Fast Response Liquid Crystal Devices

Liquid crystal (LC) has been widely used for displays, spatial light modulators, variable optical attenuators (VOAs) and other tunable photonic devices. The response time of these devices is mainly determined by the employed liquid crystal material. How to obtain fast response for the LC devices is a fundamentally important and technically challenging task. In this dissertation, we investigate several methods to improve liquid crystal response time, for examples, using dual-frequency liquid crystals, polymer stabilized liquid crystals, and sheared polymer network liquid crystals. We discover a new class of material, denoted as sheared polymer network liquid crystal (SPNLC) which exhibits a submillisecond response time. First, dual-frequency liquid crystals and polymer network methods are demonstrated as examples for the variable optical attenuators. Variable optical attenuator (VOA) is a key component in optical communications. Especially, the sheared PNLC VOA shows the best result; its dynamic range reaches 43 dB while the response time is in the submillisecond range at 1550 nm wavelength, which is 50 times faster than the commercial LC-based VOA. Second, we report a new device called axially-symmetric sheared polymer network liquid crystals (AS-SPNLC) and use it as LC devices. An axially-symmetric sheared polymer network liquid crystal has several attractive features: 1) it is polarization independent, 2) it has gradient phase change, and 3) its response time is fast. It can be used for polarization converter and divergent LC lens. In addition, a new method for simultaneously measuring the phase retardation and optic axis of a compensation film is demonstrated using an axially-symmetric sheared polymer network liquid crystal. This simple technique can be used for simultaneously measuring the optic axis and phase retardations of both A- and C-plates. These compensation films have been used extensively in wide-view LCD industry. Therefore, this method will make an important impact to the LCD industry

Transflective Liquid Crystal Display Using Separate Transmissive and Reflective Liquid Crystal Cells and Materials with Single Cell Gap

A transflective liquid crystal display (TLCD) using separate transmissive (T) and reflective (R) cells in which two liquid crystal materials with different birefringence changes are used. The birefringence change of the R region is half of the birefringence change of the T region. In this case, a single cell gap is possible and identical transmittance and reflectance for R and T is obtained. It is applicable to various reflective LC modes, and the fabrication methods are simple

Transflective liquid crystal display using separate transmissive and reflective liquid crystal cells and materials with single cell gap

A transflective liquid crystal display (TLCD) using separate transmissive (T) and reflective (R) cells in which two liquid crystal materials with different birefringence changes are used. The birefringence change of the R region is half of the birefringence change of the T region. In this case, a single cell gap is possible and identical transmittance and reflectance for R and T is obtained. It is applicable to various reflective LC modes, and the fabrication methods are simple

Pinning effect on the phase separation dynamics of thin polymer-dispersed liquid crystals

The surface pining effects on phase separation dynamics of polymer-dispersed liquid crystals (PDLCs) with thin cell gaps are demonstrated. Comparing various boundary conditions, the inner surfaces of the substrates with or without polyimide layers [but no rubbing] cannot provide enough anchoring force, so in either case the liquid crystal (LC) droplets flow and coalesce to form larger and less uniform droplets. However, if the inner surfaces of the substrates are coated with rubbed polyimide layers with anchoring energy \u3e 1 x 10(-4) J/m(2), almost all the nucleated LC droplets grow at a fixed position during phase separation. The appearance of the coalescence is not obvious and the formed LC droplets are relatively uniform. The surface anchoring has a significant effect on the morphology of PDLCs

Dual-frequency addressed hybrid-aligned nematic liquid crystal

Dual-frequency addressed hybrid-aligned nematic (HAN) liquid crystal cell is demonstrated as a variable optical attenuator at 1.55 mum wavelength. By controlling the low- and high-frequency electric field, the dual-frequency liquid crystal (DFLC) molecules can be reoriented parallel or perpendicular to the substrates so that the maximum obtainable phase modulation is doubled. In comparison to a homogeneous cell, the DFLC HAN cell shows a lower operating voltage and faster response time. Furthermore, the DFLC HAN cell exhibits three stable states that have some applications such as ternary photonic devices

Simultaneous measurement of phase retardation and optic axis of a phase compensation film using an axially-symmetric sheared polymer network liquid crystal

A new method for simultaneously measuring the phase retardation and optic axis of a uniaxial compensation film is demonstrated using an axially-symmetric sheared polymer network liquid crystal (SPNLC). By overlaying a tested compensation film with a calibrated SPNLC cell between crossed polarizers, two dark spots are clearly observed in a CCD image. From the orientation direction and distance of these two spots, the optic axis and phase retardation value of the compensation film can be determined. This method is particularly useful for those optical systems whose optic axis and phase retardation are dynamically changing

Polarization-independent and fast-response phase modulation using a normal-mode polymer-stabilized cholesteric texture

Fast-response, polarization-independent, and hysteresis-free phase-only modulation using a normal-mode polymer-stabilized cholesteric texture (PSCT) is demonstrated. Although the remaining phase change in the high-voltage regime is small, it is still useful for making microdevices. Polarization-independent tunable-focus microlens arrays using such a PSCT are demonstrated

Axially-symmetric sheared polymer network liquid crystals

An axially-symmetric sheared polymer network liquid crystal (SPNLC) device is demonstrated and its performances characterized. Through analyzing the structure of this axially-symmetric SPNLC, we constructed a 3-D model to explain the observed phenomena. The simulation results agree well with the experiment. Two potential applications of such an axially-symmetric SPNLC, namely tunable-focus negative lens and spatial polarization converter, are discussed

Polarization-independent liquid crystal phase modulator using a thin polymer-separated double-layered structure

A polarization-independent phase-only liquid crystal (LC) phase modulator using a double-layered structure is demonstrated. Two orthogonal LC layers are separated by two ultra-thin anisotropic polymer films. The anisotropic polymeric films not only separate the LC layers but also provide good molecular alignment. As a result, a polarization-independent phase modulator with 2 pi phase shift is achieved at 9V(rms) and 8.1 pi at 40V(rms) using a 12-mu m-thick E7 LC layers. This operating voltage is similar to 10X lower than that using a conventional 0.3-mm-thick glass separator

Electrically tunable wettability of liquid crystal/polymer composite films

An electrically tunable wettability in a liquid crystal/polymer composite film is demonstrated, in which liquid crystal molecules are anchored among polymer grains. The tunable wettability of the composite films originates from the reorientation of the anchored liquid-crystal molecules, which is switched by an in-plane electric field with squared pulses of voltages. These liquid crystal/polymer composite films with electrically tunable wettability have potential applications in polarizer-free displays, ink-jet printing, microfluidic devices, and lab-on-a-chip