Distinct responses of photoelastic coefficients to He+ ion implantation in LiNbO3 studied by ultrafast photoacoustic imaging

In ultrafast photoacoustics, the application of ultrashort laser pulses provides an opportunity both to generate coherent ultrashort acoustic pulses and to detect them. Probing the ultrashort acoustic pulse along its propagation path in optically transparent materials constitutes a photoacoustic modality for imaging with nanoscale resolution, which is called time-domain Brillouin scattering (TDBS). We present TDBS measurements with variable probe light polarization on He+ ion-implanted X-cut LiNbO3 to investigate the depth-dependent modification of its acoustic, optical and acousto-optic properties. An anisotropic TDBS model incorporating optical birefringence and photoelastic mode conversion was developed and validated by comparing theoretical calculations with experimental results. The analysis reveals that ion implantation predominantly modifies the photoelastic coefficients, rather than the acoustic velocity and optical refractive indices of LiNbO3. The extracted depth-resolved profiles share a peak near 0.76 μm, in close agreement with the shifted Transport of Ions in Matter (TRIM) vacancy profile and within the damaged layer visible by transmission electron microscopy (TEM), but they exhibit broader depth distributions. Moreover, the individual coefficients respond differently to implantation, with the magnitude of P41 exhibiting a significantly larger relative decrease than P21 and P31. This anisotropic modification of the photoelastic coefficients, revealed by ultrafast photoacoustics, provides insight into implantation-induced changes in LiNbO3, and confirms the high sensitivity of TDBS for nondestructive, depth-resolved imaging of ion-implanted materials.