Reversal of Photoinduced Bending of Crystals Due to Internal Refraction of Light

The well-known effect of bending of acicular crystals undergoing photochemical reactions is associated with the transformation gradient across the crystal thickness caused by the absorption of light. It is believed that the direction of bending is unambiguously dictated by the sign of the axial component of the reaction strain and due to the higher light intensity on the irradiated side of the crystal. In this study, it is shown experimentally and theoretically that thin crystals with a convex irradiated surface bend in the opposite direction if their thickness is less than the light penetration depth. The reversal of the bending direction is due to the lens effect, able to overcome the absorption of light in a thin crystal and provide a higher light intensity on the rear side of the crystal. A crystal of [Co(NO₃)₅NO₂]Cl(NO₃) experiencing nitro-nitrito photoisomerization, begins to bend in the opposite direction after it is thinned by etching to 2 µm when irradiated at the wavelengths of 523 nm, 465 nm, and 403 nm, absorbed at a depth of more than 10 µm in the crystal, but bends in the normal direction under 350 nm light absorbed at a depth of about 1 µm. The experimental results are fully confirmed by modeling the interaction of plane EM wave with crystals of various cross sections.