All-optical switching based on inverse Raman scattering in liquid-core optical fibers.

We report on a new platform for all-optical switching based on inverse Raman scattering in liquids. Narrowband switching, which could be suitable for wavelength-division-multiplexed applications, is demonstrated using integrated liquid-core optical fiber infiltrated with both neat liquids (CCl(4) and CS(2)) as well as an organic chromophore (ß-carotene) dissolved in CCl(4). Compared to standard glass optical fibers, these liquids have much larger Raman loss coefficients, which help reduce the pump power by at least an order of magnitude. Further improvements can be expected with the development of highly soluble organic compounds possessing large Raman cross sections.

Demonstration of Zeno switching through inverse Raman scattering in an optical fiber.

We report the observation of Zeno switching through an inverse Raman scattering (IRS) process in an optical fiber. In IRS, light at the anti-Stokes frequency is strongly attenuated in the presence of a pump field, allowing it to be used for all-optical switching and modulation. Our observed level of induced absorption via IRS in the optical fiber is > 20 dB in a time scale of less than 5 ps. The full Raman response spectrum was extracted experimentally and excellent agreement was found between the experimental data and theoretical modeling of IRS.

Synthesis of organic salts with large second-order optical nonlinearities.

A series of organic salts, in which the cation has been designed to have a large molecular hyperpolarizability, has been prepared. Variation of the counterion (anion) in many cases leads to materials with large powder second harmonic generation efficiencies, the highest of which is roughly 1000 times that of a urea reference.

A unified description of linear and nonlinear polarization in organic polymethine dyes.

An internal or external electric field F can drive the chemical structure, bond order alternation, and electronic structure of linear polymethine dyes from a neutral, bond-alternated, polyene-like structure, through a cyanine-like structure, and ultimately to a zwitterionic (charge-separated) bond-alternated structure. As the structure evolves under the influence of F, the linear polarizability alpha, the first hyperpolarizability beta, and the second hyperpolarizability gamma are seen to be derivatives, with respect to F, of their next lower order polarization (for alpha) or polarizability (for beta and gamma). These derivative relations provide a unified picture of the dependence of the polarizability and hyperpolarizabilities on the structure in linear polymethine dyes. In addition, they allow for predictions of structure-property relations of higher order hyperpolarizabilities.

Large first hyperpolarizabilities in push-pull polyenes by tuning of the bond length alternation and aromaticity.

Conjugated organic compounds with 3-phenyl-5-isoxazolone or N,N'-diethylthiobarbituric acid acceptors have large first molecular hyperpolarizabilities (beta) in comparison with compounds with 4-nitrophenyl acceptors. For example, julolidinyl-(CH=CH)(3)-CH=N,N'- diethylthiobarbituric acid, which has 12 atoms between the donor and acceptor, has a beta(0) of 911 x 10(-30) electrostatic units, whereas (CH(3))(2)NC(6)H(4),-(CH=CH)(4)-C(6)H(4)NO(2), with 16 atoms between its donor and acceptor, has a beta(0) of 133 x 10(-30) electrostatic units. The design strategies demonstrated here have resulted in chromophores that when incorporated into poled-polymer electrooptic modulators exhibited significant enhancements in electrooptic coefficients relative to polymers containing the commonly used dye Disperse Red-1. Poled polymer devices based on these or related chromophores may ultimately lead to high-speed electrooptic switching elements with low drive-power requirements, suitable for telecommunications applications.

Relation between bond-length alternation and second electronic hyperpolarizability of conjugated organic molecules.

The solvent dependence of the second hyperpolarizability, gamma, of a variety of unsaturated organic compounds has been measured by third harmonic generation at 1907 nanometers. It is seen that the measured gamma is a function of solvent polarity. These solvent-dependent hyperpolarizabilities are associated with changes in molecular geometry from a highly bond-length alternated, polyene-like structure for a formyl-substituted compound in non-polar solvents, to a cyanine-like structure, with little bond-length alternation, for a dicyanovinyl-substituted compound in polar solvents. By tuning bond-length alternation, gamma can be optimized in either a positive or negative sense for polymethine dyes of a given conjugation length.

Design of organic molecules with large two-photon absorption cross sections.

A strategy for the design of molecules with large two-photon absorption cross sections, delta, was developed, on the basis of the concept that symmetric charge transfer, from the ends of a conjugated system to the middle, or vice versa, upon excitation is correlated to enhanced values of delta. Synthesized bis(styryl)benzene derivatives with donor-pi-donor, donor-acceptor-donor, and acceptor-donor-acceptor structural motifs exhibit exceptionally large values of delta, up to about 400 times that of trans-stilbene. Quantum chemical calculations performed on these molecules indicate that substantial symmetric charge redistribution occurs upon excitation and provide delta values in good agreement with experimental values. The combination of large delta and high fluorescence quantum yield or triplet yield exhibited by molecules developed here offers potential for unprecedented brightness in two-photon fluorescent imaging or enhanced photosensitivity in two-photon sensitization, respectively.

Regulation of ornithine decarboxylase in cultured mouse mammary gland by the osmolarity in the cellular environment.

The biphasic increase of ornithine decarboxylase activity in mouse mammary gland in organ culture occurs with a hormone-independent first peak and a hormone-dependent second peak. The data presented indicate that a change in the osmolarity of the cellular environment is the major contributing factor for the emergence of the hormone-independent ornithine decarboxylase activity in mammary explants. Thus, incubation of mammary explants for 3 h in a medium diluted 53% with distilled water results in approx. 1000-fold stimulation of enzyme activity over the initial level, whereas a similar dilution of the medium with 0.18 M NaCl or 0.3 M sucrose blocks the increase. The increase in enzyme activity is similarly affected by a reduction of the concentration of NaCl in the culture medium. The hypoosmotic stimulation of ornithine decarboxylase activity appears to be affected at a posttranscriptional level, and is enhanced further by the actions of insulin and prolactin. The hypoosmotic enhancement of ornithine decarboxylase activity produces a large increase in the intracellular concentration of putrescine in mammary explants. However, neither the concentration of spermidine and spermine nor the activity of S-adenosyl-L-methionine decarboxylase is affected. In addition, studies of putrescine transport in mammary explants show that hypotonicity causes an increase in the rate of influx and a decrease in efflux of putrescine with enhancement of intracellular putrescine accumulation. On the other hand, the uptake of spermidine, spermine, amino acids, sugar, and a lipophilic cation, triphenylmethylphosphonium is unaffected. These data suggest a possibility that osmotic alteration in cellular environment causes an incresed need for putrescine in mammary cells, resulting in stimulation of ornithine decarboxylase activity, which may represent a cellular mechanism for maintaining the homeostasis of the intracellular cationic environment.

Putrescine and the regulation of S-adenosyl-L-methionine decarboxylase in cultured mouse mammary gland.

The activity of S-adenosyl-L-methionine decarboxylase (S-adenosyl-L-methionine carboxy-lyase, EC 4.1.1.50) purified from mouse mammary gland can be stimulated by physiological concentrations of putrescine (Ka = 5 x 10(-7) M). In contrast, the elevation of the intracellular concentrations of putrescine in cultured mouse mammary tissue by exogenous addition of putrescine or by cultivation of the tissue in hypotonic medium results in the decrease in both the activity and the amount of the enzyme in tissue. Conversely, the addition of 1 mM alpha-methyl ornithine, an inhibitor of ornithine decarboxylase, causes augmentation of the activity and the amount of S-adenosyl-L-methionine decarboxylase, despite its inhibitory action on insulin-stimulated increase in the concentration of putrescine. These data suggest that putrescine can act as a 'negative' regulator of the enzyme in this system.