Yung-Hsun Wu

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 2pi phase shift is achieved at 9Vrms and 8.1pi at 40Vrms using a 12-microm-thick E7 LC layers. This operating voltage is ~10X lower than that using a conventional 0.3-mm-thick glass separator.

Dual-frequency addressed variable optical attenuator with submillisecond response time

We demonstrated a submillisecond response time and low-voltage variable optical attenuator (VOA) using a dual-frequency liquid crystal. The dynamic range of the VOA reaches 43 dB at a 1.55 µm wavelength. A normally on VOA with a flat wavelength response is obtained using a phase compensation cell. Fast rise and decay times are achieved using low-frequency overdrive and high-frequency undershoot voltages. Other approaches to further improve the VOAs performances are discussed.

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 1.55μm 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.

Submillisecond response variable optical attenuator based on sheared polymer network liquid crystal.

We demonstrate a variable optical attenuator (VOA) at lambda=1.55 microm using a sheared polymer network liquid crystal (SPNLC). The SPNLC exhibits a fast response time and weak wavelength dependency. Comparing with other polymer-stabilized liquid crystals, the SPNLC has lower driving voltage and negligible light scattering loss when the wavelength exceeds 700 nm. A reflection type VOA with ~0.24 ms response time and -32 dB dynamic range is demonstrated at room temperature and 35 Vrms voltage.

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.

Reflective direct-view displays using a dye-doped dual-frequency liquid crystal gel

A high-contrast, fast-response, and polarizer-free reflective display using a dye-doped dual-frequency liquid crystal gel is demonstrated. The high contrast ratio originates from the combination of light scattering from the microdomain polymer gel and absorption from the black dyes. The fast response is due to the frequency modulation of the dual-frequency liquid crystal.

Generation of high-flux attosecond extreme ultraviolet continuum with a 10 TW laser

We report a laser system that delivers 15 fs pulses with 200 mJ energy at a 10 Hz repetition rate. The broadband spectrum extending from 700 nm to 900 nm was obtained by seeding a two-stage Ti:sapphire chirped-pulse power amplifier with sub-mJ white-light pulses from a gas-filled hollow-core fiber. With this laser, an extreme ultraviolet (XUV) super-continuum supporting 230 as isolated attosecond pulses at 35 eV was generated using the generalized double optical gating technique. The XUV pulse energy was ∼100 nJ at the exit of the argon gas target.

IPS-LCD using a glass substrate and an anisotropic polymer film

A lightweight in-plane-switching liquid crystal display (IPS-LCD) using a single glass substrate and an anisotropic polymeric film is demonstrated. The liquid crystal molecules are aligned by the elongated polymer grain of the film. The alignment capability of the anisotropic film is comparable to a buffed polyimide layer. Compared to the LCD using two glass substrates, our new device exhibits a comparable contrast ratio (/spl sim/514:1), driving voltage, and response time because of good LC alignment. Such an anisotropic film can also function as a phase compensation film for widening the viewing angle. This technology is particularly attractive for making single-substrate displays and also has potential for a double-layered guest-host display and a flexible display using IPS LCDs.

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 >1x10-4 J/m2, 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.