A. Haché

Quantum interference control of electrical currents in GaAs

In an earlier publication, preliminary observations of the generation of electrical currents were reported in GaAs and low-temperature-grown GaAs (LT-GaAs) at 295 K using quantum interference control of single- and two-photon band-band absorption of 1.55- and 0.775-/spl mu/m ultrashort optical pulses. Time-integrated currents were measured via charge collection in a metal-semiconductor-metal (MSM) electrode structure. Here we present detailed characteristics of this novel effect in terms of a simple circuit model for the MSM device and show how the injected current depends on MSM parameters as well as optical coherence, power, and polarization. For picosecond pulse excitation with peak irradiance of only 30 MW/cm/sup -2/ (1.55 /spl mu/m) and 9 kW/cm/sup -2/ (0.775 /spl mu/m), peak current densities of /spl sim/10 A/cm/sup -2/ at peak carrier densities of 10/sup 15/ cm/sup -3/ are inferred from the steady-state signals. This compares with 50 A/cm/sup -2/ predicted theoretically; the discrepancy mainly reflects inefficient charge collection at the MSM electrodes.

Generation of high-repetition-rate femtosecond pulses from 8 to 18 µm

We generated subpicosecond pulses from 8 to 18 mum by difference-frequency mixing in a 1-mm-thick AgGaSe(2) crystal, the 130- and 180-fs output pulses (1.45 < lambda < 1.85 mum) from an 84-MHz-repetition-rate optical parametric oscillator. Numerical simulations show that intrapulse and interpulse group velocity dispersion determine minimum pulse duration above and below 15 mum, respectively. By cross correlation (upconversion) of 10.5-mum pulses with 90-fs, 810-nm pulses in AgGaS(2), the pulse length was measured to be 310 fs in good agreement with simulations.

Three color coherent generation and control of current in low-temperature-grown GaAs

We demonstrate coherent generation and control of electrical currents in low-temperature-grown GaAs at 300 K using three phase-related, 150 fs pulses derived from a parametric process. Interference between single photon (0.8 μm) and nondegenerate two photon (1.4 and 1.8 μm) absorption amplitudes generates ballistic electrical currents whose beam polarization dependence is in agreement with a simple Fermi’s golden rule calculation.

Coherence Control of Photocurrents in Semiconductors

We present a general overview of our experimental and theoretical work on the use of the phase properties of one or more ultrashort optical pulses to generate and control electrical currents in semiconductors. This is discussed in a tutorial fashion as one manifestation of what has come to be called coherence control. Following a brief introduction to the basic concepts of coherence control phenomena, including examples of related work, we present two theoretical views of the current generation process. In the quantum viewpoint, current production occurs through generation of polar distributions of free carriers following interference of competing absorption pathways. For a monochromatic incident beam this interference is associated with different polarization components, while in the two color-case, involving harmonically related beams, it is associated with single and two photon absorption pathways. From a macroscopic viewpoint the currents arise because of divergences in imaginary parts of nonlinear susceptibilities, which are a measure of how a system provides a response to coherent light. We illustrate the main features of coherently controlled currents in the two-color case with experimental data for GaAs or LT-GaAs at room temperature using femtosecond, picosecond, or nanosecond pulses, while we demonstrate single-color current generation and control using CdSe in conjunction with cw or femtosecond lasers. A simple circuit model is used to discuss the steady-state current response of a metal-semiconductor-metal device illuminated by a train of incident pulses.

Control of photocurrent directionality via interference of single and two photon absorption in a semiconductor

Summary form only given. In quantum mechanics, if two or more perturbations induce a transition between the same initial and final states of system, the overall transition probability is determined by the modulus squared of the sum of the transition amplitudes for each perturbation. It is therefore possible for interference effects to determine the outcome as it does in the classical Youngs double slit experiment. If two coherent beams with frequencies /spl omega/ and 2/spl omega/ are applied to a system, the interference between the quantum mechanical pathways associated with single and two photon absorption events can lead to final states on the system whose properties are dependent on the relative phase of the beams.<<ETX>>

Mid- to far-infrared femtosecond pulses

The emergence of new, efficient, nonlinear crystals and the development of passively mode- locked, solid state lasers, such as the Kerr-lens mode-locked Ti:sapphire laser, has rekindled interest in optical parametric oscillators (OPOs) while opening up new regions of the spectrum to high repetition rate, high average power femtosecond pulses. With synchronous pumping techniques it is now possible to generate pulses as short as 40 fs for (nominally) 1 < (lambda) < 4 micrometers , at repetition rates of 108 Hz, and with average powers measured in 100s of mW. Tuning can be achieved in critically or non-critically phase-matched KTP or LBO via crystal angle, temperature or pump wavelength tuning each of which has its merits in terms of tuning range, ease of use, noise properties, etc.

COHERENCE CONTROL OF FREE CARRIERS IN BULK SEMICONDUCTORS

We review our recent experimental and theoretical work on the use of optically-induced quantum interference to generate and control electrical currents and free carrier populations in bulk, low-temperature-grown GaAs at room temperature. Using phase-related nanosecond, picosecond or femtosecond pulses at 1550 and 775 nm and the quantum interference between single and two photon interband absorption pathways, we produce peak current densities of ∼ 10Acm-2 for only 1014 cm-3 carriers in GaAs (001). Within a nonlinear optics context, the induced coherence effect can be understood in terms of a divergent piece of a x(3). With 150 fs optical pulses at wavelengths similar to those used in current control we are also able to use quantum interference effects to achieve control of electron-hole populations in GaAs (111). The nonlinear susceptibility responsible for this type of interference is x(2) xyz.