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Theoretical and experimental study of second harmonic generation from the surface of the topological insulator Bi2Se3

Filed in: Blog.Theoretical-and-experimental-study-of-second-harmonic-generation-from-the-surface-of-the-topological-insulator-Bi2Se3 · Modified on : Wed, 25 Oct 17

Title : Theoretical and experimental study of second harmonic generation from the surface of the topological insulator Bi2Se3 Author : J. W. McIver et al. DOI : 10.1103/PhysRevB.86.035327

The recent discovery of 3D topological insulator phases in Bi1-xSbx, Bi2Se3 and related materials has generated great interest to measure their symmetry and electrical properties at an isolated surface or buried interface. However, a major experimental obstacle has been the high density of mobile electrons in the bulk of these materials, which can overwhelm the surface or interface electrical responses. Although transport results on electrically gated samples show evidence of surface carrier modulation, the contributions to the electrical response from carriers on different surfaces and in the bulk are difficult to separate and require highly insulating samples. Moreover, contacts and gates deposited on the surface may perturb the intrinsic surface electronic structure. Optical probes have been proposed as a contact-free alternative that can be focused onto a single surface.

Recently, nonlinear second harmonic generation (SHG) of light from bulk single crystals of Bi2Se3 was shown to be highly sensitive to electrons confined to the surface and accumulation region. The underlying principle for this surface sensitivity is that SHG is predominantly generated where inversion symmetry is broken, which only occurs at the surface of the bulk inversion-symmetric Bi2Se3 and in the accumulation region, where the band-bendinginduced electric field breaks the inversion symmetry.

In this work, they develop a theoretical model that describes the SHG intensity from Bi2Se3 in terms of the second- and third-order nonlinear electric susceptibilities. By performing a symmetry analysis of Bi2Se3 they identify the susceptibility tensor elements that contribute to SHG and show that their relative magnitudes can be determined by measuring the intensity and polarization of the emitted SHG as a function of crystal orientation and incident laser polarization.

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