## 4 Results and Discussion
### Temperature-Dependent Measurements
In both cases, the stress persists when the pump is turned off at the same level as when the pump is on at its maximum intensity. In dye-doped fiber, the horizontal pump also has the largest influence on the stress change. However, the level of persistent stress after the pump is turned off is less than at its maximum value when the pump is on. These observations imply that the DR1 dopant  
Figure 7: The stress derivative as a function of temperature for PMMA and DR1/PMMA fibers that are unannealed and annealed. contributes to the stress change in the presence of light but that its contribution is reversible, while that of PMMA is not. Since these are unannealed samples, the irreversible part of the stress change is most likely due to a light-induced relaxation of the polymer chains, perhaps from photothermal heating. To observe the process directly in real-time, the light-induced stress at fixed strain is measured as a function of time as the fibers are irradiated with the maximum light intensity in the mW range for 15 min for both horizontal and vertical pump polarizations. Figure 10 shows the stress as a function of time for unannealed PMMA and DR1/PMMA fibers for 5 min with the pump off, 15 min with the pump on, then 5 min with the pump off. A slight monotonic decrease in stress over time is observed for unannealed PMMA. Upon illumination with 488-nm light, the stress in the DR1/PMMA fiber decreases by about 0.4 MPa. Blocking the pump light after 15 min returns the stress to a level about 0.1 MPa below the starting value. The behavior is the same for both polarizations and for unannealed and annealed fibers. Table 2.0 Stress versus Strain of unannealed PMMA fiber before, during, and after visible light irradiation with vertical and horizontal polarization. Stress versus strain curves are used to determine Young's modulus as a function of absorbed pump intensity for both vertical and horizontal polarization in both plain and dye-doped fibers. Figures 11 and 12 show that for both fibers, Young's modulus is constant before, during, and after irradiation and is not polarization dependent. So, while there is an offset in the strain induced by light, it does not affect the slope of the stress versus strain curve. Table 2.1 Stress versus Strain of unannealed DR1/PMMA fiber before, during, and after visible light irradiation with vertical and horizontal polarization. Table 2.2 Young’s Modulus of unannealed and annealed PMMA and DR1/PMMA fibers as a function of horizontally polarized absorbed pump intensity. The lines are fits to the non-zero-intensity data points. Table 2.3 Stress as a function of time for unannealed PMMA and DR1/PMMA fibers when the pump is on and off. Next, we illuminate each sample with maximum light intensity for almost an hour and its Young's modulus is measured before, during, and after illumination with both polarizations of light. Figures 13 and 14 show that visible light does not induce any permanent changes to the Young's modulus of all the fibers for both polarizations. Since PMMA is transparent in the visible range, no change in its Young's modulus is expected. It was shown that the elastic modulus of PMMA thin films remains constant upon irradiation with UV and visible light [48]. Adding an isomerizable dye does not result in a persistent change of Young's modulus after visible irradiation, but it does induce a persistent and transient stress. The photomechanical effect in DR1/PMMA fibers is known to be reversible; so, it restores the stress to its initial value [16]. In contrast, a DR1/PMMA film in which the DR1 dye is grafted as a side chain pendent shows a persistent decrease in the elastic modulus after irradiating the sample with 487-nm light [33]. In addition, azobenzene-containing films showed that upon visible irradiation the elastic modulus can increase or decrease [49; 50; 51; 52]. These differences can be due to differences in azobenzene molecules used, sample preparation methods, sample geometry, wavelength, the polarization of the pump light, and the experimental protocols.