David Statman

Photoinduced gliding of the surface director in azo‐dye doped nematic liquid crystals

We report photoinduced surface director gliding with azo dye‐doped nematic liquid crystals. This gliding has the same characteristics as that induced by an applied magnetic field. Fast and slow dynamic regimes are observed, the latter fitting to a stretched exponential. In addition, the gliding demonstrates a ‘sensitizing’ effect for sequential measurements.

Charge dynamics and poling in glass waveguides

Germanosilicate glass waveguides were electrically poled using interdigitated electrodes. This poling resulted in a quasiphase‐matched second‐order susceptibility. Second‐harmonic light was observed. Measured intensities were one to two orders of magnitude smaller than expected based on previous electric‐field‐induced second‐harmonic (EFISH) generation studies with the same interdigitated electrodes. The dynamics of the EFISH signals were studied. Exponentially decaying signals suggest that through charge migration, via dielectric relaxation and electronic drift, a compromising field is established during the poling. This results in a weaker second‐order susceptibility.

One-way imaging with an aberrated reference beam

We demonstrate image reconstruction by way of four-wave mixing for a signal that makes a single pass through a distorting medium. Both the signal and the reference originate from the same source and propagate through the same disturbance. In the four-wave mixing process, phase aberrations are subtracted out. A read beam from a second source reconstructs the undistorted image, yielding a phase conjugate without a double pass through the disturbance. Good reconstructed image fidelity is demonstrated for static distortions if the phase mismatch between the signal and the reference beams is minimized. If there is significant phase mismatch, reconstructed image fidelity is poor. We show that this technique can also be used to measure the autocorrelation function of the disturbance when the mismatch between the signal and the reference in the four-wave mixing scheme is varied.

Influence of the molecular weight of a polymer on the gliding of nematic liquid crystals

Gliding of the nematic liquid crystal director on polymethyl-methacrilate layers in a magnetic field is observed. It is found that the kinetics of the gliding process are strongly influenced by the molecular weight of the polymer chains. This fact supports the model of gliding based on the mutual orienting effect of the liquid crystal and polymer network.

Temporal memory effects in photorefractive two-beam coupling: a study comparing n-doped and p-doped BaTiO 3 at 514 nm and 830 nm

The photorefractive gain and time response of n-doped and p-doped BaTiO3 were compared in the visible, λ= 514 nm, and in the infrared, λ=830 nm. Two-beam coupling experiments were conducted for beam ratios between 3600:1, 5000:1, and 8000:1. In both infrared and visible it was observed that the measured gains were higher for the p-BaTiO3 by an order of magnitude or more. When the gain was high, a temporal memory effect was observed, suggesting that the time dependence of the space-charge field is determined by the history of the modulation depth. This effect is shown to be consistent with the Kukhtarev equations for a single carrier. Low-gain behavior was observed at all interaction angles for both crystals in the IR. In the visible the high-gain temporal memory effect is reported for the p-BaTiO3.

Polarization twisted nematic gratings: a study of the far field diffraction pattern

When left and right circularly polarized beams of light from a pump laser interfere in a nematic liquid crystal doped with azo dye, a polarization twisted nematic (PTN) grating is formed in the sample. The same is not true when linearly polarized light interferes, regardless of the polarization. For circularly polarized light, the easy axis is rotated toward the polarization direction of interfering beams. The irradiance is uniform so there is less contribution to refractive index variations. In the latter case the diffraction grating arises from variation in refractive index. Gratings written with Disperse Orange 3 (DO3) as the dopant disappear after removal of the pump beams, whereas grating written with Methyl Red (MR) as the dopant tend to be semi-permanent.

Temporal response of barium titanate to slowly oscillating signals

Theoretical and experimental results are presented which demonstrate that slowly oscillating signals experience a temporal delay in photorefractive two beam coupling experiments. This delay corresponds to the response time of the signal. It is determined to be dependent on the frequency of the oscillating input signal. In the limit of low frequencies, the response time is given by the product of the space charge field response time and the gain. As the frequency increases, the response time decreases. For high enough frequencies, response times faster than the space charge field response time are predicted and experimentally observed.

Memory Effects in Photorefractive Materials

We have determined that the dynamics of the signal in photorefractive two beam coupling can be described by a memory effect in the writing of the space charge field. Experimental and computational results confirm that this time dependence is dependent on the history of interference pattern responsible for the space charge field. From this memory effect it is shown that if the input signal has a sinusoidal component much smaller than its DC component, the sinusoidal component of the two beam coupled signal is shifted by the response time of the photorefractive medium. We have demonstrated, both theoretically and experimentally, that for the non-depleting pump this response time is linear with the interaction length.

Effect of Photo-Isomerization of Azo Dyes on the Photonic Bandgap in Chiral Nematics

Chiral nematic liquid crystals prepared with Grandjean texture demonstrate a photonic bandgap whose central wavelength is proportional to the pitch length, P, of the liquid crystal and whose width is given by (ne – no)P. We show that methyl red doped chiral nematics undergo a shift in the photonic bandgap upon photo-isomerization. This shift is a result of (1) photo-induced change in anchoring energy on the nematic surface, and (2) change in the natural pitch length from the photo-isomerization of the azo dye.

Anchoring strength of o-, m-, and p-methyl red-dye-doped nematics on rubbed and unrubbed polyimide surfaces

It is demonstrated that the photoinduced gliding of the easy axis for nematics doped with various azo dyes on rubbed polyimide involves the formation of a second easy axis on the polyimide surface. While some azo dyes, such as disperse orange 3, do not exhibit large surface induced nonlinear effects, other dyes, such as methyl red, do. The amount of reorientation of the easy axis on rubbed polyimide is determined by the relative anchoring strengths of the easy axis formed from adsorbed dye and that formed from rubbing. One question of interest is what is the source of the anchoring strength? In this paper, we discuss the formation of easy axes via the photo-induced adsorption of azo dye. We will compare the anchoring strengths between dyed nematic liquid crystals and the easy axes formed by photoinduced adsorption of three isomers of the methyl red azo dye, ortho, meta, and para, as well as disperse orange 3. We will also discuss the impact of the carboxyl group position in the dye molecule on the anchoring strength.