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Abstract
Metasurfaces are two dimensional arrays of artificial subwavelength resonators, which can manipulate the amplitude and phase profile of incident electromagnetic fields. To date, limited progress has been achieved in realizing reconfigurable phase control of incident waves using metasurfaces. Here, an active metasurface is presented, whose resonance frequency can be tuned by employing insulator to metal transition in vanadium dioxide. By virtue of the phase jump accompanied by the resonance frequency tuning, the proposed metasurface acts as a phase shifter at THz frequency. It is further demonstrated that by appropriately tailoring the anisotropy of the metasurface, the observed phase shift can be used to switch the transmitted polarization from circular to approximately linear. This work thus shows potential for reconfigurable phase and polarization control at THz frequencies using vanadium dioxide based frequency tunable metasurfaces.
© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Metasurfaces, consisting of two dimensional arrays of artificial subwavelength resonators, have shown the capacity to manipulate the amplitude and phase profile of incident electromagnetic waves, realizing ultrathin lenses, beam deflectors and wave plates [1]. Metasurfaces also offer opportunities for integration with reconfigurable substrates to achieve reconfigurable optical response which is not possible in the case of naturally available optical materials [2]. Reconfigurable metasurfaces are particularly important for THz frequencies (0.3 THz - 3 THz), where there is a lack of naturally available materials. Significant research efforts have been made to realize reconfigurable THz metamaterials by realizing metamaterials on various reconfigurable substrates [3–8]. However, majority of efforts have focused on amplitude modulation and fewer efforts have been devoted towards realizing phase modulation. More recently, THz phase modulation has been reported in graphene based metasurfaces in reflection mode [9]. In transmission mode, THz optical phase modulation has been demonstrated in metasurfaces combined with GaN based heterostructures and vanadium dioxide [10–12]. These metasurfaces consist of split ring resonators (SRR) or other types of resonant structures and employ reconfigurable substrates to dampen the resonance amplitude of the resonators. Metasurfaces featuring resonance frequency control have also been investigated, but the main emphasis has been towards amplitude modulation and relatively minor attention has been devoted towards the associated phase modulation characteristics [13–15].
In this work, an active metasurface design is presented. It employs resonance mode switching for metasurface resonance frequency shifting which enables phase and polarization modulation of THz waves. The active metasurface device consists of a metallic grating and cut-wire hybrid metasurface structure on top of a vanadium dioxide (VO2) thin film, as shown in Fig. 1. For incident fields polarized perpendicular to the wire grating (y-polarization), the resonance frequency of the metasurface can be tuned from 0.52 THz to 0.37 THz by varying the input bias current from 100 mA to 300 mA. This results in a large phase jump modulation from 31° to −33° at the frequencies of 0.46-0.47 THz, while having nearly constant insertion loss of about −6.5 dB. For incident fields polarized parallel to the wire grating (x-polarization), the metasurface exhibits a current independent phase shift of −45° at the same frequency range. Thus, the phase difference between orthogonally polarized incident fields is varied from 78° to 14° and the transmitted polarization can be switched from approximately circular to linear. In contrast to some of the previously reported VO2 based metasurfaces [11, 16], the proposed design does not require external heating elements and provides control of metasurface resonance frequency and phase shift. The presented work highlights design strategies for THz phase and polarization modulation by using frequency tunable metasurfaces.
Fig. 1 Schematic diagrams showing operational characteristics of metasurfaces at (a) 0 mA and (b) 300 mA where incident and transmitted polarization states are illustrated. (c) Metasurface design and dimensions. l1 = 12 μm, l2 = 8 μm, l3 = 5 μm, l4 = 20 μm, l5 = 10 μm, w1 = 100 μm, w2 = 50 μm. (d) Optical images of the fabricated device. The area of the fabricated metasurface device consisting of 480 unit cells is 2 × 2 mm2.
2. Device Design and Fabrication
A schematic diagram of the metasurface device is shown in Fig. 1(a). The metasurface consists of pairs of cut wires sandwiched between horizontal grating wires [17]. Each cut wire is connected to top or bottom grating wire via a vertical rod. VO2 thin film patches having a thickness of approximately 1 μm are placed underneath the cut wire pair. The horizontal wires forming each unit cell are alternatively connected to the left and right bias pads, placed at the periphery of the main device as shown in the Fig. 1(a). By applying a bias voltage or current to the bias pads, insulator to metal transition (IMT) [18] can be triggered in VO2 film placed beneath the cut wire pairs, which in turn modulates the transmission characteristics of the metasurface device. The mechanism of IMT in VO2 is still under active investigation and various mechanisms have been proposed [19, 20]. It has been reported that the current flow through the VO2 film sandwiched between two electrodes causes Joule heating inside the VO2 film leading to IMT [20].
The present metasurface design, which realizes large frequency shifting and phase modulation by allowing currents to be locally applied to the VO2, eliminates the need of external heating elements. As the metasurface allows local Joule heating in VO2 thin film patches, the thermal mass of the device is greatly reduced, as compared to the VO2 based metasurface devices where global heating is required to heat up the whole substrate [16]. It will lead to the greatly reduced switching power for VO2 based metasurface devices.
The variation in metasurface response as a result of IMT depends on the VO2 conductivity values in insulator and metallic state. Typical VO2 films grown on c-type or r-type sapphire substrate exhibit conductivity (resistivity) values in the range of 101 ~102 S/m (101 ~100 Ωcm) and 104 ~105 S/m (10−2 ~10−3 Ωcm) in insulator state and metallic state, respectively [21].
THz transmission, t, (the ratio of transmitted electric field over incident electric field) of thin conducting films having thicknesses d can be calculated as a function of conductivity σ using the Eq. (1) [22],
, where n is the refractive index of the substrate and Zo is the free space impedance. Figure 2(a) shows the calculated THz transmission vs. conductivity, along with the measured characteristics of two VO2 films. When the conductivity variation range of VO2 film covers the shaded region in Fig. 2(a), THz transmission characteristics of the film can be most efficiently controlled. Accordingly, the deposition condition of 1-μm-thick VO2 thin film was optimized such that its conductivity values in insulator and metallic state are in the vicinity of 103 S/m (10−1 Ωcm) and 105 S/m (10−3 Ωcm), respectively. As a result, maximum variation in THz transmission was achieved only with conductivity variation ratio of 102. The transmission values of the optimized VO2 in insulator and metallic state are highlighted in black and red circles, respectively. Figure 2(b) shows the measured THz transmission of the VO2 films with optimized conductivity levels, by using THz time domain spectroscopy (TDS). The THz transmission is close to 0.85 when the VO2 is in the insulator phase at 25°C, and it drops to 0.05 at 80°C when the VO2 is in metallic state.Fig. 2 VO2 conductivity optimization. (a) THz transmission through a thin lossy film as a function of film conductivity and thickness. Blue and green triangles indicate the insulator and metallic conductivities of orignial unoptimized VO2 film. Black and red circles indicate the insulator and metallic conductivities of optimized VO2 film (b). THz transmission spectra of grown VO2 films. (c) Metasurface equivalent circuit model for vertically (y-) polarized fields. (d) Simulated transmittance T and transmission phase ϕ.
Once the appropriate conductivity variation is realized in VO2, the metasurface design is carried out. Figure 2(c) shows a schematic diagram of the metasurface. For incident fields polarized along y-axis, the metasurface unit cell can be modeled as the equivalent circuit shown in Fig. 2(c). The equivalent inductance, L, of the unit cell originates from the vertical rods connecting cut wire and the grating wires while the capacitance, C2, originates from the gap between top and bottom grating wires of the unit cells in the same column. The vertical cut wire pair along with the VO2 acts as tunable leaky capacitor C1.
As described previously, by applying bias current to the device, IMT can be triggered in VO2 in each unit cell. When the VO2 is in insulator state, the metasurface resonance frequency is determined by the the series connected capacitances C1 and C2. When the VO2 switches to metallic state, capacitance C1, determined by the cut wire pair, is reduced and eventually shorted. This results in an increase of the equivalent capacitance leading to down shifting of the metasurface resonance frequency. The device structure and the operation principle employed here markedly differ from earlier work [12], where metasurface is based on cross aperture resonators containing VO2 patches which are used to tune the aperture length of the resonators.
The dimensions of our metasurface were optimized using full wave electromagnetic (EM) simulation to have a resonant response around 0.5 THz. EM simulations of the metasurface unit cell were carried out using HFSS by ANSYS. Periodic boundary conditions were applied to the metasurface unit cell shown in Fig. 1(c) to mimic a practical scenario containing thousands of unit cells. VO2 was simply modeled as a lossy dielectric film with optical conductivity values measured previously. The optimization of the metasurface design was carried out by using principles in the earlier work [17], where the additional details regarding the metasurface geometry and dependence of resonance frequency on individual unit cell parameters are available. In Fig. 2(d), the simulated transmittance “T (the power ratio of transmitted wave over the incident wave)” and phase “φy” for y-polarization are shown. The metasurface resonance frequency shifts from 0.55 THz to 0.35 THz, as the VO2 switches from insulator to metallic state, accompanied by large phase modulation of 86° at 0.47 THz.
The active metasurface device with the above dimensions was fabricated as follows. Firstly, the 1-μm-thick VO2 thin film was deposited on c-plane sapphire substrates (0001) by using radio frequency magnetron sputtering. The distance between VO2 solid target and substrate was kept at 35 mm. The system was evacuated to 5 × 10−6 torr prior to growth. Flow rate of Ar was kept at 20 sccm, maintaining the working gas pressure of 2.8 × 10−3 torr during deposition. The films were grown at a substrate temperature of 492°C. The metasurface fabrication started with wet-etching of as-grown VO2 film. For the patterning VO2 film, photoresist mask and a mixture of phosphoric acid, nitric acid, and acetic acid was utilized. Ti/Au (20/300 nm) metallization was e-beam evaporated and lifted off for the metamaterial patterns. Optical images of the fabricated device are shown in Fig. 1(d).
3. Results and Discussion
The active metasurface device with the above dimensions was fabricated and characterized using a THz-TDS setup based on linearly polarized photoconductive antennas [17]. During the measurement, no special measures were taken in order to synchronize the VO2 switching in all the unit cells. No burn out was observed in any area of the device during the characterization. Device operation seems to be tolerant to slight variation in VO2 switching times in different unit cells, considering that IMT in VO2 does not take place instantaneously, rather the transition happens gradually throughout the VO2 film as the input current is increased. This is supported by the measured THz transmission results described below, where the metasurface resonance frequency varies gradually as the bias current is increased. Figure 3(a) shows the measured transmittance and phase response for y-polarized incident fields. Under applied current lower than 100 mA, the metasurface resonance frequency stays invariant at 0.52 THz. As the applied current is increased above 100 mA, resonance frequency begins to shift downward along with a decrease in the resonance strength. Increasing the current above 230 mA results in an increase of resonance magnitude along with small down shifting of the resonance frequency. Beyond 300 mA, the transmittance does not show any variation, indicating that IMT in VO2 has completed. This behavior can be explained with the equivalent circuit model in Fig. 2(d). As the VO2 conductivity increases, the capacitance C1 becomes lossier and the equivalent capacitance of the unit cell also increases. It decreases resonance strength as well as resonance frequency. Further increase in conductivity makes VO2 act as lossy inductor, shorting the capacitance C1. It leads to a sharp down shifting of resonance frequency as the equivalent capacitance is now only given by capacitance C2. With the further increase of conductivity, the inductor becomes less lossier. It leads to the strengthened resonance at 0.37 THz.
Fig. 3 Metasurface transmission characteristics for y-polarized incident fields. (a) & (b) transmittance T and transmission phase ϕ. (c) & (d) 2D map of normalized transmission spectra for different values of bias current under y-polarized incident fields. T0 is transmittance at 0 mA and ϕ0 is the optical phase at 0 mA.
The resonance frequency shift described above is also accompanied by variation in phase shift as shown in Fig. 3(b). A maximum phase modulation of 64° is observed at 0.46 and 0.47 THz. In order to get a more detailed picture of the modulation characteristics at all frequencies, we plot a two dimensional map of the normalized transmittance and phase spectra while the bias current is varied, as shown in Figs. 3(c) and (d). The transmittance and phase spectra are normalized with respect to the values at 0 mA in order to emphasize the modulation characteristics. As the bias current is increased, THz transmittance decreases at frequencies below 0.45 THz. On the contrary, it increases as the bias current is increased at frequencies above 0.47 THz. This shows that combining metasurfaces with VO2 allows more diverse control of THz transmittance as compared to bare VO2 films. For bare VO2 films, insulator to metal transition always results in decrease of transmittance. Large phase modulation takes place at the frequencies between 0.52 THz and 0.37 THz, as shown in Fig. 3(d). At 0.46-0.47 THz, transmission phase shows maximum variation from 0° to 64° while the normalized transmittance, T/To, stays nearly constant, as shown in Fig. 3(c). The insertion loss at this frequency, −10 log (To), is calculated to be −6.5 dB, where To is the transmittance at 0 mA. Thus the present metasurface acts as a phase shifter for y-polarized incident fields at frequencies of 0.46-0.47 THz. The device presented in this work exhibits a resonance frequency shift of 0.15 THz, much larger than that in the previous work [12]. Furthermore, in contrast to the earlier work [12], the present design realizes a phase modulation of 64° while having a nearly constant transmittance at 0.46-0.47 THz, which is an important requirement for phase shifters.
In contrast to some of the previously reported metasurface based phase modulators [10, 11], the present metasurface does not impose any limits on the anisotropy of metasurface i.e. the metasurface response for x-polarization can be tuned independently without affecting the metasurface response for y-polarization. This property enables the metasurface to manipulate the transmitted polarization as well. Figure 4(a) shows the transmission response and metasurface equivalent circuit model for x-polarization. The metasurface unit cell acts as a parallel LC resonator [17], resulting in a bandpass resonant response at 0.81 THz. For x-polarized fields, the equivalent inductance L of the unit cell is governed by the width of horizontal grating wires while the capacitance C is determined by the distance between the horizontal cut wire pairs in the adjacent unit cells. Since both of these parameters are independent of VO2, the x-polarization response is almost independent of the variation in VO2, as shown in Fig. 4(b). At 0.46 THz, the transmittance of x-polarized wave is same as that of y-polarized wave and the transmission phase is close to −45°. Figure 4(c) shows the transmission phase difference between y and x-polarizations vs. the applied bias current at 0.46 THz. As shown in the figure, as the y-polarization phase shift ϕy varies from 33° to −31°, the phase difference (ϕy - ϕx) varies from 78° to 14°. Additionally, the ratio of transmission amplitudes for x and y-polarized fields stays close to 1, lying in the range between 0.90 and 1.05. If the metasurface is tilted by 45° with respect to an incident linear polarization as shown in Fig. 1(a), the transmitted polarization can be changed from circular to approximately linear by increasing the applied bias current. The metasurface thus acts as a switchable quarter wave plate.
Fig. 4 Transmission characteristics of metasurface for x-polarized incident field. (a) Transmittance T and (b) transmission phase ϕ. Inset shows the equivalent circuit model of the unit cell under x-polarization. (c) Axial ratio and phase characteristics of the transmitted field vs. the applied bias current at 0.46 THz. (d) Polarization ellipses of the transmitted field for selected values of the bias current at 0.46 THz. For the sake of reference, red lines are plotted to show the ideal linear and circular polarizations.
The polarization ellipses for the transmitted polarization, calculated based on the above values are shown in Fig. 4(d). At 100 mA, the transmitted polarization is nearly circular and is slightly tilted above x-axis. At higher current levels, the polarization becomes more and more elliptical and the tilt angle is also increased. At 300 mA, the polarization ellipse becomes quite narrow and is tilted further towards 45°, approaching a straight line (red curve) representing perfect linear polarization. The VO2 based metasurface presented here, realizes actively reconfigurable circular polarizer, as compared to the static circular polarizer demonstrated previously [17]. The polarization ellipses for the transmitted polarization show a little bit of deviation from ideal circular and linear polarizations (90° and 0°) shown by the red curves. However, further improvement of the fabrication process or quality factor of the metasurface, the phase modulation values of 90° can be realized. The above results thus show potential for realizing polarization control devices using frequency tunable metasurfaces. The higher phase modulation up to 180° may be achieved by realizing opposite phase modulation along both y and x-axis, instead of modulating along only one axis which is the case in the present device. This would enable switchable half wave plates and polarization rotators.
4. Summary and Conclusions
We have presented an active metasurface design for realizing THz phase and polarization modulation. The active metasurface device consists of a metallic grating and cut-wire hybrid metasurface structure on top of vanadium dioxide (VO2) thin film patches. We demonstrated that, by varying the input bias current from 100 mA to 300 mA, the metasurface resonance frequency was shifted from 0.52 THz to 0.37 THz. This resonance frequency shift resulted in a phase modulation of 64° at the frequencies of 0.46~0.47 THz for y-polarized incident fields, while having nearly constant insertion loss of about −6.5 dB. Furthermore, by appropriately tailoring the x-polarization response of the metasurface, we demonstrated that the observed phase modulation can be used to switch the transmitted polarization from circular to approximately linear. This work thus shows potential for realizing the reconfigurable phase and polarization control devices at THz frequencies by using VO2 based metasurface.
Funding
National Research Foundation of Korea (NRF) grant funded by the Korea government (Ministry of Science, ICT and Future Planning) (No. 2017R1A2B3004049) and Creative Materials Discovery Program through the National Research Foundation of Korea (NRF) funded by Ministry of Science and ICT(NRF-2017M3D1A1040828).
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