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We demonstrate an active metamaterial device that allows to electrically control terahertz transmission over more than one order of magnitude. Our device consists of a lithographically defined gold nano antenna array fabricated on a thin film of vanadium dioxide (VO2), a material that possesses an insulator to metal transition. The nano antennas let terahertz (THz) radiation funnel through when the VO2 film is in the insulating state. By applying a dc-bias voltage through our device, the VO2 becomes metallic. This electrically shorts the antennas and therefore switches off the transmission in two distinct regimes: reversible and irreversible switching.
©2011 Optical Society of America
1. Introduction
Vanadium dioxide (VO2) has been widely investigated due to its insulator-metal phase transition (IMT) near room temperature [1]. This transition can be controlled by thermal [2–4], optical [5, 6] and electrical [7, 8] means. In the terahertz frequency region, the dielectric constant of VO2 varies by several orders of magnitude when the film undergoes phase transition, which can in turn be used to control terahertz radiation transmitted through the VO2 film [3]. Advances in thin film fabrication have made it possible to grow very high quality layers of VO2 either by pulsed laser deposition [9], sol-gel method [10] or rf-magnetron sputtering technique [11]. These films have typical thicknesses around a few hundreds of nanometer which is much smaller than the effective THz frequency skin depth [3, 12–14]. Therefore, even as the VO2 film turns metallic, it still stays largely transparent at these wavelengths where the skin depth exceeds the film thickness [15].
Combining VO2 thin film with plasmonic nanostructures helps to overcome this limitation imposed on the THz wave switching capability [16–18]. We adopt a lithographically defined gold nano antenna array on top of the VO2 thin film [19–21]. The antenna structure acts as nanogap-capacitor where the incident THz waves induces the strongly localized near field which makes the THz waves funnel through while the film is in the insulating state. As the film undergoes a phase transition and becomes metallic, these antennas are electrically shorted. This switches the THz transmission off. This modulation of the transmissivity has been demonstrated by thermal [15] and optical [4, 22] control of the VO2 phase transition. Electrical control is also desired for active THz devices and applications [23–25]. In this research we establish control of THz transmission through nano antennas on VO2 thin film with an electric bias.
2. Experimental results and discussions
VO2 films of 100 nm thickness are created by pulsed laser deposition onto a 430-μm-thick c-plane sapphire substrate. In order to apply electric voltages to the film, 1 mm wide gold electrodes separated by 1 mm from each other are fabricated on the surface. We include a 25 kΩ resistance in the voltage applying circuit in series to prevent over-current flow after the phase transition that would harm the source. Figure 1 (a) shows a typical current to voltage measurement of this device. As the voltage is increased at a rate of 1 V/s the phase transition is clearly visible around 400 V. This switching is hysteretic and when ramping down the voltage the transition occurs at around 270 V. We observe that the exact transition voltage vary between different voltage sweeps. They generally start at lower values and stabilize at values between 400 and 500 V after repeated sweeps. We account this behavior to the formation of current channels [26]. These channels tend to slightly vary in position and size when we apply voltage to the sample many times [27]. We observe that by applying once a high voltage (~1000 V), the experimental results are repeatable. When a high voltage is applied, oxygen depletion happens along the current channel so that this one can be the preferred channel because the depleted area becomes less insulating when the oxygen deficiency increases [8, 28].
Fig. 1 (a) External voltage driven insulator-metal transition in VO2 thin film. The two electrodes are 1 mm apart from each other. (b) When a pulsed voltage is applied, the transition happens on a microsecond time-scale. (c) Schematic of the THz transmission measurement stage. (d) Relative transmission of THz radiation through bare VO2 in dependence of an external bias from 0 V to 500 V at 333 K. The film is largely transparent irrespective of the temperature and the intensity of the voltage bias.
In order to determine the switching speed a voltage pulse is applied to the film [29]. The current response (Fig. 1 (b)) shows a double step. During the first plateau the film is still in its insulating state and the second step marks the phase transition. The whole switching occurs in less than 10 microseconds. This timescale is in good agreement with the predicted switching speed for Joule heating induced transitions [30]. Also, we use a typical THz time-domain spectroscopy system to analyze the film’s transmissivity in the THz regime in dependence of voltage bias [31]. As illustrated in Fig. 1(c), the samples are mounted on temperature stabilized thermostage and a thermistor (SEMITEC 104JT-015) is used to record the temperature. When applying a voltage to the VO2 film from 0 V to 500 V at room temperature, there is no clear difference in THz transmission level. This is very likely due to the fact that the current channel, along which the transition to the metallic state occurs, is confined to a much smaller area than the incident THz beam cross section. To enlarge the metallic area of VO2, we raise the temperature of the thermostage close to the transition temperature (Tc ~340 K). The additional current flow can then raise the temperature of a larger area above the transition. Indeed, in Fig. 1(d) the THz transmissivity through our device, which is heated to 333 K, decreases by the current induce IMT at 500 V. However the switching capability is limited to about 10% which can be attributed to the film thickness which is much smaller than the effective THz skin depth and to the limited area over which the switching occurs.
To enable enhanced electrical THz wave modulation through nanoscale VO2 thin films, we introduce a nano antenna array pattern on top of the film. We use e-beam lithography to fabricate our device. The detailed procedure is described in Fig. 2(a) . The size of each antenna is 140 μm by 600 nm and the periods of the antenna array are 10 μm (parallel with antenna array direction) and 30 μm (perpendicular to antenna array direction). This pattern is placed between the gold electrodes and the total pattern size is 1 mm by 1 mm. There is a 10 μm wide gap between each electrode and the pattern in order to prevent current flowing solely through the gold film and not the VO2 (Fig. 2(b) and (c)).
Fig. 2 (a) Schematic of the sample fabrication procedure. The nano antenna pattern and gold electrodes are deposited on a 100-nm-thick VO2 thin film by e-beam lithography. (b) SEM image of the nano antenna pattern. (c) Schematic of the nano antenna array (width w = 600 nm, length l = 140 μm, parallel period p// = 10 μm, perpendicular period p┴ = 30 μm, distance between the electrodes d = 1 mm, gap between the electrode and nano antenna pattern g = 10 μm).
Figure 3 shows the THz transmissivity of the nanopatterned device in dependence of voltage bias at two different temperature of the thermostage. The transmission shows a broad resonance at 0.52 THz given by the geometry of the antennas. At 333 K, the relative transmission of THz waves at the resonance frequency decreases significantly by applying a voltage of 500 V (Fig. 3(a)). We find that 98% of the THz transmission can be actively controlled by the electric bias. This is a 10 fold enhancement compared to the unpatterned VO2 film. When we remove the bias, the transmission signal is recovered to its original level.
Fig. 3 (a) Nanopatterned VO2 sample shows on-off (at 333 K) and (b) memory (at 335 K) switching behavior of THz transmission. Each transmission spectrum is normalized by the maximum THz transmission intensity at each resonance frequency.
By increasing the temperature of the thermostage to 335 K, another interesting switching behavior is observed. After the electric bias returns to 0 V, the THz transmission signal still stays in the off-state (Fig. 3(b)). This effect is due to the hysteretic switching of VO2 and has been used to form a memory metamaterial [32–34]. Here we demonstrate an electrically triggered memory effect which has much potential for practical applications.
Previous research shows that among different electrical switching mechanisms of VO2, Joule heating induced by current flow is dominant for large geometries [30]. Therefore, we conclude that the IMT of our device is based on Joule heating, which is supported by the following observations [35]: (i) The transition occurs at both positive and negative bias. It only depends on the absolute value of the external bias. (ii) Raising the temperature of the thermostage lowers the transition voltage. (iii) The switching observed in Fig. 1(b) occurs on a microsecond timescale.
3. Conclusion
THz radiation through a VO2 thin film can be actively modulated by an electric bias. For a bare film switching capabilities of 10% are observed. We significantly enhance this control range by fabricating a nano antenna array on top of the 100-nm-thick VO2 film. In this geometry, a change of 98% of the THz transmissivity is observed. By operating the device at different temperature biases, two distinct switching behaviors are demonstrated: reversible and irreversible switching. This provides the potential of active metamaterial device development in the THz regime.
Acknowledgments
This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Government of the Republic of Korea (MEST) (SRC, No:R11-2008-095-01000-0) (No:2010-0029648), KICOS (GRL, K20815000003), the creative research project of ETRI and Hi Seoul Science / Humanities Fellowship from Seoul Scholarship Foundation. The authors acknowledge SEMITEC for providing thermistors.
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