Today (Friday) and yesterday, I had a chance to look at some of the data that Antoine took last week under conditions in which he filled the microscope chamber with nitrogen gas in order to remove the polar molecules (i.e. water vapor). This was done because we know that THz is absorbed quite readily into polar molecules, of which water vapor is. In almost every spectral plot that I have looked at (many of which are in past posts), we see sharp spikes, or valleys, in which a certain frequency (or band of frequencies) is not very prominent. One suspicion as to what causes these sharp regions of no frequency is absorption of THz into water. Thus, removing the polar molecules may show us some interesting results.
I do not know the conditions too well under which Antoine took the data (as I will need to talk to him about this), but he simply took a few scans at some of the more interesting points of the THz beam. The plot below shows the spectra for the four points that Antoine took scans at. The plot is labelled, and so it should be clear which spectrum corresponds to which position.
There is clearly some very strange things happening here. I do not understand why there are oscillations for each spectra, and I do not understand how a spectrum could have this shape in general. Before I worry about this too much, I would like to talk with Antoine a bit more about his procedure in taking this data and if they usually see things such as this in spectroscopy measurements under N2 conditions.
For the sake of displaying this data, I have also shown a plot of the temporal trace at each of the points that was scanned. I first show the full time-domain traces, and then for the sake of cleanliness, I chop these pulses to try and "zoom in" on the primary pulse. Both plots are shown below.
The colors for each of the specific positions correspond exactly to those in the spectral plot from above. It is clear from these time-domain traces that we still get the reflections that are so obvious in the time-domain traces done without the N2 environment. It does, however, seem as though there may be a significant decrease in the amount of reflections, though these plots do not rightfully show this.
The following plot is that of the time-domain traces at (0,0) for the N2 environment and under the measurements which I made a few weeks ago in which I took four traces and averaged their signal.
This does not seem to tell too much other than there is a lag between the time of the two pulses (due to the speed of light in the given medium) and there is a clear difference in peak amplitude between the two different environmental conditions. There does, however, seem to be a similar amount of reflections/noise after the main pulses for both scans, which might suggest the N2 did not do much to decrease said reflections.
Finally, it is interesting to look at a comparison between the two spectra -- one at (0,0) in N2 and one at (0,0) in the typical environment. Such a plot is shown below, using the same data as the time-domain trace comparison above.
We still see this oscillatory effect in the N2-environment spectrum, while we see nothing of the sort when not using N2. Also, it is clear that there is a much greater percentage of the spectrum which is transmitted through the wafer, as the amplitude of the N2-environment spectrum is greater than that of the other spectrum. Again, I am not very certain why we see such effects using the N2 as opposed to not, but I would like to investigate this a good deal more.
In order to attain a better understanding of these absorptions and anomalies in the spectral plots, I think I first need to be able to truncate the data from the time-domain pulses to hopefully result in a better, smoother-looking spectra. From here I should be able to better determine waist-size dependence on frequency.