Reduced Path Length, Translational Stage, and Measuring the Displacement of a Speaker Membrane

This afternoon (Monday), we removed the delay line from the excitation beam, therefore effectively decreasing the path length of the excitation beam. This should not matter too much since the lifetime of the charge carriers seems to be long enough for the pulses to not need to be timed. This arrangement was simple, as we just took out the delay and moved some of the mirrors.

I then took the focusing lens and attached it to a manual translational stage. This will allow me to change the incident position of the excitation beam on the Si wafer.

Once this lens was attached to the stage, I tried to optimize the signal by tweaking the final mirror that the excitation beam was reflected from. I then took a few scans:

- reference scan without the excitation beam or Si wafer and THz chopped
- reference scan without the excitation beam but with the Si wafer and THz chopped
- reference scan with excitation beam and Si wafer with THz chopped

- initial scan with potential difference maximized and excitation beam chopped

These are saved and are mainly for reference later on.

We then wanted to determine the dispalcement of a mirror attached to a speaker (i.e. the distance between all the way up and all the way down on the speaker). This is because we plan on placing this vibrating mirror in the experiment somewhere in order to modulate the wave.

We tried to determine this distance by reflecting a visible laser beam off the vibrating mirror and onto a wall. The beam on the wall was stretched due to the different incident positions that the beam was on the mirror. The problem with this approach is that the "stretching" increases with the increase in distance from the mirror to the wall. Common sense (well, at least simple ray optics) suggests that the stretching should be the same, as there would just be a "fan" of parallel light rays that all travel without diverging from each other.

We did not have enough time look into this more, but the suggestion was made to use interferometry. Also, perhaps this widening is due to some frequency shift of the incident light...