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.