G. T. Kennedy

AlGaAs BELOW HALF BANDGAP: THE SILICON OF NONLINEAR OPTICAL MATERIALS

Very few nonlinear optical materials are actually useful for high throughput all-optical devices. However, AlGaAs does satisfy all of the nonlinear optical figures of merit when used with photons of energy less than one half the semiconductor bandgap. Here we review our measurements of the pertinent nonlinear coefficients in waveguides and various device applications to all-optical switching in the communications band around 1550 nm.

Ultralow-pump-threshold, femtosecond Cr 3+ :LiSrAlF 6 laser pumped by a single narrow-stripe AlGaInP laser diode

We report what we believe to be the first demonstration of a Kerr-lens mode-locked Cr(3+):LiSrAlF(6)laser that is pumped by a single narrow-stripe AlGaInP laser diode with a diffraction-limited output beam. A novel low-loss three-mirror laser cavity design is described in which strong, localized Kerr lensing was exploited such that 75-fs-duration pulses were obtained for only 36mW of incident pump power. This pump power was maintained for 18h by just three AA batteries as the electrical power source. We have shown that mode locking can be sustained for pump powers as low as 22mW.

Polarization dependence of ultrafast nonlinear refraction in an AlGaAs waveguide at the half-band gap

We have obtained the polarization dependence of ultrafast nonlinear refraction by measuring the orientational dependence of self-phase modulation and cross-phase modulation in an Al0.18Ga0.82As waveguide at a frequency just beneath the two-photon absorption edge. It was found that nonlinear refraction exhibits a considerable anisotropy in accordance with theoretical calculations for GaAs. It was also found, as predicted, that perpendicular cross-phase modulation behaves differently from the isotropic Kleinmann result appropriate to silica fibers.

Localized Kerr‐type nonlinearities in GaAs/AlGaAs multiple quantum well structures at 1.55 μm

We report the use of a novel impurity free vacancy disordering technique which has been used to produce waveguides with different Kerr‐type nonlinear coefficients. The technique relies on standard SiO2 dielectric caps to promote disordering and Ga2O3 caps to suppress disordering. Band‐gap shifts of around 40 nm and consequent changes in n2 of more than 60% are reported.

Modification of the second-order optical nonlinearities in AlGaAs asymmetric multiple quantum well waveguides by quantum well intermixing

We demonstrate that a quantum well intermixing technique can be used to control the second-order nonlinearity χzzz(2) in an AlGaAs asymmetric coupled quantum well waveguide structure at 1.52 μm. Photoluminescence measurements also indicate that the spatial resolution of the impurity-free vacancy disordering process used for quantum well intermixing is better than 1.5 μm which should be sufficient for first-order quasiphase-matched second harmonic generation.

All‐optical modulation with ultrafast recovery at low pump energies in passive InGaAs/InGaAsP multiquantum well waveguides

We report on the observation of ultrafast (<1 ps) recovery of all‐optical modulation based on below band gap, optically induced excess absorption using pump pulse energies of less than 4 pJ. At higher pump pulse energies, the hot carriers produced by two‐photon absorption significantly reduce the recovery rate. The increase in absorption recovery times at the higher densities of hot carriers is consistent with a contribution to the nonlinear absorption change from hot electron assisted absorption. The measured hot electron assisted absorption cross section of 2×10−20 m2 is in good agreement with the theoretical value.

Exciton saturation in room temperature GaAs/AlGaAs multiple quantum wells

We report time resolved studies of optically induced circular dichroism in room temperature GaAs/AlGaAs multiple quantum wells to resolve spin-dependent and spin-independent contributions to exciton saturation. Phase-space filling and Coulomb contributions were separated by spin dependence while the effects of broadening and screening were distinguished using different pulse bandwidths. Bound and free carrier contributions were compared by observing the temporal dependence of the circular dichroism on femtosecond timescales. The spin independent contribution to exciton saturation was found to be independent of whether the carriers were bound or free.

Quantum-well intermixing for the control of second-order nonlinear effects in AlGaAs multiple-quantum-well waveguides

We present experimental evidence to demonstrate the feasibility of a promising new quasi-phase-matching technique in AlGaAs multiple-quantum-well waveguides. Non-phase-matched second-harmonic-generation measurements indicate that, for sub-half-bandgap excitation near 1.5 microm , quantum-well intermixing by impurity-free vacancy disordering results in a reduction of the nonlinear susceptibility chi((2))(zxy) (~340 pm/V) by 17%. Relatively low intermixed waveguide losses, and the high spatial resolution of the impurity-free vacancy disordering process, suggest that periodic intermixing along the direction of propagation should lead to useful frequency-conversion efficiencies.

Bright solitary pulses in AlGaAs waveguides at half the band gap

We show that bright temporal solitonlike pulses can propagate in AlGaAs waveguides excited in the half-bandgap spectral regime. The use of an antiresonant reflecting optical waveguide permits the dispersion of the waveguide to be controlled so that a mode with anomalous dispersion can propagate.

Amplitude noise reduction of a coupled-cavity mode-locked NaCl:OH- color-center laser by active synchronization.

Stable noise-free operation of a coupled-cavity mode-locked NaCl:OH− color-center laser was obtained by synchronizing it to the mode-locked Nd:YAG pump laser. This stabilization technique, based on active cavity-length control of the color-center laser, eliminated the amplitude modulation of the output power arising from the beating of the pump laser pulse train with the color-center laser pulse train.