1. Introduction

The field of terahertz (THz) science and technology has grown dramatically in recent years for its potential applications in security checking, wireless communication, nondestructive testing, imaging, and other areas [1–6]. Utilizing THz waves as a means to transmit data has attracted a lot of attention as one of the most important points of interest [7–9]. Recently, the demonstration of ultrafast tunable external spatial THz functional devices has shown great potential for applications in THz data transmission [10–17]. As an artificial designed electromagnetic (EM) structure, a metamaterial that can be engineered to exhibit exotic electric and magnetic properties provides a sufficient way to realize the tunable THz functional device. Great effort has been made and important progress achieved in this aspect [18–22]. However, as the key technique, tunability that promises to further the applicability of metamaterials in external spatial THz manipulations is still to be resolved. A metamaterial/functional-materials hybrid structure seems to be one promising approach. It is well known that, as a tunable functional material, vanadium dioxide (VO₂), which is a reversible phase transition material due to its insulator-metal transition at a critical temperature near 68°C, poses many concerns. The phase transition of VO₂ could be triggered by temperature, electric field, or light. There are some published studies on these topics such as in [23] where the authors proposed the memory metamaterial based on VO₂, or in [24–26] where the authors presented the study on electric and THz-field-induced phase transition in VO₂, or in [27–30] where the authors demonstrated the photoinduced metallic state in VO₂. It is suggested that external spatial control of a THz wave by utilizing the photoinduced characteristics of VO₂ could be an efficient way to construct a THz active device. We noticed that the sol-gel method serves to adjust the microstructure and composition of VO₂, which could lead to a giant phase transition property in THz range [31]. Therefore, in this paper, a photoinduced external spatial THz active device based on dual-resonance metamaterials combined with VO₂ film deposited by sol-gel method was presented.

2. Structure and simulation

The dual-resonance metamaterial-VO₂ film hybrid structure (DMVS) which is composed of three parts at different layers, is suggested as shown in Fig. 1. The first layer is silicon layer, which is made of high-purity single-crystal silicon with a 0.5 mm depth and acts as a substrate. The second is a film layer, wherein 220 nm thick VO₂ films were deposited on Si (100) substrates (~2000 Ω·cm resistivity) by a dip-coating process, followed by annealing at 500°C for 1.5 h in a static atmosphere of nitrogen, in order to further crystallization and reduce the V₂O₅ phase to be a VO₂ phase. The relevant parameters and material properties of VO₂ films are shown in [32]. The surface of VO₂ film is shown in Fig. 1(b). The film layer realizes the dynamic modulation function. The third layer is the metamaterial structure layer where a 500 nm aluminum film is deposited on the VO₂ film and then patterned into the designed shape by photo etching. This layer is used as a frequency-selective surface that controls the frequency band pass and band stop to make sure the THz wave modulation is at a specific frequency band [33]. Additionally, for the VO₂ film, the interface material heat conductivity and the thickness are very important for the recovery time of the phase-transition [34, 35]. So beneficial to heat loss, this metal metamaterial layer can act as a cooling fin to reduce the negative effects on temperature rising on the surface in the dynamic test. In this device, a symmetric dual-resonance unit is applied to realize a well-transmission frequency band between two strong absorptive frequencies. The photographs of a unit cell and a portion of the third layer are shown in Figs. 1(c) and 1(d). Then, computer simulation, static experiment, and a dynamic test have been carried out to study the characteristics of DMVS.

 

Fig. 1 The structure of DMVS. (a) The 3-D model of one unit. (b) The surface of VO₂ film. (c) The manufactured unit. (d) The manufactured array.

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First, the EM characteristics of the structure have been studied by applying computer simulation with the 3D EM field commercial simulation software CST. In the simulation, a THz wave with line polarization has been applied to project perpendicularly on the structure. The direction of the polarization is vertical to the gap of the metamaterial unit. In this paper, this device is aimed at a 0.34 THz working frequency, which is a very important window in the THz band, and the parameters are shown as below (Table 1). The simulation results are shown in Figs. 2 and 3, which demonstrate that the surface current stimulated by the incident wave in the structure has different characteristic distributions corresponding to different resonant frequencies. It is clear that at around 0.53 THz the surface current flows mainly through the center circular loop [Fig. 2(d)], and at 0.21 THz the surface current flows through the outermost open circular loop [Fig. 2(b)]. The field contour map shows that at the two resonant frequencies, the field intensity concentrates on the center gap and outer gap, respectively [Figs. 2(a) and 2(c)], which leads to the energy absorbance at these frequencies when the incident wave transmits the structure. More importantly, it should be noted that between the two resonant frequencies there is a high transmission pass band at around 0.34 THz, as shown in Fig. 3. Next, the experimental study of statics characterizing the performance of the structure is performed using THz time-domain spectroscopy (THz-TDS) system TPS3000, which is produced by Teraview Corporation. It is found that the experimental result (black solid line in Fig. 3) agrees very well with that of simulation (red dot line in Fig. 3). Moreover, in Fig. 3 it is clear that the transmittance is relative high at 0.28-0.36 THz, and at 0.34 THz the transmission can reach 85%.

Table 1. Parameters of the DMVS

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Fig. 2 Computer simulation results of DMVS structure. (a) The contour map at 0.21THz. (b) The surface current at 0.21THz. (c) The contour map at 0.53THz. (d) The surface current at 0.53THz.

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Fig. 3 Simulation and TDS experiment results of transmittance depends on frequency of this structure.

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3. Experiment result and discussion

In this part, the statics experiment with a TDS system and a single frequency source was carried out, respectively. In the experiment, we use an 808 nm external cw laser as the induced source for the DMVS; meanwhile, the sample was placed on the middle of a THz beam with the laser pump beam at 500 mm to the surface, as shown in Fig. 4. The diameter of the induced laser is about 5 mm, which is a little less than the THz beam.

 

Fig. 4 The 3D-sketch map of the experiment with pump laser.

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Figure 5 shows the experimental results obtained by applying the TDS system, Fig. 5(a) shows the time domain waveforms of the transmitted THz wave, and Fig. 5(b) shows the transmittance with frequency dependence. It can be found that the amplitude of the THz wave decreased with the laser power increase. Figure 5(b) demonstrates the transmittance change of the DMVS (solid lines) and bare VO₂ film (dotted lines) induced by the external pump laser. Since the VO₂ film exhibits a compact nanostructure with grain sizes ranging from 30 to 70 nm, the THz switching ratio of bare VO₂ film is more than 80% during the phase transition with the incident laser shown as dotted lines in Fig. 5(b). It also shows that for the DMVS, the dual-resonance metamaterial layer plays a very important role in the well-transmission [Au: What does ‘well-transmission’ mean here?]frequency band control, and the effect is very good. It can be found that around the specific frequency band of 0.28–0.36 THz, the transmission is close to that of bare VO₂ film. However the other frequency components decrease swiftly and the transmission is much lower than that of bare VO₂ film. Furthermore, in the well-transmission[Au: The term is used here also.] frequency band of 0.28–0.36 THz, the modulation depth is the highest with the external incident laser, so we call it the modulation region. These results demonstrate that at around 0.34 THz the DMVS could work very well. Next, the statics experiment with a single frequency source is carried out. This test system is made up of a 0.34 THz source and a detector. The results are shown in Fig. 5(c). It can be found that these experimental results have the same regularity with a TDS system and can also be verified at 0.34 THz this structure has high transmission and deep modulation. Moreover, the power of the incident laser could heat up the surface of the structure, so in the case of the temperature reaching the critical temperature the phase transition of VO₂ film triggered by temperature takes place as the transmission curve with the 2.8W incident laser power shown in Fig. 5(b). It is clear that although the amplitude of the THz wave decreases with the external laser power increase, there is no significant change of the shape of the wave observed when the condition of a temperature-induced phase transition is not fulfilled. However, when the phase transition takes place, the shape of the wave is completely changed. Therefore, it is found that there are two components of THz wave transmittance change induced by the external pump laser. One is that the transmittance decreases rapidly, which is caused by photoexcitation, and the other is a relatively slow one that is caused by the temperature-induced phase transition.

 

Fig. 5 The external 808 nm laser pump static experimental results with TDS system and single frequency source. (a) The time domain waveforms of transmitted THz wave tested by TDS system with different incident laser power. (b) The FFT of time domain waveforms tested by TDS. (c) The transmittance tested by 0.34THz single frequency source with different incident laser power.

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In order to further study the modulation characteristics of the DMVS, we have tested the resistivity change induced by temperature and an external laser as shown in Fig. 6. From Fig. 6 we can find that the resistivity modulation of the DMVS is nearly 4 orders of magnitude when cycled through the phase transition, which is due to a polycrystalline nature with high crystallinity and compact nanostructure of the VO₂ film used in the DMVS. From our study, the concentration of +4 valence vanadium of the film is 79.85%. So a giant transmission modulation ratio of about 81% was observed by THz-TDS in Fig. 5. Moreover, it should be noted that for the temperature-induced phase transition, the conductivity changed sharply around the critical temperature. In the laser-induced case, the process of conductivity change is relatively flat. These results agree well with those of the TDS test and single frequency source experiment and also verified there are two components of THz wave transmittance change induced by the external pump laser.

 

Fig. 6 Resistivity curve induced by temperature and external laser. (a) Hysteresis loop of the resistivity against temperature for the VO₂ film across the phase transition. (b) Resistivity changing with external laser power.

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Next, the dynamic experiment is carried out to test the real-time modulation depth and speed of the DMVS as shown in Fig. 7. The experimental system is composed of five key components: a cw THz source, the DMVS, the external induced laser, the acoustic optical modulator, and the detector. In the first step of the experiment, a 0.34 THz source made up of a 42 GHz amplifier and a frequency multiplier chain has been applied to generate a 0.34 THz carrier wave with nearly linear[Au: Is ‘linear’ correct here?] polarization. In the second step, the acoustic optical modulator (AOM), which has been driven by the voltage amplitude signal generated from an analogue function generator (AFG), is applied to modulate the 808 nm laser. In the third step, the amplitude-modulated 808 nm laser with about 150 mW of power is obliquely projected on the DMVS to induce the device to realize the real-time dynamic control of the THz carrier wave transmittance. Meanwhile the 0.34 THz wave transmits perpendicular to the DMVS, and then the signal is loaded on the carrier wave. In the last step, a fast-response detector is utilized to detect the 0.34 THz carrier wave with the amplitude modulation signal. The dynamic experimental results in Fig. 8 show the modulation signals of different frequencies detected by the detector displayed in the oscilloscope, applying the rectangular voltage signal with a very short-duty ratio. It is clear that up to a 1 MHz modulation signal was detected without any low noise amplifier or lock-in amplifier. Moreover, the repetition frequency of the AOM used in this experiment only can reach 1 MHz, so we believe this DMVS could realize faster modulation speed.

 

Fig. 7 The dynamic experiment setup.

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Fig. 8 Signal detected by the detector.

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Furthermore, in Fig. 8, it should be noted that the modulation frequency is limited by two factors: transition time and recovery time. It is clear that when the laser is induced on the surface, the THz transmittance is immediately changed from point A to point B, and when the laser is off the transmittance is slowly recovering from point B to point C, as shown in the first figure in Fig. 8. It means that the phase transition speed of VO₂ film is very fast with photoexcitation but the recovery time is relatively long. So if the recovery time is larger than the period of the voltage signal, when we increase the frequency of the modulation voltage signal the modulation depth will decrease. That is why at 50 kHz the amplitude of the modulation signal detected by the detector shown in the oscilloscope is one order larger than that of 1 MHz. This transition recovery time is largely caused by the heat dissipation and relaxation time of the photon-generated carrier in the VO₂ film. Reference [35] shows the relation between recovery time and thermal conductivity of different materials. It will be possible to achieve higher modulation speed by reducing the heat dissipation of the DMVS.

4. Conclusion

In conclusion, we have demonstrated a photo-induced external spatial THz wave active device with a combination of vanadium dioxide film and dual-resonance metamaterials aimed at 0.34 THz, which is the first atmosphere window. Theoretical and statical experimental studies show that there is a well-transmission[Au: The term is used here also.] pass band at around 0.28 THz–0.37 THz of the structure, and with an external induced laser the DMVS can realize an 80% modulated depth. The dynamic experimental results show that a 1 MHz modulation signal has been observed, which verified that this modulator has a relative ultra-fast response induced by the laser. The electromagnetic properties and photoinduced dynamic characteristic of this modulator will see significant advancement leading to the research of THz modulators, intelligent switches, sensors, and other items of interest.