Eric Christensen and Dr. Justin Peatross, Physics and Astronomy
Extreme ultraviolet (EUV) light can be generated through the interaction of a high-intensity laser beam with atoms. When an intense laser pulse hits an atom, its outer electron is pulled away from the atom. The electric field of the laser may then pull the electron back to the atom. When the electron and atom collide, a photon can be emitted.1 As the laser interacts with many atoms near the focus of the beam, the emitted photons build up coherently in the forward direction, creating a highly-directional beam of light. In Dr. Justin Peatross’s lab we work on ways to optimize the generation of these photons, which are high-order harmonics of the original laser beam.
One of the limiting factors in the generation of high-order harmonics is the re-absorption of the harmonics as they travel through the very gas that generates them. The more gas through which the generated harmonics travel, the more likely it is that they will be absorbed by the gas molecules. We performed experiments to directly determine the rate of absorption of high-order harmonics as they propagate through a helium- or neon-filled gas cell. Our results agreed fairly well with theoretical predictions.
In the experiment, we shot a laser through a long gas tube filled with neon or helium. The interaction between the laser and the gas generated high-order harmonics of the laser, as mentioned above. A molybdenum foil sealed the end of the tube. The laser drilled a tiny hole through the foil, allowing the laser and the harmonics to pass through while keeping the gas from leaking too much. Using a diffraction grating, a microchannel plate, and a CCD camera, we were able to take pictures of the high-order harmonics and put them into a computer.
A second cell was placed approximately 6 cm away from the molybdenum foil on the end of the glass tube. In individual trials, we filled the secondary cell with varying amounts of neon and helium. As we increased the pressure of the gas in the secondary cell, more of the high-order harmonics were absorbed. Figure 1a shows pictures of the harmonics after passing through the secondary cell filled with various amounts of helium.
One of my big tasks in this experiment (with help from Dr. Peatross and other students in the group) was to design and build the secondary cell. We had to decide what materials to use to make the cell. Also, we had to figure out how to hold the cell and be able to adjust its positioning in order to align it with the laser. After making a cell, we found that it did not work the way we wanted to, so we had to figure out a new way to make the cell. It took quite a bit of effort to make the cell, and in the end the final cell design was actually much simpler and easier to use than the first cell we made. Figure 1b is a picture of the secondary cell.
Our group has used theoretical re-absorption data in order to interpret some data in another experiment we have been doing. The results of my experiment correspond fairly well with the theoretical data (especially qualitatively—we would probably need a little nicer equipment to get more accurate quantitative data) and help confirm that our conclusions in the other experiment are valid. Figure 1c shows plots of the experimental data we obtained and the corresponding theoretical data for the re-absorption of high-order harmonics in neon.
Working in Dr. Peatross’s lab—and particularly on this project—has provided me with invaluable experience. This research experience was very helpful in preparing me for research in graduate school.2