amorphous states, respectively (see Appendix D for more details).

Second, a dynamic bifocal metalens is proposed that exhibits switchable focal lengths in amorphous and crystalline states of GST, as depicted in Fig. 3(a). Structure sets ${\mathrm{U}}_1$ and ${\mathrm{U}}_3$ are designed to obtain the acquired phase profiles for realizing two distinct focal lengths with $f_1=7\mathrm{mm}$ and $f_2=10\mathrm{mm}$, respectively, at 0.8 THz. ${\mathrm{U}}_2$ is arranged to exhibit a phase shift of $\pi$ compared to ${\mathrm{U}}_3$ in the focal plane, which results in destructive interference. The phase profiles are obtained as follows: $${\varphi }_{1-1}(x,y)=-\frac{2\pi }{\lambda }(\sqrt{x^2+y^2+f_1^2}-f_1),$$(1a)$${\varphi }_{1-2}(x,y)=-\frac{2\pi }{\lambda }(\sqrt{x^2+y^2+f_2^2}-f_2)+\pi ,$$(1b)$${\varphi }_{1-3}(x,y)=-\frac{2\pi }{\lambda }(\sqrt{x^2+y^2+f_2^2}-f_2),$$(1c)where $\lambda$ is the wavelength, and $f_1$ and $f_2$ are the designed focal lengths. For ${\varphi }_{n-m}$, index $n$ is the metasurface functionality number, and $m$ is the sub-surface particle design criteria in Eqs. (1)–(3). To experimentally verify the performance of the device, we fabricate the sample by conventional multistep lithography. The optical microscope image of part of the fabricated sample and enlarged views of three structure sets are shown in Fig. 3(b). Near-field scanning THz microscopy (NSTM) [7] is applied to characterize the device. $Y$-polarization THz waves normally illuminate on the sample from the side of the substrate and $x$-polarization THz electric field is detected. Figures 3(c) and 3(d) show the normalized $x-z$ and $x-y$ plane electric field distributions, respectively, in the amorphous state of GST. By thermal annealing at 300°C for 2 min on a hot plate, GST is switched from the amorphous to crystalline state, and by applying a single $120{\mathrm{mJ}/\mathrm{cm}}^2$ laser pulse with 1064 nm wavelength, 7 ns pulse width, and 1 cm spot diameter, reamorphization of GST is realized. The measured results are shown in Figs. 3(e)–3(h), respectively. As we can see, in both states of GST, the designed metalenses exhibit different focal lengths, and nearly all of the energy is converged to the focal point. To further characterize the performance of bifocal metalenses, we extract the THz intensity distributions along the propagation direction and the $y$ direction, as illustrated in Figs. 3(i) and 3(j). It can be seen that the measured focal lengths are 6.6 mm, 8.8 mm, and 6.6 mm in amorphous, crystalline, and reamorphization states, respectively, which are smaller than the designed results. This experimental error in focal length is caused by the incident THz beam being slightly focused instead of a perfectly collimated plane wave. The measured full width at half maximum (FWHM) values are around 0.29 μm, 0.37 μm, and 0.27 μm, respectively, which indicates the subwavelength focusing performance. The proposed bifocal metalens exhibits excellent nonvolatile dynamic switchable characteristics in amorphous and crystalline states of GST with the help of optical and thermal stimuli. The calculated amplitude ratios are 14.15 dB and $-6\mathrm{dB}$ at $z=6.6\mathrm{mm}$ and 8.8 mm, respectively (see Appendix D for more details).

Next, an FOV generator with dual-mode functionality is proposed, which exhibits switchable FOV modes in amorphous and crystalline states of GST, as illustrated in Fig. 4(a). As mentioned above, the metasurface is composed of three sets of structures, in which ${\mathrm{U}}_1$ and ${\mathrm{U}}_3$ are designed to acquire the desired phase distribution to achieve different values of topological charges $l=-1$ and $l=+1$ with a focal length of $f=10\mathrm{mm}$, and ${\mathrm{U}}_2$ is engineered to have a phase delay of $\pi$ compared to ${\mathrm{U}}_3$ on the detection plane, which results in destructive interference. The phase profiles are obtained as follows: $${\varphi }_{2-1}(x,y)=-\frac{2\pi }{\lambda }(\sqrt{x^2+y^2+f^2}-f)+l_1·\arctan \left(\frac{y}{x}\right),$$(2a)$${\varphi }_{2-2}(x,y)=-\frac{2\pi }{\lambda }(\sqrt{x^2+y^2+f^2}-f)+l_2·\arctan \left(\frac{y}{x}\right)+\pi ,$$(2b)$${\varphi }_{2-3}(x,y)=-\frac{2\pi }{\lambda }(\sqrt{x^2+y^2+f^2}-f)+l_2·\arctan \left(\frac{y}{x}\right),$$(2c)where $\lambda$ is the wavelength, $f$ is the focal length, and $l_i(i=1,2)$ is the topological charge of FOV mode. The optical microscope image of part of the fabricated sample and enlarged views of the three sets of basic structures ${\mathrm{U}}_1$, ${\mathrm{U}}_2$, and ${\mathrm{U}}_3$ are shown in Fig. 4(b). The FOV generator is also experimentally characterized by the use of the above-mentioned NSTM system. The normalized intensity distributions of $x-z$ (from 1 to 14 mm with 0.2 mm step in the $z$ direction and from $-3$ to 3 mm with 0.1 mm step in the $x$ direction) and $x-y$ (from $-3$ to 3 mm with 0.1 mm step at a height of 8.8 mm above the sample) planes in amorphous, crystalline, and reamorphization states are shown in Figs. 4(c)–4(h), respectively. Here, the transition from the amorphous to crystalline state of GST is realized by thermal annealing at 300°C for 2 min on a hot plate, and the reamorphization process is induced by a single ns-laser pulse with $120{\mathrm{mJ}/\mathrm{cm}}^2$. The intensity distributions exhibit hollow and convergent characteristics along the propagation direction and appear doughnut shaped in the $x-y$ focal plane. Due to the focusing characteristic, almost all the energy is concentrated on the focal point. The reason why the experimentally measured focal length is smaller than the designed one is the same as mentioned above. The corresponding phase profiles are illustrated in the black dotted lines of Figs. 4(i)–4(k), which exhibit helical twists patterns with a $2\pi$ phase shift around a closed loop and show the opposite values of topological charge in amorphous and crystalline states. Outside the focal point, since there is nearly no energy, the phase appears to be random. Apart from the switchable FOV mode from $l=-1$ to $l=+1$, any two modes with various topological charges can be reversibly switched in both states of GST (see Appendix C for more details). To demonstrate the switching performance of the FOV generator, the purity of the generated vortex beams is calculated, as shown in Fig. 10 (see Appendix D for more details).

The above devices are all switching between the same functions. To verify the versatility of the designed device, as a proof of concept, beam switching between different functionalities depending on the state of GST is proposed, as illustrated in Fig. 5(a). The metasurface acts as an FOV generator in the amorphous state and as a metalens in the crystalline state. Structures ${\mathrm{U}}_1$ and ${\mathrm{U}}_3$ are designed to construct an FOV generator with topological charge $l=2$ and a metalens with a focal length of $f=10\mathrm{mm}$, respectively. ${\mathrm{U}}_2$ is arranged to exhibit a phase shift of $\pi$ relative to ${\mathrm{U}}_3$ on the focal plane. The phase profiles are calculated as follows: $${\varphi }_{3-1}(x,y)=-\frac{2\pi }{\lambda }(\sqrt{x^2+y^2+f^2}-f)+l_3·\arctan \left(\frac{y}{x}\right),$$(3a)$${\varphi }_{3-2}(x,y)=-\frac{2\pi }{\lambda }(\sqrt{x^2+y^2+f^2}-f)+\pi ,$$(3b)$${\varphi }_{3-3}(x,y)=-\frac{2\pi }{\lambda }(\sqrt{x^2+y^2+f^2}-f),$$(3c)where $\lambda$ is the wavelength, $f$ is the focal length, and $l_3$ is the topological charge of FOV mode. To experimentally characterize the performance of the device, we fabricated the sample by conventional multistep lithography. The optical microscope image of part of the fabricated metasurface and enlarged views of basic structures are shown in Fig. 5(b). The normalized intensity distribution is detected in 0.1 mm steps in the $x$ and $y$ directions from $-3$ to $+3\mathrm{mm}$ on the focal plane at a height of 8.8 mm above the sample using the NSTM system, as shown in Figs. 5(c)–5(e), and the corresponding phase profiles are demonstrated in Figs. 5(f)–5(h). An FOV with $l=2$ is generated in the amorphous state of GST, which features a clear doughnut-shaped energy ring and a helical phase distribution with a $4\pi$ phase shift. By thermal annealing at 300°C for 2 min, a metalens with $f=8.8\mathrm{mm}$ is produced in the crystalline state of GST, which exhibits a uniform phase profile in the center. Subsequently, by applying a single ns-laser pulse with $120{\mathrm{mJ}/\mathrm{cm}}^2$, the device is switched back to the FOV generator. The designed device exhibits satisfactory reconfigurability with the help of thermal and optical stimuli. In our measured phase distributions of the FOV generator, two phase singularities are observed, which can also be found in some previous reports [7,50,51]. The reason is that the polarization conversion of the designed device is incomplete. In our experiments, $y$-polarization THz waves are incident on the sample, and $x$-polarization radiation is detected. Thus, there are still interference effects in the measured results due to the existence of a co-propagating Gaussian beam. Our proposed methods can also be efficiently extended to realize reconfigurable dynamic switching between other desired functions.

High-resistivity Si wafers with a thickness of 2 mm were used as substrates. Before deposition, the substrates were cleaned ultrasonically in acetone, isopropanol, and deionized water in sequence. A 100-nm-thick GST film was first sputter-deposited on the Si substrate. Conventional photolithography was used to attain desired GST patterns. The second photolithography was carried out and a 200-nm-thick Al was subsequently deposited on top of the GST film. Finally, the lift-off procedure was adopted to attain desired CR patterns.

The dynamic beam splitter is characterized using FAT-TDS. The THz waves are transmitted and detected by a pair of commercially available photoconductive antennas. The generated THz radiation is collimated and normally illuminated on the sample, which is fixed at the center of a rotation stage. Additionally, three metallic-grid-based THz linear polarizers are employed to attain the desired polarization. Two are placed before the sample and used to allow 45°-polarized and $y$-polarized transmission in sequence, and the third is after the sample to allow $x$-polarized transmission. The receiver and third polarizer are installed on the rotation stage to collect the output THz radiation. The diffraction wave is detected every 1° by rotating the rotation stage from $-80°$ to $+80°$.

The bifocal metalens, dual-mode FOV generators, and switchable metalens/FOV generators are characterized by an NSTM system. The THz waves are generated by a homemade photoconductive antenna and detected by a commercially available THz near-field probe. To enable a 3D scan of the THz electric field, the probe is mounted on a 2D translation stage, and the sample is on a 3D translation stage. Same as the FAT-TDS system, three polarizers are used to allow only $y$-polarized THz waves to illuminate on the sample and only $x$-polarized THz waves to be detected. The 3D electric fields of devices are detected in 0.1 mm steps in $x$ and $y$ directions from $-3$ to $+3\mathrm{mm}$ and 0.2 mm steps in the $z$ direction from 1 to 14 mm.

In conclusion, we have proposed dynamic Janus metasurfaces based on a multiplex scheme comprising two sets of active structures and a passive set. With the help of optical and thermal stimuli, G-CRs can be reversibly switched between amorphous and crystalline states, which contributes to switching between different functions. As a proof of concept, dynamic beam splitter, bifocal metalens, dual-mode FOV generator, and switchable beam between an FOV beam and a converging beam are successfully designed, fabricated, and experimentally characterized. Our devices exhibit great reconfigurability, which features repeatable switching between two different functions, and nonvolatility without constant stimuli to maintain compared with other PCMs (e.g., ${\mathrm{VO}}_2$). Our work provides a novel scheme for realizing a nonvolatile, reconfigurable, dynamic Janus metasurface in the THz regime and can also be extended to other wavelength regions. Furthermore, the proposed metasurfaces are easy to fabricate and compatible with semiconductor technologies, making them very promising in developing next-generation ultra-compact and multifunctional THz devices.

Acknowledgment. Z. Tian conceived the idea. S. Zhang conducted the design. H. Li and K. Liu deposited the GST films. Y. Xu and Q. Wang helped with the sample preparation. S. Zhang, X. Chen, Y. Xu, and X. Jiang performed the measurements. S. Zhang wrote the manuscript. Z. Tian and T. Cao revised the manuscript and supervised the project. All authors discussed the results and commented on the paper.