High-power 390-nm laser source based on efficient frequency doubling of a tapered diode laser in an external resonant cavity.

We frequency doubled the single-frequency beam from an external-cavity tapered laser diode operating at 780 nm in a resonant cavity containing a beta -barium borate crystal to generate an output at 390 nm with high efficiency. Output powers as great as 233 mW were obtained, corresponding to an efficiency of 65%/W . The resonant-cavity design was a low-loss three-mirror configuration that provided compensation for astigmatism and coma. The laser diode frequency was locked to the doubling-cavity resonance by use of the Hänsch-Couillaud discrimination technique.

Solid-State Cavity Lasing from Poly(p-Phenylene Vinylene)-Silica Nanocomposite Bulk.

We have fabricated inorganic-organic nanocomposite bulk samples consisting of poly(p-phenylene vinylene) (PPV) and silica by in situ polymerization of a PPV salt monomer within a porous glass using a base-catalyzed polymerization reaction and subsequent heat treatment. The samples processed at temperatures above 200 degrees C showed a sharp reduction in fluorescence. Solid-state cavity lasing was achieved from the samples processed at 150 degrees C with optical efficiency as high as 11.4%. We report characteristic narrowing of the linewidth and the temporal profile.

Optical limiting, pulse reshaping, and stabilization with a nonlinear absorptive fiber system.

Optical limiting, pulse reshaping, and stabilization effects have been demonstrated based on a two-photon absorption mechanism with a dye-solution-filled hollow fiber system. The nonlinear absorptive medium is the solution of a new dye, trans-4-[p-(N-hydroxyethyl-N-methylamino)styryl]-N-methylpyridinium iodide (ASPI) in dimethyl sulfoxide, with which we filled a 20-cm-long quartz hollow fiber of 100-mum internal diameter. The input optical signal was a laser pulse train that contained ~30 pulses of 130-ps pulse width. When the input peak intensity reached 400-1000-MW/cm(2) levels, obvious optical limiting could be observed and the envelope of the transmitted pulse train became flatter and broader. By using another new dye solution, 4-[N-(2-hydroxyethyl)-N-(methyl)amino phenyl]-4?-(6-hydroxyhexyl sulfonyl)-stilbene (APSS) in benzyl alcohol, which interacted with a series of ~800-nm laser pulses of ~8-ns pulse width, we obtained a much higher nonlinear absorption coefficient and a superior optical peak-power stabilization effect.

Two-photon resonance-enhanced refractive-index change and self-focusing in a dye-solution-filled hollow fiber system.

The general behavior of two-photon absorption-enhanced refractive-index change in a third-order nonlinear optical medium is briefly described. The nonlinear medium was the solution of a new dye, trans-4-[p-(N-hydroxyethyl-N-methylamino)styryl]-N-methyl pyridinium iodide (ASPI) in dimethyl sulfoxide as the solvent, that was poured into a 20-cm-long quartz hollow fiber of 100-mum internal diameter. This dye solution has a strong two-photon absorption and subsequent upconversion fluorescence emission when excited with 1064-nm laser radiation. When the input peak intensity reached 500-1500-MW/cm(2) levels, obvious changes in beam profiles of the output IR laser beam were observed due to a self-focusing or self-trapping process occurring inside the fiber system. As a result of this process, highly directional frequency-upconverted superradiant lasing output was obtained with a beam size ~5 times smaller than that of a linearly transmitted He-Ne probe laser beam. The demonstrated mechanism can be useful for fiber laser-amplifier and fiber-integrated optics devices.

Two-photon photodynamic therapy.

OBJECTIVE: We demonstrate the use of infrared excitation in conjunction with an efficient two-photon absorbing dye and a photosensitizer in photodynamic therapy. SUMMARY BACKGROUND DATA: An efficient two-photon absorbing dye is excited by short infrared (800 nm) laser pulses, which transfer its energy to the photosensitizer and the photosensitizer, in turn, generates the singlet oxygen. METHODS: A new approach to photodynamic cancer therapy based on the strong two-photon absorption of certain newly developed organic molecules. Near infrared pulsed laser light efficiently excites these molecules which, in turn, transfer the energy to the photosensitizer used in photodynamic therapy. RESULTS: A newly synthesized two-photon absorbing dye 4-[N-(2-hydroxyethyl)-N-(methyl) amino phenyl]-4'-(6-hydroxyhexyl sulfonyl)stilbene (APSS), which exhibits a strong two-photon absorption at 800 nm, and upconverted fluorescence at 520 nm, in solution in the presence of a photosensitizer was found to generate singlet oxygen under infrared excitation (800 nm). The generation of singlet oxygen in a reaction system containing two-photon absorbing dye and photosensitizer under infrared excitation has been chemically detected by using ADPA (9,10-anthracenedipropionic acid) as singlet oxygen detector. CONCLUSIONS: The efficient two-photon absorption of certain chromophores can be used to extend photodynamic therapy to the near infrared spectral region. Whereas the photosensitizer itself may not exhibit two-photon absorption, it could be used in conjunction with one of these new chromophores. The chromophores can act as "photon harvesters" whereby they absorb two photons of near infrared light and transfer the energy to the photosensitizer, which can generate singlet oxygen in the presence of atmospheric oxygen.

Three-dimensional laser scanning two-photon fluorescence confocal microscopy of polymer materials using a new, efficient upconverting fluorophore.

Three-dimensional confocal imaging of polymer samples was achieved by the use of two-photon excited fluorescence in both positive and negative contrast modes. The fluorophore was a new and highly efficient two-photon induced upconverter, resulting in improved signal strength at low pumping power. Because of the relatively long wavelength of the excitation source (798 nm from a mode-locked Ti:Sapphire laser), this technique shows a larger penetration depth into the samples than provided by conventional single-photon fluorescence confocal microscopy. Single-photon and two-photon images of the same area of each sample show significant differences. The results suggest the possibility of using two-photon confocal microscopy, in conjunction with highly efficient fluorophores, as a tool to study the surface, interface, and fracture in material science applications.

Three-photon-absorption-induced fluorescence and optical limiting effects in an organic compound.

Three-photon-absorption-induced frequency-upconversion f luorescence emission has been observed in a solution of 2,5-benzothiazole 3,4-didecyloxy thiophene (BBTDOT) in tetrahydrofuran, pumped with 10-ns Q-switched 1.06-microm laser pulses. The spectral peak of the observed fluorescence emission is located in the 450-480-nm range, and the intensity dependence of the visible emission on the IR excitation obeys the cubic law. At a higher solute concentration (0.18 M/L) and moderate IR excitation intensity levels (50-200 MW/cm(2)), obvious optical limiting behavior has been observed as a result of three-photon absorption. The measured nonlinear absorption coeff icient and the corresponding molecular three-photon-absorption cross section are gamma= 2.7 x 10(-18) cm(3)/W(2) and sigma'(3) = 8.8 x 10(-76) cm(6) s(2), respectively.