NONLINEAR OPTICAL RESPONSES OF ANTIFERROMAGNETIC INSULATORS

Abstract

Firstly, we show how to resolve the observed mystery in the resonance-enhancement of two-magnon Raman scattering in antiferromagnetic (AFM) insulators La2CuO4 and YBa2Cu3O6 in terms of the excitonic cluster model. Secondly, we refer to the strong third-harmonic generation (THG) in visible region from La2CuO4, and explain how we are able to understand the fine structures as quantum interference among two-photon and three-photon resonant THG. Ferroelectric (FEL)-AFM crystals RMnO3 (R=Y, Ho, Er) with a hexagonal structure show interesting two-photon resonant second-harmonic generation (SHG). Two lowest-energy excitons in the visible region are found to contribute to SHG constructively in YMnO3 and destructively in ErMnO3 depending upon the AFM spin-configuration. It is also found that the clamping of two order-parameters FEL and AFM must be introduced to explain the observation.