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.
We report on terahertz (THz) emission from a (111)-cut InAs crystal in the reflection and transmission directions, excited by femtosecond optical pulses in the direction of its surface normal. THz pulse amplitudes emitted from the crystal surface in this case were only ~20% smaller than for optimal photoexcitation at a 45° angle. This observation evidences that THz emission from InAs is caused by lateral photocurrent transients appearing due to a crystal anisotropy rather than directly by the photo-Dember effect, which creates fast changing electric polarization perpendicular to the surface. Such a simple geometry of the photoexcitation could greatly enhance the fields of surface THz emitter applications.
Polarized terahertz radiation generated from (112)-oriented InSb surfaces is investigated as a function of the sample azimuthal orientation under excitation from femtosecond Yb:KGW laser pulses. The expressions describing the optical rectification and the surface electric-field-induced optical rectification in reflection from zinc-blende crystals, such as InSb, are calculated. It is shown the contributions of both these effects should be taken into account when describing terahertz emission from InSb surfaces.
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