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Abstract
Two terahertz metamaterials were joined by a conductivity variable VO2 patch to obtain a metamaterial dimer. By applying voltage or heat to the VO2 patches, active modulation of terahertz wave could be achieved. A cut-wire metamaterial was placed adjacent to the VO2 joined dimer to affect its electromagnetic response. It was found that the cut wire could heavily impact the resonance mode of the VO2 joined dimer, which gives dual resonance dips in transmission spectrum for both insulating and conducting states of VO2 patches. As a result, by tuning the conductivity of VO2, active dual band phase modulation could be achieved with high transmission window by this dimer-cut wire coupling system.
© 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Artificial metamaterials can overcome the drawbacks of natural materials to have strong interactions with terahertz (THz) wave [1], which significantly promoted the development of THz science and technology in recent decades [2,3]. Due to their strong interaction with THz wave, THz metamaterials have been widely used to make critical electronic components such as amplitude, phase and chirality modulators [4–6] for THz imaging [7,8], absorbing [9], sensing [10,11] and high-speed communication [12]. The widely used methods to tune the electromagnetic response of terahertz metamaterials are through tuning the terahertz metamaterial itself [13] and/or controlling their mutual interactions [14–16]. As a result, delicate design of metamaterial unit and controlling their mutual interaction are two efficient pathways to obtain metamaterial-based terahertz devices with high performance and multi-functionalities [17].
For THz metamaterials design, some exotic structures of atomic molecules and nanostructures are often adopted in microscale structures to obtain analogous response to terahertz wave. For example, an atom/nanoparticle dimer bridged by a conductor junction could show very different optical properties when the junction varies its electrical conductivities [18]. By mimicking the conductor bridged dimer, subwavelength double ring dimers were studied in terahertz range, and their resonance mode transition due to the junction was also studied in Zhang’s group [19]. Similar to their nanostructure counterpart, different resonance modes were observed when the bridge junction have different conductivity. When the junction is absent or insulating, electrons oscillate within each monomer metamaterial to generate transient electrical dipoles, and monomer metamaterials are coupled head-to-tail by transient dipolar interaction, resulting in bonding dimer plasmonic (BDP) resonance mode. When the junction is very conductive, there exist two resonance modes, namely, charge transfer plasmonic (CTP) and screened bonding dimer plasmonic (SBDP) resonance modes. For CTP resonance mode, electrons transfer between monomer metamaterials through the conductive junction when driven by the electric field of terahertz wave, resulting in transient separation of positive and negative charges in the dimer. For CTP mode, electrons can mobilize throughout the dimers in contrast to that for BDP mode, which result in lower resonance frequency of CTP than BDP mode. For SBDP mode, transient dipolar interaction also forms between monomer metamaterials as in BDP mode. However, the conductive junction reduces the concentration of transient charge and electrical field at the gap between monomer metamaterials. Namely, the dipolar interaction is screened by the conductive junction. As result, by tuning the conductivity of the junction, the resonance of the dimer can be actively tuned to achieve active modulation of terahertz wave.
As pointed out above, the electromagnetic response of the THz metamaterials could be tuned by manipulating their mutual interaction. As a result, it is our curiosity to know how adjacent metamaterials could affect the resonance mode transition of the joined THz metamaterial dimers. Here a pair of C-shaped split ring THz metamaterials was joined by a patch of VO2 thin film, which was placed adjacent to a metallic cut wire (CW). Due to the metal-to-insulator transition of VO2 thin film [20], VO2 thin films have been widely used for active THz devices [21,22]. During the metal-to-insulator transition of VO2 thin film, the resonance modes of the metamaterial dimer could be strongly affected by the adjacent CW. By applying voltage or heat to the VO2 thin film patches, THz wave could be dynamically modulated by this exotic metamaterial system. By studying this system, not only novel phenomenon could be observed, but also new terahertz modulation method could be developed.
2. Methods
Figure 1 (a) shows the modulation of THz wave by the dimer-CW arrays, whose structure and dimension are shown in Fig. 1(b). The unit cell of the array is composed of a CW and a C-shaped metamaterial dimer joined by a VO2 thin film patch as shown in Fig. 1(b). To implement voltage control of the VO2 thin film patches, each end of the dimers was connected to an interdigitated electrode as show in Fig. 1 (b). As a result, here each monomer metamaterial is composed of a C-shaped split ring and a side electrode. The electromagnetic response was simulated by CST Microwave Studio software to tune the resonance frequency. The incident THz wave shines normally on the sample with electric field component along the CW as shown in Fig. 1(a). In simulation, unit cell boundary condition was used to simulate two dimensional infinite periodic arrays. The sapphire substrate (500 µm) was treated as a lossless dielectric material with ${\varepsilon _{sap}} = 9.4$, and gold as a lossy metal with conductivity of ${\mathrm{\sigma }_{Au}} = 4.561 \times {10^7}\; S/m$. According to simulation, the dimension of each part in Fig. 1(b) is determined as such ${P_x} = 90\; \mu m\textrm{, }{P_y} = 220\; \mu m\textrm{, }g = 60\; \mu m\textrm{, }w = 40\; \mu m\textrm{, }n = 80\; \mu m$.
Fig. 1. (a) modulation of THz wave by the dimer-CW arrays, and (b) the geometric structure of the dimer-CW unit cell.
The designed metamaterial arrays were fabricated by microfabrication method as shown in Fig. 2(a). Firstly, high quality VO2 thin film (∼125 nm) on sapphire substrate was fabricated by polymer assisted deposition method [23]. The cleaned VO2 thin films were spin coated by positive photoresist AZ6112, which was patterned by standard photolithography method. Using patterned photoresist as mask, VO2 thin film patch arrays were fabricated by deep reactive ion etching (DRIE) on the sapphire substrate with dimension of $\mathrm{16\;\mu m \; x \ 30\;\ \mu m}$. The power of the DRIE is 100 W using SF6 as reactive gas. After removing residual AZ6112 photoresist, negative photoresist AZ5214 was coated on the substrate to fabricate the metallic parts of the metamaterials. After aligned the mask to the VO2 thin film patch, the photoresist was patterned with undercut by photolithography. Magnetron sputtering was used to deposit a gold layer with thickness of 220 nm on the patterned photoresist. After that, residual photoresist was then washed away by acetone to finalize the fabricate of the sample. Corresponding dimer sample was also fabricated to compare with the CW-dimer coupling metamaterials.
Fig. 2. (a) the microfabrication process of the THz metamaterials, (b) the optical microscope image of the CW-dimer coupling metamaterials, and (c) the image of the whole mounted sample.
The fabricated samples were characterized by standard optical fiber coupled THz time domain spectroscopy (TDS) in dry air environment. When measuring, the electrical field of the THz pulse was aligned along the longitudinal direction of the dimer as shown in Fig. 1(a).
3. Results and discussion
The representative optical microscope image of the fabricated metamaterials was shown in Fig. 2(b). The fabricated sample was mounted on a hollowed-out PCB board for modulation test as shown in Fig. 2(c). The transmission spectrum at insulating state of VO2 was simulated by CST software using the S21 parameters. When the VO2 patches are insulating, the CW-dimer coupling metamaterials show two resonance dips at 0.572 THz and 0.79 THz, which coincide with the resonance dips of the corresponding dimer and the CW respectively as shown in Fig. 3(a).
Fig. 3. The simulation results when VO2 patches are insulating: (a) the transmittance of THz wave for CW, dimer and CW-dimer coupling metamaterials, (b) shows the electric field distribution of the dimer at 0.574 THz, (c), (d) and (e) show the electric field distribution of the CW-dimer coupling metamaterial at 0.572 THz, 0.726 THz and 0.79 THz respectively.
At resonance dip of 0.572 THz, the dimer in the coupling system show very similar electric field distribution to that of the dimer alone as compared between Fig. 3(b) and (c), and the CW in the coupling system show trivial oscillation at this frequency. Here each C-ring and the electrode together show dipolar plasmon oscillation along the vertical direction, which is head-to-tail coupled to another side of C-ring and electrode by dipolar interaction. The electrical field distribution of the dimer in the coupling system show obvious characteristics of BDP mode, same as the dimer alone. At 0.79 THz, the CW in the coupling system show vigorous resonance. The strong electric field at the end of the CW induce a minute quadrupole oscillation in each monomer metamaterial as shown in Fig. 3(e) [24]. As a result, the resonance dip at 0.79 THz of the coupling system can be attributed to the major dipolar resonance of the CW. The slight resonance frequency shift with respect to the CW alone could be attributed to its coupling to the dimer. At 0.726 THz, the transient polarization of the CW is opposite to the left arm of both monomer metamaterials as shown in Fig. 3(d), which induced opposite surface current of CW to the monomer metamaterials. This destructive interference between the CW and dimer produced metamaterial analogue of electromagnetically induced transparency with transmission maximum at 0.726 THz as shown in Fig. 3(a).
When the VO2 patches were transformed from insulating to conducting state by thermal or electrical stimuli, the electromagnetic response of the metamaterial arrays will also be alternated. Figure 4 (a) shows the simulation of the transition of the resonance modes of the dimer arrays. When VO2 patches were at insulating state with conductivities of 200 S/m, only one resonance dip was observed at position A (0.574 THz) as marked in the Fig. 4(a), which is corresponding to the BDP resonance mode as discussed above. When the VO2 patches was at conducting state with conductivity of 2×105 S/m, two resonance dips were obtained, namely a strong resonance at position B (0.414 THz) and a minor resonance at position C (0.796 THz) respectively. The electrical field distribution of the dimer is given in Fig. 4(c). For resonance mode B, it can be seen that free electrons were transferred from one C-ring to another C-ring through the conducting VO2 bridge, and the dimers behave like a large electrical dipole. As a result, the resonance mode of the dimer at position B is corresponding to the CTP resonance mode. For the resonance mode C, the electrical field distribution is similar to that of the BDP mode, however, at the conducting state of the VO2 bridge. Namely, the resonance mode at position C is corresponding to the SBDP mode. The suppression of the SBDP mode in the dimer could be attributed to the large size of the conductive VO2 junction, which significantly reduces the joint capacitance between the two C-rings.
Fig. 4. The simulation results when VO2 patches transform from insulating to conducting: the transmittance of THz wave of (a) dimer alone and (b) CW-dimer coupling system at different conductivity of VO2, (c) shows the electric field distribution of the dimer at frequencies marked in (a), (d) shows the electric field distribution of the coupling system at frequencies marked in (b).
Figure 4(b) shows the simulated transmission spectrum of the CW-dimer coupling system. When VO2 junction is at insulating state with conductivity of 200 S/m, two resonance dips were observed as discussed above at 0.572 THz and 0.79 THz, which are mainly attributed to the BDP resonance mode of the dimer and dipolar resonance of the CW. When the VO2 junction was at conductive state with conductivity of 2×105 S/m, the prior two resonance modes disappear at 0.572 THz and 0.79 THz. Instead, two new resonance dips appear at 0.405 THz and 0.686 THz respectively. Figure 4(d) shows the electrical field distribution of the coupling system at resonance dips at corresponding position in Fig. 4(b). For resonance mode at lower frequency of 0.405 THz, CTP resonance mode of the dimer was also obtained same as the dimer alone. However, the transient dipole of the CW is anti-parallel coupled to that of the dimer. As a result, the CTP mode give lower resonance frequency [25] of 0.405 THz in contrast to 0.414 THz of the dimer alone.
Striking results were observed at position C (0.686 THz). The electrical field distribution indicates the resonance mode at position C is corresponding to the SBDP mode of the dimer. However, at the SBDP resonance frequency, the CW also show strong transient dipole, which even induces strong electrical field distribution at the left arm of each C-rings. Because of that, more transient electrical charges were accumulated near the conductive junction, which is equivalent to increased joint capacitance of the dimer. As a result, strong SBDP resonance mode were observed in the coupling system, which is in contrast to the previously suppressed SBDP mode in the dimer alone. Further, due to the strong antiparallel coupling between the CW and the dimer, the resonance frequency is significantly reduced from 0.796 THz of the dimer alone to 0.686 THz for the coupling system.
To verify the simulation, both dimer and CW-dimer coupling metamaterials were fabricated by micro-fabrication method. And these samples were characterized by the all-optical fiber TDS spectroscopy. The transmitted THz pulses in time domain were transformed to frequency domain by fast Fourier transformation. Using air as reference, the transmission spectrum of these sample was calculated and shown in Fig. 5. The conductivity of VO2 junction can be varied by both thermal heating and electrical voltage applied to the electrode [20]. The phase transition upon electrical voltage is indicated by a sharp increase of electrical current as shown in the supplemental information in Figure S1. The transition voltage is ∼8 V and ∼6V for the dimer and CW-dimer coupling metamaterial respectively. The heating induced phase transition of VO2 thin film was determined by the resistance measured at different temperature as shown in Figure S2 in the supplemental information. The phase transition of VO2 thin film occurs at ∼336 K upon heating.
Fig. 5. The experimental transition of the transmission spectrum for (a) & (b) dimer alone and (c) & (d) the CW-dimer coupling system. (a) &(c) show the transition excited by heating, and (b) & (d) show the transition excited by electrical voltage.
Upon phase transition of the VO2 junction, the transition of transmission spectra was observed for both thermal and electrical excitation as shown in Fig. 5. When it is at room temperature (Fig. 5(a)) or no external voltage bias is applied (Fig. 5(b)), one resonance dip was observed at ∼0.55 THz for the dimer control sample, which is due to the BDP resonance mode of the dimer. As temperature is above 343 K, the resonance peak starts gradual shift from 0.55 THz to 0.42 THz (at 363 K). The resonance dip at 0.42 THz is owing to the CTP resonance mode of the dimer as discussed above. The resonance frequency of the BDP and CTP mode matches well with simulation. The poor quality of CTP resonance dip of the dimer could be due to the poor conductivity of the metallic VO2 patches compared to metal. Similar shift was also observed upon voltage excitation for the dimer control sample as shown in Fig. 5 (b). Further, there is no SBDP mode was observed at ∼0.8 THz for the dimer at metallic state of VO2 patches, which could be due to its own trivial effect or due to the inferior conductivity of the metallic VO2.
When CW-dimer coupling metamaterials is at room temperature or under no external electrical field, there are two obvious resonance dip at 0.56 THz and 0.77 THz, which is attributed to the BDP mode of the dimer and the dipolar resonance of the CW respectively. When VO2 patches were completely transformed to metallic state at 363 K, two obvious resonance mode was observed at 0.39 THz and 0.716 THz, which are very close to the simulated CTP resonance mode and SBDP resonance mode of the dimer in the coupling system. Compared to the mute SBDP mode for the dimer control sample, obvious SBDP resonance mode was observed for the CW-dimer coupling sample, as predicted by the simulation results. Similar transition of the transmission spectrum was also observed when voltage bias is applied to the sample. The obvious transition starts at voltage above 5.8 V for this sample.
THz amplitude modulation method have been developing quickly in last decades [26,27]. However, solution of THz phase modulation is still lacking, especially for special applications [28,29]. According to above discussion, dimer control samples have one obvious resonance peak at both insulating and metallic state of VO2 patches respectively, and the two resonance peaks shift ∼0.15 THz with respect to each other. The shifted resonance frequency warrants a high transmission window for phase modulation [29,30], which is marked by the yellow area in Fig. 6. For the dimer, the simulated transmission is ∼40% at 0.52 THz with phase modulation of ∼101°, which is 61° for experimental result. The inferior experimental result could be due to the poor conductivity of metallic VO2 patches compared to metal.
Fig. 6. The phase modulation of THz wave by the dimer and CW-dimer sample respectively. (a) & (d) show the transmission window of the phase modulation by the yellow range, (b) & (e) show the simulated phase modulation, and (c) & (f) show the experimental result of phase modulation.
The CW-dimer coupling sample have two obvious resonance dips at both insulating and metallic state of VO2 patches. More importantly, the dual resonance frequency shift with each other during the metal-to-insulator transition of VO2 patches, as shown in Fig. 6 (d). As a result, the CW-dimer coupling sample could be very useful for dual band phase modulation. For this sample, there are two high transmission windows for phase modulation, which is at ∼0.5 THz and ∼0.64 THz respectively. According to simulation, phase modulation of 103° and 55° were obtained at the two transmission windows as shown in Fig. 6 (e), and the experimental result is shown in Fig. 6(f). The quality of phase modulation could be improved by replacing VO2 patches with other electronic structures such as high electron mobility transistor (HEMT) [31] to enhance the modulation depth and speed.
4. Conclusions
The coupling effect of VO2 joined metamaterial dimer and CW was investigated for thermal and electrical modulation of THz wave. By varying the conductivity of VO2 junction, the resonance mode of the coupling system can be actively tuned. Compared to the dimer control sample, the CW could heavily enhance the SBDP mode of the dimer in the coupling sample when VO2 is metallic. Dual resonance dips were obviously obtained for both insulating and conductive VO2, which have large relative frequency shift. As a result, the CW-dimer coupling system could be very useful for dual band phase modulation of THz wave. The phase modulation could be further improved using advanced electronic structures instead of VO2 thin films.
Funding
National Key Research and Development Program of China (2018YFE0115500); National Natural Science Foundation of China (51772045, 61734002); National Key Scientific Instrument and Equipment Development Projects of China (51827802); Sichuan Province Science and Technology Support Program (2021JDTD0026).
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
Supplemental document
See Supplement 1 for supporting content.
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