Linear and Nonlinear Photoinduced Deformations of Cantilevers

Abstract

Glassy and elastomeric nematic networks with dye molecules present can be very responsive to illumination, huge reversible strains being possible. If absorption is appreciable, strain decreases with depth into a cantilever, leading to bend that is the basis of micro-opto-mechanical systems (MOMS). Bend actually occurs even when Beer's law suggests a tiny penetration of light into a heavily dye-doped system. We model the nonlinear opto-elastic processes behind this effect. In the regime of cantilever thickness giving optimal bending for a given incident light intensity, there are three neutral surfaces. In practice such nonlinear absorptive effects are very important since heavily doped systems are commonly used.

Curvature in nematic elastica responding to light and heat

“…Furthermore, if the top and bottom of the cantilever are free surfaces, then s zz vanishes on the boundaries in z and thus s zz (z) is identically zero (as are also s xz and s yz by the same argument). For spontaneous rather than the usual imposed deformations, we have bend with no net force or torque acting through any section of the cantilever, in particular through the xx-and yy-sections (Warner & Mahadevan 2004;Corbett & Warner 2007;van Oosten et al 2007;Jin et al 2010):…”

Bleaching and stimulated recovery of dyes and of photocantilevers

“…We return to this problem elsewhere [19] and here take the timeindependent, equilibrium state. Thus, using n c = 1 − n t , dividing through by the incident intensity I 0 , and letting d t = 1/(γ t Γ t ) and d c = 1/(γ c Γ c ) denote the characteristic lengths for optical attenuation by trans and cis chromophores respectively, one obtains [16,17] …”

“…Differentiating eqns (16) and (17) with respect to the cantilever thickness w and solving between the resulting equations, we obtain the relationship:…”

“…An initial attack on this problem [16] invoked photobleaching, that is depletion of the trans form, letting light through to greater depths than would be expected by Beer's law. Here we derive these non-linear effects fully, paying attention to two additional influences that are potentially important.…”

Photomechanical Bending of Azobenzene-Based Photochromic Molecular Fibers

“…Recently, photomechanical effects observed for photochromic materials due to reversible photochemical reactions have been attracted a great deal of attention. For example, photomechanical bending motions of a variety of azobenzene-based polymers and azo-dye doped polymer systems which related the trans-cis photochromic reactions of azo-moiety have been reported [3][4][5][6][7][8][9][10][11]. With regard to materials composed of low molecular-mass compounds, needle-and plate-shaped microcrystals of diarylethene, anthrathene, and azobenzene derivatives have been reported to exhibit reversible bending motions by photoirradiation [12][13][14].…”