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A multi-wavelength laser is demonstrated using a dye-doped cholesteric polymer film whose reflection bandwidth is broadened with several oscillations. Due to the abrupt change of the density of state between oscillation peak and valley, each oscillation functions as a photonic band gap for generating a laser wavelength under the excitation of a pumping laser. As a result, a multiple wavelength laser is generated. Results indicate that the dye-doped cholesteric liquid crystal polymer film is a good candidate for fabricating broadband lasers such as white light lasers. Potential applications include experimental testing of laser materials, identification markers, information displays, and inertial confinement laser fusion.

We demonstrate an enhanced circularly polarized laser emission whose handedness is opposite to the cholesteric helix by the stacked cholesteric polymeric films. The dye-doped right-handed cholesteric polymer film is sandwiched between a mirror and a cholesteric polymer reflector. Due to the stimulated amplification and light recycling effects, the original laser emission from the middle active layer is not only dramatically enhanced but also converted to a left-handed circularly polarized emission with high purity. Moreover, a single 15 mum dye-doped cholesteric film is found to lase more efficiently when the top side faces the pump source than when the bottom side does. This phenomenon is attributed to the band gap broadening of the bottom side.

Low threshold laser action of dye-doped cholesteric liquid crystals (CLCs) is demonstrated using an input circularly polarized light whose handedness is the same as the cholesteric helix of the sample at the high-energy band edge of the reflection band. The mechanism originates from the dramatic increase of the photon density of state at the band edges. We also demonstrate an enhanced laser action of a CLC in a dielectric multilayer cavity. In such a device configuration, the band-edge excitation at high-energy band edge improves the lasing performance not only for the same handedness circularly polarized pump beam as the cholesteric helix but also for the opposite one. It stems from the polarization independence of the dielectric multilayers.

We demonstrate a method for enhancing the laser efficiency by stacking multiple dye-doped chiral polymer films. No laser emission was observed from a single 8 mum film. By stacking two films together, the laser efficiency is dramatically enhanced. Further increasing the number of stacked films, the output laser power is further increased. It is also observed that the output laser power in the forward direction is almost the same as that in the backward for the two- and three-layered films. However, in the six-layered film the output laser power is much stronger in the backward direction than the forward one. This is due to the absorption of the laser dyes and the distributed feedback in the chiral polymer films.

We demonstrate a direction controllable linearly polarized laser from a dye-doped cholesteric liquid crystal (CLC) in a homogeneous cell coated with a metallic mirror on the inner side of a glass substrate. Due to coherent superposition of two orthogonal polarization states, the output laser light becomes linearly polarized and its output energy is greatly enhanced. Moreover, the linear polarization direction angle is proportional to the product of the CLC effective birefringence and cell gap. Hence direction tunable laser devices can be demonstrated by controlling the cell gap and the operating temperature.

We have investigated the optical properties of the polymer-stabilized cholesteric liquid crystal (CLC). It was observed that the reflectance was decreased and the transmittance was increased with the increase of the applied voltage. Based on this property, a polarization independent two-way variable optical attenuator (VOA) has been demonstrated by sandwiching a lambda/2 film between two left handed polymer-stabilized CLC films. Different from a conventional VOA, the VOA based on our developed polymer-stabilized CLC can continuously change the optical intensity in both the reflection and transmission directions by applying voltage on it. This unique property will allow it to be widely used in many applications, such as optical communications.

A scattering-free broadband (approximately 120 nm bandwidth) circular polarizer is demonstrated by stacking three chiral polymer films with different pitch lengths. Using 4 x 4 matrix method, we have theoretically simulated the transmission spectra of each chiral polymer film and the three stacked films. Simulation results agree well with experiment. A broadband circular polarizer with bandwidth ranging from 400 to 736 nm can be achieved by stacking 8 such chiral polymer films together. Simulation results indicate that if a high birefringence (Deltan approximately 0.35) polymer film is employed the number of films can be reduced to three. Potential applications of these circular polarizers for liquid crystal displays, optical communications, and optical remote sensors are discussed.

We have investigated the physical and optical properties of the left-handed chiral dopant ZLI-811 mixed in a nematic liquid crystal (LC) host BL006. The solubility of ZLI-811 in BL006 at room temperature is ~24 wt%, but can be enhanced by increasing the temperature. Consequently, the photonic band gap of the cholesteric liquid crystal (CLC) mixed with more than 24 wt% chiral dopant ZLI-811 is blue shifted as the temperature increases. Based on this property, we demonstrate two applications in thermally tunable band-pass filters and dye-doped CLC lasers.

A high performance dye-doped cholesteric liquid crystal (CLC) laser is demonstrated by incorporating a passive CLC reflector to the active cell. The polarization-conserved CLC reflector effectively increases the distributed feedback cavity length which, in turn, results in a significant enhancement in lasing efficiency and a dramatic reduction in beam divergence. The lasing characteristics are still dominated by the circularly polarized light with the same sense as the cholesteric helix.

We experimentally and theoretically investigated the optical switching characteristics of bacteriorhodopsin (bR) at lambda=633 nm using the pump-probe method. A diode-pumped second harmonic YAG laser (lambda=532 nm which is located around the maximum initial Br state absorption) was used as a pumping beam and a cw He-Ne laser (lambda=633 nm which is around the peaks of K and O states) was used as a probe. Due to the nonlinear intensity induced excited state absorption of the K, L, M, N, and O states in the bR photocycle, the switching characteristics are sensitive to the intensity of the probe and pump beams. Based on this property, we have demonstrated an all-optical device functioning as 11 kinds of variable binary all-optical logic gates.