1. Introduction

Over the past decades, remarkable advances have been made in terahertz [1–3] functional devices such as modulators [4, 5], absorbers [6, 7], splitters [8, 9] and polarizers [10–12]. In particular, terahertz polarization devices play a significant role in almost all branches of terahertz application systems. For example, in practical terahertz sensing and imaging systems, it is necessary to control the efficiency and bandwidth of polarization devices to improve the sensitivity and resolution of these systems [13, 14]. Traditional polarization conversion devices usually require large crystals to accumulate enough phase [15], which are not applicable in practical systems, so small size polarization conversion devices should be designed to develop the next generation of compact and lightweight terahertz systems. Artificially constructed subwavelength metasurfaces [16–18] are attractive due to their ability to manipulate electromagnetic waves arbitrarily. In previous single-layer metasurfaces [19, 20], it has shown that they can produce polarization conversion in the terahertz band, but their bandwidths and polarization conversion ratios are limited. Currently, multilayer metasurfaces remain important options for achieving broadband and efficient terahertz polarization conversion [21, 22]. Recently, it has been discovered that wideband terahertz polarization conversion and asymmetric transmission based on coupled dielectric-metal gratings can be realized experimentally [23]. Highly efficient and broadband linear polarization to circular polarization conversion can also be accomplished at terahertz frequencies by the judicious design of birefringent metasurfaces [24]. Although the above mentioned multilayer metasurfaces have reported broadband polarization conversions, their bandwidth ranges are still limited. Moreover, they all rely on static geometry to produce the desired optical responses, which is also detrimental to practical applications.

Recently, switchable and multi-functional metasurface devices have gradually flourished and can modulate the electromagnetic responses via thermal, electrical, optical stimuli. Different means of modulating metasurfaces have been reported, such as semiconductor materials (silicon and conductive oxides) [25–27], 2D materials (graphene and perovskite) [28, 29], phase change materials [vanadium dioxide (VO₂) and chalcogenide GeSbTe (GST)] [30, 31] and nonlinear materials [32, 33]. All these methods have shown excellent ability to modulate the electromagnetic response. Among the known transition metal oxides so far, VO₂ has been widely used because of its low insulator-to-metal phase transition temperature. For example, a recent demonstration of a VO₂ based frequency tunable metasurface filter realized terahertz phase and polarization modulation [34]. In addition, a switchable VO₂ integrated coding metasurface for wavefront and polarization manipulation of terahertz beams has been demonstrated numerically [35]. Most recently, temperature-controlled chirality, optical activity, and negative refractive index were observed in a terahertz metamaterial based on 3D-chiral metallic resonators and achiral VO₂ inclusions [36]. Yet, most of these reported studies on VO₂-based metamaterials/metasurfaces are for the control of the resonant frequency, field amplitude or the phase (polarization). All of these designs give no active control of the polarization conversion bandwidth.

Here we demonstrate a dynamically controllable broadband linear polarization converter at THz frequencies. The device is a metasurface consisting of a five-layer structure, including a sapphire substrate, a layer of VO₂ and gold split-ring resonators (SRRs), a polyimide dielectric layer, and a grating structure. By designing the structural parameters and utilizing the insulator-to-metal phase transition of VO₂, the dynamic tunability of the polarization conversion function from dual-broadband (0.45∼0.77 THz and 0.97∼1.2 THz) to ultra-broadband (0.38∼1.20 THz) can be realized with a high polarization conversion ratio (PCR) via thermal stimuli. Simulation and experimental results show that there are two spike-like high-efficiency polarization conversion bandwidths on both sides of 0.85/0.92 THz, and the PCR can reach up to 95% at low conductivity of VO₂ and room temperature. With the increase of the VO₂ temperature and conductivity, the gold SRRs gradually change from asymmetric to symmetric, so that the amplitude of the valley and the PCR in the 0.85/0.92 THz band gradually increase until reaching the level of the two adjacent peaks. This process enables dynamic modulation of the bandwidth of the polarization conversion, changing from dual-band at low temperature to ultra-broadband at high temperature. Furthermore, for the proposed metasurface, the polarization conversion amplitude at the resonance frequency between the two transmission windows is greatly sensitive to the temperature, and the resonance frequency can be flexibly adjusted as well. These properties enable the designed metasurface promising in the fields of terahertz sensing, secret communication and imaging.

2. Structure and design

Figure 1(a) shows the unit cell, where the period is P = 80 µm. To fabricate the structure, we first grow a 150-nm-thick VO₂ film on a sapphire substrate with a thickness of h = 2000 µm and fabricate a layer of VO₂ SRRs with two obliquely symmetrical openings by lithography. Second, we engrave a layer of three-opening gold SRRs with a 200-nm-thickness on the processed VO₂ structure, in which the opening O along the -45° direction is key to producing cross-polarization conversion, and O₁ is key to generating the resonance. The specific parameters of the gold SRRs are R = 35 μm and R₀ = 30 μm, and the split line of the two symmetrical O = 10° openings forms an angle of 45° with respect to the x-axis, and the split line of O₁ forms C₁ = 30° with respect to the x-axis, as shown in Fig. 1(c). The two symmetrical openings of the gold SRRs are the same in size and direction as those of the VO₂ SRRs. In a second lithography step, a gold grating is formed on top an isotropic polyimide layer with a thickness of t = 35 μm that is coated on a 5-inch SiO₂ substrate. Then the gold grating and polyimide layer separated from the SiO₂ substrate is assembled with the prefabricated sample consisting of sapphire substrate, VO₂ and gold SRRs under a microscope. The grating structural parameters are: d₁ = 10 µm, d₂ = 22 µm, and d = 200 nm. A schematic diagram of the vertical view of the metasurface unit cell is given in Fig. 1(b). Figures 1(d)–1(f) show the optical microscope images of the fabricated gold SRR array, gold grating and their combined structure, respectively. We use a transmission-type THz time-domain spectroscopy system (THz-TDS) [see Fig. 1(g)] to characterize the spectral response of the metasurface. To characterize the fabricated sample, four broadband terahertz wire grid polarizers are required. A wire grid polarizer is placed between the first two off-axis parabolic mirrors to obtain y-polarized wave as the incidence. The metasurface sample is placed at the focus of the second and third off-axis parabolic mirrors.

 

Fig. 1. Schematic diagram of the metasurface structure and experimental system. (a) Unit cell, composed of sapphire substrate, VO₂ and gold SRRs layers, polyimide dielectric layer and a gold grating, with geometrical parameters h = 2000 µm, P = 80 µm, d = 200 nm, t = 35 µm, d₁ = 10 µm, and d₂ = 22 µm. (b) Vertical view of the unit cell. (c) Gold SRR with geometrical parameters O = 10°, O₁ = 10°, C₁ = 30°, and C₂ = 45°. (d)-(f) Optical microscope images of the fabricated VO₂ and gold SRRs, gold grating and their combined structure, respectively. (g) Sample characterization by THz-TDS.

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We use the electromagnetic simulation software CST Microwave Studio to design and optimize the spectral performance of the polarization conversion metasurface. In the simulation, we set the electric field along the y direction (y-linear polarization) as the excitation source, and use periodic boundary conditions in the x and y directions, and open boundary conditions in the + z direction. VO₂ is modeled by the Drude model, where the relative permittivity ε_VO2 = 9, the temperatures are 296 K and 363 K corresponding to the fully insulated and metallic states respectively, and the two correspond respectively to the frequency-independent conductivity σ_low = 10 S/m and σ_high = 2.6 × 10⁵ S/m. The dielectric layer is composed of lossy polyimide with a relative permittivity of ε_polymide = 2.93 + 0.044 i. The relative permittivity of sapphire is ε_sapphire= 9.67. The amplitude transmittance is defined as $|{T_{yy}}(\omega )|= |E_{yy}^{tr}(\omega )|/|E_{yy}^{re}(\omega )|$ and $|{T_{xy}}(\omega )|= |E_{xy}^{tr}(\omega )|/|E_{yy}^{re}(\omega )| , $where $E_{yy}^{tr}(\omega )$ and $E_{xy}^{tr}(\omega )$ represent the electric fields transmitted through the metasurface (superscript tr), and $E_{yy}^{re}(\omega )$ and$E_{xy}^{re}(\omega )$ are the electric fields through the reference sample (superscript re). In addition, the phase difference ${\delta _{\rm diff}} = \phi xy - \phi yy = \arg ({T_{xy}}(\omega )) - \arg ({T_{yy}}(\omega ))$ and the PCR ${\rm PCR}_{xy}(\omega ) = |{T_{xy}}(\omega ){|^2}/(|{T_{xy}}(\omega ){|^2} + |{T_{yy}}(\omega ){|^2})$ and ${\rm PCR}_{yy}(\omega ) = |{T_{yy}}(\omega ){|^2}/(|{T_{xy}}(\omega ){|^2} + |{T_{yy}}(\omega ){|^2})$ between the two orthogonal polarizations are extracted from the data.

3. Results and discussion

3.1 Results

We have characterized the transmission characteristics of the metasurface, simulated VO₂ as a function of its conductivity [Fig. 2(a)], and carried out experimental measurements as a function of temperature [Fig. 2(b)]. In the simulation shown in Fig. 2(a), when VO₂ has a low conductivity (10 S/m), the cross-polarization |T_xy| forms a valley in the vicinity of 0.85 THz frequency due to the existence of the O₁ opening in the gold SRRs, and the formed two windows have 3 dB bandwidth of 0.32 THz (0.45∼0.77 THz) and 0.23 THz (0.97∼1.2 THz) respectively, showing a peak-valley-peak state as a whole. The PCR_xy in this dual-band can reach up to 95%. With the gradual increase of the VO₂ conductivity, the VO₂ film state gradually changes from the insulator to the metal phase, and the opening O₁ in each gold SRRs is gradually connected. Therefore, the cross-polarization amplitude |T_xy| gradually increases, while the co-polarization transmission amplitude |T_yy| basically does not change. When the VO₂ conductivity reaches 2.6×10⁵ S/m, the amplitude |T_xy| at 0.77∼0.97 THz has changed from the initial valley to the current peak, and is basically equal on the two sides. Now the polarization conversion 3 dB bandwidth is 0.82 THz (0.38∼1.20 THz). The corresponding PCR_xy can reach more than 97%. The transition process from dual-broadband to ultra-wideband operation fully embodies the dynamic control function of VO₂ on the polarization conversion bandwidth.

 

Fig. 2. Transmission amplitude and PCR for simulation and experiment. (a) Simulated frequency dependence of co- and cross-polarization amplitudes and PCR for y-linear polarization incidence with VO₂ conductivity ranging from 10 S/m to 2.6 × 10⁵ S/m. (b) Measured frequency dependence of co- and cross-polarization amplitudes and PCR for y-linear polarization incidence at temperatures ranging from 296 K to 363 K.

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We have modeled a metasurface with a conductivity from 10 to 2.6 × 10⁵ S/m for VO₂ and compared the performance of the metasurface measured in the experimentally achievable temperature range from 296 K to 363 K. The experimental results are shown in Fig. 2(b), where at room temperature (296 K), the VO₂ structure is equivalent to a transparent insulator and hardly affects the electromagnetic response characteristics of the structure. Therefore, the combined VO₂ and gold structures are equivalent to gold SRRs functioning only. As the temperature increases, VO₂ undergoes an insulator-to-metal phase change. At this time, the O₁ opening in each gold SRRs is connected, and the electromagnetic response also changes. The cross-polarization |T_xy| near 0.92 THz changes from a valley to a peak, and the co-polarization |T_yy| is basically unchanged. Therefore, the polarization converter is converted from low-temperature dual-broadband to high-temperature ultra-wideband. The electromagnetic response measured in the experiments and the calculated PCR are in good agreement with the simulation. However, compared with the simulation, there are some deviations in the actual measurement. For example, the transmission amplitude in the 0.92 THz∼1.4 THz frequency band is significantly reduced, and the valley of the transmission amplitude near 0.92 THz has a blue shift, which may be due to sample processing or experimental measurement. In general, the polarization conversion metasurface achieves dynamic control of the polarization conversion bandwidth and broadens the band of polarization conversion.

In order to visually describe the transformation of the polarization state in the process of electrical conductivity and temperature changes, we use the following Stokes parameters [10]:

As shown in Fig. 3(a), as the VO₂ conductivity increases, the absolute value of the normalized ellipticity χ evolves from a low value (0.85, -0.22) to a maximum (0.84, 0.40) first at 0.85 THz, which clearly shows that the initial y-linear polarization gradually converts to elliptical polarization (T_xy and T_yy coexist). Then χ gradually decreases to near 0 (0.85, -0.08), indicating that the elliptical polarization gradually changes to x-linear polarization. The change of the overall normalized ellipticity χ vividly demonstrates the dynamic control function of VO₂ on the valley around 0.85 THz. Figure 3(b) shows the normalized ellipticity χ calculated in the experiment. It is found that the trend of evolution is basically the same as the simulation. The only difference is that χ is significantly higher at room temperature. It may be caused by measurement or sample processing.

 

Fig. 3. Normalized ellipticity χ in simulation (a) and experiment (b).

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3.2 Physical mechanism

In order to better understand the physical mechanism of the polarization conversion process in the metasurface, we extract the surface current distribution on the microstructure to reveal the linear polarization conversion effect and its temperature-controlled modulation process. Vertical magnetic fields are generated by the currents in the metasurface according to Ampere's law. Our simulation reveals two surface current distributions, namely the electric and magnetic dipoles) (see Fig. 4). The electric dipole mode is controlled by the current in one direction (at a certain moment). It radiates, generates the orthogonal component, and leads to effective linear polarization conversion, thereby generating a field orthogonal to the incident y-linear polarization. In contrast, the magnetic dipole mode is dominated by antisymmetric charge oscillations in the SRRs, which results in a magnetic moment perpendicular to the metasurface. The magnetic dipole mode traps energy on the metasurface, and this energy is eventually dissipated, causing absorption instead of polarization conversion. At resonance, the y-linear polarization incidence on the substrate side of the metasurface excites the magnetic dipole mode [Fig. 4(b)] which does not induce the polarization conversion effect at low VO₂ conductivity (σ_low = 10 S/m). However, when increasing the conductivity of VO₂ (σ_high = 2.6×10⁵ S/m), the insulator-to-metal phase change of VO₂ causes the gold SRRs opening O₁ to be connected, and excite the electric dipole mode, which can be indicated by the symmetric current distributions as shown in Fig. 4(e), thereby greatly enhancing the polarization conversion effect. Moreover, the proposed metasurface at frequencies of 0.6 and 1.1 THz both excites electric dipole modes and exhibits good polarization conversion effects whether VO₂ is in an insulating or metallic state [see Figs. 4(a), 4(c), 4(d), and 4(f)].

 

Fig. 4. Physical mechanism of linear polarization conversion in multi-layer metasurface. Modes and surface currents (black arrow) excited on the gold SRRs layer by normally incident y-linear polarization wave at 0.6 THz (a) and (d), 0.85 THz (b) and (e), 1.1 THz (c) and (f) when VO₂ is in insulator (σ_VO2 = 10 S/m) and metal phase (σ_VO2= 2.6 × 10⁵ S/m).

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We used the principle of current resonance to explain the mechanism of polarization conversion of the gold SRRs, and then investigated the electromagnetic response of the structure composed of only the sapphire substrate, VO₂ and gold SRRs (see Fig. 5). At low VO₂ conductivity, the polarization conversion effect is obviously produced in the ranges of 0.45∼0.77 THz and 0.97∼1.2 THz. As the VO₂ conductivity increases, the valley near 0.85 THz gradually turns into a peak and becomes flat with the peaks on both sides to form an ultra-wide band. However, the gold SRRs only obviously has a very low PCR. Therefore, we introduce a grating structure along the y-axis to reduce the co-polarization transmission |T_yy| to improve the PCR_xy.

 

Fig. 5. Unit structure and transmission amplitude in simulation (a) Unit structure composed of sapphire, VO₂ and gold SRRs. Transmission amplitudes in simulation under VO₂ conductivity of 10 S/m (b) and 2.6 × 10⁵ S/m (c).

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3.3 Discussion

As we have shown in Figs. 2 and 4, at room temperature (σ_low = 10 S/m), resonant magnetic dipole mode is excited around 0.85 THz, exhibiting a resonance valley in the cross-polarization spectrum. When VO₂ is heated to 363 K (σ_high = 2.6×10⁵ S/m), the heating-induced dielectric-to-metal phase transition of VO₂ short-circuits the opening O₁ in the gold SRRs. The observed behavior corresponds to a thermally switchable polarization convertor exhibiting different polarization conversion bandwidths. Here, we also investigate the influence of the different included angle C₁ between the opening O₁ in the gold SRRs and the x-axis on the electromagnetic response. As shown in Fig. 6, it can be clearly observed that as the included angle increases from 0 to 45°, the position of the valley has a blue shift, which shows that the included angle C₁ determines the valley position. But as C₁ increases from 45° to 90°, a red shift occurs. These two processes show a symmetrical change of the valleys. From this change, we can infer that the position of C₁ in the gold SRRs may be shifted during the processing of the sample, leading to a blue shift of the spectrum in the experiment.

 

Fig. 6. Transmission amplitudes for different C₁ when the angle between the split line of O₁ and the x-axis in the gold SRRs is 0° (a) and (e), 15° (b) and (f), 30° (c) and (g), and 45° (d) and (h).

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In the designed metasurface, the gold grating mainly inhibits the co-polarization components, and therefore enhances the polarization conversion. Due to the sub-wavelength size of the unit structure, the transmittance of different grating gap ratios for cross-polarization should be basically the same. To verify this conjecture, we numerically studied the effect of the grating gap ratio on the transmission spectrum. As shown in Fig. 7, in the structural changes with the grating gap ratios of 1:2, 1:1, and 2:1, the transmission spectra are nearly unchanged, indicating that the spacing between the grating does not affect the structural response. Therefore, the grating has good robustness and also provides favorable conditions for sample processing and multilayer alignment.

 

Fig. 7. Transmission amplitudes when the grating gap ratio is 1:2 (a) and (d), 1:1 (b) and (e), and 2:1 (c) and (f).

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4. Conclusion

We show a THz polarization conversion metasurface with dynamic control function. The metasurface device is composed of multilayer junctions, including a sapphire substrate, VO₂ and gold SRRs, a polyimide dielectric layer, and a grating. This design uses the principle of current resonance to produce an efficient polarization conversion effect, and then uses the phase change property of VO₂ to achieve the dynamic control function of the polarization conversion device. The metasurface-based polarization converter can realize the dynamic control function from dual broadband (0.45∼0.77 THz and 0.97∼1.2 THz) to ultra-wideband (0.38∼1.2 THz) operation with a high polarization conversion ratio (above 95%). By changing the opening position of the gold SRRs, the position of the resonance valley can be easily tuned, which makes the metasurface structure more flexible; the grating gap has almost no effect on the structure response, so the metasurface has good robustness, which provides favorable conditions for sample processing and alignment. The proposed active polarization converter is expected to be applied in technical fields such as integrated terahertz systems, imaging, and communications, thus enriching THz functional devices.