Pao H. Chen

Optically induced space-charge fields, dc voltage, and extraordinarily large nonlinearity in dye-doped nematic liquid crystals

We have observed extraordinarily large optical nonlinearity in Methyl Red-doped nematic liquid-crystal film. Grating diffraction can be generated with an optical intensity as low as 40 microW/cm(2) , and a refractive-index change coefficient of more than 6 cm(2)/ W is obtained. The effect is attributed to formation of an optically induced dc space-charge field and to the resulting reorientation of the highly birefringent nematic director axis.

Extremely nonlinear photosensitive liquid crystals for image sensing and sensor protection

Optically induced nematic liquid crystal axis reorientation results in extraordinarily large nonlinear refractive index changes that could find practical applications in conjunction with cw or long-pulse lasers. We discuss the origins of these nonlinearities, and present the results of recent experimental studies of image conversion, optical limiting and sensor protection using aligned dye-doped nematic liquid crystal films in all-optical configurations. These processes are characterized by unprecedented low threshold laser powers, thus presenting nonlinear photosensitive nematic liquid crystals as promising next generation image processing and optical switching/limiting material.

Supra optical nonlinearities (SON) of methyl red- and azobenzene liquid crystal - doped nematic liquid crystals

Abstract In nematic liquid crystal films doped with methyl-red dye or azobenzene liquid crystal, we have observed extremely efficient optically induced director axis reorientation effects, and refractive index change coefficients >> 1 cm2/W. The basic mechanisms involved are determined to be optically induced space charge fields and flow reorientation, order parameter modification and molecular torques by the photo-excited dopant molecules. These supra-optical nonlinearities (SONs) enable nonlinear image conversion, optical limiting and holographic grating formation with μW power lasers.

All-optical image processing with a supranonlinear dye-doped liquid-crystal film.

We demonstrate a dynamic all-optical image-intensity-inversion technique that uses self- and mutual-phase-modulation effects with a highly nonlinear nematic liquid-crystal film placed in an intermediate focal plane. This process requires submilliwatt optical power, responds in a few milliseconds, and can be realized over a very broad spectral range.

Liquid crystal photorefractivity - towards supra-optical nonlinearity

Abstract A review of the basic mechanisms of optically induced refractive index modification in liquid crystalline materials is presented. We describe how the use of methyl-red dye or an azobenzene liquid crystal will dramatically enhance the nonlinear optical responses of typical nematic liquid crystals (NLC). The intensity dependent refractive index changes in these materials are characterized by nonlinear coefficients of over 1 cm 2 / W . The principal contributing mechanisms are orientational photorefractivity associated with photo-induced space charge fields, dopant-nematic molecular torque, and order parameter modification caused by trans–cis isomerization of the azo-dopants.

Extremely nonlinear photosensitive nematic liquid crystal film

Abstract Optically induced nematic liquid crystal (NLC) axis reorientations result in extraordinarily large nonlinear refractive index changes. We discuss the origins of these nonlinearities, and recently, obtained results of experimental studies of photovoltage generation, dynamic grating diffraction, and image conversion using dye-doped NLC films in all-optical configurations are presented. These processes are characterized by unprecedented low threshold laser powers. For example, we have demonstrated incoherent–coherent image conversion with laser intensity on the order of 70 microWatts/cm 2 .

Novel liquid-cored fiber and liquid crystal film for sensor protection against visible ps-cw lasers

We have developed compact millimeter thick optical fiber arrays with nonlinear optical liquid guiding cores. These image transmitting fiber arrays are capable of passive optical limiting action against frequency agile short (picosecond- nanosecond) laser pulses in the visible spectrum. We present the results of further studies of their limiting performance. We also discuss how a recently discovered extraordinarily large nonlinear optical response of methyl-red doped nematic liquid crystal film may be employed for limiting action on longer pulse or cw lasers with nanowatt threshold power.

Self-defocusing and optical limiting of nano-Watt cw laser and image processing at μ-Watt/cm2 intensity with nematic liquid crystals

Using nematic liquid crystal films doped with photocharge producing dyes, we have demonstrated the feasibility of optical limiting of long pulse - cw lasers at nanowatt power, and clamped transmission of < < 1(mu) J. We also demonstration incoherent-coherent image conversion capability with optical intensity in the (mu) Watt/cm2 level.

Orientational optical nonlinearity of nematic liquid crystals for image conversion and optical limiting

We present a review of the basic mechanisms for laser induced refractive index changes associated with director axis reorientation in nematic liquid crystals. In methyl-red doped nematic liquid crystal, an index change coefficient of more than 10 cm2/GW has been obtained, allowing us to perform image conversion and optical limiting processes at very low optical intensities. In particular, we demonstrated a very large dynamic range twisted nematic liquid crystal limiter that will protect sensor from cw or long-pulse laser jamming.

Modeling nonlinear photonics of nonlinear fiber core for optical limiting application

A quantitative model for nonlinear photonic absorption processes in a liquid and their effects in the nonlinear transmission and limiting of a laser pulse are studied. The theoretical simulation results accounting for two-photon- and excited-state absorption processes, and the dynamic evolution of the molecular level populations and propagation of picosecond and nanosecond laser pulses through a fiber core formed by such liquid are in good agreement with experimental observations. In pump-prove studies using picosecond and nanosecond laser pump pulses and cw probe, we have also observed oscillatory transmission associated with laser induced thermal/density effects. We also discuss applications of the supra optical nonlinearity of nematic liquid crystal film for anti-laser jamming and optical limiting operation against long-pulse or cw lasers.