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Advanced optical materials | Wang tianwu team successfully developed a flexible terahertz carrier envelope phase shifter based on metamaterials

Recently, the R & D results of terahertz (THz) carrier envelope phase shifter by Wang tianwu’s research team in Guangzhou Park of Aerospace Information Innovation Research Institute were published in advanced optical materials. His research paper is entitled”flexible THz carrier envelope phase shifter based on metamaterials”.

As the core component of terahertz scanning tunneling microscope system, the carrier envelope phase shifter is composed of Different Artificial Microstructure arrays (Fig. 1). For the first time, the research team used”metamaterials” to realize achromatic controllable phase shift of wideband THz carrier envelope phase (CEP) without changing THz electric field polarization, and the phase shift of CEP is up to 2. Compared with the traditional THz carrier envelope phase shifter, this phase shifter has the advantages of ultra-thin, flexible, low insertion loss, easy installation and operation.

CEP of ultrashort pulses determines the instantaneous electric field intensity of pulses, which plays an important role in many nonlinear physical processes. For example, in recent years, THz scanning tunneling microscope (THz STM), developed by coupling sub picosecond THz pulses to nano tips to modulate the bias of tunnel junctions, has achieved atomic resolution on ultrafast time scales. The active control of near-field THz time waveform in tunnel junction by controlling the CEP of THz pulse in a simple and efficient way is essential to promote the ultrafast nano scale manipulation of electrons. A complex device composed of several THz polarization elements has been used to control the CEP of THz pulse, but its insertion loss is very large due to Fresnel reflection loss. In addition, natural materials have weak dispersion response and small birefringence coefficient in terahertz band, so it is not easy to be designed for CEP control of terahertz pulses with wide frequency components. Metamaterials are artificial materials with special optical properties derived from sub wavelength structures. Compared with natural materials, they can customize the dispersion response and birefringence index of electromagnetic waves. Despite the rapid development of metamaterial technology, the CEP control of THz pulses is still challenging due to the broadband characteristics of near single period THz pulses.

Based on this, the research team proposed a flexible THz carrier envelope phase shifter based on metamaterials to control THz CEP, and simulated and experimentally characterized the performance of the phase shifter. Researchers use the specific geometric phase and resonant phase of metal split ring resonator to control the CEP of THz pulse, and use orthogonal directional grating to improve the transmission efficiency. When the incident THz pulse is modulated by different microstructure arrays in the carrier envelope phase shifter in turn, the change of the time waveform of THz pulse under different CEP values is clearly observed through the THz time domain spectroscopy system (THz TDS), which is very consistent with the simulation results (Fig. 2a, b). In addition, experiments verify that the phase shifter has good robustness under wide-angle incidence and large deformation (Fig. 2c, d), and by appropriately scaling the structural parameters of the model. The design scheme can also be applied to other bands.

The above research work has been supported by the National Natural Science Foundation of China, Guangzhou and Guangzhou Development Zone projects. Research partners include Beijing Institute of condensed matter physics, Songshanhu materials laboratory and University of Chinese Academy of Sciences.

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Figure 1 (a) Schematic diagram of unit structure of phase shifter, (b) overall and local optical photos

Figure 2 (a, b) THz time waveform changes corresponding to different CEP values obtained by simulation and experiment, (C, d) effects of wide-angle incidence and sample bending deformation on device performance