Flexible Terahertz Metamaterials Are Optically Tuned

Fitting high-performing electronic components into complex structures and geometries could open the doors for terahertz metamaterial devices like detectors, imagers, switches, and filters. The gallium-arsenide (GaAs)-based metamaterials in current literature are built on rigid substrates, which limit applications to planar geometries. Using polymeric substrates, flexible and optically tunable terahertz sheets made of metamaterials have been presented by Kebin Fan, Xiaoguang Zhao, Jingdi Zhang, Kun Geng, George Keiser, Huseyin Seren, Grace Metcalfe, Michael Wraback, Xin Zhang, and Richard Averitt.

Their device is composed of stacked layers of polyimide, electric split-ring resonators (ESRRs), and thin GaAs patches. A photoactive material can be used to short the capacitance of the inductor-capacitor (LC) ESRR or modify the inductance. As photons react with the photo-induced polyimide, the carrier density is increased. This results in capacitive shunting, which reduces the LC resonance.

Optical-pump-terahertz-probe (OPTP) spectroscopy was used to test and characterize the responses of the experimental structures. Modulation depth beyond 60% was seen for a range of 1.1 to 1.8 THz, which could be viable for broadband transmission. Simulations using CST commercial software were also performed to analyze the amplitude tuning and blueshift of the resonance. This finite-difference-time-domain (FDTD) simulation used a Drude model for homogeneous photo-excitation of carriers in both layers. The simulation correlated with the experimental results. In doing so, it revealed that the optical tuning also induced modulation of the effective permittivity of the metamaterials, causing the resonant-frequency blue shifting. See “Optically Tunable Terahertz Metamaterials on Highly Flexible Substrates,” IEEE Transactions On Terahertz Science And Technology, Nov. 2013, p. 702.