Wednesday, July 1, 2009

Horizontal and Vertical Profiles of THz Beam

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).


  1. Are you using a current pre-amp after the photoconductive receiver and before the lock-in? It seems that you have quite a bit of noise, expecially for the experiments where you used a 1 s time constant on the lock-in. Can you take multiple time-domain traces of the THz pulses, and then average them together? This is a handy way to increase the SNR, by the SQRT of the number of waveforms averaged together.

  2. We are indeed using a current pre-amp between the photoconductive receiver and the lock-in. The thing is, though, that these profiles were not done using a time-domain trace. Basically we took a reference scan and from there found where the maximum of the pulse was (i.e. the displacement of the receiver THz pulse stage which gave the greatest amplitude in transmitted signal).

    Once we located this maximum position (which was in fact done by taking a temporal scan), that is where we set receiver pulse (to interfere with the signal) for the duration of the measurements. Then, once the excitation beam was incident on the Si, we could chop that beam and read the difference in signal from the reference before.

    All that was moved after this was the x- and y-position of the incident excitation beam. At each new location, there was a different value of difference in transmitted THz signal. These different values are all I used to create the profiles -- which are shown in the plots of the next day's post (tomorrow from this post).

    Since I am not taking time-domain traces, there should be a different way to reduce this noise (and this is talked about in a post for about a week from today).

    I hope this clears up a little about the oscillating signal, and sorry about the ambiguity from before.

  3. Ok, I see what you mean. Note, however, that even though you are not actually scanning the delay rail in time, you are still taking data from a time-domain trace - it is just frozen at one particular point on the delay-rail travel and one particular point of synchronization between the laser pulses illuminating the emitter and receiver. Thus, increasing the time constant of the lock-in, taking multiple measurements of the peak amplitude and averaging them, etc., will all serve to reduce noise. There are all sorts of things that can jitter in this experiment, from the laser to reference frequency to the mechanical components to the electrical components. e.g., if the path length should change between the laser pulse trains on the emitter and receiver, even a very slight amount, then the sampling gate will move off the peak of the THz signal. That is one reason to be cautious about using the sort of fixed-delay-path-at-the-peak-of-the-THz-signal technique that you have. i.e., if there is any jitter in the path difference, it directly equates to a change in the amplitude you are observing. One thing to periodically check to ease your mind about this is the amplitude under the reference condition - that is, when the THz beam is chopped and the excitation beam is blocked. If you're not changing the delay-rail position during the day, then anytime you check this reference condition, you should observe the same THz amplitude on the lock-in. If it has changed, it doesn't mean necessarily that the optical beam paths have changed relative to one another (e.g., it could be the laser power has changed), but if you can move the delay rail slightly and observe an increase in the lock-in signal, then that path length difference has somehow changed by the amount you had to move the delay rail.

    Hope that's somewhat clear. And have a good weekend.

  4. This is in fact very clear... and I understand better now about the frozen delay rail and how my technique could use maybe some more precise monitoring. I hope to have some time to do this tomorrow (Monday), when I hope to take one final, good roster scan of the THz signal.