Femtosecond Laser Beam on an Si Wafer

I spent this morning (Friday) trying to update the blog and follow up some more of John's comments. I also found a plethora of interesting articles that were cited on the original Japanese paper on spatial profiles. One of the articles was by the RPI group that John mentioned in a comment from a few days ago. The other articles that I found look rather interesting (most of which are different techniques of spatially profiling a beam, etc.).

I then worked with Antoine a bit in the lab. We knew that the Si wafer did in fact transmit THz and so the next logical thing to do was try to create a local semi-metallic area on the wafer to see if we can change the amount of THz beam that transmits through. To do this, we took part of the fs laser beam from one of the other microscopy setups and reflected it off two mirrors and onto the Si wafer. This Si wafer had been placed directly in the THz beam and the reflected fs beam was centered on the wafer.

We then put a chopper in front of the fs beam (non-THz beam) and tried looking at the signal. The signal that we found was just a lot of noise. All we wanted to accomplish in this demonstration was finding if, when the fs beam was blocked, the THz signal changed. The problem is that the THz signal (photocurrent) was varying slightly both with and without the fs beam incident on the wafer, meaning that if there was indeed a small change in photocurrent, we could not detect it.

Maybe there was too much noise in the surrounding environment, and so maybe having everything inside of a box could decrease this – though I seem to think we should be able to at least see a slight change in signal. Also, perhaps the beam is too wide or too focused… but we tried focusing the beam and still found nothing. We would like to try switching this Si wafer with a GaAs wafer to see if this semiconductor might work better.