Tailoring second-harmonic generation in KTP via femtosecond laser microstructuring

Abstract

Potassium titanyl phosphate (KTP) is a widely used nonlinear optical crystal due to its large second-order susceptibility and broad transparency range. However, second-harmonic generation (SHG) at Ti:Sapphire laser wavelengths (~800 nm) cannot be achieved through birefringent phase matching, typically requiring quasi-phase-matching techniques. Here, we demonstrate that femtosecond-laser-induced microstructures in KTP enable spatially localized enhancement of SHG under phase-mismatched conditions. Subsurface damage tracks were inscribed using 60 fs Ti:Sapphire laser pulses at 800 nm. The nonlinear optical response of the inscribed microstructures was investigated through two complementary approaches: microscale analysis using second-harmonic and Raman microscopy, revealing localized enhancement and structural modifications, and large-beam excitation under loose focusing conditions, showing periodic SH modulation consistent with the microstructure pattern. These results demonstrate a local enhancement of the nonlinear optical response in KTP, opening new possibilities for the design of integrated nonlinear photonic devices.