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

Perovskites, particularly BaTiO3 (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 BaTiO3. 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 BaTiO3 perovskite materials' nonlinear optical properties by adjusting the particle size and lattice structures, providing beneficial insights for advanced optical and medical imaging applications.