Observation of Nondegenerate Two-Photon Gain in GaAs.

Two-photon lasers require materials with large two-photon gain (2PG) coefficients and low linear and nonlinear losses. Our previous demonstration of large enhancement of two-photon absorption in semiconductors for very different photon energies translates directly into enhancement of 2PG. We experimentally demonstrate nondegenerate 2PG in optically excited bulk GaAs via femtosecond pump-probe measurements.

Ultrafast time-resolved quantum beats in the polarization state of coherent emission from quantum wells.

Dual-channel spectral interferometry is used to measure the ellipticity, the orientation of the polarization ellipse, and the sense of rotation of the four-wave-mixing signal from a GaAs-AlGaAs multiple quantum well. Each parameter is observed to oscillate dramatically at the heavy-hole-light-hole quantum beat frequency. During each beat period (in the strong quantum beat regime), the ellipticity oscillates twice between linear and almost-circular polarization, the orientation of the polarization ellipse rotates through a complete 180 degrees , and the sense of rotation changes from left- to right-circular polarization.

Enhanced second-harmonic generation in AlGaAs microring resonators.

Highly efficient second-harmonic generation can be achieved by harnessing resonance effects in microring resonator structures. We propose an angular quasi-phase-matching scheme based on the position dependence of polarization inside the ring resonator.

Coherence control of Hall charge and spin currents.

Using two-color optical coherence control techniques in intrinsic GaAs at 80 K with orthogonally polarized 70 fs, 1430 and 715 nm pulses, we generate a pure spin source current that yields a transverse Hall pure charge current; or alternatively, with parallel polarized pulses, we generate a pure charge source current that yields a pure spin current. By varying the relative phase or polarization of the incident pulses, one can effectively tune the type, magnitude and direction of both the source and transverse currents without application of electric or magnetic fields.

Distortionless light pulse delay in quantum-well Bragg structures.

We describe a reflection scheme that allows Bragg-spaced semiconductor quantum wells to be used to trap, store, and release light. We study the temporal and spectral distortion of delayed light pulses and show that this geometry allows multibit delays and offers a high degree of distortion compensation.

Quantum interference control of ballistic pure spin currents in semiconductors.

We demonstrate all-optical quantum interference injection and control of a ballistic pure spin current (without an accompanying charge current) in GaAs/AlGaAs quantum wells, consisting of spin-up electrons traveling in one direction and spin-down electrons traveling in the opposite direction. This current is generated through quantum interference of one- and two-photon absorption of approximately 100 fs phase-locked pulses that have orthogonal linear polarizations. We use a spatially resolved pump-probe technique to measure carrier movement of approximately 10 nm.

Characterization of the polarization state of weak ultrashort coherent signals by dual-channel spectral interferometry.

We demonstrate that dual-channel spectral interferometry in conjunction with a well-characterized reference pulse can be used to time resolve the polarization state of extremely weak ultrashort coherent signals from linear-and nonlinear-optical experiments by measuring the intensity and the phase of two orthogonal polarization components. In this way the signal is completely characterized.

Hot-carrier enhancement of photorefractive space-charge fields in zinc-blende semiconductors.

We observe an order-of-magnitude enhancement of the photorefractive effect in undoped CdTe on ~1-ps time scales that is caused by an overshoot in the two-photon-produced Dember space-charge field associated with hot-carrier transport.

Photorefractive nonlinearities caused by the Dember space-charge field in undoped CdTe.

The photorefractive nonlinearity associated with the Dember space-charge field between electrons and holes produced by two-photon absorption is unambiguously isolated and studied in undoped CdTe by using a nondegenerate, forward-probing, polarization-sensitive, transient-grating technique with a temporal resolution of <5 ps.

Picosecond separation and measurement of coexisting photorefractive, bound-electronic, and free-carrier grating dynamics in GaAs.

A novel transient-grating geometry, which is nondegenerate, copropagating, phase matched, and polarization sensitive, is used to isolate and measure independently the ultrafast dynamics of multiple coexisting gratings in GaAs:EL2 with a temporal resolution of <5 psec. This technique permits the measurement of the evolution of the photorefractive grating in materials with zinc blende symmetry, where the photorefractive grating is usually obscured by the stronger free-carrier and instantaneous bound-electronic gratings.

Observation of fundamental dark spatial solitons in semiconductors using picosecond pulses.

We report what is to our knowledge the first observation of fundamental dark spatial solitons. The solitons are launched in bulk semiconductors using an initial condition with odd symmetry. In order to form the initial condition, a glass platelet is positioned in one half of an incident 30-psec pulse, creating a pi phase shift in the spatial profile.

Picosecond photorefractive response of GaAs:EL2, InP:Fe, and CdTe:V.

Measurements and theoretical calculations are presented for the photorefractive effect in three semi-insulating semiconductors (GaAs:EL2, InP:Fe, and CdTe:V) using 29-psec pulses at a wavelength of 1.06 microm. The photorefractive gain is largest in the CdTe crystal and smallest in our InP sample.

Wavelength-tunable synchronous amplification of picosecond dye-laser pulses near 1 microm.

We report use of the dye Styryl 13 for picosecond synchronous amplification in a noncollinear longitudinally pumped dye-amplifier arrangement. In this manner we obtained pulses ~5 psec in duration, tunable from 900 to 990 nm, with energy up to 300 microJ, and focusable to near the diffraction limit.

Polarization-rotation switch using picosecond pulses in GaAs.

Photorefractive and free-carrier nonlinearities in GaAs transfer energy from a strong picosecond pump, which is linearly polarized at an arbitrary angle to a weak s-polarized probe, into a p-polarized beam propagating in the probe direction. By placing the GaAs between a crossed pair of high-quality polarizers that are set to extinguish the probe in the absence of the pump, an efficient, high-contrast, high-speed optical switch is realized. By varying the pumpprobe ratio, delay, fluence, and polarization, we can maximize the intensity of the p-polarized component.

Characterization of a high-gain picosecond flash-lamp-pumped Nd:YAG regenerative amplifier.

We evaluate the performance of a novel 10-Hz picosecond Nd:YAG regenerative amplifier configured as a selffiltering unstable resonator. Pulse energies of ~60 mJ in ~100 psec at 1.06 microm are achieved using a single 6-mm-diameter Nd:YAG rod, while mode-matching requirements are minimized.

Picosecond and femtosecond pulse generation near 1000 nm from a frequency-doubled Nd:YAG-pumped cw dye laser.

We report efficient synchronous mode locking of a 532-nm pumped cw dye laser tunable from 840 to 1030 nm, using a styryl compound as the gain medium. Hybrid synchronously pumped, passive mode locking of the laser using the saturable absorber DaQTeC generated 230-fsec pulses near 975 nm.

Picosecond photorefractive effect in BaTiO(3).

We report the observation of photorefractive index gratings written and read out in BaTiO(3) using single pulses of 30-psec duration and fluences of 1 to 15 mJ/cm(2) at a wavelength of 0.532 microm. While the photorefractive gratings are masked during formation by large free-carrier grating, they are clearly seen from 50 psec to many minutes following the peak in the writing beams.

Femtosecond synchronously pumped Pyridine dye lasers.

Optical pulses as short as 103 fsec at 695 nm and 263 fsec at 733 nm have been generated in synchronously pumped hybrid Pyridine 1 and Pyridine 2 cw dye lasers, respectively, both using the saturable absorber 1,1'-diethyl-2,2'-dicarbocyanine iodide (DDI). These results combine with other data from the same linear cavity to give direct femtosecond generation over the spectral range 560-840 nm.

Variable intracavity spectral windowing in a synchronously pumped femtosecond cw dye laser.

An aperture of variable width has been placed within an intracavity prism sequence in a synchronously pumped femtosecond dye laser. This is shown to permit direct control of the output pulse width in the range approximately 100-500 fsec. Wavelength tunability is achieved by translating the aperture transversely.