Dynamic Optical Grating Based on a Photomechanical Molecular Crystal

Abstract

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 ≈5% and changes the diffraction angle of a 633 nm probe laser beam by 0.7° (first order) and 1.3° (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.

Uniaxial photomechanical motion in large diameter polydimethylsiloxane optical fibers with fluoropolymer claddings

“…The effect can be reversible 14 – 16 or irreversible 17 – 19 Some well-studied mechanisms responsible for the photomechanical effect in solids include microscopic mechanisms such as photoisomerization 20 – 26 and photoreorientation 27 – 30 as well as macroscopic mechanisms such as electrostriction 31 – 33 and photothermal heating 34 – 39 Many materials can have multiple photomechanical response mechanisms 15 , 40 .…”

“…photoisomerization [20][21][22][23][24][25][26] and photoreorientation [27][28][29][30] as well as macroscopic mechanisms such as electrostriction [31][32][33] and photothermal heating. [34][35][36][37][38][39] Many materials can have multiple photomechanical response mechanisms.…”

Distinct photomechanical responses of two new 1,3-dimethylbarbituric acid derivative crystals

“…35 There has been considerable progress in determining how individual molecular movement can be gainfully combined to achieve targeted photomechanical responses. 36 The release of strain energy depends on several factors such as steric constraint, packing modes, topochemical alignment of molecules in the lattice and presence of non-covalent interactions. The photomechanical responses in molecular crystals can be tuned using crystal engineering approach.…”