Q. Z. Wang

Excited-state nonlinearity in polythiophene thin films investigated by the Z-scan technique

Measurements of the third-order optical nonlinear susceptibility χ(3) have shown that nonlinearity around the single-photon absorption region is dominated by the saturation absorption in polythiophene thin films. The sign and the size of the real and the imaginary part of χ(3) have been measured at 532 nm to be −6.1 × 10−9 and −1.9 × 10−9 esu, respectively, by the Z-scan method.

Third-order optical nonlinearity in polycondensed thiophene-based polymers and polysilane polymers

The relaxation kinetics and size of the third-order nonlinear susceptibility χ(3) of a homologous series of polycondensed thiophene-based polymers and polysilane polymers were determined by ultrafast nonlinear laser techniques. The wavelength dependence over 530–1060 nm of χ(3) was measured for polycondensed thiophene-based polymers. The resonant χ(3) was determined to be of the order of 10−9 esu. For polysilane polymers, the measured nonresonant χ(3) value was ~10−12 esu. The response time of χ(3) for both polymers was determined to be less than the resolution of optical pulse durations, <3 psec for polysilane and <12 psec for polycondensed thiophene-based polymers.

Fourier spatial filter acts as a temporal gate for light propagating through a turbid medium

The temporal profiles of ultrashort light pulses propagating through turbid media of different lengths were measured with a streak camera. Various Fourier spatial filters were used to select the spatial frequencies of the scattered pulses. The temporal profiles of the pulses scattered by a 0.4% Intralipid solution in a cell of 5-cm thickness were signif icantly narrowed because of the removal of the higher-frequency components by a Fourier spatial filter.

ULTRAFAST OPTICAL PULSE DIGITIZATION WITH UNARY SPECTRALLY ENCODED CROSS-PHASE MODULATION

A method to digitize the intensity of ultrashort laser pulses for high-speed optical signal processing is described. This digitization was based on the spectral broadening of a weak probe (carrier) pulse by a more intense pump (signal) pulse through the nonlinear optical process of cross-phase modulation (XPM). The signal pulse intensity was varied to generate different spectral widths that can be encoded into digital form. Using a 50-ps time-divided multiplexing pulse train with a waveguide splitter, combiner, and an array of fibers with variable lengths, a unary XPM encoding approach is demonstrated. The spectral encoding scheme can be used to achieve a 5-GHz sampling rate at a 16-level accuracy.

Ultrafast resonant optical Kerr effect in 4-butoxycarbonylmethylurethane polydiacetylene

The resonant optical Kerr effect in the isotropic phase of 4-butoxycarbonylmethylurethane polydiacetylene was measured to determine the third-order optical nonlinearity χ3 and the relaxation time.

Two-photon excitation of fluorescence from chicken tissue

We have measured UV fluorescence excited through two-photon absorption from native chicken tissue, using 600-nm, 500-fs pulses from a R6G dye laser. The observed emission signal was found to depend quadratically on the excitation intensity. The two-photon excitation-induced fluorescence spectrum is attributed to tryptophan residues in proteins.

Noninvasive two-photon-excitation imaging of tryptophan distribution in highly scattering biological tissues

Abstract We report on noninvasive two-photon fluorescence (TPF) image mapping of tryptophan distribution in highly-scattering native biological tissues. Fluorescence from tryptophan molecules in tissue samples was excited by two-photon absorption of 625 nm, 100 fs light pulses from a colliding-pulse-modelocked dye laser. A two-dimensional map of the tryptophan distribution in a chicken-tissue sample was obtained by scanning the laser beam over the tissue surface and accumulating the fluorescence signal point-by-point with a photomultiplier tube and a lock-in amplifier system. Images of a tissue at an axial position up to 250 μm from the surface were recorded with a lateral resolution of ∼10 μm. The technique is wavelength selective, morphology dependent, and molecule specific. This method offers potential for a less invasive method by giving the analysis of certain molecules in tissue structures.

High-resolution spectra of self-phase modulation in optical fibers

We present theoretical calculations and experimental measurements of self-phase-modulation spectra generated with picosecond laser pulses at 532 and 1064 nm propagating in optical fibers. The spectral structures of self-phase-modulation spectra are observed with a high-resolution optical spectral analysis system. The experimental observations are in good agreement with the theoretical predictions.

Single-pulse degenerate-cross-phase modulation in a single-mode optical fiber.

Using a single ultrashort optical pulse propagating through a short nonbirefringent single-mode optical fiber, we have observed degenerate-cross-phase modulation of the two orthogonal circularly polarized components of the pulse. The spectral broadening of the two polarization components in the single laser pulse is attributed to self-phase modulation (SPM), degenerate-cross-phase modulation (DXPM), and interference between SPM and DXPM.

Ultrahigh repetition rate picosecond pulses from a passively mode-locked neodymium glass laser by solid-state saturable dye films

Ultrashort pulses with ultrahigh tunable repetition rates up to 50 GHz are produced by the use of a solid-state saturable absorber film of polymethine dyes in a polymer matrix to mode lock a neodymium glass laser. The energy and the pulse duration of the ultrashort pulses and the stability of the train are measured. The shortest duration pulses produced are ~4 ps with no noticeable change in the optical properties of solid-state dye films after several hundred shots of operation.