## 3 Experiment
### Apparatus
In this configuration, the force sensor has by far the lowest spring constant; so, it is responsible for most of the displacement of the stepper motor. The slope of a plot of the force as a function of displacement gives the force sensor's spring constant [16]. After the spring constant is determined for a particular fiber using Equation (13), its Young's modulus is determined from  
\[E_{fiber}=\frac{kL_{0}}{A}, \tag{14}\]  
where \(L_{0}\) is the resting length of the part of the fiber between the clamps and \(A\) is the cross-section area of the fiber. The spring constant of the fiber in its resting and excited state can be each determined using Equation (13) by performing the measurement without and with illumination, respectively. Light-induced response measurements require uniform illumination across the entire face of the sample between the mounting clamps. The 488 nm-wavelength krypton/argon laser beam is imaged with a combination of lenses to uniformly illuminate the fiber samples and then to collect the light transmitted through the sample. The resulting force along the vertical axis of the fiber is measured with the force sensor. Figure 3 shows a schematic diagram of the experiment. Figure 3a shows the configuration used for the polarization-dependent Young's modulus measurements and Figure 3b shows the configuration for polarization-dependent photomechanical stress measurements. The pump polarization is varied between vertical and horizontal for a range of pump intensities. The pump power is in the milliwatt range. In Figure 3a, the polarizer is used to select the desired polarization axis and the intensity is varied by rotating the half-wave plate. The beam splitter picks off a small portion of the beam to monitor the incident power. The power of the picked-off light and the light transmitted through the sample are measured with a power meter. A cylindrical lens images the beam onto the length of the fiber sample. The transmitted beams are focused onto the power meters with convex lenses to ensure that all of the light is collected. The force is measured as a function of translation stage displacement for a range of intensities for the two orthogonal polarizations at each intensity. These data, when corrected for the effects of the force sensor's elasticity, determine [PERSON]'s modulus as a function of absorbed light intensity at each pump polarization. The configuration in Figure 3b is identical to that of Figure 3a, except that the beam splitter is removed and a shutter is added in front of the sample to modulate the pump light. There are two separate experiments as follows:  
First, at the start of the experiment while the vertically polarized laser beam is blocked by the shutter, the sample is mounted between the clamps and the data acquisition system is turned on to record the reading of the power meter and the strain. The start of data acquisition is marked as \(t=0\). At \(t=5\,\mathrm{min}\), the shutter is open to illuminate the sample. At \(t=20\,\mathrm{min}\), the beam is blocked again and the recording continues until \(t=25\,\mathrm{min}\). The above protocol is repeated for horizontally polarized pump light. These measurements are used to assess long-term effects on samples under stress and light exposure. In the second set of experiments, the pump light with fixed polarization and intensity is turned on for 30 s and off for 30 s while the transmitted power and stress are recorded. These provide the step photomechanical response. The intensity is then increased by a small increment and the on/off cycle repeated. The intensity is repeatedly incremented to obtain the intensity-dependence of the photomechanical response. The experiment is repeated for pump light of the orthogonal polarization.