## 3 Results
Figure 2b shows the results of an analogous experiment performed with a two-dimensional grating with unit cell dimensions \(a=\) 13.5 \(\upmu\)m and \(D=\) 19.1 \(\upmu\)m. Due to stretching, the square lattice is transformed to a diamond (rhomboidal) lattice with different unit cell diagonals \(D_{\mathrm{L}}\) and \(D_{\mathrm{T}}\) in directions parallel and perpendicular to \(\mathbf{n_{0}}\), respectively (see insert). The observed relative modifications \(\Delta D_{\mathrm{L}}/\mathrm{D}\) and \(\Delta D_{\mathrm{T}}/\mathrm{D}\) as a function of \(\varepsilon_{\parallel}\) are very similar to those obtained for one-dimensional gratings. Figure 3 shows the relative intensity of diffraction peaks \(I_{\mathrm{d}}/I_{\mathrm{d}0}\) as a function of \(\varepsilon_{\parallel}\), where \(I_{\mathrm{d}0}\) denotes diffracted intensity at \(\varepsilon_{\parallel}=0\). The peaks associated with diffraction from diagonal lattice planes separated for \(D_{\mathrm{L}}/2\) (denoted in the insert as peaks 11 and \(\overline{11}\)) and \(D_{\mathrm{T}}/2\) (peaks \(\overline{1}1\) and \(1\overline{1}\)) were analyzed. For longitudinal (L) peaks, the intensity is practically constant, while for transversal (T) peaks, it slightly decreases with increasing strain. At this point, it should be mentioned that the datashown in Figure 3 are already renormalized for the decrease of intensity associated with the _cis_-to-_trans_ back isomerization, i.e., the original measured intensity is multiplied with the factor \(\mathrm{e}^{(t/\tau)}\), where \(\tau\) is the _cis_-to-_trans_ back-relaxation time measured at \(\varepsilon_{\parallel}=0\), and \(t\) is the time passed since switching off the recording beams [30], so they solely reveal the effects associated with strain. If the film is strained in the direction perpendicular to \(\mathbf{n}_{0}\), strain-induced modifications of the grating periodicity show similar behavior as observed for straining along \(\mathbf{n}_{0}\). Figure 4 shows the  
Figure 3: Relative modification of intensity \(I_{\mathrm{d}}\) of optical diffraction peaks from a two-dimensional square grating as a function of the tensile strain \(\varepsilon_{\parallel}\) applied in the direction parallel to the alignment direction \(\mathbf{n}_{0}\). The insert on the left shows the polarization optical microscopy (POM) image of the sample with a superimposed unit cell. The insert on the right shows the far field diffraction pattern observed on a white screen. The transmitted beam is blocked. The diffraction peaks \(11\) and \(\overline{11}\) (longitudinal (L) peaks) and \(\overline{1}1\) and \(1\overline{1}\) (transversal (T) peaks) are marked. Figure 2: (**a**) Relative modification of grating periodicity \(\varDelta\) of a one-dimensional grating as a function of tensile strain \(\varepsilon_{\parallel}\) applied parallel to the alignment direction \(\mathbf{n}_{0}\). Solid symbols correspond to increasing strain and open symbols to decreasing strain. The drawings illustrate the orientation of the grating lines with respect to \(\mathbf{n}_{0}\). The arrows indicate the direction of strain. The insert in the lower left corner shows the far field diffraction pattern observed on a screen. The transmitted beam was blocked, so that only the first-order diffraction peaks can be seen. (**b**) Relative modification of unit cell diagonals \(D\) of a two-dimensional square grating as a function of \(\varepsilon_{\parallel}\). The drawing on the left side shows a rhomboidal unit cell with the longitudinal diagonal \(D_{\mathrm{L}}\) and the transversal diagonal \(D_{\mathrm{T}}\). The drawing on the right side shows the orientation of the 2D lattice with respect to \(\mathbf{n}_{0}\). dependence of a relative elongation of the period of a one-dimensional grating with \(\Lambda=2.5\)\(\upmu\)m as a function of the tensile strain \(\varepsilon_{\perp}\) applied in the direction perpendicular to \(\mathbf{n_{0}}\).