Novel law of nonlinear optical response in strongly correlated electron systems

In the Mott insulator Ca2RuO4, which is a strongly correlated material material, we observed for the first time the generation of high-order harmonics that convert a strong infrared laser beam into visible light. Our experiments show that the intensity of the generated signal follows a surprisingly simple rule.

In recent years, with the development of ultrashort pulse light generation technology, it has become possible to shine strong light on materials. This causes phenomena such as the generation of high-order harmonics that change the color of the emitted light to the visible range when an invisible infrared laser beam is applied to an object. The mechanism of high-order harmonic generation is roughly understood in well-known materials such as semiconductors. On the other hand, in the strongly correlated materials in which electrons strongly interact with each other and show complicated properties, high-order harmonics are hardly observed, and the mechanism of generation is unknown. We succeeded in observing high-order harmonics for the first time in the typical Mott insulator Ca2RuO4, which is a strongly correlated electron system.

Stronger when colder

Surprisingly, the harmonic signal increased sharply as the material was cooled. This effect has never been observed in any material and has never been theoretically predicted. In addition, we found that this effect follows a surprisingly simple law (upper right in the figure) for insulator gap energy and harmonic photon energy that are opened by electron correlation. This is an example of how complex physical phenomena show a simple and beautiful law, and our research group believes that a new harmonic generation mechanism derived from electron correlation is involved in this law.

When the temperature T is changed, the harmonic signal IHH changes significantly. Behind it is the law relating to the gap energy Δ and the harmonic photon energy hνHH described by the equation in the upper right of the figure.

This result will help elucidate the non-equilibrium dynamics in strongly correlated materials. It also suggests the possibility of providing new design guidelines for nonlinear optical materials used in laser pointers and optical modulators that are familiar to us in our daily lives.

The results of this research were published online in the international academic journal “Physical Review Letters” on March 23, 2022.

Article information

Uchida, K; Mattoni, G; Yonezawa, S; Nakamura, F; Maeno, Y; Tanaka, K

High-order harmonic generation and its unconventional scaling law in the Mott-insulating Ca2RuO4 Journal Article

In: Physical Review Letters, vol. 128, no. 127401, 2022.

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