On September 7, Advanced Functional Materials published online a research paper titled "Second-Harmonic-Generation Circular-Dichroism in Chiral Perovskite Single Crystals Enhanced by Self-Trapped Excitonic State" by Professor Han Yibo's research group at the National Pulsed High Magnetic Field Science Center.

Chiral organic-inorganic hybrid perovskites (OIHPs) possess intrinsic chirality and a series of unique properties, such as circular dichroism (CD), circularly polarized luminescence (CPL), chirality-induced spin selectivity (CISS), and strong spin-orbit coupling (SOC). These characteristics make chiral OIHPs promising for diverse applications including circularly polarized light detectors, solar cells, spin light-emitting diodes, circularly polarized light sources, ferroelectric materials, and nonlinear opti cs. Given that chiral OIHPs combine both CD and nonlinear optical properties, they can exhibit optical activity in both linear and nonlinear optical processes, including circular dichroism in second-harmonic generation (SHG-CD). In nanomaterials and thin film systems, the SHG effect primarily arises from interface symmetry breaking. However, in bulk single crystals, the presence of multiple chiral domains and long-range crystalline order tend to suppress the SHG intensity and the g_SHG-CD value. Since the theoretical upper limit for g_SHG-CD is 2, designing and synthesizing chiral OIHPs with g_SHG-CD values approaching this theoretical limit remains a significant challenge.

Figure 1. (a) Crystal structure of (R/S-BrsBA)₂PbBr₄ chiral crystals. (b) Wavelength-dependent SHG and photoluminescence (PL) intensities. (c) Incident light polarization-dependent SHG intensity.

The research group studied (R/S-BrsBA)₂PbBr₄ (where BrsBA is 4-bromo-2-butylammonium) single crystals with an optically chiral structure (Fig. 1a). Using a self-built polarization-controlled SHG measurement system, they discovered that this type of single crystal exhibits a wavelength-dependent, highly polarized SHG signal due to its structural chirality (Fig. 1b). Under circularly polarized incident light, the SHG intensity of the crystal showed a highly sensitive response to the degree of circular polarization of the incident light. Furthermore, the g_SHG-CD value exhibited an anomalous enhancement below the band edge, reaching a maximum of 0.64 (Fig. 1c).

Figure 2. (a) Modulation of different types of exciton luminescence by a high magnetic field. (b) Schematic diagram of the mechanism related to self-trapped exciton state enhancement of the SHG circular polarization response.

Studies of temperature-dependent (5-300 K) and magnetic field-dependent (0-44 T) photoluminescence spectra revealed the existence of self-trapped excitons (STEs) in this wavelength range. A resonance enhancement relationship was observed between the photon energy of the STE luminescence and the photon energy position corresponding to the maximum g_SHG-CD value. Notably, the band-edge exciton luminescence at low temperatures increased with increasing magnetic field strength, indicating characteristics of spin-singlet dark excitons. In contrast, the STE luminescence intensity at the resonance wavelength remained almost unchanged with the magnetic field, exhibiting characteristics of spin-triplet bright excitons (Fig. 2a). Therefore, these results suggest that spin-triplet excitons are beneficial for enhancing the SHG-CD effect (Fig. 2b). This discovery not only deepens the understanding of the second-harmonic generation mechanism in chiral perovskites but also opens new avenues for their applications in nonlinear chiral optics and spin-photonics.

Link to the paper:

https://advanced.onlinelibrary.wiley.com/doi/10.1002/adfm.202508904