Birefringence modulation via intense coherent phonons engineering with asymmetric VO₂/TiO₂ heterostructures

The ultrafast modulation of optical crystal birefringence, involving rapid deformations of crystalline lattices, holds significant scientific and technological importance. High-frequency coherent phonons, through transient perturbation of lattice order, have emerged as a powerful tool for modifying the properties of materials. Here, we systematically investigate coherent phonons in the asymmetric crystal directions [011], [110], and [100] of VO₂/TiO₂ heterostructures. Notably, in the (011)-VO₂/TiO₂ system, a remarkable shear mode coherent phonon signal was excited, exhibiting a marginally higher intensity compared to the longitudinal mode. By changing the probe light polarization, we observed a fascinating reversal in the birefringence sign induced by the giant coherent phonons. Density functional theory (DFT) calculations indicate that TiO₂ possesses excellent photoelastic properties, with strong coherent phonons efficiently modulating refractive index anisotropy, accounting for this phenomenon. This finding provides novel insights into the development of ultrafast acousto-optic devices.