## 4 Results and Discussion
### Temperature-Dependent Measurements
The “marked” samples are those whose surfaces are painted black to prevent light from entering that sample, resulting in surface heating only. \[T_{c}^{\text{Unannealed PMMA}}=T_{c}^{\text{Unannealed DR1}}\]2 Time constant \(t_{2}\)—light off—as a function of absorbed intensity for unannealed and annealed PMMA and DR1/PMMA fibers for vertical and horizontal pump polarization. The insets show the average time constants over all intensities as a function of the fiber’s composition. The “marked” samples are those whose surfaces are painted black to prevent light from entering that sample, resulting in surface heating only. The inset of Figure 7 shows the photomechanical response for the dye-doped fiber and the painted plain PMMA fiber, unannealed and annealed. There are several important features of the data. First, in all of the fibers, the thermal contribution to the photomechanical response is approximately independent of the polarization of the pump beam. The difference between the two polarizations for each fiber is at most about \(\pm 5\%\) from the mean. As such, the largest contribution from other mechanisms is bounded by \(\pm 5\%\). Secondly, the photothermal response is smaller when the fibers are annealed. This could be due to \begin{table}
\begin{tabular}{c c c c} \hline
**Sample** & \(\rho\) **(kg/m\({}^{3}\))** & \(c\) **(J/kg K)** & \(w\) **(mm)** \\ \hline PMMA-Unannealed & \(1.19\times 10^{3}\) & \(1.42\times 10^{3}\) & 0.14 \\ PMMA-Annealed & \(1.19\times 10^{3}\) & \(1.42\times 10^{3}\) & 0.15 \\ DR1/PMMA-Unannealed & \(1.19\times 10^{3}\) & \(1.42\times 10^{3}\) & 0.13 \\ DR1/PMMA-Annealed & \(1.19\times 10^{3}\) & \(1.42\times 10^{3}\) & 0.14 \\ \hline \end{tabular}
\end{table} 
Table 4: Density \(\rho\), the specific heat \(c\), and thickness \(w\) of the fibers used in the models in this paper are from the literature [23; 41]. Figure 18: The photothermal heating contribution to the photomechanical response as a function of the static absorbed intensity \(I_{0}\) derived using Equation (7). The “marked” fibers are painted with black marker ink. The inset shows the contribution of each fiber averaged over all intensities. \begin{table}
\begin{tabular}{c c c c c} \hline
**Sample** & \(\sigma_{0}\) **(MPa)** & \(\sigma_{1}\) **(MPa)** & \(T_{0}\) **(K)** & \(n\) \\ \hline PMMA-Unannealed & \(-10\) (\(\pm 0\)) & \(12.25\) (\(\pm 0.01\)) & \(327.02\) (\(\pm 0.04\)) & \(13.99\) (\(\pm 0.04\)) \\ \hline PMMA-Annealed & \(-10\) (\(\pm 0\)) & \(11.86\) (\(\pm 0.01\)) & \(330.64\) (\(\pm 0.03\)) & \(14.29\) (\(\pm 0.04\)) \\ \hline DR1/PMMA-Unannealed & \(-10\) (\(\pm 0\)) & \(11.21\) (\(\pm 0.01\)) & \(327.91\) (\(\pm 0.01\)) & \(18.80\) (\(\pm 0.04\)) \\ \hline DR1/PMMA-Annealed & \(-10\) (\(\pm 0\)) & \(11.36\) (\(\pm 0.01\)) & \(331.16\) (\(\pm 0.02\)) & \(15.83\) (\(\pm 0.04\)) \\ \hline \end{tabular}
\end{table} 
Table 5: The parameters determined from fitting the temperature-dependent stress data in Figure 6 to Equation (6). the fact that after annealing, the internal stress that adds to the photomechanical response has been released. Further, the photomechanical response for pump beam polarization perpendicular to the strain direction is consistently lower than for the parallel polarization. This effect, though small, could be from the broken symmetry by the uniaxial strain direction. Finally, we find the plain PMMA fibers that are marked with ink have the same photomechanical response as the dye-doped ones when comparing the unannealed ones as well as the annealed ones. The temperature-dependent figure of merit is also measured to determine if elevated temperature can increase their efficiencies. It is derived using Equation (11) and is shown in Figure 19. The result shows that increasing temperature increases the figure of merit and indicates that controlling the temperature can improve the response of such materials. The largest change is for unannealed DR1-doped PMMA, which shows a linear dependence with a slope of 10% K\({}^{-1}\)--a dramatic increase.