Two-photon driven magnon-pair resonance as a signature of spin-nematic order

We theoretically study the nonlinear magnetic resonance driven by intense lasers or electromagnetic waves in a fully polarized frustrated magnet near a less-visible spin-nematic ordered phase. In general, both magnons and magnon pairs (two-magnon bound state) appear as the low-energy excitation in the saturated state of spin-nematic magnets. Their excitation energies are usually in the range between 10 GHz and 10 THz. Magnon pairs with angular momentum $2\ensuremath{\hbar}$ can be excited by the simultaneous absorption of two photons, and such multiphoton processes occur if the applied THz laser is strong enough. We compute laser-driven magnetic dynamics of a frustrated four-spin system with both magnon ($\ensuremath{\hbar}$) and magnon-pair ($2\ensuremath{\hbar}$)-like excitations, which is analogous to a macroscopic frustrated magnet with a spin-nematic phase. We estimate the required strength of the magnetic field of a laser for the realization of two-photon absorption, taking into account dissipation effects with the Lindblad equation. We show that an intense THz laser with an ac magnetic field of 0.1--1.0 T is enough to observe magnon-pair resonance.