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1Second harmonic generation due to the spatial structure
of radiation beam
M.V. Durnev*, A.A. Gunyaga, S.A. Tarasenko
Ioffe Institute, 194021, St. Petersburg, Polytechnicheskaya 26
Nonlinear transport and optical effects in two-dimensional (2D) electronic systems are at the focus of modern research in the condensed-matter physics. Among them, the second harmonic generation (SHG) is of particular interest, both fundamentally and in terms of applications. This effect emerges in structures with broken space inversion symmetry and, therefore, has been established as a sensitive tool to probe structural inhomogeneity, crystalline symmetry, the staking and twist of 2D crystal flakes, etc. [1].
The main SHG mechanisms, which are studied at the moment, are related to the absence of inversion center in 2D crystals, photon wave vector at oblique incidence of radiation or the sample geometry, for example, the presence of an edge [2,3]. In our work, we show that the second harmonic generation is also possible in an isotropic 2D medium, provided that the radiation itself is spatially inhomogeneous. From the symmetry point of view, this effect is analogous to the effect of generation of a direct electric current by an inhomogeneous field of structured radiation [4].
We have studied the SHG induced by the spatial structure of incident radiation in a 2D electron gas (2DEG). We have shown that structured radiation with intensity, polarization, or phase, which vary in the 2DEG plane, leads to the generation of local electric currents oscillating at double frequency. These currents, in turn, emit secondary waves at double frequency, see Fig. 1. We have developed a kinetic theory of this effect for the incident field of the terahertz spectral range with an arbitrary spatial profile and have derived a general analytical expression for the current density. As an example, we have considered the SHG by a beam of twisted photons carrying an orbital angular momentum. It is shown that the secondary radiation is also twisted and characterized by a double angular momentum.
The work is supported by the Russian Science Foundation (Project No. 22-12-00211).
Fig. 1. Second harmonic generation due to the spatial structure of radiation beam. Nonuniform electromagnetic field of the twisted radiation beam generates electric currents oscillating at double frequency. These currents, in turn, emit twisted electromagnetic wave at double frequency.
[1] L. Zhou, et al, Nonlinear optical characterization of 2D materials, Nanomaterials, 10, 2263 (2020).
[2] M.M. Glazov and S.D. Ganichev, High frequency electric field induced nonlinear effects in graphene, Physics Reports, 535, 101 (2014).
[3] M.V. Durnev and S.A. Tarasenko, Second harmonic generation at the edge of a two-dimensional electron gas, Phys. Rev. B, 106, 125426 (2022).
[4] A.A. Gunyaga, M.V. Durnev, S.A. Tarasenko, Photocurrents induced by structured light, Phys. Rev. B, 108, 115402 (2023).
