1. Introduction

Recently, metasurfaces have been attracting enormous interest in controlling the polarization and propagation states of light [1–8 ]. Being as thin as only a fraction of the wavelength, metasurfaces enables the reduction of ohmic loss of the constituent metal resonators, and also can be made using straightforward fabrication processing, which are the challenges that occur in bulk metamaterials. Especially, the concept of phase discontinuities has been applied to metasurfaces, demonstrating light bending in directions characterized by generalized laws of reflection and refraction [9–23 ]. This opens up a new route to control the wavefront, such as generation of vortex beam, ultrathin flat lenses, light bending, and coding of holograms. However, most metasurfaces passively manipulate the output electromagnetic properties by varying the geometrical parameters. As the amount of application proposals blooms, it is necessary to develop tunable metasurfaces [24–29 ]. In this article, we present the design, fabrication, and modulation characterizations of an active and broadband terahertz deflector metasurface. The broadband response of the C-shape split-ring resonators (SRRs) was demonstrated in ref. 16 for the orthogonally polarized terahertz wave.Its complementary configuration investigated here on the other hand is expected to perform similar characteristics due to the Babinet principle and in addition enable the active switching functionality. Comprised of the complementary C-shape SRR unit building blocks with an equal phase increment of π/4 and identical transmission amplitude, the metasurface device enables the generation of an orthogonally polarized wave in the frequency range from 0.48 to 0.93 THz. The metasurface arrays made from gold film were fabricated on a doped semiconductor substrate, the resulted Schottky diode effect between the metasurface arrays and substrate enables active modulation in amplitude for the anomalously diffractive terahertz waves through the application of an external voltage. Furthermore, the modulation speed of this active metasurface is investigated by applying a rectangular AC voltage to the structure, which is strongly related to the capacitance and resistance of depletion region. This electrically controlled metasurface with phase discontinuities opens up opportunities for novel tunable terahertz devices.

2. Fabrication and experiment

As illustrated in Fig. 1(a) , the complementary C-shape SRR is employed as a unit building block to constitute the entire phase-gradient profile of the metasurface. The phase of one such unit ranges from 0 to 2π at a step of π/4 at the interface; the uniform transmission amplitude is achieved for the y-polarized wave. In the unit cell, there are eight complementary C-shape SRRs with changed geometric parameters – the arm length γ and the open angle α, whose symmetry axes are along either + 45° or −45° with respect to the x-axis [see Fig. 1(a, bottom)]. Those eight complementary C-shape SRRs as a synthetic unit element were patterned on a n-doped gallium arsenide (n-GaAs) layer of 1-μm-thick with free carrier density of 1.6 × 10¹⁵ cm⁻³, forming a Schottky junction between these two layers. The n-GaAs layer was grown on a 500 μm-thick semi-insulating GaAs (SI-GaAs) wafer by molecular beam epitaxy, as shown in Fig. 1(b). The optical image of the sample is given in Fig. 1(c). An extra metal line built near the metal structure and the C-shape SRR hole arrays were used as two electrodes to apply the voltage bias, making the depletion region be actively controlled, as illustrated in Fig. 1(d). In order to reduce the resistance between the metal electrode layer and the n-GaAs layer as much as possible, the ohmic contact was fabricated by electron-beam deposition of 20 nm of nickel, 20 nm of germanium, and 150 nm of gold in sequence on the n-GaAs layer, followed by a rapid thermal annealing at 350° C for 1 min in a nitrogen atmosphere.

 

Fig. 1 (a) Calculated transmission amplitude and phase of the cross-polarized radiation from eight complementary C-shaped SRRs in a unit cell on a 500-μm-thick SI-GaAs substrate. The geometrical parameters characterizing the elements from 1 to 4 are the arm length γ = 34.4, 33.1, 34.5, 29.7 μm, and the open angle α = 17°, 59°, 118°, 137°. Elements from 5 to 8 are obtained by rotating the first set of elements by an angle of 90° counterclockwise, in which the width and period of the individual complementary C-shaped SRRs are fixed at ω = 5 μm and p = 80 μm. The eight complementary C-shaped SRRs are assembled as a synthetic unit with periods P _x = 640 μm and P _y = 80 μm. (b) Diagram of the substrate and depletion region under the C-shape SRR hole. (c) Microscopic image of the fabricated metasurface sample. (d) An extra metal line built near the metal structure and the C-shape SRR hole arrays were connected to serve as a Schottky gate. (e) Experimental diagram of the angular resolved terahertz time-domain spectroscopy system.

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The proposed metasurface operates in the linear cross-polarization scheme, in which anomalous diffraction is observed in the transmitted beam that is orthogonally polarized to the incident beam. In the measurement, a fiber-based angular resolved terahertz time-domain spectroscopy system was employed to characterize the biased metasurface sample [see Fig. 1(e)]. The terahertz pulses were transmitted and detected by a pair of commercially available fiber-based terahertz photoconductive antennas (Menlon System). The radiant terahertz beam from the transmitter was collimated and illuminated on the metasurface arrays with the phase gradient along the incident polarization direction. The metasurface sample was electrically connected to an external voltage source using conducting wires and fixed in the center of a rotated stage. Meanwhile, the receiver was installed on the rotation stage, collecting the scattered signal with polarization orthogonal to the incidence from different angles in the rotation process. Additionally, two polarizers were employed to perfect the desired wave polarization: one in front and one behind the sample. Under different bias voltages, the deflected wave was detected every 1° by rotating the rotation stage from 0° to 90°.

The measured results with respect to the scan angles at three reverse voltage biases 0, −3 and −10 V are shown in Figs. 2(a)-2(c) , respectively. Without the applied voltage, the deflected waves with orthogonal polarization propagated from the metasurface reveal a broad frequency response from 0.48 to 0.93 THz in a broad angular range from 26° to 81°, but a relatively weak amplitude. With the increase of the reverse voltage bias, the anomalous diffraction amplitude distinctly enhances [see Figs. 2(b) and 2(c)], while the frequency range and deflected angle range remain unchanged. This enhancement originates from the fact that the increasing reverse voltage bias depletes the free charge carriers of the n-GaAs layer under the metal structure film and therefore, significantly reduces the dissipation near the C-shape SRR gaps, consequently strengthens the resonant behavior. The green lines in Figs. 2(a)-2(c) are calculated based on the generalized Snell’s law [16]:

 

Fig. 2 (a)-(c) The measured results with respect to the scan angles (y-axis) and frequency (x-axis) at three reverse voltage biases 0, −3 and −10 V, with the x-polarized wave incidence and y-polarized wave detection. The color represents the relative anomalous diffraction amplitude, and the green lines are calculated results using Eq. (1). (d) and (e) are the transmitted spectra of the deflected waves as a function of reverse and obverse voltage biases, respectively.

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3. Simulation and discussion

When a reverse bias is applied, the depletion of free carriers in the n-GaAs layer results in variation of the conductivity at the C-shape SRR gaps. To acquire a clear understanding of the modulation mechanism, 3D numerical simulations using the commercial finite-integration package CST Microwave Studio are performed. In the simulation, the x-polarized terahertz wave is normally incident from the substrate side onto the metasurface and the y-polarized electric field distributions transmitting through the metasurface are probed and plotted at three representative frequencies 0.55, 0.68 and 0.83 THz, when the conductivity of the n-GaAs layer is set to 0 and 1000 Sm⁻¹, respectively, as shown in Figs. 3(a) and 3(b) . The black arrows indicate the incident and outgoing direction of the simulated wave. We observe that the wavefronts of the three dispersed waves with y-polarization transmitted from the metasurface are deflected obviously, further demonstrating the broadband property of the metasurface. The deflected angles of three frequencies are 59.09°, 44.27°, 34.88°, respectively, from left to right in Fig. 3(a), agreeing well with the theoretically calculated angles of 58.46°, 43.58°, 34.39°. Furthermore, the amplitude of the deflected waves recedes as the conductivity increases. Significantly, the deflected angles of the corresponding three frequencies remain unchanged. Figure 3(c) shows the transmission intensities with respect to the conductivities of 0, 50, 100, 150 and 210 S m⁻¹, corresponding to the applied external voltages of −10, −6, −3, −1 and 0 V, respectively, displayed a good agreement with the measurements. Numerical results also show that higher conductivity leads to a weaker transmission. Thus, it can be predicted that if the carrier density of the n-GaAs layer reaches 1.6 × 10¹⁶ cm⁻³, namely, the conductivity is up to 2600 S m⁻¹, the deflected dispersive waves may almost vanish at the bias voltage of 0 V, the intensity modulation depth will improve drastically [see Fig. 3(c)].

 

Fig. 3 (a)-(b) Simulated y-polarized electric field distributions at 0.55, 0.68, 0.83 THz with respect to the conductivity σ = 0 and 1000 Sm⁻¹, respectively. The arrows indicate the wave propagation direction. The color represents the intensity of cross-polarized anomalous refraction beam. (c) Simulated transmission spectra as a function of conductivity.

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In order to further understand the performance of this active metasurface device, we also characterized the modulation speed of anomalous diffraction by applying a rectangular AC voltage instead of a DC voltage to the structure. The rectangular AC voltage changed alternately between −10 and 0 V. Meanwhile, the modulation frequency was given to the lock-in amplifier as a reference signal, which had been used to measure the modulation speed in ref. 24. In our measurement, the angular resolved terahertz time-domain spectroscopy system was still used, but the receiver was fixed along 45° with respect to the incident direction of the terahertz wave, collecting the scattered signals with cross-polarization. The peak-to-peak values of the time-domain pulses were measured, which was receding with increasing modulation frequency. The normalized amplitude as a function of modulation frequency was shown in Fig. 4 . We take the 10% of normalized amplitude as the available baseline of device, and the corresponding modulation frequency is 3 kHz. The modulation speed is strongly related to a quick formation and dissolution of the charge depletion zone at the gaps, depending on the device RC time constant, where R is the contact resistance of the Schottky diode, C is its depletion capacitance. Reducing the device resistance and/or capacitance would improve the operation speed of the active terahertz wave deflector. This can be achieved by increasing the doped density of the n-GaAs layer or optimizing the contact between the metal structure and substrate.

 

Fig. 4 Normalized modulation magnitude of the terahertz wave deflector as a function of operation frequency of rectangular AC voltage.

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4. Conclusion

An electronically switchable terahertz diffracted wave metasurface comprised of complementary C-shape SRR arrays was experimentally demonstrated. The active metasurface achieved an intensity modulation depth as high as 46% with varying the reverse voltage bias from 0 to −10 V, while the broadband frequency response characteristics and constant deflected angles were well maintained. The formation of a Schottky diode structure between the metal film and doped semiconductor layer enables real-time modification of the substrate conductivity and loss, resulting in the switchable resonance and intensity modulation of the deflected wave. The modulation speed reaches several kilohertz, which can be further improved through increasing the doped density of the n-GaAs layer and reducing the contact loss between the metal structure and substrate. Our approach can be utilized to engineer terahertz wave deflections in amplitude, which opens up an opportunity for the design of active metasurface devices.