Terahertz emission from two-dimensional Ruddlesden-Popper halide perovskites driven by electric dipole and quadrupole under below-band-gap excitation

Understanding the nonlinear polarization mechanism of two-dimensional (2D) halide perovskites is essential for developing various nonlinear optical applications. Herein, the terahertz (THz) emission from Ruddlesden-Popper type ${(\mathrm{BA})}_{2}\mathrm{Pb}{\mathrm{I}}_{4}$ and ${(\mathrm{iso}\ensuremath{-}\mathrm{BA})}_{2}\mathrm{Pb}{\mathrm{I}}_{4}$ are investigated by both above-band-gap (400 nm) and below-band-gap (800 nm) femtosecond laser excitation. Under above-band-gap excitation, the dominant mechanism is the linear photogalvanic effect instead of the common photo-Dember effect and surface depletion field effect, suggesting the carrier transport between adjacent conductor layers is forbidden by the organic layers. Under below-band-gap excitation, the THz emission from orthorhombic ${(\mathrm{BA})}_{2}\mathrm{Pb}{\mathrm{I}}_{4}$ exhibits a remarkable quadruple rotational symmetry on the azimuthal angle, indicating that the nonlinear polarization is driven by a higher-order electric quadrupole term instead of the bulk electric dipole term, which is forbidden by the centrosymmetric structure. In comparison, the low symmetry of THz emission on the azimuthal angle of monoclinic ${(\mathrm{iso}\ensuremath{-}\mathrm{BA})}_{2}\mathrm{Pb}{\mathrm{I}}_{4}$ is attributed to the influence of symmetry breaking in the surface region, which raises a surface electric dipole contribution. In this paper, we reveal THz emission spectroscopy as a powerful contactless technique in investigating the nonlinear polarization of advanced materials.