Quadratic optical responses in a chiral magnetic metal

Chiral magnets breaking both spatial inversion and time reversal symmetries carry a potential for novel quadratic optical responses, for instance, photovoltaic effect (PVE) and second harmonic generation (SHG) [1]. Such nonlinear optical responses have attracted much attention in the aspect of an application for the next-generation optical electronic devices, such as unconventional solar cells and optical sensors. Despite the possibility of enhanced and controlled responses through the magnetic degree of freedom, the systematic understanding remains yet to be developed.
In this study, we theoretically investigate quadratic optical responses in electrons coupled with a one-dimensional chiral magnet, as a prototypical example. The magnetic texture changes from a chiral helimagnetic state to a chiral conical magnetic state with spin canting, and to a forced ferromagnetic state with increasing magnetic field along the helical axis. By using the second-order response theory [2], we show that the chiral conical magnetic state exhibits the PVE and SHG through the asymmetric modulation of the electronic band structure due to breaking of both spatial inversion and time reversal symmetries. We find that these quadratic responses vary drastically depending on the frequency of the incident lights, the magnetization, and the spin-charge coupling [3]. In particular, we find that the coefficient of the PVE changes in not only the magnitude but also the sign by these parameters. Furthermore, the PVE and SHG in the chiral magnet can be colossal in comparison with those in the conventional semiconductors and ferromagnetic materials and be comparable to those in the topological materials such as Weyl semimetals [4]. Our results suggest that chiral magnetic textures provide a new platform for nonlinear optical responses with flexible controllability.

[1] Y. Tokura and N. Nagaosa, Nat. Commun. 9, 3740 (2018).
[2] D. E. Parker, T. Morimoto, J. Orenstein, and J. E. Moore, Phys. Rev. B 99, 045121 (2019).
[3] S. Okumura, T. Moritomo, Y. Kato, and Y. Motome, in preparation.
[4] G. B. Osterhoudt et al., Nat. Mater. 18, 471 (2019); L. Wu et al., Nat. Phys. 13, 350 (2017).