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11THz-I-9
Generation of the second harmonic of optical radiation in the presence of a strong terahertz field
A. Stepanov1
institute of Applied Physics, Nizhny Novgorod, Russian Federation
The generation of the second harmonic of optical radiation (SHG) is a non-linear process widely used, in particular, for the diagnosis of various media, especially their surfaces and boundaries [1]. In centrosymmetric media, SHG is forbidden in the dipole approximation and therefore very weak. The application of an additional DC or low frequency field to the SHG process leads to the removal of the restriction and allows the use of SHG for diagnostic purposes, etc. In this paper, we will present the results of the study of the generation of the second harmonic (SH) in various materials, using the powerful short-pulse terahertz radiation as an auxiliary field. THz pulses were generated in a nonlinear LiNbO3 crystal when it was irradiated with femtosecond laser radiation in intensity tilt front scheme. Then they were focused on the sample using an off-axis parabolas; the maximum field at the focus reached 300 kV / cm. A portion of the femtosecond laser pulse was also focused on the sample. Registration of SH was carried out using a photomultiplier with a photon counting system. Semiconductor materials Si, InAs, and GaP were used as samples. In part of the experiments, studies were carried out with samples of single-layer graphene. SHG studies were also carried out in gaseous media (air, Ar, He).
In the experiments, the dependences of the SH on the azimuthal angle of crystalline semiconductor samples, the signals of the second harmonic with different combinations of polarizations involved in the generation of SH were recorded. A phenomenological theory of SHG in crystalline semiconductors was developed. For graphene, a theory has been developed that links the generation of SH with the "field" broadening of interband resonance in the region of production of electron-hole pairs in k-space, resulting in the appearance of anisotropy of the electric dipole response [2].
References
[1] Aktsipetrov O.A., Baranova I.M., Evtukhov K.N. Nonlinear optic of silicon and silicon nanostructures. M. Fizmatlit, 2012 (in Russian).
[2] M. Tokman, S. Bodrov, Y. Sergeev, A. Korytin, I. Oladyshkin, Y. Wang, A. Belyanin, and A. Stepanov. Second harmonic generation in graphene dressed by a strong terahertz field, Phys. Rev. B 99, 155411 (2019).
