I began the day (Tuesday) with trying to center the excitation beam on the Si wafer. I did this by first roughly guessing where the maximum x- and y- displacements were and then performing two manual scans: one along the horizontal with the vertical displacement constant at maximum and then the same thing for the vertical scan (horizontal position constant at about maximum value).
These initial scans I did in increments of 0.5 mm and scanned about 7 mm in the x-direction and about 5 mm in the y-direction. Also, note that I have set x to be horizontal and y to be vertical. The idea of these scans was pretty simple: move the excitation beam across the wafer and record the change in THz signal from the lock-in. The greater the change, the more effective this local mirror was.
Plotting the points from these scans showed some pretty nice looking curves for such quick/crude measurements. Both curves were Gaussian in shape, with some minor anomalies.
Note: These initial measurements were done with an aperture which did two things: limited the amount of THz signal and made it possible to see a Fabry-Perot effect between the aperture and the Si wafer. This is why it was removed in later measurements.
Note: The signal tended to be rather noisy (the signal would oscillate in upwards of 0.1 mV at each position, with the relative maximum being about 4 – 5 mV). To reduce the effective noise, I looked for the signal to ‘oscillate’ and took the maximum and minimum values, later to be averaged together.
I then decided to take more precise measurements of this signal and so I changed the step size to 0.25 mm. I first did this with the x – position and plotted it. I found it to be slightly skewed, but still Gaussian. After this measurement, I removed the aperture and took scans in both the x- and y- directions. I found some very nice looking Gaussian curves for both of these scans, with the x- still being slightly skewed.
These scans ended up being pretty convincing that the THz beam is indeed Gaussian in distribution. I approximated the FWHM of the x- and y- directions to be 2.5 mm and 1.5 mm, respectively. I found the maximum signal difference to be 4.548 mV. The integration constant was 1 s, with 24 dB and 10 mV sensitivity, which were all set on the lock-in.
I also need to check if the excitation beam is stretched too much in the horizontal direction because of its angle of incidence on the wafer. I also need to see if the translation on the stage corresponds to the same translation on the wafer (i.e. if moving the stage 2 mm moves the beam 2 mm).