Nonlinear optical effects have exhibited substantial application potential in areas such as electro-optic technology, photonics, and biomedical imaging. Second-harmonic generation (SHG), a representative second-order nonlinear optical process, is pivotal for laser frequency doubling, optoelectronic modulation, and integrated photonic devices. Conventional nonlinear optical devices predominantly rely on bulk crystal materials, including lithium niobate (LiNbO₃) and potassium dihydrogen phosphate (KDP). Nevertheless, their relatively large size hinders their integration into highly compact photonic systems. Although low-dimensional materials offer structural tunability, their atomic-scale thickness limits the interaction length between light and matter, thus affecting the overall nonlinear conversion efficiency. To address this challenge, ferroelectric nematic liquid crystals have emerged as a promising platform for nonlinear optics research. They combine the fluidity and processability of liquid crystals with the macroscopic polar ordering characteristics of ferroelectric materials. Their inherent non-centrosymmetric structure endows them with the potential for SHG effects. However, the absence of rigid lattice constraints in liquid crystal systems results in localized dispersion and disorder in molecular orientation, which restricts the enhancement of the second-harmonic response.

Recent research has achieved remarkable SHG performance by constructing ferroelectric nematic liquid crystal microdroplets. Under excitation from an 800 nm femtosecond laser, the effective SHG coefficient reaches 56.9 pm/V, approximately an order of magnitude higher than previously reported ferroelectric nematic liquid crystal systems. The second-harmonic conversion efficiency reaches 142.2×10⁻⁹ W⁻¹, outperforming reference samples of lithium niobate thin films of equivalent thickness. This system can generate second-harmonic responses across a wavelength range of 375-475 nm when excited by fundamental frequencies of 750-950 nm. Combined with high optical transmittance above 380 nm, it shows potential for broad-wavelength-band nonlinear optical applications. Additionally, the polar topological structure of the liquid crystal microdroplets confers significant spatial optical modulation capabilities. The effective SHG coefficient exhibits radial distribution characteristics, with a spatial modulation contrast ratio of up to 330%, enabling passive spatial light modulation without the need for an external electric field. This provides new insights for developing tunable nonlinear photonic devices in soft matter systems.

Through characterization techniques such as angle-resolved SHG imaging and polarization Raman spectroscopy, the research uncovered that surface-anchoring-induced polar orientation ordering is the key factor in enhancing the nonlinear optical response of ferroelectric nematic liquid crystals. The findings highlight the application prospects of ferroelectric nematic liquid crystals with polar topological structures in integrated nonlinear photonics and tunable optical devices. They offer experimental evidence for achieving high nonlinear optical coefficients, excellent conversion efficiency, broad-wavelength-band response potential, and significant spatial light modulation capabilities in solution-processable liquid crystal systems.