Reversible Diffraction Efficiency of Photochromic Polymer Gratings Related to Photoinduced Dimensional Changes

Abstract

In this Full Paper, the possibility of reversibly changing the diffraction efficiency of gratings, fabricated by soft molding lithography on polymer films, containing photochromic molecules, is demonstrated. In particular, alternating UV and visible laser irradiation of the gratings causes the doped photochromic molecules to undergo transformations, which induce reversible dimensional changes to the samples. As a result, reversible changes are monitored in the intensity of the beams of a diode laser, transmitted and diffracted from the gratings. These changes affect the diffraction efficiency, which is increased upon irradiation with UV and decreased after irradiation with visible laser light. Such gratings are promising candidates for the fabrication of modern optical components such as optical switching devices.

Photocontrolled Reversible Dimensional Changes of Microstructured Photochromic Polymers

“…The experimental results are confirmed by a theoretical diffraction model. It is proved that the diffraction efficiency changes are attributed exclusively to the reversible dimensional changes of the imprinted structures (Fragouli et al 2008), and not to the refractive index changes as is the case in the majority of previous works. Specifically, most of the gratings with similar thickness as the here presented, are produced after irradiation with interfering beams that cause variation of the refractive index inside the samples mainly consisting of azobenzene polymers and liquid crystals.…”

“…(α is the period and β is the distance between the stripes, measured at the full width at half-maximum of each feature. (Fragouli et al 2008) …”

“…7.4±3.0%) values is notable, taking into account the experimental error due to factors such as the exact value of the laser beam spot size, which leads to an approximate value of the observed stripes, the imperfections of the surface introduced during the grating formation, and so on. The theoretical calculations presented, demonstrate that the decrease of the dimensions of the stripes of the gratings and of the period, are the main parameters that define the change in the DE upon irradiation (Fragouli et al 2008). Fig.…”

“…Diffraction efficiency changes of the grating upon UV-green irradiation. (Fragouli et al 2008) In order to examine the effect of the refractive index (n) change of the photochromic polymer sample upon UV-green irradiation on the observed change to the DE, ellipsometric measurements on a similar sample were conducted. The results show that before any irradiation for λ=822 nm, the refractive index of the sample is n=1.509.…”

Photoinduced variable stiffness of spiropyran-based composites

“…A proper choice of the polymer and PMs is necessary in order to tailor effectively the mechanical properties of the material, due to the usually quite low efficiency of the photomechanical effect. So far the photochromic blends of SP and polymers were proposed for applications in rather few papers, e.g., the works of Athanassiou et al 8,9 and Fragouli et al, 10 where it was shown that SP-polymeric films (5 to 10% wt.) can change reversibly their volume due to the isomeric transformations of the embedded photochromic molecules, acting as light-controlled mechanical actuators or as optical gratings with reversible diffraction efficiency.…”