- Improvements on the charge control measurements of the torsion pendulum, electron gun, and autocollimator system were conducted at the University of Washington in preparation for the fabrication of the final prototype of the Laser Interferometer Space Antenna (LISA) gravitational wave detector for NASA. One main cause of transient forces on LISA is solar charges in space that can cause unwanted torque on the test masses. Using a torsion pendulum as a geometric equivalent of the test masses, charge control measurements were conducted by producing a sinusoidal curve of the charge. The pendulum could be negatively or positively charged by an electron gun or a UV LED by increments as small as a pico-coulomb. The average charging for the UV LED is 3.5*10⁻¹⁴ C/s, and electron gun is -4.4*10⁻¹⁴ C/s, which allows for very precise control.
Photocurrent of the UV LED and electron gun were measured by fabricating two new electron guns, one including an Einzel lens. Preliminary results showed that a current leakage powering the UV LED was a major source of error. To compensate for this error a battery system was built to power the UV LED separately from the electron gun. Subsequent results showed that, even with the battery improvement, no difference in current could be measured. To cut down on further current leakage a data acquisition program (DAQ) was designed to electronically control the power of the system. The results from varying the voltage using the DAQ, showed that the photocurrent linearly increases and asymptotically approaches a maximum current. However, by moving the battery system, the current jumps drastically, which means that the production of photocurrent is yet to be conclusive.
Finally, the autocollimator was used to measure the torque on the pendulum with heterodyne interferometry. A system of thermometers was fabricated to measure the temperature inside and outside the autocollimating system. It was shown that the thermal noise fluctuation is 0.14C° in the system. It is hypothesized that this fluctuation can be eliminated which will be enough to improve the sensitivity of the autocollimator by an order of magnitude.
“Lab on a Chip” - Prototypes of a “Lab on a Chip” system that can make the sensors in situ in the device and then directly measure changes within a cell could greatly aid biologist in investigating changes within a variety of cells. At Oregon State University, an experimental setup was designed to trap and excite fluorescence of a particle or nano-sensor with and without a microfluidic device. An excitation laser at 532 nm and an optical trapping laser at 800 nm were used. The power of the excitation laser must be ~10 mW, which is low enough to cause minimal photo bleaching of the particles, and the power of the trapping laser must be ~5 mW for a 1 μm particle. One particle was trapped at a time by using confocal setting and aligning both lasers to hit the sample on the inverted microscope to within a few nano meters.
Proof of concept of the setup was then conducted using commercial Fluorospheres. The spectrum measured by this design matches the theoretical spectra provided by the particles’ manufacturer demonstrating that the experimental design performs to specifications. Measurements were taken on the nano-sensors made by the collaborators at the chemistry department to verify that the setup is compatible with the new particles. First using only the excitation laser to excite and trap the sensors, different fluorescence peaks were measured for two samples of particles in pH ~2 and ~6 solutions. Because of photo bleaching, the behavior of the spectra are indistinguishable from that of deprotonated peaks, however there was notable difference between the particles. The trapping laser was then reintroduced and verified that fluorescence measurements of a nano-sensor can be obtained with this experimental design.
Two other experiments were conducted, which confirm the proof of concept of this experimental design. Fluorescence data of K+ ion sensitive sensors, which switch from deprotonated peaks, to protonated, using widefield excitation, was measured. This shows that the sensors behave as expected. Also, using this experimental technique developed, the setup was able to trap and measure the fluorescence of pH 2 and pH 7.5 sensors inside a microfluidic trap, and that the correct measurements can be taken with the setup even within a microfluidic trap.