Frank W. Wise

Self-Similar Evolution of Parabolic Pulses in a Laser

Self-similar propagation of ultrashort, parabolic pulses in a laser resonator is observed theoretically and experimentally. This constitutes a new type of pulse shaping in mode-locked lasers: in contrast to the well-known static (solitonlike) and breathing (dispersion-managed soliton) pulse evolutions, asymptotic solutions to the nonlinear wave equation that governs pulse propagation in most of the laser cavity are observed. Stable self-similar pulses exist with energies much greater than can be tolerated in solitonlike pulse shaping, and this has implications for practical lasers.

In vivo three-photon microscopy of subcortical structures within an intact mouse brain

We demonstrate non-invasive, high-resolution, in vivo imaging of subcortical structures (the external capsule (EC) and hippocampus) within an intact mouse brain using three-photon fluorescence microscopy at the new spectral window of 1700 nm.

Electronic structure and optical properties of PbS and PbSe quantum dots

The electronic structure of spherical PbS and PbSe quantum dots is calculated with a four-band envelope-function formalism. This calculation accounts for both exciton energies and wave functions with the correct symmetry of the materials. The selection rules and the strength of the dipole transitions of lead-salt quantum dots are derived accounting for the symmetry of the band-edge Bloch functions of the lead salts. The calculated energies of the optically allowed exciton states are found to be in good agreement with experimental data. The effects of many-body perturbations, such as Coulomb interactions and intervalley scattering, are also discussed.

Electron Injection from Colloidal PbS Quantum Dots into Titanium Dioxide Nanoparticles

Injection of photoexcited electrons from colloidal PbS quantum dots into TiO(2) nanoparticles is investigated. The electron affinity and ionization potential of PbS quantum dots, inferred from cyclic voltammetry measurements, show strong size dependence due to quantum confinement. On the basis of the measured energy levels, photoexcited electrons should transfer efficiently from the quantum dots into TiO(2) only for quantum-dot diameter below approximately 4.3 nm. Continuous-wave fluorescence spectra and fluorescence transients of PbS quantum dots coupled to titanium dioxide nanoparticles are consistent with electron transfer for small quantum dots. The measured electron transfer time is surprisingly slow ( approximately 100 ns), and implications of this for future photovoltaics will be discussed. Initial results obtained from solar cells sensitized with PbS quantum dots are presented.

Spatiotemporal optical solitons

In the course of the past several years, a new level of understanding has been achieved about conditions for the existence, stability, and generation of spatiotemporal optical solitons, which are nondiffracting and nondispersing wavepackets propagating in nonlinear optical media. Experimentally, effectively two-dimensional (2D) spatiotemporal solitons that overcome diffraction in one transverse spatial dimension have been created in quadratic nonlinear media. With regard to the theory, fundamentally new features of light pulses that self-trap in one or two transverse spatial dimensions and do not spread out in time, when propagating in various optical media, were thoroughly investigated in models with various nonlinearities. Stable vorticity-carrying spatiotemporal solitons have been predicted too, in media with competing nonlinearities (quadratic–cubic or cubic–quintic). This article offers an up-to-date survey of experimental and theoretical results in this field. Both achievements and outstanding difficulties are reviewed, and open problems are highlighted. Also briefly described are recent predictions for stable 2D and 3D solitons in Bose–Einstein condensates supported by full or low-dimensional optical lattices.

All-normal-dispersion femtosecond fiber laser with pulse energy above 20 nJ.

We report a study of the scaling and limits to pulse energy in an all-normal-dispersion femtosecond fiber laser. Theoretical calculations show that operation at large normal cavity dispersion is possible in the presence of large nonlinear phase shifts, owing to strong pulse shaping by spectral filtering of the chirped pulse in the laser. Stable pulses are possible with energies of tens of nanojoules. Experimental results from Yb-doped fiber lasers agree with the trends of numerical simulations. Stable and self-starting pulses are generated with energies above 20 nJ, and these can be dechirped to <200 fs duration. Femtosecond pulses with peak powers near 100 kW are thus available from this simple and practical design.

Highly nonlinear As–S–Se glasses for all-optical switching

We have synthesized a series of chalcogenide glasses from the As-S-Se system that is designed to have strong nonlinearities. Measurements reveal that many of these glasses offer optical Kerr nonlinearities greater than 400 times that of fused silica at 1.25 and 1.55mum and figures of merit for all-optical switching greater than 5 at 1.55mum .

Properties of normal-dispersion femtosecond fiber lasers

We report a systematic study of all-normal-dispersion mode-locked fiber lasers. Spectral filtering of a chirped pulse in the cavity is a major component of the pulse shaping in these lasers. We identify the nonlinear phase shift accumulated by the pulse, spectral filter bandwidth, and group-velocity dispersion as the key parameters that determine the behavior and properties of these lasers. Trends in the performance as these parameters are varied are summarized. A wide range of pulse shapes and evolutions can occur. Experimental results from Yb-doped all-normal-dispersion fiber lasers agree reasonably well with the results of numerical simulations.

Synthesis and characterization of PbSe quantum dots in phosphate glass

The controlled synthesis of PbSe nanocrystal quantum dots with narrow size distributions was achieved through phase decomposition of the PbSe solid solution in a phosphate glass host. Structural characterization by electron microscopy and x-ray diffraction shows that the dots have mean diameters between 2 and 15 nm. The exciton Bohr radius aB=46 nm in PbSe, so these quantum dots provide unusual and perhaps unique access to the regime of strong quantum confinement. The optical absorption spectra are compared to the predictions of a theoretical treatment of the electronic structure. The theory agrees well with experiment for dots larger than ∼7 nm, but for smaller dots there is some deviation from the theoretical predictions.

Highly nonlinear Ge-As-Se and Ge-As-S-Se glasses for all-optical switching

We have synthesized Ge-As-Se and Ge-As-S-Se chalcogenide glasses designed to have large optical nonlinearities. Measurements reveal that these glasses offer optical Kerr nonlinearities greater than 500 times that of fused silica and figures of merit for all-optical switching >5 at 1.25 and 1.55 /spl mu/m.