Huan Xu

High figure-of-merit compact phase shifters based on liquid crystal material for 1–10 GHz applications

© 2017 The Japan Society of Applied Physics. A liquid crystal (LC) based tunable microstrip line (ML) phase shifter featuring high performance is presented. The experimental results show an electrically tunable differential phase up to 360° at 10GHz with an overall insertion loss <8.5 dB. The device possesses a high figure-of-merit (FoM) of 64°/dB at 9.8GHz and 60°/dB between 7-10GHz. This is achieved by simultaneously considering both of the LC tuned phase and overall loss in the design. The proposed device utilizes the inverted meander ML technology to minimize its size. Taking into account the real fabrication procedure, a novel impedance matching structure is applied, and the measured return loss is considerably improved. The FoM and phase tuning property of the fabricated device as optimized are compared with the state-of-Art results published recently and show better performance for both of them.

Fast bistable intensive light scattering in helix-free ferroelectric liquid crystals.

A new type of ferroelectric liquid crystal (FLC) is considered, where the reorientation of the director (main optical axes) at the interaction of an electric field with the FLCs spontaneous polarization is due to the movement of spatially localized waves with a stationary profile: solitons arise at the transition due to the Maxwellian mechanism of energy dissipation. Under certain conditions, the appearance of such waves leads to the formation of a structure of transient domains, and as a consequence, to the scattering of light. The Maxwellian mechanism of energy dissipation allows one to reduce the electric field strength at which the maximum efficiency of light scattering is achieved down to 2-3 V/μm and to increase the frequency of light modulation up to 3-5 kHz. Intensive bistable light scattering in an electro-optical cell filled with a specially designed helix-free FLC was studied, and a stable scattering state can be switched on and off for a few tens of microseconds and memorized for a few tens of seconds.

Minimising the losses due to EM field discontinuities in a compact LC tunable microstrip line phase shifter

We present here our recent result on reducing the losses of a multiple-folded tunable microstrip line phase shifter based on liquid crystal (LC) for the regime of low microwave (MW) frequencies. It is shown that the insertion and return losses can be decreased through the minimization of the electromagnetic (EM) wave discontinuities in the folded microstrip line structure. Based on a modified 6-folded microstrip line structure, the return loss of the currently developed LC phase shifter was reduced by 10 dB and the insertion loss was limited to <;10 dB in the frequency range up to 10GHz. In addition, the impact of LC material loss in low MW frequency regime is discussed.

A tunable wideband microstrip bandstop filter based on liquid crystal materials

A compact liquid crystal (LC) based tunable wideband microstrip bandstop filter (BSF) is presented in this paper. To realize a compact size and wide stopband bandwidth, a double-stub bending structure was designed. The BSF stopbands can be tuned by applying a voltage between 0-20 V to regulate the LC dielectric permittivity. A tunability >7% and 3dB bandwidth > 3 GHz and 2.5 GHz for the first and second stopbands respectively were achieved. The rejection level was higher than 41 dB. Such a LC based BSF device with a high tunability, wide stopband bandwidth, low driving voltage and compact size are useful in a wide range of applications.

UV durable colour pigment doped SmA liquid crystal composites for outdoor trans-reflective bi-stable displays

High brightness trans-reflective bi-stable displays based on smectic A (SmA) liquid crystals (LCs) can have nearly perfect transparency in the clear state and very high reflection in the scattered state. Because the LC material in use is stable under UV radiation, this kind of displays can stand for strong day-light and therefore be ideal for outdoor applications from e-books to public signage and advertisement. However, the colour application has been limited because the traditional colourants in use are conventional dyes which are lack of UV stability and that their colours are easily photo bleached. Here we present a colour SmA display demonstrator using pigments as colourant. Mixing pigments with SmA LCs and maintain the desirable optical switching performance is not straightforward. We show here how it can be done, including how to obtain fine sized pigment nano-particles, the effects of particle size and size distribution on the display performance. Our optimized pigments/SmA compositions can be driven by a low frequency waveform (~101Hz) to a scattered state to exhibit colour while by a high frequency waveform (~103Hz) to a cleared state showing no colour. Finally, we will present its excellent UV life-time (at least <7.2 years) in comparison with that of dye composition (~2.4 years). The complex interaction of pigment nano-particles with LC molecules and the resulting effects on the LC electro-optical performances are still to be fully understood. We hope this work will not only demonstrate a new and practical approach for outdoor reflective colour displays but also provide a new material system for fundamental liquid crystal colloid research work.