used in the range of millimeterwave and microwave, do not offer switching ability in THz frequency. [4] To overcome the problem, metamaterial modulators [5] and integrally-gated graphene parallel-plate waveguides [6] have been proposed. The farmer technology provides a continuous phase shift by bias voltage, but has drawbacks that the modulators change both amplitude and phase simultaneously and are available only at a specific frequency. The latter technology offers low-loss THz electronic switches, but the phase shift is discontinuous and the devices also are designed to be available only at a specific frequency. Compared with these devices, the phase shifters based on LC materials have advantages that a single cell can be used in a wide THz frequency range and the phase shift is continuously controlled by electric field. [2] Therefore, liquid crystals (LCs) are expected to be useful in THz frequency.LCs are one of the soft materials, exhibiting fluid nature and molecular order. [7] Owing to the softness of the materials, LCs realign the molecular alignment easily by the electric field, which provides a larger refractive index change (≈0.3) and lower voltage driving than other electro-optical materials such as Pockels cell and Kerr cell. The high responsiveness to the field has been beneficial in display technology demanding thinness and power saving performance. However, because of the softness, LCs have a drawback that the decay response speed is slow. Considering technological transfer from visible wavelength range to THz frequency, the drawback in response speed is one of the most significant problems to overcome. For example, if one fabricates a half-wave plate, the retardation Δnd is required to be 250 nm for visible light at a wavelength of 500 nm, while to be 250 µm for a THz wave at a wavelength of 500 µm (0.6 THz), where Δn is the birefringence of LCs and d is the cell thickness. To increase the retardation Δnd, one has to increase the cell thickness d because Δn is a specific parameter of the LC materials and is usually up to 0.30. Namely, the cell thickness d is required to be more than 1 mm for THz wave, which is 10 3 times larger than that required for visible light. In general, the decay time τ d of nematic LCs (NLCs) is proportional to the square of the cell thickness d. [8,9] Consequently, the decay time of the NLC cell for THz wave is 10 6 times longer than for visible light. C. F. Hsieh et al. reported a long decay time of τ d = 190 s in a thick LC cell of 0.57 mm, although the cell demonstrated a phase shift of approximately 20° for THz wave at 0.31 THz. [2] Toward the practical use as the Liquid CrystalsAn ideal polymer/liquid crystal (LC) composite structure for terahertz phase shifters is proposed, and the fabrication method of the ideal structure using polymerization induced phase separation is reported. The ideal structure consists of four significant factors: i) the polymer structure surrounds LC droplets like polymer dispersed liquid crystals; ii) the size of the LC droplets is on the...
We report refraction-type non-mechanical beam steering using a 100-μm-thick swelling liquid crystal gel film with a polymer concentration gradient, in which an electrically-induced, large refractive index gradient along the uniaxial direction causes the deviation of a laser beam incident perpendicularly to the LC gel film. The swelling LC gel film is fabricated by polymerizing a LC-monomer/LC mixture while cooling it at a low temperature of -20 °C, and exhibits a short decay response time on the order of tens of microseconds. Thus, our device demonstrates non-mechanical beam steering with scan rate greater than 1 kHz.
Back Cover: In article 1800766 by Yo Inoue and co‐workers, an electrically tunable terahertz phase shifter using a polymer structure embedded with liquid crystal droplets is demonstrated. The device exhibits a short response time on the order of tens of milliseconds, while performing a phase shift of 30° for a terahertz wave at a frequency of 0.4 THz.
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