## 2 Experimental  
Macroscopically aligned (single domain) polysiloxane-based LS-LCE films were prepared by the two-step \"Finkelmann cross-linking procedure\" [24]. The cross-linker concentration was 5 wt% and the photosensitive azomesogen concentration was 9 wt%. The azomesogens and also the conventional mesogens were attached to the polysiloxane backbone as side groups. The details of the synthesis method and sample preparation procedures are described elsewhere [6]. The thickness of the films was 150 \(\upmu\)m. Samples of rectangular shape with a typical surface area of around 5 \(\times\) 5 mm\({}^{2}\) were cut out from the films. For probing strain-induced modifications of diffractive properties, one edge of the sample was clamped to the frame of the sample holder, while the other edge was attached to the translation stage used for imposing a controlled tensile strain (see Figure 1). The straining was performed either along or perpendicular to the initial alignment direction \(\mathbf{n_{0}}\). For probing illumination-induced stress modifications, the sample was slightly prestrained, and then clamped to the strain gauge that was used to measure the tensile force. The force was measured via two resistive type sensors, in which resistance is very sensitive to contraction/expansion. They were attached to the frame of the sample holder, as indicated in Figure 1b. The investigated one-dimensional and two-dimensional holographic transmission gratings were recorded either with two or four intersecting UV laser beams from an argon ion laser operating at the recording wavelength of \(\lambda_{R}\) = 351 nm. All of the beams were expanded to the area with a diameter \(d_{R}\)\(\sim\) 10 mm, which was considerably larger than the LCE film, and were linearly polarized in the direction parallel to \(\mathbf{n_{0}}\) (extraordinary polarization). The average power density of UV radiation on the sample during the recording process was \(\sim\)20 mW/cm\({}^{2}\), and the typical illumination time was several minutes. In the two-beam configuration, the intensity pattern is described as [7]:  
\[I(x)=I_{0}(\cos(k_{\perp}x))^{2}, \tag{1}\]  
where \(k_{\perp}\) = \(k_{0}\) sin\(\alpha\) is the transversal component of the wave vector of incident beams, \(\alpha\) is the incidence angle (with respect to surface normal), and \(k_{0}\) is the wave vector in vacuum. This pattern induces a one-dimensional modulation of the optical properties of the sample with a periodicity of \(\Lambda=\pi/k_{\perp}\) along the \(x\) axis. In the four-beam configuration, the intensity pattern is given as [25]:  
\[I(x,y)=\mathrm{I}_{0}(\cos(k_{\perp}x)+\cos(k_{\perp}y))^{2}, \tag{2}\]  
which corresponds to the quadratic lattice oriented at 45\({}^{\circ}\) with respect to the \(x\) and \(y\) axes. The lattice distance (unit cell dimension) is \(a=\sqrt{2}\pi/k_{\perp}\) and the diagonal of the unit cell is \(D=2\pi/k_{\perp}\). The diffraction properties of the recorded grating structures were probed with a low-power beam (\(<\)1 mW) from a HeNe laser operating at a probing wavelength of \(\lambda_{P}\) = 633 nm. As this wavelength is far from the absorption peaks of the material, it is affected mainly by the phase-type modulation of optical properties [26]. The probe beam was linearly polarized in the direction of extraordinary polarization, and entered the film at normal incidence. The spot size of the probe beam in the film was \(d_{p}\)\(\sim\)0.2 mm, which is much smaller than the diameter of the recording beams [27]. The far field diffraction pattern was detected either with a CCD camera (strain-induced effects) or by a set of photodiodes (experiments with fixed sample length) placed at the distance of 0.5 m behind the sample. Figure 1: Schematic drawing of the sample mount used for (**a**) investigations of the effects of stretching and (**b**) investigations with a fixed sample length. Yellow rectangle denotes the liquid crystalline elastomers (LCE) film, blue denotes the adhesive tape (Capton tape), and black denotes the metallic holders.