Brett D. Guenther

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.

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.

Photo-induced space charge fields, photo-voltaic, photorefractivity, and optical wave mixing in nematic liquid crystals

Abstract We present a detailed discussion of the fundamental mechanisms for recently observed nonlinear electro-optical wave mixing effects in nematic liquid crystal films. Experimental results on photoconductivity, photorefractive-like two beam coupling, beam amplification, self-diffraction, and storage holographic grating formation in dye- and Fullerene C60 and C61 doped nematic films are presented along with a review of two- and four- wave mixing theories. Nematic liquid crystal films are shown to be potentially useful for adaptive and optical storage applications.

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.

Liquid Crystals for Fast Infrared Laser Switching and Optical Limiting Application

The nonlinear optical effects of chlorophyll ‘a’ have been determined using an array of measurement techniques. Regions of spectral interest exhibiting induced excited state absorption were identified with transient absorption spectroscopy measurements. At specific wavelengths within these regions, the dye was characterised using the Z-scan technique. Additional measurements were made to complement the latter results including a focused aperture experiment in which thermal defocusing and scattering mechanisms were identified.

Nonlinear Optical Fiber Core Materials For Optical Limiting Application

Several liquid phase liquid crystals and liquids are found to possess broadband nonlinear absorption characteristics suitable for optical limiting applications. Using these liquids as fiber waveguiding cores, we have developed fiber arrays that provide excellent limiting performance characteristics against frequency agile visible laser pulses in the nanosecond - picosecond time scale. These fiber arrays are compact, low cost/weight, and are capable of transmitting high quality images throughout the entire visible spectrum.

Photorefractivity, phase conjugation, and thermal wave mixing effects in the visible and near-IR spectral region for dye and fullerence C60-doped nematic liquid crystals

The results of a recent study of a nonlinear electro-optical effect in dye and fullerene C60-doped nematic liquid crystal films are presented. In particular, photo-induced conduction, self diffraction effects, and optical phase conjugation in the films form near-UV to near-IR wavelengths are explained.

Photorefractivity and Optical Wave Mixing Effects in Nematic Liquid Crystals Doped with Fullerene Derivatives and Dyes

The photorefractive effect has been observed and documented for decades1,2. This effect is typically observed in impurity doped electrooptic crystals. When the crystal is illuminated, charge carriers are excited from occupied donor states to the conduction band. These excited charge carriers migrate under the influence of diffusion, and any internal or external electric fields, until they are trapped by empty donors thereby causing a space-charge field to be formed within the crystal. The electric field that results from the charge separation acts to modulate the index of refraction through the electrooptic Pockels effect.