Terahertz and High-Harmonic Radiation from Ultrafast Light Subgap or Above-Gap Driving of Spin-Orbit Proximitized...

Ultrafast light-driven strongly correlated antiferromagnetic insulators, such as prototypical NiO with a large Mott energy gap ≃4  eV, have recently attracted experimental attention using photons of both subgap [H. Qiu et al., Nat. Phys. 17, 388 (2021)NPAHAX1745-247310.1038/s41567-020-01061-7] and above-gap energy [K. Gillmeister et al., Nat. Commun. 11, 4095 (2020)NCAOBW2041-172310.1038/s41467-020-17925-8]. In the former context, which is also of great interest to applications, emission of terahertz (THz) radiation is observed from NiO/Pt bilayers, where heavy metal (HM) Pt introduces strong spin-orbit coupling (SOC) effects. However, in contrast to amply studied spintronic THz emitters using femtosecond laser pulse (fsLP)-driven FM/HM (where FM represents a ferromagnetic metal of the conventional type, such as Fe, Ni, or Co) bilayers, where ultrafast demagnetization takes place and is directly related to THz emission, microscopic mechanisms of electromagnetic (EM) radiation from NiO/HM bilayers remain obscure, as the total magnetization of NiO is zero before fsLP application. We employ the two-orbital Hubbard-Hund-Heisenberg model and study, via numerically exact quantum many-body methods, the dynamics of its Néel vector and nonequilibrium magnetization. This reveals nonclassical (i.e., not describable by the Landau-Lifshitz equation) dynamics of Néel vector and nonequilibrium magnetization, changing only in length while not rotating, where the former is substantially reduced in the case of above-gap fsLPs. Additionally, we compute EM radiation by time dependence of magnetization or of local charge currents, finding that both contributions are significant in the THz frequency range only in NiO with proximity SOC introduced by the HM layer. Outside the THz range, we find an integer high-harmonic generation, as well as unusual noninteger harmonics for the above-gap fsLP pump.