Dynamic Optical Grating Based on a Photomechanical Molecular Crystal

Organic photomechanical crystals transform molecular-scale photoisomerization events into largescale crystal shape changes. The results in this paper demonstrate that this photomechanical motion can be harnessed to reconfigure reflective elements on the crystal surface to actively control light propagation. This is accomplished by using a single organic photomechanical crystal 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-closed photoisomerization. Electron beam lithography patterning followed by a sublimation development step leaves periodically positioned, 120 nm deep grooves that are coated with gold metal to form a reflective grating on the crystal surface. Under UV irradiation, expansion of the bulk crystal increases the grating period by approximate to 5% and changes the diffraction angle of a 633 nm probe laser beam by 0.7 degrees (first order) and 1.3 degrees (second order). The grating period change can be reversed by visible light exposure and repeated for multiple cycles. The crystal expansion and thus the diffraction angle can also be continuously tuned by controlling the duration of light exposure. Anisotropic crystal expansion and contraction due to molecular photoisomerization enables low-power light sources to steer light beams continuously over a range of diffraction angles.