High harmonic generation in strongly correlated materials

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High order harmonic generation (HHG) is the most typical extreme nonlinear optical phenomenon, where integer multiples of driving laser photon energy are emitted, and gives us the fundamental insight of non-perturbative light-matter interaction in solids. So far, HHG have been mainly studied in semiconductors which can be described with single electron approximation. However, little has been elucidated about HHG in strongly correlated electron systems such as Mott-insulators, where electron-electron correlation plays a dominant role. Here, we firstly report HHG from the Mott-insulating phase of Ca2RuO4. By changing sample temperature, we controlled the gap energy of Ca2RuO4 and investigated the relation between HHG emission properties and gap energy. We found that yields of high harmonics are well scaled by the gap energy: exponential increase of HHG yields with an increase of gap energy for the harmonic order greater than third [1]. We also study HHG theoretically in Mott-insulators and conclude that this anomalous behavior should originate from a strong coupling between charge and other degree of freedom (spin, orbital, etc…) with the cooperation of the thermal ensemble, and the resulting strongly temperature-dependent coherence between charge carriers [2]. Our results demonstrate that correlations between different degrees of freedom, which are a characteristic feature of strongly correlated solids, have significant and nontrivial effects on nonlinear optical responses. [1] K. Uchida et al., arXiv:2106.15478 (PRL accepted). [2] Y. Murakami et al., arXiv:2203.01029.