Magnetic Field Effects in Optical Harmonics Generation by Excitons
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Magnetic Field Effects in Optical Harmonics Generation by Excitons V. V. Pavlov* Ioffe Institute, St. Petersburg, Russia *e-mail: [email protected] Received March 26, 2020; revised March 26, 2020; accepted April 2, 2020
Abstract—Mechanisms that are responsible for the optical harmonics generation at exciton transitions are analyzed in several classes of materials. In the cubic GaAs semiconductor, the magneto-induced optical second-harmonic generation second optical harmonic is observed in the region of orbital quantization of the valence and conduction bands. An unusually strong amplitude of the optical third-harmonic generation in an external magnetic field in the region of 1s exciton due to exciton–polariton resonance is found. The optical second-harmonic generation due to 1s, 2s, and 2p exciton resonances in a magnetic field is revealed in a hexagonal wide band-gap ZnO semiconductor. Depending on the symmetry of the exciton states, the mechanisms of the optical second-harmonic generation involve the spin and orbital Zeeman effects, and the magneto-Stark effect. The magneto-induced contribution to second harmonic generation (SHG) in the region of exciton transitions in the Cr2O3 antiferromagnet placed in an external magnetic field is studied. Keywords: optical harmonics generation, magneto-induced effects, exciton states, GaAs and ZnO semiconductors, Cr2O3 magnetic dielectric DOI: 10.1134/S1063783420090243
1. INTRODUCTION In linear optics, the propagation and absorption of light in solids are determined by one-photon processes with preserving the photon energy. The optical second- and third-harmonics generation (SHG and THG, respectively) are the simplest nonlinear optical processes of the second and third orders [1]. The harmonics generation in a nonlinear optical medium is controlled by multiphoton processes (a three-photon process in the case of SHG and a four-photon process in the case of THG). A whole class of magneto-optical phenomena, such as the well-known Faraday effect, magnetic circular dichroism, magnetic linear birefringence, and magnetic linear dichroism, arises when light passes through a magnetized medium [2, 3]. The linear and quadratic Kerr effects are also observed when light is reflected from a magnetized medium [4, 5]. With optical harmonics generation, a number of new nonlinear effects are possible for a magnetized medium [6–8]. Linear and nonlinear optical effects are determined by different types of optical susceptibilities, which, in turn, are determined by the electron spin states of the material under study and, consequently, can be considered as complementary tools for studying the solids. The nonlinear SHG and THG optical processes are described by the second-order (χ(2)) and thirdorder (χ(3)) nonlinear susceptibilities, which are very
sensitive to crystallographic symmetry determined by the spatial arrangement of ions in the crystal lattice and to magnetic symmetry determined by spatial distribution of magnetic moments of ions [9, 10]. The unique features of the metho
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