Second-Harmonic Generation in Perovskite Particles for Bioimaging: Unveiling the Effects of Particle Size and Structural Modification

Abstract

Perovskites, particularly BaTiO 3 (BT), are renowned for their nonlinear optical applications, notably second harmonic generation (SHG). This study investigates the interplay among the particle size effect, lattice distortion, and electron−phonon coupling, focusing on their collective impact on the second-order nonlinear optical susceptibility (χ (2) ) of BaTiO 3 . BT particles of varying sizes were synthesized using the solid-state reaction method and subsequently subjected to different thermal treatments. Using Raman spectroscopy and X-ray diffraction, the SHG response's intrinsic relationship with χ (2) was analyzed concerning lattice parameters, particle size, and other pivotal factors. The Raman modes show a systematic blueshift of the phonon mode (Δω = 15 cm −1 ) and asymmetrically broadened Raman lines, understood in terms of the lattice strain associated with the particle size effect induced by thermal treatment. A correlation was identified between lattice distortion and SHG response. The increase of the tetragonality ratio, associated with shifts in the Ti−O bond length, correlated with net dipole moments spanning from 1.9 × 10 −31 to 2.5 × 10 −31 C m and significantly influenced χ (2) values. Besides, the spectroscopy studies showed that Mie resonances can enhance the conversion efficiency of SHG approximately 2.4 times. At last, the improved SHG in modified BT particles, with higher χ (2) and biocompatibility, shows potential for high-resolution, low-interference bioimaging applications. The findings highlight the feasibility of optimizing BaTiO 3 perovskite materials' nonlinear optical properties by adjusting the particle size and lattice structures, providing beneficial insights for advanced optical and medical imaging applications.