J. M. Fraser

THz emission from coherently controlled photocurrents in GaAs

We report broadband terahertz radiation from ballistic photocurrents generated via quantum interference of one- and two-photon absorption in low-temperature-grown and semi-insulating GaAs at 295 K. For 90 fs, 1550 and 775 nm optical pulses, we obtain phase-controllable near-single cycle 4 THz radiation. Higher frequency THz emission should be achievable with shorter pulses. At a 250 kHz repetition rate and average powers of 10 mW (1550 nm) and 400 μW (775 nm), we measure 3 nW of THz power, limited mainly by phase walkoff of the optical beams within the 1.5-μm-thick sample and collection efficiency.

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.

THz emission from coherently controlled currents in GaAs

Summary form only given. Coherence control (CC) makes use of the phase of optical beams to alter the properties of matter via interference of quantum mechanical pathways. Earlier it was shown that CC electrical currents can be generated in GaAs and LT-GaAs at 295 K by interfering single and two photon transition amplitudes coupling the same valence and conduction band states. The current temporal profile is expected to follow the optical pulse envelope before decaying to zero on a subpicosecond time scale due to relaxation effects. However in Ref. 1 time-integrated photocurrents were measured using a train of femtosecond pulses and a metal-semiconductor-metal charge collector. Here we report observation of THz radiation from the transient currents. CC currents permit tailoring of THz radiation through the bandwidth of optical pulses, e.g., producing single cycle high frequency radiation and overcoming limitations of biased antenna emitters. The radiation is also a diagnostic of intraband coherence dynamics of nonequilibrium carriers.

Coherent control of electron–hole populations in GaAs

The electron–hole generation rate in (111)-oriented GaAs is coherently controlled through the relative phase of absorbed 1550 and 775 nm, 120 fs pulses. Quantum interference between single- and two-photon transitions underlies this process which, in general, only occurs in noncentrosymmetric semiconductors since macroscopically it is related to a second-order susceptibility (χ(2)).

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.

Coherent control of free-carrier density in GaAs

Coherent control is the exploitation of interference between two or more optically excited quantum mechanical pathways to manipulate the final state of a physical system. Recently it has been used to control exciton populations in GaAs-AlGaAs quantum wells at 4 K via phase-related 805-nm 100-fs pulses and to generate electrical current in bulk GaAs at 295 K via 1.55 /spl mu/m and 0.775 /spl mu/m, 175 to 1000-fs pulses. We consider here the coherent control of electron-hole density in GaAs at 295 K via the interference of single- and two-photon band-to-band generation processes.

Quantum Interference Control of Free Carriers in Semiconductors

Optically-induced quantum interference is used to generate and control electrical currents and free carrier populations in bulk semiconductors at room temperature. In a macroscopic picture this coherent response can be described in terms of nonlinear optical susceptibilities. We demonstrate current control in GaAs by simultaneous single and two photon absorption processes involving picosecond or femtosecond pulses via a divergent piece of χ (3) process. Using a polarization- controlled single beam and a χ (2) process we demonstrate coherent control of current in CdSe, a wurtzite structure material. Finally we partially control carrier populations via a χ (2) process.