We have studied terahertz ͑THz͒ emission from InAs and GaAs in a magnetic field, and find that the emitted radiation is produced by coupled cyclotron-plasma charge oscillations. Ultrashort pulses of THz radiation were produced at semiconductor surfaces by photoexcitation with a femtosecond Ti-sapphire laser. We recorded the integrated THz power and the THz emission spectrum as a function of magnetic field at fields up to 5.5 T, and as function of temperature for Tϭ10-280 K. The maximum observed THz power is ϳ1.6ϫ10 Ϫ13 J/pulse ͑12 W average power͒ from n-InAs (1.8ϫ10 16 cm Ϫ3 ͒ at Bϭ3.2 T. We compare our results to semiclassical models of magnetoplasma oscillations of bulk free carriers and damped motion of free carriers in a twodimensional electron gas. The bulk model describes THz emission from n-GaAs at all magnetic fields, and InAs at Bϭ0. It fails to describe THz emission from InAs at nonzero magnetic fields. We show that a model including both bulk plasma oscillations and THz emission from a surface accumulation layer describes THz emission from InAs in a moderate magnetic field, but this model does not completely describe emission at fields ͉B͉Ͼ1.0 T.

No abstract

Ultrafast terahertz spectroscopy can be used to probe charge and spin dynamics in semiconductors. We have studied THz emission from bulk InAs and GaAs and from GaAs/AlGaAs quantum wells as a function of magnetic field. Ultrashort pulses of THz radiation were produced at semiconductor surfaces by photoexcitation with a femtosecond TiSapphire laser, and we recorded the THz emission spectrum and the integrated THz power as a function of magnetic field and temperature. In bulk samples the emitted radiation is produced by coupled cyclotron-plasma oscillations: we model THz emission from n-GaAs as magneto-plasma oscillations in a 3-D electron gas. THz emission from a modulation-doped parabolic quantum well is described in terms of coupled intersubband-cyclotron motion. A model including both 3-D plasma oscillations and a 2-D electron gas in a surface accumulation layer is required to describe THz emission from InAs in a magnetic field.