M. V. Wood

Dye-doped photorefractive liquid crystals for dynamic and storage holographic grating formation and spatial light modulation

The basic mechanisms of photo-induced space charge field formation, director axis re-orientation, and refractive index changes in fullerene C60- and dye-doped nematic liquid crystals films are presented. In particular, in aligned methyl-red-doped nematic liquid crystal film, we observe a nonlinear index change coefficient as high as 10 cm/sup 2//W, associated with purely optically induced liquid crystal director axis re-orientation. Experimental observations of dynamic and high-resolution storage holographic grating formation, two beam coupling with gain of nearly 3000 cm/sup -1/, optical limiting action at nanowatt cw laser power, and incoherent-coherent image conversion at /spl mu/W/cm/sup 2/ light intensity level are discussed.

Coherent beam amplification with a photorefractive liquid crystal.

Coherent amplification of a signal beam by a strong pump beam is observed in thin films of fullerene-doped nematic liquid crystal. Exponential gain constants as high as 2890 cm(-1) with no phase cross talk are achieved at low applied dc bias voltage and pump beam intensity. The underlying mechanism is the electro-optically induced spatially reorientation of the liquid-crystal axis and the resultant phase-shifted index grating required for two-beam coupling.

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.

Nolinear optical liquid cored fiber array and liquid crystal film fo ps-cw frequency agile laser optical limiting application.

The molecular nonlinear photonic absorption processes of two nonlinear fiber core liquids are discussed in the context of nonlinear propagation and optical limiting of short pulses. These fiber arrays are capable of limiting threshold and clamped output below 1 micro J for picosecond and nanosecond pulses. We also discuss the observation of perhaps the largest optical nonlinearity in some dye-doped nematic liquid crystal films. These films will provide limiting action with a threshold power of 100 nWatt and limited transmission of << 1 microJoule for ms - cw laser.

NONLINEAR LIQUID-CRYSTAL FIBER STRUCTURES FOR PASSIVE OPTICAL LIMITING OF SHORT LASER PULSES

Optical limiting of nanosecond and picosecond laser pulses through millimeter-length isotropic liquid-crystalcored fiber structures is reported. Low limiting threshold and clamped transmitted outputs are observed. The underlying nonlinear mechanisms are nonlinear photoabsorptions and scattering and lossy waveguiding caused by laser-induced thermal-density index fluctuations.

Nonlinear Optical Effects in Nematic Liquid-Crystal Films in the 1.55 µm Spectral Region

We have studied the optical nonlinearities of nematic liquid crystals in the near IR communication spectral region [1.55 µm]. The origins of the refractive index changes are thermal indexing effect and director axis reorientations. Very large refractive index and phase modulation of several πs can be generated with mW laser power in micron thick dye-doped nematic liquid crystal films.

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 .