Double-probe atomic force microscopy for observing spatiotemporal dynamics in a photochromic thin film

Photochromic diarylethenes undergo reversible isomerization upon alternate irradiation with ultraviolet and visible light, having excellent thermal stability and durability. Thus, in combination with localized light, they are expected to exhibit intelligent functions based on light–matter composite systems at the nanometer-scale. In previous work, the formation of a clear open-ring/closed-ring boundary was observed on the surface of photochromic diarylethene thin-film crystals by simultaneously irradiating visible and ultraviolet light. To examine dynamic processes of the boundary formation, we developed a multi-probe atomic force microscope allowing simultaneous and continuous observations at different spatial positions. In this study, we developed atomic force microscopy containing two independent probe tips that allow simultaneous observation at spatially different positions. We used a unique method to control two probes in close proximity to achieve measurements at arbitrary positions. We have correlated the structural changes on the surface of photochromic thin films with the movement of the phase boundary at the crystal surface caused by photoisomerization under simultaneous broad visible and ultraviolet light irradiation. We also measured inhomogeneous expansion of nanometer-scale photoisomerizations due to local photoexcitation. This study paves the way for the elucidation of phenomena in the dynamic optical processes of crystals and for understanding versatile spatiotemporal dynamics in two- and three-dimensional optoelectronic structures within a nanometer scale.