Bob L. Epling

Optical limiting in solutions of diphenyl polyenes

The optical-limiting behavior of a series of trans-alpha, omega-diphenyl polyene compounds was observed in solutions of chloroform. The influence of planarity, the substitution of donor and acceptor groups, and the extent of pi-electron delocalization on the nonlinear thresholds of the diphenyl polyenes in an optical-limiting geometry were examined. A saturation effect of optical-limiting nonlinear thresholds, consistent with the theoretical work, was observed at 10-11 pi-electron bonds. The temporal profiles of the transmitted laser pulses and the power dependence of the nonlinear thresholds as a function of spot size and wavelength were examined. These examinations led to the conclusion that the predominant nonlinear mechanism was quasi-steady-state self-focusing. Nonlinear thresholds an order of magnitude lower, and thus effective n(2) and X(3) values an order of magnitude higher, than the well-known self-focusing medium CS(2) were observed. Our studies demonstrate that this series of polyenes consists of efficient broadband nonresonant optical-limiting materials.

Relation of electro-optical characteristics to materials properties and morphology in polymer-dispersed liquid crystal holographic gratings

Switchable holograms in polymer-dispersed liquid crystals are now being considered for a variety of applications in which transmission loss, cross-talk, and on/off dynamic range are important issues, particularly as devices are stacked in tandem. Switching voltage and speed also remain as important considerations. All of these characteristics are related to the properties of materials comprising the switchable grating as well as the microscopic morphology. In this paper we explore these material-structure-property relations and examine trade-offs in device parameters. It appears that low optical scattering and wide dynamic range approximately 40 dB are achievable while retaining microsecond switching times. Further reductions in switching voltage will require research in the interfacial anchoring and order parameter of the liquid crystal droplets.

Holographic PDLCs for spatial light intensity modulation

Holographic polymer-dispersed liquid crystals are switchable holograms exhibiting electrically controllable diffraction efficiency. Such devices form the building blocks for several applications under active consideration including spatial light modulators. Several demanding requirements are placed on switchable holograms for these applications, such as high diffraction efficiency, wide on/off dynamic range, low optical scatter, low switching voltage and power consumption, high speed, uniformity and repeatability, low cost, and manufacturability. We describe the challenges and progress in meeting many of these goals.

Second harmonic generation in corona-poled polypeptides

In order to develop polypeptide-based second order nonlinear optical materials, we have derivatized succinylated poly(l-lysine) with a spiropyran. Spin coated thin films (2 (mu) thickness) of this material on indium tin oxide-coated glass were corona-poled (6 X 105 V/m) under conditions of variable temperature. The degree of alignment and nonlinear optical properties were monitored by measurement of the intensity of second harmonic signal resulting from irradiation at 1064 nm. The dark-adapted polypeptide was in the spiropyran form. The second harmonic signal intensity was 20 times greater than that of the dark-adapted sample. The results demonstrate the feasibility of development of a light- activated nonlinear optical material.