Surface relief gratings in azo-polymers revisited

Abstract

We use the concept of vectorial photochemical tweezing to rationalize the experimental observations of surface relief gratings in azo-polymers, e.g., the photochemically induced motion of the polymer in a one-dimensional intensity gradient produced by two-laser beam interference. Vectorial motion of matter occurs when photochemically active, polarization sensitive molecules are photo-selected in a gradient of light intensity. Directional motion is imposed parallel to the gradient vector with an efficiency that depends on the respective orientations of the vectors of light polarization and intensity gradient. Different combinations of polarizations of the interfering beams leading to differing efficiencies of matter motion are revisited and discussed. We show that the magnitude of photoisomerization force dictates the efficiency of the observed matter motion. We also show that the spatial distribution of the photo-moved matter is Gaussian, the height and width of which exhibit an intensity dependence which is predicted by the theory of photochemical vectorial tweezing; both theory and experiments indicate that the photoisomerization force, which acts on thin films of azo-polymers, is in the mN range.

Surface Enhanced Visible Absorption of Dye Molecules in the Near-Field of Gold Nanoparticles

“…Gold NPs (Au-NPs) have been used for enhancing photovoltaic properties of dye-sensitized solar cells 28,29 and perovskite solar cells 30 , as well as in photocatalysis and environmental remediation [13][14][15]31,32 , and plasmonic photothermal therapy of cancer 33 as well as applications in imaging, sensing, biology and medicine 20 . Thin films containing photoresponsive units act as transducers that can convert incident light into molecular motion, resulting in numerous applications such as holography and data storage, as well as photomechanics and mass motion and micro-nano machines [34][35][36][37][38][39] , and so on 40,41 . For optically thin films, increasing the absorption results in improved transduction.…”

Model for athermal enhancement of molecular mobility in solid polymers by light

“…4]. The SE equation is applicable to macromolecules and polymers and therefore it is applicable to azopolymers as well [6,7,9]. In the original paper by Cohen and Turnbull, it was assumed that the SE relationship held around T g ; however, it is well known from the literature that the SE relationship breaks down close to T g in both supercooled liquids and polymer glasses owing to dynamic heterogeneity, i.e., to the presence of particles with significantly different mobility relative to the average mobility [17,18] (while our paper does not intend to resolve this discrepancy in the literature, we thought it is worth mentioning this caveat)…”

“…Thus the dependence of ln(τ /τ r ) on u is obtained by replacing η by τ in Eqs. (5) and (6) for the linear and general cases of the dependence of f on u, respectively, and the analysis pertaining to η with respect to u holds for τ . That is, a small increase in the fractional free volume considerably decreases τ .…”

“…The reciprocal phenomenon occurs upon heating. Exceptional enhancement of molecular mobility and a reversible change from solid to viscous regimes occur athermally in photoactive polymers when units of the polymer undergo structural rearrangement upon light absorption [2][3][4][5][6]. The photoactive units are smart molecular machines, which are fueled by light, and they switch between isomeric configurations at high frequency (gigahertz) [7], driving polymer chains in motion upon light action, as has been observed in the solid phase in a number of polymers by several researchers [8].…”

“…For a 532-nm photon, the absorbed energy is 2.331 eV and the 3.5-Å change in length of the long axis of the molecule yields an estimate for the force per molecule in the nanonewton range. Such a force magnitude has been measured in a single polymer chain [12] and in an ensemble of chains in a solid film configuration when a molecule is addressed by a single photon on average [6]. The potential that drives the polymer motion scales with the density N of the active molecules (molecules cm −3 ) and the total energy absorbed by the dyes reads…”