Joseph W. Perry

Dynamics of optical limiting in heavy-atom substituted phthalocyanines

Picosecond and nanosecond nonlinear transmission measurements were used to determine the excited singlet and triplet state absorption cross sections at 532 nm for group IVA metalloid phthalocyanines. A five-state rate-equation model is used to analyze the nonlinear transmission data. The successful simulations of the nonlinear transmission data on this series of phthalocyanines for widely differing pulse durations provide strong evidence for the validity of the excited state absorption model. Use of the heavy atom effect has allowed engineering of phthalocyanines with strongly enhanced optical limiting performance.

Second-order non-linear optical properties of diironalkenylidyne complexes; crystal structure of {(η-C5H5)2Fe2(CO)2(μ-CO)(μ-(E)CCHCHC6H4(p)-NMe2)}+BF4−

Abstract The second-order non-linear optical properties of a series of diironalkenylidyne complexes have been examined by the Kurtz powder technique. One compound, {(η-C5H5)2Fe2(CO)2(μ-CO)(μ-(E)ue5f8Cue5f8CHue5fbCHue5f8C6H4ue5f8(p)-N(CH3)2)}+CF3SO3−, exhibited a powder efficiency for second harmonic generation 3.6 times that of a urea reference standard (ω = 1907 nm, 2ω = 953.5 nm). The crystal structure of {(η-C5H5)2Fe2(CO)2(μ-CO)(μ-(E)ue5f8Cue5f8CHue5fbCHue5f8C6H4ue5f8(p)-N(CH3)2)}+BF4− has been determined.

Synthesis and nonlinear optical properties of sol-gel materials containing phthalocyanines

Optical limiters are devices that have the ability to limit the intensity of light that passes through them. They function like optical surge protectors, blocking a greater fraction of photons when the incident light intensity is very high than when the incident intensity is low. This might allow an electronic sensor or a human eye to function under ambient lighting conditions and yet be protected from being blinded by direct laser illumination. Phthalocyanine (Pc) dyes which exhibit nonlinear optical behavior known as reverse saturable absorption (RSA) have been demonstrated to be effective in solution-based optical limiters. There is considerable interest in the possibility of fabricating optical limiters based on solid-state materials containing phthalocyanine dyes. Sol-gel processing is a solution based technique for preparing porous metal oxide monoliths at low temperatures. By adding an organic dye to the precursor solution, one can trap these organic molecules in the pores of a silica host. These potentially thermally stable materials are of interest for the fabrication of monolithic lens/limiter optical components. Our study of sol-gel synthetic approaches to phthalocyanine doped glassy materials show that phthalocyanine dyes in silica hosts are stable and exhibit optical nonlinearities comparable to those of the corresponding dye solutions.

4-N-Methylstilbazolium toluene-p-sulfonate salts with large second-order optical non-linearities

Donor-substituted 4-N-methylstilbazolium toluene-p-sulfonate salts and related salts containing cations with extended conjugation exhibit an unusually high propensity to crystallize in non-centrosymmetric space groups. The new compound 3′,4′-dihydroxy-4-N-methylstilbazolium toluene-p-sulfonate crystallizes in the space group P1 and exhibits a powder second-harmonic generation efficiency 106 times that of urea measured at 1.907 µm.

Organic salts with large electro-optic coefficients

Single crystals of the organic salt 4-dimethylamino-N-methyl-4-stilbazolium tosylate (DAST) are shown to exhibit large second harmonic generation and electro-optic coefficients. Maker fringe second harmonic generation measurements at 1907 nm give d11 equals 600 +/- 200 pm/V and electro-optic modulation measurements at 820 nm give r11 equals 400 +/- 150 pm/V. These initial results as well as preliminary results on the refractive indices, dielectric constants, transparency, and thermal stability of DAST suggest that it and related organic salts are promising materials for electro-optic device applications.

Solid state optical limiting materials based on phthalocyanine-containing polymers and organically-modified sol-gels

We demonstrate optical limiting in phthalocyanine doped solid host materials such as poly- (methylmethacrylate) and organically modified sol-gels. It is shown that the nanosecond nonlinear absorptive properties and the excited-state properties, such as the triplet-triplet absorption spectrum and the triplet quantum yields, of the metallophthalocyanines in these solid hosts are very similar to those in solutions. The key figures-of-merit for these materials suggest potential for the realization of high-performance solid-state optical limiters based on phthalocyanines.

Three-dimensional microfabrication using two-photon polymerization

Photopolymerization initiated by the simultaneous absorption of two photons is unique in its ability to produce complex three-dimensional (3D) structures from a single, thick photopolymer film. Strong 3D confinement of the polymerization process is not possible in other polymer microfabrication techniques such as LIGA, rapid prototyping, and conventional photoresist technology. Two-photon polymerization also permits the fabrication of 3D structures and the definition of lithographic features on non-planar surfaces. We have developed a wide array of chromophores which hold great promise for 3D microfabrication, as well as other applications, such as two-photon fluorescence imaging and 3D optical data storage. These materials are based on a donor- (pi) -donor, donor-acceptor-donor, or acceptor-donor-acceptor structural motif. The magnitude of the two-photon absorption cross-section, (delta) , and the position of the two-photon absorption maximum, (lambda) (2)max, can be controlled by varying the length of the conjugated bridge and by varying the strength of the donor/acceptor groups. In this way, chromophores have been developed which exhibit strong two- photon absorption in the range of 500 - 975 nm, in some cases as high as 4400 X 10-50 cm4 s/photon-molecule. In the case of donor-(pi) -donor structures, quantum-chemical calculations show that the large absorption cross-sections arise from the symmetric re-distribution of charge from the donor end-groups to the conjugated bridge, resulting in an electronic excited-state which is more delocalized than the ground state. For many of these molecules, two-photon excitation populates a state which is sufficiently reducing that a charge transfer reaction can occur with acrylate monomers. The efficiency of these processes can be described using Marcus theory. Under suitable conditions, such reactions can induce radical polymerization of acrylate resins. Polymerization rates have been measured, and we show that these two-photon chromophores display increased sensitivity and recording speed over conventional photoinitiators. Complex 3D structures can be fabricated in acrylate films doped with these chromophores using tightly focused near-infrared femtosecond laser pulses. A 3D periodic array of polymeric columns has been produced for use in photonic bandgap applications. Tapered waveguide structures for interconnecting disparate-sized optical components have been constructed. More traditional MEMS structures, such as cantilevers, have also been produced. Such structures may be useful for organic vapor sensors. The two-photon photopolymerization process can be extended to other material systems, such as metallic, ceramic, and composite materials, by templating the photopolymer structures.

Self-protecting optical limiters using cascading geometries

An increase in the dynamic range of an optical limiting device is achieved by using two different nonlinear elements cascaded within a single limiting device. A comparison between the results of optical limiting experiments for a single nonlinear element limiter and a two element limiter shows a significant increase in the damage threshold of the latter with only a small increase of the limiting threshold.

Development of electro-optic polymers for high-voltage instrument transformers

This paper describes some poled electrooptic bulk polymers (EOP) of the guest/host type having a cured epoxy resin as the host. The electrooptic polymers of typical dimensions 13 X 13 X 3 mm are characterized with respect to application as Pockels materials in an optical high voltage sensor. The electrooptic coefficients obtained here are of the same order of magnitude as those which are required for high voltage applications. An optical sensor based on disperse red 1 dye and an epoxy polymer is developed. It is able to measure voltages up to 10 kV AC. It is shown that bulk EOP can be produced with relatively large physical dimensions comparable to commercially available Pockels crystals. A technique is described which compensates for the inevitable intrinsic birefringence built into most bulk polymers. The epoxy based EOP shows high orientational stability, even when compared with a polysulfone based EOP having a higher glass transition temperature.

Second-order nonlinear optical properties of saccharide materials

Saccharide materials are potential candidates for frequency conversion applications. In addition to being chiral, which ensures crystallization in a space-group relevant for three-wave mixing processes, they generally possess useful physical and optical properties. We have examined the powder second harmonic generation efficiencies of both saturated saccharides and sugars with simple polar (pi) -functionalities. Powder efficiencies of up to 5 times that of sucrose were observed for simple saturated sugars, whereas values of 18 times sucrose (or 0.45 X urea) were observed for unsaturated saccharide derivatives. We have noted that for both classes of material, there is a tendency for more efficient nonlinear compounds to reside in a monoclinic rather than an orthorhombic space-group. We have also noted that there appears to be a correlation between the phase-matching potential and the crystal symmetry.