## 4 Results and Discussion
### Intensity-Dependent Photomechanical Measurements
The data for vertically polarized pump light, thus, shows that heating is responsible for at least 90% of the observed photomechanical response. Molecular reorientation is the process by which molecules reorient away from the pump light's polarization axis, which should lead to an increase in the measured stress as the material's equilibrium length decreases for a vertical pump polarization. Reorientation thus opposes heating for vertical polarization, which tends to decrease the stress as the  
Figure 12: The intensity-dependent photomechanical response as a function of intensity calculated from Equation (36) at the measured intensities using the fit parameters \(\kappa_{\sigma}^{(1)}\) and \(\kappa_{\sigma}^{(2)}\) from Table 3. \begin{table}
\begin{tabular}{c c c c c c} \hline
**CTA Volume** & **Polarization** &
\begin{tabular}{c} **Pre-Stress** \\ **(MPa)** \\ \end{tabular} & \(\kappa_{\sigma}^{(1)}\) **(s/m)** & \(\kappa_{\sigma}^{(2)}\) **(s-mm/W2)** & **E (Gpa)** & **FOM (10\({}^{-4}\) s\({}^{2}\)/N )** \\ \hline \(1.33\times 10^{-3}\) & Vertical & 0.47 & \(240\pm 63\) & \(-5\pm 8\) & \(3.12\pm 0.05\) & \(0.19\pm 0.05\) \\ \(1.33\times 10^{-3}\) & Horizontal & 0.47 & \(249\pm 35\) & \(-2\pm 0.03\) & \(3.12\pm 0.05\) & \(0.20\pm 0.03\) \\ \(2\times 10^{-3}\) & Vertical & 0.47 & \(243\pm 15\) & \(-5\pm 3\) & \(3.02\pm 0.04\) & \(0.20\pm 0.01\) \\ \(2\times 10^{-3}\) & Horizontal & 0.47 & \(266\pm 16\) & \(-10\pm 2\) & \(3.02\pm 0.04\) & \(0.23\pm 0.01\) \\ \(2.66\times 10^{-3}\) & Vertical & 0.38 & \(221\pm 30\) & \(-2\pm 4\) & \(2.87\pm 0.02\) & \(0.17\pm 0.02\) \\ \(2.66\times 10^{-3}\) & Horizontal & 0.38 & \(235\pm 48\) & \(-13\pm 4\) & \(2.87\pm 0.02\) & \(0.19\pm 0.04\) \\ \(2.66\times 10^{-3}\) & Vertical & 0.76 & \(263\pm 42\) & \(6\pm 4\) & \(2.87\pm 0.02\) & \(0.24\pm 0.08\) \\ \(2.66\times 10^{-3}\) & Horizontal & 0.76 & \(266\pm 20\) & \(1.3\pm 3\) & \(2.87\pm 0.02\) & \(0.25\pm 0.04\) \\ \hline \end{tabular}
\end{table} 
Table 3: Summary of the room-temperature properties of Disperse Red 1 doped poly (methyl methacrylate) (DR1-doped PMMA) fibers as a function of CTA volume fraction, pre-stress and light polarization at a pump wavelength of 488 nm. sample elongates. As a result, the response is lower than it would be with heating alone. When the light is horizontally polarized, reorientation toward the vertical direction leads to a decrease in the stress as the material's length increases, thus adding to photothermal heating, which also results in the sample elongating. For all three polymers of varying polymer chain length, Figure 12 shows the vertically polarized pump to have a smaller response than the horizonal one in the limit of small intensity, as we would expect for a small reorientational contribution to a response that is dominated by photothermal heating. The difference between the two diagonal tensor components at room temperature is on the other of 10 s/m to 20 s/m, suggesting that the reorientational contribution is 5 s/m to 10 s/m--a 2%-to-5% contribution. The intensity dependence is governed by the \(\kappa_{\sigma}^{(2)}I\) term, which is more difficult to explain. It is due to both the temperature dependence of photothermal heating--which originates in the temperature dependence of material parameters such as the heat capacity, and molecular reorientation--which depends on the viscoelastic properties of the polymer and the temperature-dependence of the isomer populations. Accounting for these contributions are beyond the scope of our present work but will be the topic of future studies. Figure 13 shows the turn-on time constants, \(t_{1}\)--when the pump is on--and Figure 14 shows the turn-off time constant, \(t_{2}\)--when the pump is off--as a function of intensity, determined from a fit of the data to the functions in Equations (33) and (34). If heating is the only mechanism, then \(t_{1}\) and \(t_{2}\) should be the same as the Newton cooling time constant \(t_{N}\). The insets show the time constants as a function of CTA volume fraction averaged over all the intensities.