D. J. Dougherty

Measurement of the Raman gain spectrum of optical fibers

The stimulated Raman gain spectrum of optical fibers has been measured down to 6 cm(-1) by means of short pulses. Results for parallel and perpendicular polarizations are reported. With this technique, we observe spectral oscillations arising from a Brillouin mediated coupling between cw and pulsed light.

ULTRAFAST CARRIER DYNAMICS AND INTERVALLEY SCATTERING IN ZNSE

Intraband carrier dynamics were measured in ZnSe films by a two-wavelength pump probe technique with 180 fs resolution. A below-band pump pulse was used to heat carrier distributions in N-type samples by free carrier absorption. The electron cooling time constant was observed to be 500 fs. Intervalley scattering was seen to play a significant role in the electron dynamics and the scattering time from the L back to the Γ valley was measured to be 1.8 ps. By examining the ratio of the intervalley component to the total response as a function of pump photon energy, the bottom of the L valley was determined to lie 1.30 eV above the Γ-valley minimum.

Phototransformation in visible and near‐IR femtosecond pump‐probe studies of C60 films

The ultrafast non‐linear optical dynamics of C60 thin films were investigated at low excitation levels. The pump‐probe response observed at low power is found to differ significantly from the ultrafast non‐exponential behavior observed at higher intensity. It is determined that the ultrafast relaxation processes measured at higher fluence levels are affected by phototransformation of C60.

Coherent phonons in alkali metal-doped C60

We report the femtosecond impulsive excitation of coherent phonons in alkali metal-doped fullerenes. These excitations appear as small oscillations of the reflectivity (ΔR/R∼10−7) produced by the pump in an optical pump-probe experiment. The observed phonons include previously unobserved highly damped modes near 150 cm−1 as well as long-lived Ag(1) modes near 490 cm−1.

Femtosecond fiber laser pulses amplified by a KCl:Tl + color-center amplifier for continuum generation in the 1.5-μm region

A diode-pumped stretched-pulse additive-pulse mode-locked Er-doped fiber laser is used to seed a KCl:Tl + color-center amplif ier crystal. The initial l-nJ chirped pulses are double passed through the amplif ier, which is pumped by a 1-kHz Q-switched Nd:YAG laser. The resulting 10-microJ pulses are then chirp compensated to 250-fs duration and are used to generate a spectral continuum. A few high-n(2) materials are investigated for continuum generation, and spectral slicing of the continuum is demonstrated.

Growth and characterization of ZnSeGaAs single quantum well structures

Abstract The integration of II–VI and III–V semiconductors into a single II–VI/III–V heterojunction device enables the exploitation of the many similarities, as well as the many differences, in material properties (energy bandgap, lattice constant, dielectric constant, etc.) to create new devices exhibiting unique optical and electronic properties. The epitaxial growth of dielectric quantum wells (QWs) composed of ZnSe and GaAs is under investigation. One of the critical factors affecting the properties of the ZnSe GaAs QW structure is the formation of the heterovalent interfaces, particularly the formation of the inverted interface formed by GaAs nucleated onto a ZnSe epitaxial surface. The stoichiometry of each interface is engineered by using various growth techniques. Due to the severe mismatch in optimal growth temperatures for the two material systems (600°C for GaAs and 300°C for ZnSe), additional emphasis has been placed on the reduced temperature growth of GaAs on ZnSe. The effects of the growth parameters and nucleation methodology are examined by in situ surface reconstruction analysis using reflection high energy electron diffraction and ex situ using transmission electron microscopy. The optical properties of the reduced temperature GaAs and ZnSe GaAs QWs are further investigated with photoluminescence.