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Dynamic Photo-induced Controlling of the Large Phase Shift of Terahertz Waves via Vanadium Dioxide Coupling Nanostructures Yuncheng Zhao, Yaxin Zhang, Qiwu Shi, Shixiong Liang, Wanxia Huang, Wei Kou, and Ziqiang Yang ACS Photonics, Just Accepted Manuscript • DOI: 10.1021/acsphotonics.8b00276 • Publication Date (Web): 12 Jul 2018 Downloaded from http://pubs.acs.org on July 16, 2018

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ACS photonics Article

Dynamic Photo-induced Controlling of the Large Phase Shift of Terahertz Waves via Vanadium Dioxide Coupling Nanostructures Yuncheng Zhao1, Yaxin Zhang1*, Qiwu Shi2, Shixiong Liang3*, Wanxia Huang2, WeiKou1,and Ziqiang Yang1 1

Terahertz Science and Technology Research Center, School of Electronic Science and Engineering, University of

Electronic Science and Technology of China, Chengdu 610054, China 2

College of Materials Science and Engineering, Sichuan University, Chengdu 610065, China

3

National Key Laboratory of Application Specific Integrated Circuit, Hebei Semiconductor Research Institute，

Shijiazhuang 050051, China

ABSTRACT Utilizing terahertz (THz) waves to transmit data for communication and imaging places high demands on phase modulation. However, until now, it is difficult to realize a more than 100 degree phase shift in the transmission mode with one-layer structure. In this paper, a ring-dumbbell composite resonator nested with VO2 nanostructures is proposed to achieve the large phase shift. It is found that in this structure a hybrid mode with an enhanced resonant intensity, which is coupled by the L-C resonance and dipole resonance has been observed. Applying the photo-induced phase transition characteristics of VO2, the resonant intensity of the mode can be dynamically controlled, which leads to a large phase shift in the incident THz wave. The dynamic experimental results show that controlling the power of the external laser can achieve a phase shift of up to 138 degrees near 0.6 THz using this one-layer VO2 nested composite structure. Moreover, within a 55 GHz (575GHz-630GHz) bandwidth, the phase shift exceeds 130 degrees. This attractive phase shift modulation may provide prospective applications in THz imaging, communications, etc. KEYWORDS: insulator-metal phase transition, coupling mode, phase shift, terahertz, vanadium dioxide. INTRODUCTION With the development of terahertz (THz) technologies, the demand for devices in the THz regime is rapidly increasing for applications in multiple areas [1-11]. Among THz devices, tunable modulators capable of actively manipulating the phase or amplitude of a THz wave have become the subject of substantial attention due to its potential applications in THz communication, imaging and radar systems. However, the vast majority of natural materials exhibit only weak electric and magnetic responses and hence cannot be used to control THz radiation. Meanwhile, because of

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ACS photonics Article the specific characteristics of the THz wave, devices for millimeter and visible-light bands cannot be directly used. Thus, considerable efforts have been devoted to developing tunable THz modulators based on different methods. In recent years, the discovery of metamaterials has greatly improved our capabilities to manipulate electromagnetic radiation in the THz band. The combination of metamaterials and semiconductor technologies has led to significant breakthroughs in dynamic THz functional devices [12-31], great achievements have been obtained in THz amplitude modulation. Such as by applying the bulk Schottky diode composed of the metallic split-ring resonators metamaterial array and n-doped GaAs or n-doped Si, the L-C resonance mode can be changed by the external voltage, thereby amplitude modulation can be achieved [12-14]. Combining metamaterial array with photosensitive semiconductor substrate, the mode conversion between the L-C resonance and the dipole resonance results in THz wave amplitude modulation under some kind of optical-pump beam [15-20]. In addition, the 2DEG in the heterostructure of HEMT [21-24] and graphene [25-31] have been proposed to realize high speed THz modulator. Utilizing an equivalent collective dipole array or the split-ring resonator (SRR) metamaterial structure with GaAs/GaN-HEMT heterostructure or graphene, more than 1GHz modulation speed has been obtained and more than 90% modulation depth has been achieved. With the rapid development of THz amplitude modulator, recently, increasing attentions have been paid to the dynamic phase control of THz radiation [14, 20, 22, 26]. In the year 2009, the first THz phase modulator based on an undoped GaAs-metallic split-ring resonators metamaterial structure was developed. In this work, by electrically tuning of the carrier density near the gaps of the SRR, L-C resonance mode is dynamically regulated, leading to a 30 degree phase shift [14]. Later, using the photo characteristics of the GaAs, external ultrashort laser pulse has been utilized to induced the change of the dipole-like resonance in order to achieve 40~50 degree phase shift [20]. Furthermore, an electrically gate-controlled active graphene-Fanolike asymmetric double split rings composite structure with a 32.2 degree phase shift was proposed [26]. Although, considerable progresses have been achieved, a phase shift of over 100 degrees has not been observed in transmission mode until now. From the previous work mentioned above, it is known that the metamaterial structure plays a very key role which determine the performance of the devices. It is known that by designing the electromagnetic unit-cell structures of metamaterials, there are different kinds of resonant modes could be realized, such as the inductance-capacitance (L-C) resonance, dipole resonance, quadrupole resonance, FANO resonance and so on. In most of previous published paper, L-C resonance and


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ACS photonics Article dipole resonance have been studied in the dynamic devices by using different ways to control the resonant intensity to achieve THz wave amplitude and phase modulation [12-31]. For the L-C resonance, the structures are usually made of typical split ring resonators (SRRs) which contain an equivalent circuit including inductance and capacitance. The dipole resonance is always composed of a straight metallic wire whose current distribution is along the wire. These resonances are the fundamental modes in the meta-surface for the relevant structures are very simple. However, the resonant intensities of these mode are not strong enough to achieve over 100 degrees phase shift. Therefore, in this paper, a hybrid resonant mode which is coupled by the L-C resonance and dipole resonance has been proposed to obtain large phase shift. The meta-unit is composed of a metallic wire and a split-ring resonator nested with VO2 nanostructures. In this structure, the dipole resonance can efficiently couple with the L-C resonance so that the resonant intensity and phase jump around the resonant frequency peak has been distinctly improved. Moreover, utilizing the photo-induced phase transition properties of VO2, a mode conversion in this metamaterial structure can be dynamically implemented, which leads to a phase shift of over 130 degrees in transmission mode with external laser illumination. Furthermore, the phase shift can be controlled dynamically by adjusting the power of external laser. RESULTS

Model and Structure In this structure, the ring-dumbbell composite resonator (RDCR) unit cell which is

composed of a circular split-ring resonator (CSRR) and a perpendicular dumbbell dipole resonator (DR) is proposed as Fig.1a. The VO2 nanostructure is nested within two side gaps in the center of unit-cell. This photoconductive composite structure (PCS) is deposited on a quartz substrate. The mechanism is shown in Fig. 1b, wherein the external laser is projected onto the surface of the VO2 nanostructure to induce the phase transition, which leads to the reconfiguration of the whole structure. Accordingly, the resonant mode is converted, thus causing a large phase shift. Fig.1c show the scanning electron microscope images of the manufactured structure: The widths of the metallic line and the gap in the CSRR are 4 µm. The diameter of the CSRR is 36 µm, and the vertical wire of dumbbell is 90 µm. The horizontal and vertical periods of the unit cell are 70 µm and 130 µm, respectively. The VO2 nanostructure is a 11µm ×10 µm square.


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Fig. 1.PCS structure. (a) Schematic of a unit cell. (b) 3D sketch of the experiment with the pump laser. (c) The SEM images of manufactured structure.

VO2 Film Preparation and Sample Fabrication The VO2 film was fabricated by an inorganic sol-gel method, followed by a

annealing process [32-34]: Firstly, 10.0 g of the V2O5 powder mixed with ammonium molybdate

((NH4)6Mo7O24·4H2O,

>99.0%

pure)

and

ammonium

tungstate

((NH4)5H5[H2(WO4)6]·H2O, >99.0% pure) was heated to 800 °C in a crucible until molten (about 30 min), and then poured it into 300 ml deionized water at room temperature. After vigorous stirring of 2 hours, a brownish V2O5 sol was formed. Then the precursor film was deposited on a 500 µm thick quartz substrate by dip-coating method. By controlling the coating times, the final thickness of the V2O5 film is 284nm. Next, the film was annealed at 500℃ for 1.5 h under a nitrogen atmosphere (purity of 99.999%). The annealing process was accompanied by crystallization and reduction of the film from the V2O5 phase to the VO2 phase [35]. By using this VO2 film, a series of micromachining processes were performed during the following procedures to fabricate the device. First step, the photoresist on a VO2 membrane was photo-etched to define the photosensitive area. This area was defined by photolithographically protecting the two 11×10 µm regions that coincide with the two side gaps of the metallic RDCR and subsequent inductively coupled plasma etching (ICPE) of the VO2 film outside the two side gap regions, which


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ACS photonics Article resulted in two 11µm×10µm×0.284µm VO2 nanostructures per unit cell. The second step, layers of the metal films were defined by photolithography. Next, 50 nm of Ti and 200 nm of Au were deposited by e-beam evaporation, followed by a lift-off process to form the entire device. The fabricated PCS is shown in Fig. 1c and 1d.

Photo-induced Phase Transition of the Nanoparticle VO2 Film The VO2 Film has been tested by XRD and THz time-domain spectroscopy

(THz-TDS). Fig. 2(a) shows the XRD pattern of the VO2 film, which was collected under grazing incidence at an angle of 1.5°. The film exhibits a strong peak at around 2θ=27.98°, referring to diffraction from the (011) planes of the monoclinic VO2 phase (JCPDS card no. 72-0514). The XRD pattern indicates well crystallinity with a preferred orientation for the film. The SEM image of the VO2 film shown in Fig.2b illustrated that the VO2 film is compact without obvious pores and consists of uniform, continuous nanoparticles whose size ranges from 50 to 90 nm with a mean value of approximately 70 nm. As mentioned in the reference [36, 37], the morphology of VO2 film fill strongly affect its phase transition properties, so that this compact nanostructure in VO2 film achieved a giant THz switching property during the phase transition which observed in our earlier work [33, 34]. Particularly, the nanoparticle film is compact without obvious cracks. It will be helpful for further microfabrication of the film in this work.

Fig.2. (a) The XRD pattern of the VO2 film (b) SEM image of the VO2 film-quartz. Original time


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ACS photonics Article domain data (c) and transmission spectra (normalized by substrate) (d) of VO2 film.

The transmission and phase spectra of all the samples were tested by a THz time-domain spectroscopy (THz-TDS). The THz-TDS system is produced by Teraview company (Teraview TPS 3000). In this system, the Terahertz transmitter and detector are laser gated photo-conductive semiconductor devices. The femtosecond pulse generated by Ti: Sapphire ultrashort pulsed laser is split into two beam by splitter, one beam is immediately received by detector and another beam, passing through an optical delay system, is focused onto a photoconductive antenna to generate THz radiation with a frequency range of 0.06–3 THz. A series of parabolic mirrors are set to make sure that THz beam is accurately focused on the sample and detector. All the measurements were performed in super-clean laboratory with 40% relative humidity. The samples were placed orthogonally on the THz beam path and at the focus of the THz beam. Meanwhile, an external cw laser (800 nm) with a tunable power (0~5W) was projected onto the surface of the sample with a 45 degree angle to induce the phase transition of the VO2 elements. As shown in Fig. 1b, the diameter of the induced laser is about 5 mm, which is nearly the same as that of the THz beam. The time domain waveforms of the transmitted THz wave with different laser powers are illustrated in Fig. 2c, Fig. 3a and Fig. 4a. By performing a fast fourier transformation (FFT) of the time domain waveform, the frequency-dependent amplitude t(ω) of the transmitted THz pulse through the metamaterial (Fig. 2d, 3b, and 4b)and frequency-dependent phase p(ω) (Fig. 4c, 8b, 8d, and 8f) are obtained. The substrate without any metallic structures and VO2 is considering as the reference which can be applied to normalize the electric field intensity and the phase spectra as below:

t(ω)= Esample(ω) / Ereference(ω) p(ω)= Psample(ω) -Preference(ω) where Esample/reference(ω) is the electric field intensity obtained from the time domain data of sample (reference) and Psample/reference(ω) is the phase spectra of sample (reference). The TDS test results prove the prominent photosensitivity of the VO2 film. The original time domain signals of the pure VO2 film is shown in Fig. 2c. The transmission rate of VO2 film with different laser energy intensities is shown in Fig. 2d. Without an external laser, the film stays in the semiconductor state, and the THz transmission is nearly 0.95, which shows the VO2 nano layer has very high transparency in the THz band. When the external laser is projected onto the surface of the structure, the VO2 film transforms into a metallic state so that the incident THz wave is reflected and absorbed. Thus, with increasing the laser power, the transmission decreases. It can be found that the transmission rate is only 0.16 while


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ACS photonics Article the laser power is 2W, which illustrates that the VO2 film has an excellent photo-induced switching property for THz waves. Therefore, as the key tunable element for modulating THz transmission properties, such a VO2 film nanostructure plays a very important role in dynamic phase manipulation.

Dynamic test for the phase modulation of PCS First of all, as mentioned above, the VO2 film has been etched into nanostructures

embedded in the RDCR metallic array structure. Such nanostructure-elements just act as switches for the mode-conversion in the unit. So if we only consider the etched VO2 nanostructures array, as shown in Fig.3b, because the surface area of the VO2 nanostructures is too small (only 2.4% of the quartz substrate) and the spacing between adjacent structures is too large, the phase transition of such nanostructures array will affect the Terahertz transmission slightly, as shown in Fig. 3b. The transmission nearly stays the same with different laser power.

Fig.3. Original time domain data (a) measured by THz-TDS and transmission spectra (normalized by substrate) (b) of etched VO2 nanostructures. The inset in Fig.3b is SEM image of the etched VO2 nanostructure array and unit.

If we consider the RDCR combining with such VO2 nanostructures, the phase transition will induce the mode conversion in the unit-cell, so the resonant frequency peak will shift. The dynamic original time domain TDS results are shown in Fig.4a. The transmission relation (Fig. 4b) depicts that without external laser, a resonant peak appears at approximately 0.56 THz, which is the eigenmode of the PCS. With increasing laser power, the phase transition of the VO2 film nanostructure is gradually induced so that the resonant mode has been converted, which leads to transmission distinctly changes. The resonant peak blue-shifts towards another peak at approximately 0.63THz. More importantly, such a blue-shift leads to a large phase shift as high as 138 degrees at approximately 0.6 THz (shown in Fig. 4c). Furthermore, from the 575GHz-630GHz wide band, the phase shift can be greater than 130 degrees, which is a very attractive value. Moreover, according to the references [38-41], since


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ACS photonics Article photo-induced phase transition of VO2 could be completed on the order of about 100 fs, this device is expected to achieve high speed phase modulation.

Fig.4.THz-TDS experimental results of PCS with the variation in laser energy. (a) Original time domain data. (b) Transmission spectra. (c) Phase spectra.

DISCUSSION

Analysis of the coupling resonant mode In this section, the analysis of the hybrid mode is carried out to illustrate the large

phase shift observed in the experiments. As the basic resonances of the metasurface, the L-C resonance can be supported by the SRR unit and the dipole resonance can be observed in the long metallic wire unit [12-31]. In this structure, the unit-cell composed both of the CSRR and long metallic elements so that the incident electromagnetic wave can induce a hybrid mode which is coupled by the L-C and dipole resonances. In the Fig.5, it is clear that, in the CSRR unit cell, the induced mode is a typical L-C resonance. The electric field is focus on the gaps and the surface current is a circuit around the unit (shown as ①in Fig. 5b). For the independent metallic wire, the field intensity concentrates on the two sides meanwhile the current flows along the wire (②in Fig. 5b) which are the distinctive features of the dipole resonance. The transmission spectrum (Fig.5c) demonstrates the L-C resonance has narrower bands while the dipole resonance has stronger resonant intensity.


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ACS photonics Article In the case of combing the CSRR and long metallic wire together as a ring-wire composite resonator, these two different kinds of modes are coupled with each other to form a new hybrid mode that possesses the electromagnetic characteristics of both L-C resonance and dipole resonance. Hence, the electric field focuses on both sides and the center gaps of the unit (Fig. 5a). The surface current has three different pathways as shown in Fig.5b. Pathways① and ②are generated by the eigenmodes of the CSRR and DR, respectively. Pathway ③ is derived from the coupling effect. More importantly, such coupling mode leads to a much stronger resonant intensity than L-C and dipole resonances shown in Fig.5c. As mentioned in references [42-43], the terahertz transmission amplitude and phase are related by Kramers-Kronig (K-K) relations. The phase is proportional to the derivative of the amplitude with respect to the frequency and there is a phase jump around the resonant frequency [14, 44]. By comparing the resonant intensities and the phase jump value, it can be found that, the stronger the resonant intensity is, the larger phase jumps can be achieved. Therefore, for the RWCR, due to the strong resonant intensity the phase jump could reach to nearly 180 degree around the frequency peak.

Fig.5. (a) Electric fields (absolute value of electric field) of CSRR, DR and RWCR. (b) Surface current distributions (absolute value of current) of CSRR, DR and RWCR. Simulated transmission spectra (c) and phase spectra (d) for CSRR, DR and RWCR.

Analysis of the ring-dumbbell composite resonator Based on the analysis of the coupling mode, the switching elements with varying

equivalent conductivity have been considered in the structure to simulate the phase shift physical process. In the simulation, two independent statuses have been considered by setting a switching component in the gap of the composite resonator (the red parts in the small graph of the Fig.6a and 6c).


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Fig.6. Comparison of transmission spectra (a, c) and phase spectra (b, d) of RWCR and RDCR.

It can be found that in Fig.6, the dynamic phase shift is caused by a frequency shift between two different resonance frequencies. When the switch is off, the resonant intensity of RWCR can reach to -57dB with 179 degree phase jump value. In the case of the switch turned on, the resonance has blue shift while the intensity is only -25dB with 123 degree phase jump. According to K-K relations, shown as green line in Fig.6a and 6b, the largest phase shift takes place around the region where the amplitude change is the smallest [14, 44]. Therefore, during the switching process, the largest phase shift is only 120 degree. Based upon the above analysis, in order to achieve large shift, on the one hand a single large phase jump is a necessary factor, on the other hand, two resonant modes with similar strong resonant intensity at two statuses is also very important. Under this consideration, we have optimized the ring-wire resonator to a ring-dumbbell composite resonator (RDCR) unit as shown in Fig.6c. Compared with RWCR (Fig.6), although the independent resonance at off status in RDCR is not as strong as that of the RWCR, the amount of phase shift can reach to 140 degrees. Thus, the RDCR units have been applied in the device. Next, in order to further study the EM characteristics of the hybrid resonant mode of the RDCR, the influences of the geometric parameters have been analyzed. In the simulation, a fixed variable method was used to study the influences from the geometric parameters such as dumbbell length, split ring diameter and the period of a unit cell. Firstly, the vertical dimensions (h and b) have been considered. Fig.7a shows when adjusting these parameters, both resonant frequency and intensity changed. The


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ACS photonics Article surface current distribution illustrates that the decrease of the vertical dimensions will weak the dipole resonance with the frequency peak blue shift. As the value of h decreases, the main mode becomes L-C resonance, so the coupling is weaker and weaker. Particularly, when h is equal to D (diameter of the CSRR), the dipole resonance completely disappears and the coupling becomes the weakest. Meanwhile, increasing the vertical dimensions could lead to obvious enhancement in the dipole resonant intensity while weakening the L-C resonance. In this case, the coupling also is reduced. Therefore, there is a best match state (h:D=2.5) corresponding to the largest resonant intensity where both L-C and dipole resonance could be clearly observed that means the coupling is the strongest. Secondly, when adjusting the horizontal dimensions (a and D), as shown in Fig.7c, the strongest resonant intensity also corresponds to the h:D=2.5, which means the ratio value of h/D plays an very important role in the mode coupling. Only at the optimum match state, the L-C and dipole resonance can couple with each other efficiently.


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Fig.7. Simulation results of RDCR with the change of different parameters value. Surface current distributions (absolute value of current) and geometrical parameters: (a) h and b change in the same proportion while other parameters are unchanged. (c) D and a change in the same proportion while other parameters are unchanged. (e) Only a changes. (g) Only D changes. (b, d, f, k) Simulated transmission spectra and phase spectra corresponding to different geometric parameters.


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ACS photonics Article Next, the independent analysis of dumbbell length (a) and diameter of the CSRR (D) on the resonance strength are considered. When the dumbbell length increasing, the surface currents dispersively distribute along the dumbbell-arms so that the frequency peak red shift and resonant intensity is weaken. Thus, as shown in Fig.7c, in the 110

µm length case, both the L-C and dipole resonances become very weak so that the resonant intensity is only -25dB. With the length decreasing, the dipole current becomes stronger and stronger and dipole resonance becomes the dominate mode. Besides, the diameter of the CSRR also affects the resonant mode as demonstrated in Fig.7d. With the diameter D increasing, the L-C resonance is the dominate mode. On the contrary, the dipole resonance will become stronger while the D is decreasing. Therefore, consistent with the above rule, only at the optimum match state, the resonant intensity is the largest. In all, coupling between the L-C and dipole resonances plays a crucial role in large phase shift as the four figures show. Only at the equilibrium and optimum position where the L-C resonance could be coupled with the dipole resonance greatly, the intensity is the largest, then resulting in largest phase shift (Fig.7b, 7d, 7f and 7h).


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Fig.8. Simulated and experimental results of the RDCR with different geometric dimension in unconnected and connected status. Simulation phase results for h:D=2.5 (a), 1.39 (c) and 1 (e). Experimental phase results for h:D=2.5(b), 1.39(d) and 1(f). Simulation (g) and experimental (h) phase difference for different values of h:D. The insets in Fig.8b, 8d and 8f are microscope images of the samples.

Based on the simulation, a series of micro fabrications and tests we performed to verify the electromagnetic characteristics of the coupling mode. Fig. 8a-f depict three typical samples of various ratio of h/D. As in the simulation, only in the matched state, resonant intensity is strongest. Fig. 8h shows that the phase shift is the largest when ratio of h/D is 2.5.


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Analysis of Photo-induced THz Phase Shift Based on the study of the coupling mode in the unit cell, combined with the characteristics of VO2, the dynamic controlling process is studied. In the simulation, the equivalent conductivity change is applied to equal the phase transition of VO2. In the original state, without an external laser, the VO2 particles are uniformly in the monoclinic insulating phase and exhibit semiconductor-like behavior [45-48]; the eigenmode is only the coupling mode that has been analyzed above, here we call it the off-coupling mode in the Fig. 9a. While the external laser induces the phase transition, the VO2 nanostructure functions as a conductor-like material. Therefore, the unit has been reconfigured, and the center gaps have been connected. Under low laser power, only some of the VO2 particles transform into the rutile metallic phase and form discrete metallic domains [45-48]. With the increasing power of the external laser, the discrete metallic domains gradually diffuse and eventually fill the entireVO2 nanostructure [45-48]. Benefiting from the prominent compactness of VO2 particles, a strong, continuous current is stimulated in the VO2 nanostructure and connects the center gaps. Thus, the resonant mode has been converted to another coupling mode (on-coupling mode) wherein the surface current flows through the center long wire and both sides of the half circle, as shown in Fig.9b. During this dynamic conversion process, the transmission resonant frequency of the original status is approximately 0.56 THz, as shown in Fig. 9c. Under the external laser illumination, the phase of VO2 changes, and the equivalent conductivity increases with the diffusion of the metallic domains. Hence, the resonant intensity and frequency have been gradually changed. When the equivalent conductivity is sufficiently large to connect the gap, the resonant frequency blue-shifts to 0.64 THz. As a result, a large phase shift of more than 150 degrees can be observed in the simulation in a one-layer metamaterial structure in the transmission mode. Despite some deviations between simulated and experimental results, which may be mainly due to imperfections from the sample preparation, overall, they are in good agreement.


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Fig.9. Simulation results of the PCS. Electric field (absolute value of electric field) and surface current distributions (absolute value of current) for the off- (a) and on-coupling modes (b), respectively. Transmission (c) and phase spectra (d) with the conductivity of VO2 from 1 S/m to 3×105 S/m, respectively.

CONCLUSIONS In conclusion, this paper proposed a ring-dumbbell composite resonator nested with VO2 nanostructures to achieve a large phase shift in the THz band. In this structure, the dipole resonance and capacitive inductance resonance are coupled together to construct a hybrid mode with enhanced resonant intensity and a large phase jump change. The photo-induced phase transition of VO2 can convert the coupling mode in the structure, which leads to a large phase shift. The THz-TDS experimental results show that the external induced laser can induce a 138 degree phase shift at approximately 0.6 THz. The bandwidth where the phase shift is greater than 130 degrees is up to 55GHz, which represents a considerable improvement over the previous results in transmission mode. Moreover, by cascading 3 layers based on the PCS, a 2π phase shifter may be achieved. Therefore, the concept of an active VO2-metamaterial may provide a potential way to fabricate dynamic THz functional devices in THz communication system, imaging system, etc., in the future.


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ACS photonics Article AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected] *E-mail:[email protected] Notes The authors declare no conflict of interest. ACKNOWLEDGMENTS This work is supported by the National Natural Science Foundation of China under Contract Nos. 61270011, 91438118, 61741121and 61501094, the National Key Basic Research Program of China under Contract No. 2014CB339806, the Program for New Century Excellent Talents in University of Ministry of Education of China (Grant No. NECT_13_0100) and the Sichuan Youth Science and Technology Foundation No. 2014JQ0013.

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