Optical trapping of photochromic microcrystals by a dual fiber tweezers

Upon light irradiation, photochromic materials exhibit drastic changes in absorbance in addition to molecular and mechanical changes. In this study, we demonstrated optical trapping of photochromic microcrystals in a double fiber optical tweezers system combined with two external lights for photoisomerization and observed changes in the trapping force by photoisomerization. In situ photoisomerizations of the single crystal trapped in the system were performed thrice revealing a reversible change in the trapping force. The trapping stiffness for the colored crystal was one-third of that for the decolorized crystal in the axial direction, whereas almost no difference was observed in the transverse direction. Furthermore, based on the intermittent motion and asymmetry of the positional distribution of the trapped crystals, we inferred that the motion includes transitions through multiple stable sites, and the ease of the transition is related to the isomerization state of the crystals. Such switching of optical trapping indicates the possibility of revealing the physical properties of trapped materials through trapping schemes, and the nonlinearity in the trapping is also expected to be applied to advanced computing with physical phenomena.