Symmetry driven control of optical properties in WO₃ films

In this work, we demonstrate that the optical bandgap of WO₃ films can be continuously controlled through uniaxial strain induced by low-energy helium implantation. The insertion of He into epitaxially grown and coherently strained WO₃ films can be used to induce single axis out-of-plane lattice expansion of up to 2%. Ellipsometric spectroscopy reveals that the optical bandgap is reduced by about 0.18 eV per percent expansion of the out-of-plane unit cell length. Density functional theory calculations show that this response is a direct result of changes in orbital degeneracy driven by changes in the octahedral rotations and tilts.