## 5 Conclusions  
We have presented an exhaustive study on the photomechanical and thermomechanical response of DR1-doped PMMA fibers of varying CTA volume fraction as a function of temperature, intensity, polarization, and prestress along the two principle axes of the uniaxial material. The data has been analyzed with two models to determine the underlying mechanisms. The studies start with characterizing the thermo-rheological properties of the DR1-doped PMMA polymer as a function of time, temperature, pre-stress, and polymer chain length. Measurements of the time-dependence of the photomechanical stress in response to a pump laser determine the steady state photomechanical constants, as well as the time constant of the response. The measured thermo-rheological properties and photomechanical time constant are used as input parameters to a model of the heating contribution to the photomechanical response, which predicts its absolute steady state value with no adjustable parameters. The measured results agree with the model's predictions, suggesting that photothermal heating is the dominant mechanism. The maximum temperature increase above ambient induced by the light at the highest intensity is determined to be on the order of \(\Delta T\approx 10\,\mathrm{K}\). The photomechanical experiment determines the stress response to light of a fiber sample that is constrained between a fixed clamp and a force sensor. The force is measured as a function of time, temperature, pump light polarization, and pre-stress. The photomechanical response is found to be well-modeled by a quadratic function of pump intensity and the fit functions are used to determine the effective intensity-dependent linear photomechanical coefficient. The time response is found to follow a single exponential, suggesting that one mechanism is at work. Furthermore, all time constants appear to be independent of intensity and pump light polarization, but the turn-on and turn-off time constants might be slightly different with a small dependence on polymer chain length. We apply our second independent model of the uniaxial photomechanical stress tensor to photothermal heating, angular hole burning, and molecular reorientation with the strategy of using polarization-dependent pump measurements to separate the mechanisms. Such measurements also show that photothermal heating is the dominant mechanism, with a hint of molecular reorientation at the edge of statistical significance. Finally, we use the parameters measured here to determine the photomechanical figure or merit (FOM), which is a measure of the conversion efficiency of light to mechanical energy. The FOM is found to be approximately independent of polarization and polymer chain length. We also evaluate the intensity- and temperature-dependent FOM using the intensity- and temperature-dependent photomechanical constant and Young's modulus. We find that the FOM decreases with increased intensity but increases with increased temperature by a factor of two or three, with a temperature increase of \(15\,\mathrm{K}\). Thus, soaking a material at elevated temperature might be a method for making existing materials more efficient. To conclude, we find that the photomechanical effect in dye-doped polymers is dominated by photothermal heating, and that molecular reorientation plays a negligible, though perhaps a statistically significant, role. The approach presented here has been applied to dye-doped polymers but is generally applicable to any material that absorbs light and is made of molecules that change shape or orientation when excited with light. Future studies will focus on more novel materials and sample geometries. **Author Contributions:** Conceptualization, M.G.K. ; methodology, M.G.K. ; software, B.Z. ; validation, B.Z., Z.G. and M.S.K. ; formal analysis, M.G.K. and Z.G. ; investigation, Z.G., M.S.K, M.G.K., B.Z. and R.O. ; resources, M.G.K. ; data curation, B.Z., Z.G. and M.G.K. ; writing--original draft preparation, Z.G. and M.G.K. ; writing--review and editing, M.G.K., Z.G., B.Z., R.O., M.S.K.