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Abstract
In this paper, we propose an asymmetric split-loop resonator with an outer square loop (ASLR-OSL) based on vanadium dioxide (VO2) which can actively control the transmission characteristics of a terahertz wave while maintaining a high quality factor of the asymmetric split-loop resonator (ASLR) by adding an outer square loop. The proposed ASLR-OSL demonstrated transmission characteristics similar to those of ASLR, and the transmission characteristics of ASLR-OSL were successfully controlled by directly applying a bias voltage. These results show a simple method for imposing active properties on a common metamaterial having a high quality factor by adding a loop structure.
© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Terahertz (THz) technology has received considerable interest because of its potential in a wide variety of applications such as wireless communication, spectroscopy, imaging, and sensing [1–3]. In the past several decades, cost-effective and compact THz sources and detectors have been intensively developed to use the THz wave in industrial applications. For more practical THz applications, various active and passive devices such as THz filters, modulators, phase shifters, switches, and mirrors must be developed. However, the development of THz devices is very deficient compared to microwave and light wave bands because the electromagnetic properties of most natural materials are not suitable to be used in the THz frequency range. To overcome the limitations of natural materials in the THz band, research on the utilization of metamaterials, which can artificially control electrical and magnetic properties, as devices in the THz band has attracted much attention [4–9]. The response of metamaterials to electromagnetic waves is determined by the structure of metallic resonators with periodic patterns whose unit cells are smaller than the wavelength of the wave. The resonances of the metamaterials are characterized by both the LC resonance of the unit cell and the periodicity of the structures. The controllable resonances of artificially engineered metamaterials can offer opportunities to realize novel THz devices for a wide variety of THz applications.
Numerous research studies on the realization of tunable characteristics for THz metamaterials have been reported by using semiconductors, graphene, and tunable functional materials [10–13]. Tunable metamaterials based on vanadium dioxide (VO2) present a promising approach to spatially manipulate the THz wave thanks to easy fabrication and high tunability. Several studies have researched tunable THz metamaterials based on the phase transition of VO2 by applying temperature, a THz field, or light [14–17]. However, these methods require external devices such as a heater or a source of THz waves or light, and the external devices cause these THz tunable devices to be expensive and bulky. Thus, electrical control of the phase transition of VO2 is preferred for practical applications [18–20]. The metamaterial that can be electrically controlled but has low Q-factor is limited in improving the performance when applied as a device. To improve the performance of THz devices using metamaterials, it is very important to increase the quality factor of the metamaterials. The main factor that lowers the quality factor of a metamaterial is radiation loss in the resonance state, so research on metamaterial structures with low radiation loss has been actively carried out [21–24]. In one example, an asymmetric structure of the metamaterial, which has an asymmetric resonance, can reduce the radiation loss and improve the quality factor of the metamaterial [25–28].
In this paper, we propose an asymmetric split-loop resonator with an outer square loop (ASLR-OSL) based on VO2, which can actively control the transmission characteristics of a THz wave while maintaining a high quality factor of the asymmetric split-loop resonator (ASLR). The outer square loop was designed with the ASLR to play the role of a microheater that can control the temperature through a directly applied bias voltage without degrading the high quality factor of the ASLR. The proposed ASLR-OSL showed transmission characteristics similar to those of the ASLR, and the transmission characteristics of the ASLR-OSL can be successfully controlled by directly applying a bias voltage to the outer square loop.
2. Asymmetric split-loop resonator with an outer square loop (ASLR-OSL)
2.1 Design and fabrication of an ASLR-OSL
Figure 1 shows the structure and a photograph of the ASLR-OSL. The outer square loop was designed with the ASLR having a high quality factor to work as a microheater to electrically control the electromagnetic characteristics of the ASLR while maintaining the high quality factor of the ASLR [29].
Fig. 1 (a) Schematic of asymmetric split-loop resonator with outer square loop (ASLR-OSL) based on VO2, and (b) photographs of fabricated ASLR-OSL.
Epitaxial and stoichiometric single-phase VO2 thin films were grown on a single-crystal Al2O3 (0001) substrate by using ion reactive radio-frequency (RF) sputtering. The thickness of the VO2 films was 100 nm, which was controlled by changing the deposition time. This was examined by a cross-sectional scanning electron microscope. The ASLR-OSL on VO2 thin film was fabricated to prove that the proposed structure can work well as an electrically tunable metamaterial having a high quality factor. A gold electrode (200 nm) with a Ti adhesion layer (20 nm) was deposited on top of the VO2 thin film by the dc sputtering method. The designed metamaterials were patterned by using general photolithography and a lift-off process.
For the 1 THz band resonance of the metamaterial, the unit cell length (cell_w) was set to 60 μm, and the width and length (L) of the ASLR were set to 50 μm. Since the square loop is formed on the outer side of the metamaterial unit cell, its horizontal and vertical lengths are equal to the unit cell length. The line width (loop_w) of the square loop formed at the outer periphery of the unit cell is 2 μm, but the actual thickness is 4 μm since it is connected to the adjacent unit cell. The linewidth (line_w) and gap size (g) of the metal line forming the metamaterial were set to 5 μm for simple fabrication. The ASLR-OSL was fabricated and analyzed by changing the offset length from 0 μm to 17.5 μm. The offset length is the length between the center of the metamaterial unit cell and the center of the gap, and it controls the asymmetric ratio of the ASLR.
2.2 Terahertz transmission characteristics of an ASLR and ASLR-OSL
To verify the effect of an outer square loop added to the ASLR on the transmission characteristics of the ASLR, we compared the transmission characteristics of the ASLR-OSL with those of the ASLR. Figures 2 and 3 show a THz transmission comparison between the ASLR-OSL and ASLR according to the variation of the offset length. The ASLR-OSL and ASLR were simulated using an electromagnetic simulator HFSS, as shown in Figs. 2(a) and 2(c), respectively. These were measured by using the Advantest TAS 7400 THz time domain spectroscopy system, as shown in Figs. 2(b) and 2(d), respectively. The metamaterial has different transmission characteristics according to the polarization of the incident THz electric field. As shown in Fig. 2, mode 1 means the polarization of the incident electric field is perpendicular to the gap, and mode 2 means the polarization is parallel to the gap, as shown in Fig. 3.
Fig. 2 Comparison of THz transmittance of ASLR-OSL and ASLR operating in mode 1 according to variation in offset length: (a) simulated results of ASLR-OSL, (b) measured results of ASLR-OSL, (c) simulated results of ASLR, and (b) measured results of ASLR.
Fig. 3 Comparison of terahertz transmittance of ASLR-OSL and ASLR operating in mode 2 according to variation of offset length: (a) simulated results of ASLR-OSL, (b) measured results of ASLR-OSL, (c) simulated results of ASLR, and (b) measured results of ASLR.
In mode 1, the THz transmission characteristics of the ASLR-OSL are very similar to those of the ASLR, as shown in Fig. 2. The transmittance of the ASLR-OSL is slightly lower than that of the ASLR in the passband, and a new low-frequency resonance appears in the frequency band below 0.3 THz owing to the existence of the square loop. When the offset length is 0 μm, the ASLR and ASLR-OSL have symmetrical structures, so they are formed as basic split-loop resonators (SLRs) and have only eigen resonance, which has a wide bandwidth and low quality factor. As the offset length affecting the asymmetry increases, Fano resonance 1 and 2 appear in a lower frequency band than the eigen resonance frequency and in a higher frequency band than the eigen resonance frequency, respectively. The resonance frequency of the eigen resonance is shifted to a higher frequency band, and the resonance frequency of Fano resonance 1 and 2 is shifted to a lower frequency band. The square loop added to the ASLR resulted in a slight transmittance decrease in the passband and new low-frequency resonance characteristics. However, the ASLR-OSL effectively maintained the transmission characteristics of the ASLR in mode 1. In mode 2, the ASLR-OSL and ASLR with an offset length of 0 μm exhibit only eigen resonance that is equivalent to that in mode 1, and new Fano resonance occurs owing to the asymmetry of the structure as the offset length increases, as shown in Fig. 3. At an offset length of 5 μm, the ASLR-OSL shows both Fano resonance 1 and 2, while ASLR shows only Fano resonance 2 in the higher frequency band. At offset lengths greater than 5 μm, Fano resonance 2 of the ASLR and ASLR-OSL disappears as the resonance intensity sharply diminishes. Fano resonance 1, which appears only in the ASLR-OSL, is a newly emerged resonance owing to a combination of the square loop and the ASLR. As the offset length increases, the resonance frequency of Fano resonance 1 decreases, and the intensity of the resonance increases.
Figure 4 shows a comparison of the quality factors of the ASLR and ASLR-OSL with offset length variation. The quality factor of each resonance was calculated by the ratio of the average frequency of the peak and the antipeak to the frequency width between the two peaks [30]. The peak and antipeak indicate the points having the highest and lowest THz transmittances near each resonance, respectively. As the offset length increases, the bandwidth of the eigen resonance of the ASLR and ASLR-OSL operating in mode 1 decreases and the quality factor increases because of the influence of the newly generated Fano resonance. The ASLR and ASLR-OSL operating at Fano resonance 1 have maximum quality factors of 10.67 and 8.79 when the offset length is 5 μm, respectively, and the quality factor decreases as the offset length increases. The quality factor of the ASLR and ASLR-OSL operating at Fano resonance 2 is slightly different at an offset length of 10 μm, but converges to a similar value as the offset length increases, resulting in a maximum quality of 18.9 and 18.5, respectively, at an offset length of 17.5 μm. The ASLR-OSL operating in mode 1 has similar quality factor characteristics without degradation of the ASLR quality factor even though a square loop is added. In mode 2, the ASLR and ASLR-OSL operating at eigen resonance have slightly different quality factor values when the offset length is less than 5 μm, but they are almost similar at 10 μm or more. In addition, the ASLR and ASLR-OSL clearly show Fano resonance 2 when the offset length is 5 μm, and their quality factors were 29.5 and 29.3, respectively. Fano resonance 1, which appears only in the ASLR-OSL, has the highest quality factor of 10.50 when the offset length is 5 μm.
Fig. 4 Comparison of quality factor of ASLR and ASLR-OSL with offset length variation operating in (a) mode 1 and (b) mode 2.
Figure 5 shows the surface current density generated at each resonance of the ASLR and ASLR-OSL to analyze their resonance characteristics. The ASLR and ASLR-OSL in mode 1 exhibit the same type of dipole resonance characteristics in the eigen resonance state, as shown in Fig. 5(a). The surface current of the ASLR and ASLR-OSL in the eigen resonance state concentrate on the relatively small upper structure. Therefore, the resonance frequency of the eigen resonance is shifted to the higher frequency band as the offset length increases because the upper structure dominates the eigen resonance characteristics. The surface current density of Fano resonance 1 generated in the frequency band lower than the eigen resonance frequency exhibits trap mode resonance characteristics in both the ASLR and ASLR-OSL. The surface current density of Fano resonance 2 generated in the frequency band higher than the eigen resonance frequency shows quadrupole resonance characteristics in both the ASLR and ASLR-OSL. The surface currents are concentrated on the relatively large lower structure in both Fano 1 and 2 resonances, and the resonance frequencies are shifted to the lower resonance frequency band as the length of the lower structure increases according to the increase in the offset length. The surface currents of the ASLR and ASLR-OSL operating in mode 2 are shown in Fig. 5(b). The ASLR and ASLR-OSL in mode 2 eigen resonance states have the same dipole resonance characteristics. The resonance frequency of the eigen resonance is shifted to the lower frequency band as the offset length is increased owing to the concentration of the surface current in the relatively large upper structure.
Fig. 5 Surface current density of ASLR and ASLR-OSL at each resonance operated in (a) mode 1 and (b) mode 2.
The surface currents of the ASLR and ASLR-OSL at Fano resonance 2 generated in a frequency band higher than the eigen resonance frequency have the same quadrupole resonance characteristics. Fano resonance 1 of the ASLR-OSL, which is not generated in the ASLR, exhibits quadrupole resonance characteristics. As the offset length increases, the resonance frequency shifts to the lower frequency band because the surface current is concentrated in the relatively large lower structure. In both modes 1 and 2, the ASLR-OSL has a surface current density and resonance characteristics that are similar to those of the ASLR because the surface current in the outer square loop of the ASLR-OSL is much smaller than that of the ASLR.
3. Electrical active-control characteristics of the ASLR-OSL
3.1 Electrical control using a square loop of an ASLR-OSL
The transmission characteristics of a VO2 thin-film-based ASLR-OSL can be electrically controlled by adjusting the characteristics of the VO2 thin film. The VO2 thin film deposited on the substrate for the active control of the ASLR-OSL has an insulator-metal phase transition at a temperature of 340 K, and the conductivity of the VO2 thin film is changed during the phase transition. Therefore, the ASLR-OSL can control the characteristics of the THz transmission by thermal changes in the VO2 thin film caused by applying the voltage directly to the square loop. Figure 6 shows the photograph of the experimental setup and configuration of a bias engagement for the electrical active-control of the ASLR-OSL. The current induced by the applied voltage flows uniformly in the square loop along the bias line and generates heat owing to the joule heating effect. Therefore, it is possible to control the THz transmission characteristics by changing the material properties of the VO2 thin film deposited on the metamaterial substrate.
Fig. 6 (a) Photograph of the experimental setup and (b) configuration of a bias engagement for the electrical active-control of the ASLR-OSL
Figure 7 shows the transmission characteristics of the ASLR-OSL at an offset length of 12.5 μm, in which eigen resonance and two Fano resonances are clearly visible, to verify the active characteristics of the ASLR-OSL by directly applying bias voltage to the outer square loop. Figure 7(a) shows the transmittance of the ASLR-OSL operating in mode 1 by increasing the bias voltage from 0.0 V to 5.5 V. As the applied bias voltage increases, the conductivity of the VO2 thin film becomes higher, so the eigen resonance frequency of the ASLR-OSL is shifted to the lower frequency band and the strength of the resonance becomes weaker. At voltages above 4.7 V, the resonance becomes very weak and the resonance almost disappears. In the Fano resonance 1 and Fano resonance 2, the transmission characteristics of the THz wave owing to the increase of the applied voltage of the ASLR-OSL are similar to those in the eigen resonance. The transmission characteristics of the THz wave in the passband of 0.7 THz band formed between the Fano resonance 1 and the eigen resonance are as follows: the pass-band shifts to the lower frequency band as the applied bias voltage increases, and the THz wave transmittance decreases from 43.16% to 2.41%. This is because as the applied bias voltage increases, the conductivity of the VO2 thin film becomes higher, so that the effective permittivity of the substrate on which the metamaterial is formed becomes higher. Therefore, the resonance intensity of the metamaterial is weakened and the resonance frequency is shifted to the low-frequency band. Figure 7(b) shows the transmittance of the ASLR-OSL operating in mode 1 by decreasing the bias voltage from 5.5 V to 0.0 V. When the voltage applied to the ASLR-OSL is reduced, the resonance characteristics of the ASLR-OSL are gradually recovered, and the original transmission characteristics of the ASLR-OSL are completely restored at 0.0 V. Figures 7(c) and 7(d) shows the transmittance of the ASLR-OSL operating in mode 2 when the bias voltage is increased from 0.0 V to 4.0 V and decreased from 4.0 V to 0.0 V, respectively. As the applied bias voltage increases, the resonance frequency of the ASLR-OSL moves down to the lower frequency band, as in mode 1. The transmittance of THz waves decreases from 30.86% to 4.23% at 1.1 THz as the bias voltage increases. When the voltage applied to the ASLR-OSL is decreased, the resonance characteristics of the ASLR-OSL are also completely restored at 0.0 V in mode 2, as in mode 1.
Fig. 7 Transmittance of ASLR-OSL at offset length of 12.5 μm operated (a) in mode 1 by increasing bias voltage, (b) in mode 1 by decreasing bias voltage, (c) in mode 2 by increasing bias voltage, and (d) in mode 2 by decreasing bias voltage.
3.2 Hysteresis of the VO2 thin film
Figure 8 shows the change in THz transmittance of the ASLR-OSL operating in modes 1 and 2 at the passband according to the variation in the applied voltage. The transmittances of the ASLR-OSL in the mode 1 and 2 passbands were measured at 0.7 THz and 1.1 THz, respectively. The THz transmittance characteristics of the ASLR-OSL based on VO2 thin film according to the variation in the bias voltage show hysteresis characteristics that are the same as the dc characteristics of the VO2 thin film. The inset in Fig. 8 shows an enlargement of the phase transition range of the transmittance. An ASLR-OSL operating in mode 1 shows hysteresis characteristics; the insulator-to-metal and metal-to-insulator phase transition of the VO2 thin film occurred between 4.1 V and 4.8 V and between 4.5 V and 3.8 V, respectively. The ASLR-OSL operating in mode 2 exhibits the same hysteresis characteristics as mode 1, and insulator-to-metal and metal-to-insulator phase transitions of VO2 were observed between 3.3 V–3.9 V and between 3.8 V–3.15 V, respectively. The resonant characteristics of the ASLR-OSL operating in mode 1 and 2 including the surface currents are very different so that the phase transition voltages of the hysteresis in two modes are different from each other.
Fig. 8 Hysteresis of THz transmittance of ASLR-OSL operating in modes 1 and 2 at passband according to variation in applied voltage. Inset: An enlargement of the phase transition range of the transmittance.
4. Conclusion
This paper proposed an asymmetric split-loop resonator with an outer square loop (ASLR-OSL) based on vanadium dioxide (VO2) thin film, which can actively control the transmission properties of a terahertz (THz) wave while maintaining a high quality factor of the asymmetric split-loop resonator (ASLR). The outer square loop was combined with the ASLR to be used as a microheater capable of controlling the temperature of the VO2 thin film through a directly applied bias voltage. Therefore, the transmission characteristics of the ASLR-OSL based on VO2 thin film were successfully controlled by directly applying a bias voltage. In addition, the ASLR-OSL could well maintain a high quality factor of the ASLR. The transmittance of the ASLR-OSL based on VO2 thin film was changed from 43.16% to 2.41% in mode 1 (0.7 THz) and 30.86% to 4.23% in mode 2 (1.1 THz). Based on these results, it is possible to impose active properties on a common metamaterial having a high-quality factor by adding a simple loop structure that works as a microheater.
Funding
Basic Science Research Program through the National Research Foundation (NRF) funded by the Ministry of Education (grant no. NRF-2016R1D1A1B03936140).
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