Nonvolatile reconfigurable terahertz wave modulator

Miniaturized nonvolatile reconfigurable optical components with a subwavelength thickness, extremely compact size, high-speed response, and low power consumption will be the core of next-generation all-optical integrated devices and photonic computing to replace traditional bulky optical devices and integrated circuits, which are reaching physical limitations of Moore’s law. Metasurfaces, as ultrathin planar surfaces, have played a major role in controlling the amplitude, phase, and polarization of electromagnetic waves and can be combined with various active modulation methods to realize a variety of functional devices. However, most existing reconfigurable devices are bounded in volatile nature with constant power to maintain and single functionality, which restricts their further extensive applications. Chalcogenide phase change materials (PCM) have attracted considerable attention due to their unique optical properties in the visible and infrared domains, whereas in the terahertz (THz) regime, research on the reversible phase transition in large-scale areas and applications of Ge2Sb2Te5 (GST) are still under exploration. Here, we achieved reversible, repeated, and large-area switching of GST with the help of optical and thermal stimuli. Large-area amorphization with a 1 cm diameter of GST is realized by using a single laser pulse. Then, we incorporate GST into metasurface designs to realize nonvolatile, reconfigurable, multilevel, and broadband terahertz modulators, including the anomalous deflector, metalens, and focusing optical vortex (FOV) generator. Experimental results verify the feasibility of multilevel modulation of THz waves in a broadband frequency range. Moreover, the modulators are reusable and nonvolatile. The proposed approach presents novel avenues of nonvolatile and reconfigurable metasurface designs and can enable wide potential applications in imaging, sensing, and high-speed communications.

, security inspection [11], and high-speed communication [12,13]. The development of miniaturized and integrated devices that control the wavefront, phase, and polarization of terahertz waves is essential to enable these applications [14][15][16]. However, the existing THz modulation devices have few types and are bulky and cannot meet the increasing demand.
The chalcogenide phase change material (PCM) Ge 2 Sb 2 Te 5 (GST) has attracted increasing attention due to its unique phase-change characteristics [47][48][49][51][52][53][54][55][56][57][58][59]. Structural transformation can be achieved through thermal, electrical, and optical stimuli, enabling nonvolatile random storage memory devices. Compared with other active media, the reversible transition between the amorphous state and the crystalline state of GST is nonvolatile and does not require constant energy to maintain. In addition, GST exhibits a dramatic difference in optical properties between amorphous and crystalline states, and there are many intermediate states, i.e., part crystalline and part amorphous, which can be switched quickly (nanosecond) and repeatedly (up to 10 15 cycles) [57] between these states. Moreover, these states are stable for years at room temperature [59]. Given these excellent properties, GST has been used in the development of integrated all-optical memory devices [53,54], photonic neuromorphic devices [55,56], color displays [57], and plasmonic and dielectric metasurfaces [58].
However, in the terahertz band, there are few studies on the reversible phase transition of GST on large-scale areas and studies on the realization of terahertz reconfigurable, nonvolatile, and functional devices by controlling the phase, amplitude, and wavefront of terahertz waves, which is crucial for enabling a wider range of terahertz applications [47][48][49]. We note that a coding metasurface with reconfiguration capabilities based on GeTe has been proposed in the terahertz band. In this work, the device needed mask alignment to achieve encoding and worked in reflection mode, which is limited to complex processing procedures and working modes [59].
In this work, we combine the phase change material GST and phase-discontinuities metasurfaces to realize nonvolatile, reconfigurable, multilevel, and broadband terahertz functional devices, including anomalous deflectors, metalenses, and focusing optical vortex generators. In the terahertz band, GST exhibits dramatic differences in conductivity between amorphous (2000 S/m) and crystalline (2 × 10 5 S/m) states, which exhibit high and low transmittance, respectively (Supporting information, Fig. S1). Here, by adopting C-shaped slit resonators (CRs) with phase discontinuities, we propose terahertz wavefront-shaped modulation devices. We design and fabricate samples and experimentally examine the characteristics of modulation devices by a fiber-based angleresolved terahertz time-domain spectroscopy (FAT-TDS) system and a near-field scanning terahertz microscopy (NSTM) system. We induce crystallization by heating the entire device on a hot plate at 300 °C for 2 min and reamorphization by a single 7-ns laser pulse with 120 mJ/cm 2 to realize repeated switching. In addition, by tuning the laser influence, intermediate states are realized, which are applied for multilevel modulation. Compared with the existing tunable approaches, our proposed THz modulators exhibit following several distinctive advantages: (i) The devices are nonvolatile and energy-efficient. By applying optical or thermal stimulus, amorphous, crystalline or intermediate states are formed, which are stable in a certain phase state for years at room temperature after removing the external energy. (ii) There is dramatic contrast in conductivity at both states (amorphous and crystalline) of GST, which results in a large difference in THz transmission. In the amorphous state, the THz functional devices are effective, and in the crystalline state, the devices are turned off. By switching between both states, we can realize 'ON' and 'OFF' states. (iii) There are many intermediate states between amorphous and crystalline states, which can be exploited to enable multilevel modulators. (iv) The devices exhibit broadband feature from 0.4 THz to 0.8 THz. The proposed functional metasurfaces combined with GST pave a novel way to realize reconfigurable and nonvolatile terahertz devices and might have essential applications in terahertz integrated devices, imaging, and communications.

Results and discussion
To realize diverse active tuning devices using phase discontinuities metasurfaces, it is necessary to achieve a set of unit cells that enable phase shift covering the 2π range and nearly constant transmission amplitude in a wide frequency range. C-shaped split-ring resonators (CSRRs), which perform strong resonance responses to terahertz radiation, have been used to realize broadband wavefront manipulation [19,20,29,35,60] , and their complementary configuration CRs have similar characteristics due to the Babinet principle [61]. Here, CRs are designed as basic unit cells whose symmetry lines are oriented either + 45° or − 45° with respect to the x-axis, as shown in Fig. 1a. Due to the symmetry, the incident y polarization will be partially converted into x-polarized components, whose phase and amplitude could be modulated by varying the opening angle α, line width w, and radius r. In addition, by mirroring the structure along the y-axis, a phase shift of π will be achieved while maintaining a constant amplitude. Numerical simulations are performed to choose the appropriate structures with computer simulation technology (CST) Microwave Studio. CRs made from aluminum (Al) are fabricated on a 2 mm-thick silicon substrate. Y-polarized plane waves are normally incident on a structure with varying geometrical parameters and scattered x-polarized radiations are detected. Here, an eight-level phase distribution is adopted, and four CRs are selected with a π/4 phase change interval and uniform transmission amplitude. The geometric parameters are as follows: (r, w, α) = (37 μm, 8 μm, 120°), (37 μm, 15 μm, 40°), (38 μm, 9 μm, 30°), and (38 μm, 5 μm, 12°) and the period size of a single unit P = 80 μm. By mirroring the structure, eight units are obtained covering the 2π phase shift from unit cells #1 to #8, as shown in Fig. 1b, which exhibit a broadband resonance response range from 0.4 THz to 0.8 THz, as shown in Fig. 1c and d. This characteristic allows the design of various functional devices that can modulate the wavefront of the cross-polarized components. Broadband, nonvolatile and reconfigurable THz modulation devices could be realized by combining 100 nm-thick GST deposited between silicon and Al. Here, several A cross-polarized anomalous deflector with a linear phase profile is first designed. Figure 2a shows microscopic images of the fabricated sample. A fiber-based angle-resolved terahertz time-domain spectroscopy (FAT-TDS) system [30] is applied to characterize the sample. The THz waves are transmitted and received by a pair of photoconductive antennas. The generated THz radiation is collimated by a lens and normally incident on the metasurfaces from the structure side, and a 45° polarizer and a y polarizer are placed before the sample so that the incident polarization is y polarization, which is perpendicular to the phase gradient. The scattered terahertz wave is focused on the detector by the second lens, and the detector and the second lens are installed on a rail of the rotating stage to collect deflected radiation at different angles. During the measurement, the stage is gradually rotated from − 80° to 80°, and the scanning accuracy is 1°. The measured normalized intensity as a function of frequency and angle is shown in Fig. 2b  when GST is in the amorphous state. It can be seen from the results that the deflector exhibits broadband deflection characteristics from 0.5 THz to 1 THz. Nearly all of the energy is steered towards a certain angle under a fixed frequency. Then, we heated the sample at 300 °C for two minutes to transition GST from an amorphous state to a crystalline state, corresponding to low conductivity/high transmittance and high conductivity/low transmittance, respectively. The scanning results are shown in Fig. 2c. Most of the energy is reflected and absorbed; thus, the deflection effect is completely closed. Figure 2d shows the extracted normalized intensity profile at 0.8 THz corresponding to the two states. The deflection angle is 36° at the amorphous state, which is in good agreement with the result calculated by the generalized Snell's law, θ = sin −1 [λ/(8P)] = 35.87 ∘ , where λ is the wavelength and P is the unit period. In the crystalline state, the deflection intensity is almost zero across the entire scanning angular range. An acceptable crosspolarized deflection switching device is realized. In the "ON" state, nearly all the energy is deflected to the first diffraction order while it is reflected and absorbed in the "OFF" state. Figure 2e shows the deflection angle at the different frequencies by theoretical calculations and experimental measurements, which are in good agreement. In addition to the two states of "ON" and "OFF", we also demonstrate the effect of multilevel modulation by irradiation with ns laser pulses, as shown in Fig. 2f. By tuning the laser influence from 60 mJ/cm 2 to 120 mJ/cm 2 with a step of 10 mJ/cm 2 , different deflection intensities are realized, which exhibit varying brightness in the inset of Fig. 2f. With increasing laser influence, the deflection intensity gradually increases while the frequency range and deflection angle remain constant. This effect comes from the increase in the amorphization degree of GST, which significantly increases the transmission. Thus, broadband, multilevel, nonvolatile, and reconfigurable THz deflectors are achieved.
Next, to demonstrate the versatile abilities of the proposed metasurface, a metalens is designed and experimentally characterized. The basic design strategy of a metasurface lens is shown in Fig. 3a. The phase distribution of the metasurface is designed to enable the radiation scattered by unit cells to constructively interfere at the focal plane, similar to the waves that emerge from the conventional lens. The hyperboloidal phase profile is designed according to where λ is the wavelength and f is the designed focal length. Here, we set f = 10 mm at 0.8 THz. According to the calculated phase distribution, we arrange the suitable CRs according to position, as illustrated in Fig. 3a. Figures 3a and b show microscopic images fabricated by conventional photolithography, and the diameter of the lens is designed to be 5.12 mm with a numerical aperture (NA) of 0.26.
A near-field scanning terahertz microscopy (NSTM) system [32], which can realize three-dimensional scanning of the x-polarized electric field, is applied to characterize the performance of the metasurface lens in the target range from 0.4 to 0.8 THz. Figure 3c demonstrates the measured normalized intensity distribution at the z-x planes in 0.2 mm steps from 1 mm to 14 mm under y-polarized incidence when GST is in the amorphous state. It can be seen from the results that the lens exhibits a broadband focusing effect from 0.4 to 0.8 THz, and with decreasing frequency, the focal length gradually (1) φ x, y = − 2π x 2 + y 2 + f 2 − f decreases from 8.8 mm to 4.2 mm. This reduction law can be inferred from Eq. (1). The difference between the experimentally measured and designed focal lengths is mainly due to the focused instead of parallel terahertz waves incident on the sample. To further characterize the broadband focal performance of the lens, the intensity distributions of the x-y focal plane at the corresponding frequency range are measured in 0.1 mm steps from − 3 mm to 3 mm, as shown in Fig. 3d. The focal spots appear to have good round shapes in a broadband frequency range, and nearly all the energy is concentrated in the focal spots. The obtained horizontal full widths at half maximum (FWHM) at 0.8 THz is 0.43 mm. Figure 3e illustrates the intensity distribution of the horizontal (red line) and vertical (blue line) cross-sections in Fig. 3d. The reasons for the difference in FWHM of different frequencies are mainly due to the variation in wavelength λ, focal length f, and diameter of the incident terahertz beam L, which can be expressed as D ∝ λf/L. To illustrate the remarkable characteristics of the designed device, the repeatability of the reversible transition between amorphous and crystalline states is measured experimentally. Figure 4a shows the normalized peak focusing intensity for 4 writeerase cycles, and the corresponding focal plane intensity distributions are shown in the illustration. Here, we heat the entire sample on a hot plate to induce crystallization, and the focal effect is measured after baking the sample at 300 °C for 2 min. As we can see from the results, the focal spot has completely disappeared, the peak intensity is close to zero, and the device works in the "OFF" state. Then, a single 7-ns laser pulse with 120 mJ/cm 2 is applied to reamorphize the GST, the device is turned on again, which performs as an excellent lens, and another write-erase cycle is carried out, which demonstrates the repeatability of the modulator. The slight increase of intensity is due to the instability of laser fluence and measurement system. In addition to binary modulation, by tuning the laser influence from 60 to 120 mJ/cm 2 at a 10 mJ/ cm 2 step, we experimentally measure the multilevel modulation of the focused intensity by partially or completely reamorphizing the crystalline GST. Figure 4b shows the normalized intensity distribution of the horizontal cross-section at the focal plane. When the laser fluence is smaller than 120 mJ/cm 2 , the GST are at intermediate states composed of amorphous and crystalline states. As the laser fluence increases from 60 to 120 mJ/cm 2 , the proportions of amorphous GST gradually increase, so does the THz transmission and the intensity at the focal spot, as shown in the inset of Fig. 4b.
Finally, a focusing optical vortex (FOV) generator is further proposed. It is necessary to combine two distinct functionalities of focusing and spiral phase plates into one metasurface to generate FOVs, whose phase distribution profiles can be expressed as follows: where (ρ, θ) is the polar coordinate of any point on the metasurface, λ is the design wavelength, f is the focal length and l is the topological charge. Then, we can obtain the spatial distribution of the phase profile, and the corresponding phase distribution is realized by eight-order phase discretized CRs. First, the FOV generator with topological charges  Fig. 5a. The above mentioned NSTM system is used to experimentally characterize the FOV, a y-polarized terahertz beam is incident on the metasurface, and x-polarized radiation is received. The normalized intensity distribution of the z-x cross-section is measured in 0.2 mm steps from 1 mm to 14 mm in the z-direction and 0.1 mm steps in the x-direction from − 3 mm to 3 mm, as shown in Fig. 5b. It can be seen from the results that the intensity distributions exhibited hollow focused characteristics with different focal lengths from 0.4 THz to 0.8 THz, which is a representative behavior of FOVs and is the same as the principle of focusing metalenses, the focal lengths are distinct at different frequencies. To further demonstrate the characteristics of the FOV, the x-y plane intensities and phase distributions are measured at corresponding focal planes in 0.1 mm steps in the x-direction and y-direction from − 3 mm to 3 mm, as shown in Fig. 5c and d. Good doughnut shape intensity distributions with a hollow center can be seen, which is a  Figure 5d shows that there is a spiral phase distribution in the central area (black dotted line) with the phase singularity in the center, and the phase has undergone a 2π shift around the closed-loop, which is the clear feature of the +1st-order vortex beam.
To evaluate the repeatability of the designed FOV, write-erase cyclability is performed. As shown in Figs. 6a and b, normalized intensity distributions and phase distributions at 0.8 THz are characterized. First, the metasurface is heated on a hot plate at 300 °C for 2 min to fully transition GST from an amorphous to a crystalline state, which erases all the information and turns off the FOV. The intensity distribution appears as a uniform zero value with an irregular phase distribution. By applying a 7-ns single laser pulse with 120 mJ/cm 2 , the GST is reamorphized, and the FOV is turned on again with a constant focal length, intensity, and phase distributions. Five switching cycles are carried out, which demonstrate that the FOV can be written and erased repeatedly and that more cycles can be performed. In addition to the 1st-order FOV device, higher-order FOV devices could be realized. In the supplementary information, we show the designed and experimentally measured -2nd-order FOV device.

Conclusion
We demonstrate nonvolatile, reconfigurable, multilevel and broadband terahertz modulators that exploit large conductivity changes induced by phase changes in GSTs and phase-discontinuities metasurface. By varying the geometric parameters of CRs, eightlevel phase distributions are realized. Then, the anomalous deflector, metalens and focusing optical vortex generator are designed, fabricated and experimentally measured. By gradually tuning the fluence of laser pulses, multilevel intensity modulation is realized. The states of GST can be repeatedly switched between amorphous, crystalline states, and intermediate states on a large scale with the help of nanosecond laser pulses and thermal annealing. Specifically, we believe that this is the novel attempt to combine phase change materials with wavefront modulation metasurfaces to achieve terahertz modulators that are nonvolatile without constant energy to maintain, reconfigurable, multilevel, including many intermediate states, and broadband from 0.4 to 0.8 THz. We expect that the proposed novel THz modulators will be helpful for THz Fig. 6 Measured repeatability of FOV switching. a and b Experimentally measured normalized intensity distribution and phase distribution of 5 write-erase cycles imaging, sensing, and high-speed communication. Furthermore, the proposed metasurfaces are easy to fabricate and compatible with semiconductor technologies, making them very promising in developing next-generation ultracompact and multifunctional THz devices.

Sample fabrication
Two-millimeter-thick high-resistivity silicon (Si) wafers were taken as the substrates and cleaned ultrasonically in acetone, isopropanol, and deionized water in sequence before deposition. Then, the 100-nm-thick GST film was direct current magnetron sputterdeposited on the Si substrate. Al (200-nm-thick) was deposited on the GST film, and the CRs were fabricated by conventional lithography procedures.

Experimental characterization
A fiber-based angle-resolved terahertz time-domain spectroscopy (FAT-TDS) system (Supporting information, Fig. S3 a) is used to characterize the dynamic beam splitter. The generated THz waves are first collimated by a lens L1, which is then normally illuminated on the sample. Then, the scattered terahertz waves are collected on the receiver by a second lens L2. The sample is fixed at the center of a rotation stage. To realize crosspolarization characterization, three metallic grid-based terahertz linear polarizers P1, P2 and P3 are applied. Two of them (P1 and P2) are placed before the sample, which are used to allow 45°-polarized and y-polarized transmission in sequence. The third (P3) is after the sample to allow x-polarized transmission. The receiver, the second lens L2, and the third polarizer P3 are fixed on a rail mounted on the motorized rotator to collect the output THz radiation.
A near-field scanning terahertz microscopy (NSTM) system (Supporting information, Fig. S3 b) is used to characterize the metalens and FOV generator. Femtosecond laser pulses with 1550 nm central wavelength and 50 fs pulse width are split into two beams to generate and detect the terahertz waves. A homemade photoconductive antenna is used to generate THz waves, and a commercially available THz near-field probe is used for detection. The probe is mounted on a 2D translation stage in the x-y plane, and the sample is on a 1D translation stage in the z-axis direction. 3D electric field scanning can be realized. The generated THz waves are collimated by a lens L1 and then detected by the probe. The metasurface is placed between the lens and the probe. Similar to the FAT-TDS system, two polarizers P1 and P2 are used to allow only the y-polarized THz waves to illuminate the sample. The 3D electric field of devices is detected at 0.1 mm step in the x-direction and y-direction from − 3 mm to + 3 mm and 0.2 mm step in the z-direction from 1 mm to 14 mm.