THz emission from coherently controlled photocurrents in GaAs

We report broadband terahertz radiation from ballistic photocurrents genera ted via quantum interference of one- and two-photon absorption in low-tempe rature-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 radiat ion. 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 40 0 mu W (775 nm), we measure 3 nW of THz power, limited mainly by phase walk off of the optical beams within the 1.5-mu m-thick sample and collection ef ficiency. (C) 1999 American Institute of Physics. [S0003-6951(99)01751-9].