Optically Tunable Terahertz Metasurfaces Using Liquid Crystal Cells Coated with Photoalignment Layers

 Shih, Yi-Hong, Xin-Yu Lin, Harry Miyosi Silalahi, Chia-Rong Lee, and Chia-Yi Huang. "Optically Tunable Terahertz Metasurfaces Using Liquid Crystal Cells Coated with Photoalignment Layers." Crystals 11, no. 9 (2021): 1100.

for full paper see https://www.mdpi.com/2073-4352/11/9/1100

Abstract

An optically tunable terahertz filter was fabricated using a metasurface-imbedded liquid crystal (LC) cell with photoalignment layers in this work. The LC director in the cell is aligned by a pump beam and makes angles θ of 0, 30, 60 and 90° with respect to the gaps of the split-ring resonators (SRRs) of the metasurface under various polarized directions of the pump beam. Experimental results display that the resonance frequency of the metasurface in the cell increases with an increase in θ, and the cell has a frequency tuning region of 15 GHz. Simulated results reveal that the increase in the resonance frequency arises from the birefringence of the LC, and the LC has a birefringence of 0.13 in the terahertz region. The resonance frequency of the metasurface is shifted using the pump beam, so the metasurface-imbedded LC cell with the photoalignment layers is an optically tunable terahertz filter. The optically tunable terahertz filter is promising for applications in terahertz telecommunication, biosensing and terahertz imaging.

Experimental

A linearly polarized pump beam (center wavelength = 365 nm) was incident to the metasurface imbedded LC cell from the top substrate. The pump beam with an intensity of 0.3 mW/cm2  irradiated the photoalignment layers for 100 s, aligning the LC director in the cell. The LC director made angles θ of 0, 30, 60 and 90° with respect to the gaps of the SRRs of the metasurface by changing the polarized direction of the pump beam via a polarizer. The metasurface-imbedded LC cell was placed in the chamber of a terahertz spectrometer (TPS 3000, TeraView) for studying the effect of the polarized direction of the pump beam on the resonance frequency of the metasurface. Terahertz waves that were polarized in a direction parallel to the x axis of figure 1a were normally incident to this cell. TPS 3000 had a frequency resolution of 3 GHz in this work.

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