Dynamic Optical Grating Based on a Photomechanical Molecular Crystal

摘要

Organic photomechanical crystals transform molecular-scale photoisomerizationevents into largescale crystal shape changes. The results in thispaper demonstrate that this photomechanical motion can be harnessed toreconfigure reflective elements on the crystal surface to actively control lightpropagation. This is accomplished by using a single organic photomechanicalcrystal composed of 1,2-bis(2,4-dimethyl-5-phenyl-3-thienyl)-3,3,4,4,5,5-hexafluoro-1-cyclopentene, which undergoes a reversible ring-open to ring-closedphotoisomerization. Electron beam lithography patterning followed by asublimation development step leaves periodically positioned, 120 nm deepgrooves that are coated with gold metal to form a reflective grating on thecrystal surface. Under UV irradiation, expansion of the bulk crystal increasesthe grating period by ≈5% and changes the diffraction angle of a 633 nmprobe laser beam by 0.7° (first order) and 1.3° (second order). The gratingperiod change can be reversed by visible light exposure and repeated for multiplecycles. The crystal expansion and thus the diffraction angle can also becontinuously tuned by controlling the duration of light exposure. Anisotropiccrystal expansion and contraction due to molecular photoisomerization enableslow-power light sources to steer light beams continuously over a rangeof diffraction angles.