Theory of nonlinear optical responses in Kitaev spin liquids

THz laser technology has developed in the last decade. Intense THz light enables us to observe nonlinear optical phenomena in magnetic insulators. Among magnetic phases, quantum spin liquids (QSLs) are less-visible ones. Most thermodynamic quantities in QSLs are featureless, and their specific nature can often be found in the dynamical quantities. Motivated by these backgrounds, we theoretically study THz-laser responses of the QSL phase in the Kitaev model which is a frustrated spin-1/2 system on honeycomb lattice with three-type Ising interactions. The excitations on the Kitaev QSL are described by Majorana fermions and its candidate materials such as α-RuCl₃ have been intensively investigated. We focus on THz-laser driven high harmonic generation (HHG) of the Kitaev model with magneto-striction (MS) interaction, through which the AC electric field is coupled to the Ising terms. Applying quantum master equation, we numerically show that the AC electric field induces HHG via the MS coupling, and the HHG spectra exhibit characteristic dependencies of laser frequency, laser intensity, and external DC fields. We report essential aspects of these spectra.