Hongkyu Park

Evaluating liquid crystal properties for use in terahertz devices

Despite the wide application of liquid crystals (LCs) in the visible frequency range, their properties in the terahertz range have not yet been extensively investigated. In this paper we have investigated the terahertz properties of LCs E7, BL037, RDP-94990 and RDP-97304 using terahertz time-domain-spectroscopy. We find that RDP-94990 has the largest birefringence and smallest absorption in the terahertz range compared to E7 and BL037. We highlight the importance of investigating all parameters, not just the birefringence, when designing fast, efficient and transmissive terahertz LC devices.

Subsurface nanoimaging by broadband terahertz pulse near-field microscopy.

Combined with terahertz (THz) time-domain spectroscopy, THz near-field microscopy based on an atomic force microscope is a technique that, while challenging to implement, is invaluable for probing low-energy light-matter interactions of solid-state and biomolecular nanostructures, which are usually embedded in background media. Here, we experimentally demonstrate a broadband THz pulse near-field microscope that provides subsurface nanoimaging of a metallic grating embedded in a dielectric film. The THz near-field microscope can obtain broadband nanoimaging of the subsurface grating with a nearly frequency-independent lateral resolution of 90 nm, corresponding to ∼ λ/3300, at 1 THz, while the AFM only provides a flat surface topography.

Robust Thin-Film Wire-Grid THz Polarizer Fabricated Via a Low-Cost Approach

A robust thin-film wire-grid terahertz (THz) polarizer was fabricated via a low-cost, mass-producible manufacturing approach. This polarizer is built on a very thin silica layer structurally supported by a silicon substrate. In addition, the metal grating is protected by a polymer thin film, which eliminates the multireflection effect and enhances the robustness of the polarizer for easy packaging. The polarizer can be easily mounted onto a Newport rotation holder for immediate use. A THz time-domain spectrometer is used to characterize its performance, and an excellent agreement is found between the FDTD-simulated results and the experimental results. The polarizer offered 20-40 dB and 0.8 dB of extinction ratio and transmission loss over a frequency range of 0.2-2.0 THz, respectively.

High extinction ratio and low transmission loss thin-film terahertz polarizer with a tunable bilayer metal wire-grid structure.

A thin-film terahertz polarizer is proposed and realized via a tunable bilayer metal wire-grid structure to achieve high extinction ratios and good transmission. The polarizer is fabricated on top of a thin silica layer by standard micro-fabrication techniques to eliminate the multireflection effects. The tunable alignment of the bilayer aluminum-wire grid structure enables tailoring of the extinction ratio and transmission characteristics. Using terahertz time-domain spectroscopy (THz-TDS), a fabricated polarizer is characterized, with extinction ratios greater than 50 dB and transmission losses below 1 dB reported in the 0.2-1.1 THz frequency range. These characteristics can be improved by further tuning the polarizer parameters such as the pitch, metal film thickness, and lateral displacement.

Accurate photoconductive antenna characterization using a thin film polarizer

The horizontal and vertical polarizations of the terahertz radiation emitted from a small-gap dipole photo-conductive antenna are characterised using a single detector and a homemade thin-film wire grid polarizer. The two polarizations are seen to be temporally distinct. In addition, the dependence of the position of the excitation spot on the dipole with the polarity of the horizontal polarization is studied where a reversal in the horizontal polarization is observed between the two edges of the antenna anode.

Tailoring liquid crystals to become fast and efficient terahertz devices

Liquid crystals have been employed for several decades in devices such as phase shifters, Fabry-Perot filters, polarizers, phase gratings, and Bragg switches at optical frequencies. However it is only recently that such devices have been demonstrated at terahertz frequencies. This is because of several fundamental frequency dependent relationships between device properties and frequency of operation. When designing liquid crystal devices, we need to find liquid crystals with high birefringence, low viscosity and low absorption at terahertz frequencies. In this paper we will present some measurements and simulations of potentially suitable liquid crystal mixtures.

Liquid crystals for terahertz technology

New terahertz (THz) optical devices which can control the optical properties of THz waves are needed to broaden the application area of THz technology. Liquid crystals (LCs) are very attractive materials for developing such devices because they have outstanding properties such as sensitivity to applied electric fields, chemical stability, relatively large birefringence and moderate absorption in the THz range. LCs need to be optimized to have a large birefringence and small absorption in the THz range. In this paper, we have investigated optical properties of a set of LCs in the THz range: E7, BL037, and RDP-97304. Optical parameters for the ordinary and extraordinary axis of LCs were acquired using THz time-domain spectroscopy and THz air-biased coherent detection system. We found that RDP-97304 has the largest birefringence and smallest absorption compared to E7 and BL037 in the THz range. It is thus a good candidate to design fast and efficient THz optical devices.