A major milestone towards the ion clock was reached today: we trapped the first Strontium ions!
This involves first heating a dispenser that contains Strontium atoms with about 4-5 W (4 A and 1 V). The atoms that fly out of the dispenser are then photoionized using two blue lasers, one at 461 nm and another at 405 nm (we use a laser from a blue-ray drive for this!). We now have Sr+ ions that can be trapped in a Paul trap. A high-voltage (300 Vpp) ~10 MHz sine-wave is applied to the electrodes of the trap. Another blue laser at 422 nm is then used both for laser-cooling and as a means to detect the ion. What we see in the video is the ion fluorescence at 422 nm when it jumps down to the ground state from an excited state. The excited state can also decay into a dark state and we need a re-pump laser at 1092 nm to keep the ion in the Doppler cooling cycle. The fluorescence emitted from one or a few ions is very weak, and we used an image-intensifier and a CCD camera with 500 ms to 1 s exposure time for this video.
The camera software produces FITS frames as output. I used these commands to make the video:
mogrify -format png *.fts # convert to PNG mogrify -crop "640x480+436+315" +repage *.png # crop to the interesting area mogrify -contrast-stretch 10x100 *.png # improve contrast ffmpeg -r 3 -f image2 -i 'myframes_%02d.png' -qscale 1 'video.avi'
On the last line "3" is the desired frame-rate of the output. I then concatenated a few of these videos together with
mencoder video1.avi video2.avi -mf fps=3 -oac copy -of lavf -ovc copy -lavcopts aglobal=1:vglobal=1:coder=0:vcodec=mpeg4:vbitrate=4500 -vf scale=1280:720 -o output2.mp4
This is the same experiment we did back in April, but now "in-situ" in another vacuum system that houses our RF endcap trap. The thin ray of light that slowly switches on and off is a 461nm blue laser-beam that excites Strontium atoms that fly in from the lower right corner of the picture. The on/off switching happens because we slowly scan the laser frequency back and forth and as a result different parts of the atom-stream absorb and emit.
Next stop: ionizing the atoms and trapping a single ion in the RF trap.
We've been playing with a blue laser at 461 nm in the lab lately. If tuned to just the right frequency (wavelength) neutral Strontium atoms will strongly absorb the laser light. Shortly (5 nanoseconds) after that the atoms emit at 461nm also, allowing us to see them:
The atoms originate from a hot "oven" at the right. It glows dark red because it's heated by driving a 5 A to 7 A current through it. The cloud of absorbing atoms glows at 461nm in the centre of the picture.
We can scan the laser frequency by adjusting the current through the diode-laser that produces the light. If the frequency is too low or too high we'll see nothing as the light will just pass through the cloud of atoms without interacting. On each side of the correct absorption frequency we'll see different parts of the atom cloud light up. This happens because the atoms stream out of the oven in slightly different directions, so they experience a different Doppler shift and will react to light with a wavelength slightly to the blue or red from the centre of the absorption-line at 461nm.
When slowly scanning the laser frequency over the absorption-line we got these nice videos. One with a narrow beam and one where the laser beam was expanded.
These were shot with a Canon DSLR so be sure to view them in HD on youtube!