Andrew David Ludlow and Dr. Scott Bergeson, Physics
Laser cooling and trapping has become one of the most widespread and successful research tools within atomic physics in the last twenty years. The idea is to “hit” an oncoming atom with photons of laser light, which slow the atom down. Then, using the appropriate magnetic field in conjunction with the laser light, atoms are not only slowed down to a near stop, but also become trapped and suspended in a small region of space. Since temperature is really only the measure of atoms’ speed, these slow, trapped atoms are very, very cold – near absolute zero. In the last five to ten years, interest has increased in laser cooling alkaline earth atoms – those found in the second column of the periodic table. We were interested in laser cooling calcium, an alkaline earth atom, in order to study ultra-cold, strongly coupled plasmas. Plasmas are most familiar in astrophysical environments like stars. The study of an ultra-cold plasma could offer more detailed insight into fundamental plasma dynamics than a star. This is because stellar plasmas are so hot that the atoms’ high speed motion overwhelms other plasma processes.
In order to laser cool and trap, we needed laser light that was carefully controlled to be stable at just the right wavelength. As part of my research this past year, I designed and built a laser system at 671nm to be used in our laser cooling setup. I used a laser diode (the same kind of laser device used in laser pointers or bar code scanners, only a little nicer). Unfortunately laser diodes on their own emit too broad a range of wavelengths and lack inherent stability to be useful in laser cooling. I implemented optical feedback in the Littrow mount to overcome these properties. The idea is to reflect a fraction of the original laser light intensity and wavelengthspread back into the laser diode. Doing so properly can force the diode to operate more stable and with a more narrow wavelength band. But unfortunately, this is still not stable enough. In order to have just the right light to be absorbed by a calcium atom (corresponding to an electron jumping from one energy level to another), a technique known as Pound-Drever-Hall frequency locking is implemented. I constructed the electronic analyzing circuits that would allow the laser to be locked at just the right wavelength. All of this must be done in a carefully controlled environment, with a stable laser diode driving current, a stable diode operating temperature, and sufficient isolation from external perturbations. This required quiet, well-tuned electronic drivers, which I constructed from the appropriate electronic components.
In designing and constructing this laser system, there were two major difficulties I encountered. First of all, in order to have proper optical feedback I needed to evaporate a thin film, antireflection coating on my laser diode. This was done in an ultra-high vacuum and required a well-designed setup which electrically and thermally isolated the laser diode from the rest of the vacuum system and from the heat of the evaporating silicon monoxide (which was used as the anti-reflection coating). This took a fair amount of time to put together. The second difficulty I had was in the locking circuitry described above. After putting it together, it did not operate exactly as it should have. After extensive troubleshooting, I discovered that the operational amplifiers that I was using had a slightly smaller bandwidth than what I needed. Although this was not overly difficult to fix, it was a time consuming process to troubleshoot.
After finishing my laser system, I characterized it using a device known as a spectrum analyzer (a confocal, scanning Fabry–Perot cavity). I saw that the overall laser system was stable and quite tunable (I could continuously tune its frequency by a range of over 15 GHz). It will serve as a useful part of our laser cooling setup and facilitate our ultra-cold plasma research.
References
- I wish to thank the Office of Research and Creative Activitites for their support. Additionally, I offer special thanks to Dr. Scott Bergeson, my research mentor.