Dynamics of photo-induced terahertz optical activity in metal chiral gratings

Photo-Excited Metasurface for Tunable Terahertz Reflective Circular Polarization Conversion and Anomalous Beam Deflection at Two Frequencies Independently

In this paper, a photo-excited metasurface (MS) based on hybrid patterned photoconductive silicon (Si) structures was proposed in the terahertz (THz) region, which can realize the tunable reflective circular polarization (CP) conversion and beam deflection effect at two frequencies independently. The unit cell of the proposed MS consists of a metal circular-ring (CR), Si ellipse-shaped-patch (ESP) and circular-double-split-ring (CDSR) structure, a middle dielectric substrate, and a bottom metal ground plane. By altering the external infrared-beam pumping power, it is possible to modify the electric conductivity of both the Si ESP and CDSR components. By varying the conductivity of the Si array in this manner, the proposed MS can achieve a reflective CP conversion efficiency that ranges from 0% to 96.6% at a lower frequency of 0.65 THz, and from 0% to 89.3% at a higher frequency of 1.37 THz. Furthermore, the corresponding modulation depth of this MS is as high as 96.6% and 89.3% at two distinct and independent frequencies, respectively. Moreover, at the lower and higher frequencies, the 2π phase shift can also be achieved by respectively rotating the oriented angle (α_i) of the Si ESP and CDSR structures. Finally, an MS supercell is constructed for the reflective CP beam deflection, and the efficiency is dynamically tuned from 0% to 99% at the two independent frequencies. Due to its excellent photo-excited response, the proposed MS may find potential applications in active functional THz wavefront devices, such as modulators, switches, and deflectors.

Spiral Chiral Metamaterial Structure Shape for Optical Activity Improvements

We report on a spiral structure suitable for obtaining a large optical response. We constructed a structural mechanics model of the shape of the planar spiral structure when deformed and verified the effectiveness of the model. As a verification structure, we fabricated a large-scale spiral structure that operates in the GHz band by laser processing. Based on the GHz radio wave experiments, a more uniform deformation structure exhibited a higher cross-polarization component. This result suggests that uniform deformation structures can improve circular dichroism. Since large-scale devices enable speedy prototype verification, the obtained knowledge can be exported to miniaturized-scale devices, such as MEMS terahertz metamaterials.

Out-of-plane actuation with a sub-micron initial gap for reconfigurable terahertz micro-electro-mechanical systems metamaterials

We propose a reconfigurable terahertz (THz) metamaterial that can control the transmittance by out-of-plane actuation with changing the sub-micron gap distance between electrically coupled metamaterial elements. By using the out-of-plane actuation, it was possible to avoid contact between the coupled metamaterial elements across the small initial gap during the adjustment of the gap size. THz spectroscopy was performed during actuation, and the transmission dip frequency was confirmed to be tunable from 0.82 to 0.92 THz for one linear polarization state and from 0.80 to 0.91 THz for the other linear polarization; the two polarizations were orthogonal. The proposed approach will contribute to the development of tunable metamaterials based on structural deformations.

All-photoinduced terahertz optical activity

We proposed and demonstrated active control of terahertz optical activity via chiral patterned photoexcitation in a semiconductor with a spatial light modulator (SLM). Arbitrary patterns can be generated by a SLM, including completely symmetric enantiomer pairs. This technique provides a new route to terahertz polarization modulators.