Sheng Lee and Dr. Scott Bergeson, Department of Physics and Astronomy
Our experiment tested the method of producing sub-thermal atomic beams using a back-illuminated laser ablation target suggested in a recent publication.
Creating a sub-thermal atomic beam has a lot of potential for researching fundamental physics. After reading and discussing several articles in the field of atomic physics with my mentor Dr. Scott Bergeson, I decided to examine a questionable method suggested by Alti & Khare for creating sub-thermal atomic beams 40-100 m/s(3-16Kelvin) using two-stage laser ablation. They claim that that achieved low velocities by using an unfocused low intensity laser. Laser ablation is the removal of material from a solid target by irradiation with a laser beam. Alti & Khare placed a He-Ne laser across from a photodiode so that the atomic beam they created crossed the laser’s path. The He-Ne laser was deflected as a result of the production of a pulsed atomic beam via rear side laser illumination. The velocity of the atomic beam was determined by measuring the peak position of the deflected signal for known distances from the thin film target. The method they used to measure the velocity of the beam was indirect and problematic. First, did they observe low velocities because large clumps of material were ablated off and not individual atoms? If we examine their results from a practical point of view, there is no known physical mechanism by which a sub-thermal beam could be created using their setup. A YAG laser knocks off atoms from an indium target, and then the atoms suddenly slow down to ~100 m/s(16Kelvin) for no reason. It breaks the law of conservation of energy.
In order to test their surprising results, we have built a laser-induced fluorescence experiment. With our design and set up, we can directly study the particle beam and its thermal velocity. Our design uses a number of optics and motors as well as two YAG laser beams and a blue probe laser. We focus YAG1 down to ~0.1 millimeter to ablate material off of a Calcium target, and deposit it onto a transparent disc creating a thin film. We then use YAG2 to back ablate the film, forming an atomic beam. In order to take long runs of data, the entire two-stage ablation process is placed in a vacuum chamber, which also prevents the Ca from oxidizing. The longevity is achieved in part by using stepper motors to rotate and advance the calcium target and disc to get a fresh region for each shot. We employ a sapphire disk as our transparent substrate to minimize the chance of laser damage.
For back ablated beam, we used velocity selective Doppler time of flight method to obtain the velocity distribution, and no one has actually measured the way we measure it; people merely assume the velocity distribution. We were the first to perform such measurement.
From the florescence signal obtained from digital oscilloscope, the earlier signal and late signal each correspond to fast atoms and slow atoms. And from all the data we took. Those fast atoms were moving well above 1,000m/s, approximately 4,000 degree Kelvin, well above the temperature measured in that recent publication. (16 degree Kelvin)
Though, we were not sure where that late signal came from. So at the very last stage of our experiment, we were trying really hard to figure out the sources of that signal shown in the late time. Unavailingly, we were running out of ideas and couldn’t figure out the exact origin of that late signal. So this is the part where we can go to town and do more research on.
Overall, our result rules out the possibility of creating sub-thermal atomic beam by two stage laser ablation.