Terahertz emission from the surfaces of narrow-gap semiconductors excited by femtosecond laser pulses was described in terms of a transient interband photoconductivity. It has been found that the nonparabolicity of the electron dispersion law as well as the optical alignment of the photoexcited carrier momenta result in anisotropic photocurrent with a component perpendicular to the surface dc electric field even in semiconductors with a cubic symmetry. This lateral transient photocurrent component is the strongest during the first few hundreds of femtoseconds after the photoexcitation and causes the emission of terahertz radiation pulses with an amplitude dependent on the angle between the optical field and the crystallographic axes. In the case of InAs the contribution of this component explains experimental results of both the azimuthal anisotropy of the emitted terahertz pulse amplitude and its dependence on the exciting photon energy. V C 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4758181] JOURNAL OF APPLIED PHYSICS 112, 073115 (2012) [This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP: 128.235.251.160 On: Mon, 22 Dec 2014 20:48:54 073115-3 Malevich et al. J. Appl. Phys. 112, 073115 (2012) [This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP: 128.235.251.160 On: Mon, 22 Dec 2014 20:48:54 073115-8 Malevich et al.

Terahertz radiation pulses emitted after exciting semiconductor heterostructures by femtosecond optical pulses were used to determine the electron energy band offsets between different constituent materials. It has been shown that when the photon energy is sufficient enough to excite electrons in the narrower bandgap layer with an energy greater than the conduction band offset, the terahertz pulse changes its polarity. Theoretical analysis performed both analytically and by numerical Monte Carlo simulation has shown that the polarity inversion is caused by the electrons that are excited in the narrow bandgap layer with energies sufficient to surmount the band offset with the wide bandgap substrate. This effect is used to evaluate the energy band offsets in GaInAs/InP and GaInAsBi/InP heterostructures.

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An analytical model describing the onset of a surface ballistic photocurrent in cubic semiconduc tors under femtosecond laser excitation is proposed. It is shown that the contribution of the photocurrent component parallel to the surface to the generation of terahertz pulses may be comparable to the contribution of the perpendicular component. Consideration of the cubic symmetry of a semiconductor leads to the azi muthal anisotropy of terahertz generation.

The dynamics of the depletion field screening induced by photoexcited carriers and THz generation caused by the electric-field-induced optical rectification are simulated for GaAs surface excited by femtosecond laser radiation on the basis of an ensemble Monte Carlo method. The results show that the photocarrier-induced screening occurs on a subpicosecond time scale and THz pulse essentially changes its wave form depending on excitation pulse duration and fluence. The possibility to use the depletion electric field induced THz generation for study of subpicosecond electric field screening dynamics is discussed.