Richard L. Sutherland

Cephalopod coloration model. I. Squid chromatophores and iridophores

We have developed a mathematical model of skin coloration in cephalopods, a class of aquatic animals. Cephalopods utilize neurological and physiological control of various skin components to achieve active camouflage and communication. Specific physical processes of this coloration are identified and modeled, utilizing available biological materials data, to simulate active spectral changes in pigment-bearing organs and structural iridescent cells. Excellent agreement with in vitro measurements of squid skin is obtained. A detailed understanding of the physical principles underlying cephalopod coloration is expected to yield insights into the behavioral ecology of these animals.

Nonlinear measurements on AF-50

The nonlinear optical properties of N,N-diphenyl-7-[2-(4- pyridinyl) ethenyl]-9,9-di-n-decylfluoren-2-amine [AF- 50] have been investigated. The nonlinear absorption of a saturated solution of this material in acetone was investigated with 430 femtosecond pulses at 790 nm. From these results, the two-photon absorption cross-section was determined to be 25 X 10-50 cm4sec/photon molecule. This number is in agreement (within a factor of 2) with theoretical calculations. Nonlinear absorption and optical limiting measurements were also made using a Nd:YAG pumped dye laser with 4.3 ns pulses at 694 nm. These results suggest inherent differences in the performance of two-photon absorbing materials in these two different geometries.

Holographic PDLCs for optical beam modulation, deflection, and dynamic filter applications

A new type of electro-optical device, a switchable hologram, has emerged from the combination of two technologies, namely, photopolymer holographic materials and polymer- dispersed liquid crystals (PDLCs). Starting from a simple homogeneous mixture containing a polymerizable monomer, a liquid crystal (LC), and a photoinitiator dye, this system cures under holographic illumination to form well defined channels of PDLCs interspersed between dense, LC-free polymer channels. These periodic PDLC planes produce diffraction of light in the Bragg regime with surprisingly good optical quality. LC droplets within the PDLC channels can be remarkably small, ranging in size from 20 nm to 200 nm. Even within this small domain, the molecules can be reoriented by an applied field, leading to electro-optical modulation of the diffraction efficiency. The polarization properties and sharp threshold switching of the diffracted light may be partially explained by the unique shape and consequent nematic ordering of the LC droplets. Complex holograms can be written in this material, and electro- optical switching at < 5 V/micrometers with response and relaxation times in the 20-40 microsecond(s) regime have been consideration. Spatial light modulators which modulate the intensity can make use of these switchable holograms. Some of the potential applications are in fiber optic switches, programmable optical interconnects, digital zoom lenses, optically assisted true-time-delay phased array radar, flat panel display, and dynamic filters.

Switchable holograms for displays and other applications

Switchable holograms exhibit an electrically controllable diffraction efficiency. Such devices form the building blocks for several applications under active consideration, including optical image and information displays. 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 these goals in a single-step photopolymer/liquid crystal composite system. We also discuss the multiple uses of this material and potential applications.

Cephalopod coloration model. II. Multiple layer skin effects

A mathematical model of multiple layer skin coloration in cephalopods, a class of aquatic animals, is presented. The model incorporates diffuse and specular reflection from both pigment and structural photonic components found in the skin of these animals. Specific physical processes of this coloration are identified and modeled utilizing available biological materials data. Several examples of combination spectra are calculated to illustrate multiple layer and incident light effects as well as the potentially rich repertoire of color schemes available to these animals. A detailed understanding of the physical principles underlying cephalopod coloration is expected to yield insights into their possible functions.

Investigating the nonlinear optical properties of molten organic materials

The nonlinear absorption of two substituted thiophene compounds containing benzithiazole units was investigated using the Z-scan technique. The experiments were performed using a Nd:YAG laser frequency doubled to 532nm with 24ps laser pulses. The samples were prepared as solutions in THF and as melts by heating above their melting points into the isotropic liquid phase. Both compounds show strong nonlinear absorption under all conditions. Solutions of these materials show little or no linear absorption at the laser wavelength. However, the melts do show significant linear absorption. The nonlinear absorption observed has been analyzed using methods for two-photon absorption and two-photon assisted excited state absorption and two-photon absorption cross-sections are reported.

Droplet deformation and alignment for high-efficiency polarization-dependent holographic polymer-dispersed liquid-crystal reflection gratings

Droplet deformation and alignment are achieved in holographic polymer-dispersed liquid-crystal reflection gratings by applying an in situ shear during recording. High diffraction efficiency (99%) is obtained for light polarized parallel to the shear, with nearly zero efficiency for perpendicular polarization, and no increase of incoherent scattering. Permanent polarization dependence is related to stress-induced morphology changes of liquid-crystal droplets that are frozen by polymerization. The system is studied by electron microscopy and modeled by anisotropic coupled-wave and scattering theory. The morphology is consistent with the theory of small deformations of liquid droplets in fluid flow. Diffraction efficiency measurements are in agreement with theory incorporating this morphology as well as concomitant orientation and alignment of liquid-crystal molecules.

Holographic polymer-dispersed liquid crystals in display applications

Holographic polymer-dispersed liquid crystal (HPDLC) gratings have been employed in various display applications. Two prototypes that have been developed are switchable lenses in wearable displays and electronic filter wheels in video projectors. Acrylate-based HPDLCs have been utilized for the most part. These have many desirable properties, including fast switching speeds, but have contributed to stability problems, including long-term diffraction notch blue shift (shrinkage) and voltage creep due to post-polymerization effects. Thiol-ene based HPDLCs have been investigated and show potential for overcoming these shortcomings. We present a comparison of acrylate and thiol-ene HPDLCs and discuss the implications for long-term stability in display applications.

Guest-host optical limiters with high-laser-damage threshold

Conventional guest-host optical limiting materials utilize either a liquid solvent or solid as the matrix for nonlinear absorbing chromophore dopants. Concentration gradients of the chromophore in the matrix can improve optical limiting performance. However, low viscosity liquid solutions can not retain a concentration gradient while polymer solid matrices damage at low laser fluences. We report on a novel approach of using an elastic polymer and viscoelastic gels for guest- host optimal limiting matrices. We achieve high bulk laser damage thresholds in the hosts and maintain a concentration gradient of the chromophore. By softening the epoxy we significantly enhance its bulk laser damage threshold. We characterize this effect by measuring the damage threshold as a function of viscoelastic properties. In addition, optical limiting was demonstrated in all the hosts doped with nonlinear phthalocyanine chromophores.

Multiphoton Absorption and Optical Limiting

Optical limiting requires the development of materials exhibiting strong nonlinear absorption. Multiphoton absorption in materials can take the form of instantaneous two-photon absorption or time-integrating excited state absorption. We examine both types of mechanisms for optical limiting. Several experimental methods are used to study these phenomena and characterize materials. Often new or unusual effects arise in these experiments that require careful analysis to elucidate desired parameters. We describe several of these and give results for different materials relevant to optical limiting.