The Aerospace Academy et al. published research results on terahertz carrier envelope phase shifters

Recently, wang Tianwu’s research team at the Guangzhou Campus of the Institute of Aerospace Information Innovation of the Chinese Academy of Sciences (THz) carrier envelope phase shifter[Flexible THz Carrier-Envelope Phase Shifter Based on Metamaterials]published in Advanced Optical Materials) on.

As the core component of the terahertz scanning tunneling microscope system, this carrier envelope phase shifter consists of different artificial microstructure arrays (Figure 1). For the first time, the research team used “metamaterials” to achieve achromatic controlled phase shift of the broadband THz carrier envelope phase (CEP) without changing the sub-wavelength thickness and without changing the polarization of the THz electric field, with a phase shift of up to 2 in CEP. Compared with traditional THz carrier envelope phase shifters, this phase shifter has the advantages of ultra-thin, flexible, low insertion loss, easy installation and operation.

The CEP of ultrashort pulses determines the instantaneous electric field intensity of the pulses and plays an important role in many nonlinear physical processes. For example, in recent years, the THz scanning tunneling microscope (THz-STM), which couples subpicosecond THz pulses to nano-tip to modulate the bias voltage of the tunnel junction, achieves atomic-level resolution on ultrafast time scales. The active control of the near-field THz time waveform in the tunnel junction is achieved by controlling the CEP of the THz pulse through a simple and efficient way, which is essential for the ultrafast nanoscale manipulation of the propulsion electrons. A complex device consisting of multiple THz polarization elements has been used to control the CEP of the THz pulse, but Fresnel reflects the loss, resulting in a large insertion loss. In addition, natural materials have a weak dispersive response and a small birefringence coefficient in the terahertz band, so they are not easily designed for CEP control of terahertz pulses with wide frequency components. Compared with natural materials, metamaterials, as an artificial material with special optical properties derived from sub-wavelength structures, can be artificially customized for the dispersive response and birefringence coefficient of electromagnetic waves. Despite the rapid development of metamaterial technology, it is still challenging to control the CEP of THz pulses due to the broadband nature of near-single-cycle THz pulses.

Based on this, the scientific research team proposed a method for controlling the THz CEP with a flexible THz carrier envelope phase shifter based on metamaterials, and simulated and experimentally characterized the performance of the phase shifter. The study uses the geometric phase and resonance phase of a specific metal split-ring resonator to control the CEP of the THz pulse, and uses a quadrature directional grating to improve transmission efficiency. When the incident THz pulses are modulated sequentially by different microstructure arrays in the carrier envelope phase shifter, the time waveform of the THz pulses at different CEP values is clearly observed through the THz time domain spectroscopy system (THz-TDS), which is consistent with the simulation results (Figures 2a, b). In addition, experiments have verified that the phase shifter has good robustness under wide-angle incidence and large deformation (Figures 2c, d). By properly scaling the structural parameters of the model, the design can also be applied to other bands.

Figure 1. (a) Schematic diagram of the element structure of the phase shifter, (b) overall and partial optical photographs.

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

The research work is supported by the National Natural Science Foundation of China and Guangzhou Municipality. Beijing Institute of Condensed Matter Physics, Songshan Lake Materials Laboratory and University of Chinese Academy of Sciences participated in the research. (Source: Institute of Aerospace Information Innovation, Chinese Academy of Sciences)

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