Depth profile of optically recorded patterns in light-sensitive liquid-crystal elastomers

Abstract

We investigated nonlinear absorption and photobleaching processes in a liquid-crystal elastomer doped with light-sensitive azobenzene moiety. A conventional one-dimensional holographic grating was recorded in the material with the use of two crossed UV laser beams and the angular dependence of the diffraction efficiency in the vicinity of the Bragg peak was analyzed. These measurements gave information on the depth to which trans to cis isomerization had progressed into the sample as a function of the UV irradiation time. Using a numerical model that takes into account the propagation of writing beams and rate equations for the local concentration of the absorbing trans conformer, we computed the expected spatial distribution of the trans and cis conformers and the shape of the corresponding Bragg diffraction peak for different irradiation doses. Due to residual absorption of the cis conformers the depth of the recording progresses logarithmically with time and is limited by the thermal relaxation from the cis to trans conformation.

Light‐Induced Refractive Index Modulation in Photoactive Liquid‐Crystalline Elastomers

“…At the beginning of the illumination with the UV intensity pattern, the photoisomerization reaction takes place only in a few micrometers thick surface layer of the film, but later on, photobleaching of the azobenzene dye molecules allows UV light to penetrate deeper and deeper into the sample depth . The associated periodic refractive index modulation causes optical diffraction of the probe beam.…”

“…Details of the mechanism of UV light‐induced periodic modifications of the refractive index in azo‐doped LCE films are described in our previous papers . The modifications are in the range Δ n ≈ 10 −2 , while the modulation depth is in the range of few micrometers to few tens of micrometers.…”

Chemical-Physical Characterization of a Binary Mixture Made of a Photosensitive Azobenzene Derivative and a Smectogen

“…For instance, the incorporation of azobenzene derivatives as dopants [10], chemically bonded pendant units [11], as well as photo-crosslinkers [12,13] into liquid crystalline polymers [14] and elastomers [15] has been widely explored with the purpose to enhance existing material properties or to enable new effects. Examples include the photo-induced alignment [16] and photo-induced optical anisotropy [17], the light-induced nanopatterning and nanowriting [18] and various properties related to optical and holographic applications [19,20,21,22].…”

“…These two molecules are basic components of liquid crystalline elastomers having optical properties [19][20][21][22]23]. The mesomorphic behaviour and self-assembling properties of the binary system, by varying the relative percentage of the two compounds, J7 and M11, were studied by differential scanning calorimetry (DSC) and polarized optical microscopy (POM).…”

Mechanical Manipulation of Diffractive Properties of Optical Holographic Gratings from Liquid Crystalline Elastomers

“…As optical holographic materials, they show many intriguing properties. For instance, they exhibit recording sensitivity that is several orders of magnitude larger than conventional (non-liquid crystalline) elastomers containing similar photoisomerizable moieties [4][5][6][7][8][9]. The recording process takes place in a very nonlinear manner that allows an intricate control over the spatial structuring of the refractive index modulation.…”