It's Not Science Without Graphs

After months and months of nothing, behold! Signal!

Explanation below the fold.

What you're looking at here is a graph showing the signal from our optically excited metastable krypton source prototype. The red dots are fluorescence detected by a photomultiplier tube (PMT) from metastable atoms created in the source, and excited by a probe laser passing through the source region. The black dots show the intensity of another beam from the same laser passing through a plasma discharge in a vapor cell, which creates metastbles that absorb the light when the laser is at the right frequency.

The laser frequency is swept back and forth across the atomic transition a few times a second, and the signals from these two beams are recorded on an oscilloscope, so the time axis is really a measure of the frequency. The upward-going peaks in the red trace (at +/- 0.08 s) indicate that atoms in the source are absorbing light from the probe laser and re-emitting it out to the side (where the PMT is). The downward-going peaks in the black trace indicate that atoms in the vapor cell are absorbing light from the laser, and reducing the intensity seen on the photodiode.

The fact that these two signals occur at the same frequency tells us that we're really seeing fluorescence from metastable atoms, and not just some weird glitch in the PMT signal. There's even a hitch in the PMT trace at about +0.1 s that matches the jump in the vapor cell signal, corresponding to a point where the diode laser frequency hopped backwards for an instant.

The PMT trace is very preliminary data, of course. The signal is really small-- you can see the digitization from the oscilloscope-- and it sits on top of a very large background of scattered light. The peaks in the signal are less than one volt high, on top of a five-volt background, which isn't exactly ideal. There are some things we can do to clean up that background, though, and we also have some adjustments we can make to attempt to increase the size of the peaks.

There's a lot of work yet to do, but the important thing is that we have a signal. Now we're doing science, not just plumbing.