Liquid crystal hyperbolic metamaterial for wide-angle negative-positive refraction and reflection.

We show that nanosphere dispersed liquid crystal (NDLC) metamaterial can be characterized in near IR spectral region as an indefinite medium whose real parts of effective ordinary and extraordinary permittivities are opposite in signs. Based on this fact we designed an electro-optic effect: an external electric-field-driven switch between normal refraction, negative refraction, and reflection of TM incident electromagnetic wave from the boundary vacuum/NDLC. A detailed analysis of its functionality is given based on effective medium theory combined with a study of negative refraction in anisotropic metamaterials and finite elements simulations.

Random lasing in blue phase liquid crystals.

Random lasing actions have been observed in optically isotropic pure blue-phase and polymer-stabilized blue-phase liquid crystals containing laser dyes. Scattering, interferences and recurrent multiple scatterings arising from disordered platelet texture as well as index mismatch between polymer and mesogen in these materials provide the optical feedbacks for lasing action. In polymer stabilized blue-phase liquid crystals, coherent random lasing could occur in the ordered blue phase with an extended temperature interval as well as in the isotropic liquid state. The dependence of lasing wavelength range, mode characteristics, excitation threshold and other pertinent properties on temperature and detailed make-up of the crystals platelets were obtained. Specifically, lasing wavelengths and mode-stability were found to be determined by platelet size, which can be set by controlling the cooling rate; lasing thresholds and emission spectrum are highly dependent on, and therefore can be tuned by temperature.

Nonlinear optical grating diffraction in dye-doped blue-phase liquid crystals.

Low-power cw lasers are employed to study grating formation in blue-phase liquid crystals. We observed that undoped samples exhibit vanishingly small optical nonlinearities whereas methyl-red-dye doped samples produce strong nonlinear self-diffraction effects. The nonlinearities are attributed to director axis reorientation, disorder, and lattice distortion by the laser-excited dye molecules. The magnitude of the observed intensity-dependent index coefficient is in the range of 10(-4)-10(-3) cm2/Watt.

Infrared cylindrical cloak in nanosphere dispersed liquid crystal metamaterial.

We present a design of an infrared cylindrical cloak using nanosphere dispersed nematic liquid crystal (NLC) metamaterial following the approach of Smith's group [Science 314, 977 (2006)]. Cloaking conditions require spatial distribution of liquid crystal birefringence with constant extraordinary index of refraction and radially dependent ordinary index of refraction. An approximate analytical formula for the latter is derived. Finite element (FE) simulations confirm the cloaking effect. Owing to the tunable birefringence of the liquid crystal component, such cloaking material offers the interesting possibilities of real-time control of invisibility. The possibility of experimental realization is briefly discussed.

Nanosphere dispersed liquid crystals for tunable negative-zero-positive index of refraction in the optical and terahertz regimes.

An analysis of aligned nematic liquid crystal cells containing core-shell nanospheres shows that it is possible to devise a new type of metamaterial whose index of refraction is tunable from negative, through zero, to positive values. The design parameters for the constituents can be scaled for application in the optical as well as very long wavelength (e.g., terahertz and microwave) regions.

Finite-size effect on one-dimensional coupled-resonator optical waveguides.

We study the finite-size effect on the dispersion relation, group velocity, and transmission curves of one-dimensional finite-size coupled-resonator optical waveguide (CROW) structures. Both the dispersion relation and the group velocity curves of a finite-size CROW oscillate along those of the corresponding infinite-extended ones. The oscillations can be suppressed by matching the equivalent admittance of the surrounding medium to that of the unit cell. Thelen's method is used to find the parameters of the matching layer to reduce oscillations on the group velocity and transmission spectra, and to analyze the structure parameters that determine the bandwidth and the group velocity.

All-optical neural-net-like image processing with photosensitive nonlinear nematic film.

Received June 30, 2003 We demonstrate that some neural-net image-processing operations can be realized in an all-optical manner by use of the natural diffraction property of light in conjunction with the photosensitivity and optical nonlinearities of a transparent thin film. An example of image edge enhancement is illustrated with photosensitive supranonlinear nematic liquid-crystal films.

Nonlinear dynamics in laser polarization conversion by stimulated scattering in nematic liquid crystal films.

We present a study of the nonlinear dynamics of a coherent polarization conversion process in nematic liquid crystal films. This effect is mediated by two-beam coupling between the incident polarized laser and its orthogonally polarized (lower-frequency) noise component scattered by the director axis fluctuations. A complete model for the director dynamics derived from the basic equations is presented. The existence of complex, time-, and intensity-dependent dynamics of the director motion such as oscillations and various bifurcations including period doubling is revealed.

All-optical image processing by means of a photosensitive nonlinear liquid-crystal film: edge enhancement and image addition-subtraction.

We demonstrate two simple yet efficient all-optical image-processing techniques that use nonlinear photosensitive dye-doped nematic liquid-crystal films, namely, edge enhancement and image addition-subtraction operations. These films require no external bias and function at much lower optical powers and shorter response times than other conventional methods.

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.

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.

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.

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.

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.

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.

Observation of stimulated orientational scattering and cross-polarized self-starting phase conjugation in a nematic liquid-crystal film.

We have observed stimulated extraordinary-ordinary wave scattering and cross-polarized self-starting optical phase conjugation effects in a nematic liquid-crystal film. The basic mechanism involved is the nonresonant laser-induced nematic crystal axis reorientation. These processes require low optical power and are characterized by millisecond response speed.

Holographic grating formation in dye- and fullerene C(60)-doped nematic liquid-crystal film.

Transient and persistent holographic gratings have been observed in dye- and fullerene C(60)-doped nematic liquid-crystal films. The nature and time evolution of the underlying mechanisms, such as space-charge field production, flows, and dielectric- and conductivity-induced torques, and the resultant director axis reorientation and refractive-index gratings are examined.

Observation of optical limiting and backscattering of nanosecond laser pulses in liquid-crystal fibers.

We report the recent observation of nonlinear-optical phenomena occurring in isotropic liquid-crystal-cored optical fibers, including passive all-optical self-limiting effects, stimulated backscattering, and phase conjugation. These processes occur at relatively low thresholds and short fiber lengths.

Observation of orientational photorefractive effects in nematic liquid crystals.

We report the observation of nonlinear photorefraction in pure aligned nematic liquid crystals in the presence of an applied dc electric field. The effect is attributed to nematic axis reorientation created by the photoinduced space-charge field in combination with the applied dc field. Strong optical nonlinearity, self-diffraction, and beam-coupling effects are observed.

Onset dynamics of self-pumped phase conjugation from speckled noise.

By accounting for both the speckle nature of the coherent noise and the depletion of the pump beam in a semilinear oscillator, we show that the buildup of phase conjugation from the noise level is characterized by a buildup time. The dependence of the buildup time on the noise level is in good agreement with experimental results.