Electrically Switchable Chiral Second-Harmonic Generation in an Achiral Ferroelectric 2D Halide Perovskite

Two-dimensional (2D) van der Waals (vdW) semiconductors are attracting attention as promising materials for efficient nanoscale nonlinear optical devices. Because the second-order nonlinearity allowed by broken inversion symmetry plays a key role in nonlinear optics, 2D vdW semiconductors lacking inversion symmetry are of great scientific and technological importance for nonlinear nanophotonics. 2D Ruddlesden-Popper lead halide perovskites (RPPs) are an emerging class of 2D vdW semiconductors with excellent photonic [1,2], optoelectronic [3,4], and spintronic [5,6] properties. In addition, 2D RPPs possess high structural flexibility and tunability [7,8]. Due to the high structural flexibility, ferroelectricity and chirality emerge and lead to unique second-order nonlinear optical responses: ferroelectricity enables electrical switching of the second-order optical nonlinearity and chirality leads to chiral second-order nonlinear responses. Along with the efficient and unique nonlinear optical properties of lead halide perovskites [9-11], these properties show that 2D RPPs can expand the potential of using 2D vdW semiconductors in nonlinear optical applications.
In particular, if ferroelectricity and chirality coexist and electrically switchable chiral nonlinear optics can be realized, it would offer a new paradigm for nonlinear nanophotonics based on 2D vdW semiconductors. However, so far, this has not been possible in 2D vdW semiconductors because it is difficult to electrically manipulate chiral structures in solid-state materials. Moreover, in contrast to the large family of 2D RPPs exhibiting either chirality or ferroelectricity, 2D RPPs having both the properties are very scarce. Therefore, a novel and versatile approach is desirable for developing electrically switchable chiral nonlinear optics and enriching the nonlinear optical functionalities of 2D vdW semiconductors.
In this study, we solved these problems by focusing on biaxial ferroelectricity of an achiral ferroelectric 2D RPP. We synthesized achiral 2D RPP (BA)₂(EA)₂Pb₃I₁₀ single crystals, which exhibit biaxal ferroelectricity at room temperature [12]. We demonstrated reversible and continuous electrical switching of chiral second-harmonic generation (SHG) in exfoliated flakes of ferroelectric (BA)₂(EA)₂Pb₃I₁₀. The chiral SHG is characterized by SHG circular dichroism (SHG-CD), a second-order nonlinear chiroptical effect, and we observed large SHG-CD. To further investigate the chiral nonlinear optical responses, we performed polarization-resolved SHG imaging. We revealed that there exist the electrically induced ferroelectric domains with perpendicular spontaneous polarizations, which reflects the biaxial ferroelectricity of the material. The ferroelectric multidomain structure breaks the in-plane glide mirror symmetry and the resulting planar chirality leads to the chiral SHG. Our findings can be used to develop a simple and versatile electrical control of nonlinear chiroptical responses without having to design and modulate a chiral crystal structure and open new avenues for novel chiral photonic applications based on ferroelectric 2D RPPs.
Part of this study was supported by JSPS KAKENHI (Grant No. JP19H05465 and JP23K13623) and NEDO-GI (JPNP21016).

References
[1] A. Fieramosca et al., Sci. Adv. 5, eaav9967 (2019).
[2] C. Qin et al., Nature 585, 53-57 (2020).
[3] H. Tsai et al., Nature 536, 312-316 (2016).
[4] C. Qin et al., Nat. Photon. 14, 70-75 (2020).
[5] D. Giovanni et al., Sci. Adv. 2, e1600477 (2016).
[6] G. Yumoto et al., Sci. Adv. 8, eabp8135 (2022).
[7] Y. Fu, Adv. Mater. 34, 2108556 (2022).
[8] C. Higashimura et al., J. Phys. Chem. Lett. 14, 8360-8366 (2023).
[9] K. Ohara et al., Phys. Rev. B 103, L041201 (2021).
[10] G. Yumoto et al., Nat. Commun. 12, 3026 (2021).
[11] G. Yumoto and Y. Kanemitsu, Phys. Chem. Chem. Phys. 24, 22405-22425 (2022).
[12] S. Han et al., J. Am. Chem. Soc. 141, 12470-12474 (2019).