Researchers Get First Glimpse of Light-boosting Effect in a Solid
Interactions between an atom and an intense laser field sometimes result in the atom emitting a photon with a higher energy than the incoming laser photons. This process, known as high harmonic generation, has been well-studied in gases. Researchers at the joint SLAC-Stanford PULSE Institute for Ultrafast Energy Science and the Ohio State University decided to test this phenomenon in a solid, where many atoms are packed closely together. Their results, published online this week in Nature Physics, detail the first observations of high harmonics in a crystal, and could help explain fundamental interactions between light and matter.
"No one has ever seen this type of harmonic generation from inside a crystal," said PULSE Deputy Director David Reis, who participated in the experiment led by PULSE research associate Shambhu Ghimire. "This is also the first time the strength of the light field has been comparable to the forces in a solid. We had to think of the light and the crystal as a whole."
The photons emitted by the crystallized zinc oxide that was used in the study had energies much higher than the energies that bind electrons to atoms. Photons with such high energy would normally be absorbed by a material, but in this case were shot from the crystal as flashes of light.
When an intense laser beam comes in contact with an atom, it can rip electrons away. An electron "rides" the electric field of the laser pulse, gaining energy while accelerating away from the atom, then suddenly pullling a U-turn and hurtling back toward the atom as the direction of the electric field oscillates.
Due to the acceleration by the electric field, the electron has more energy when it crashes back into the atom than it had when it left. Since electrons in an atom can only have specific amounts of energy, this extra energy must be dumped before the electron can settle back into place. During the recombination, the excess is released as a high-energy burst of light, which can be as brief as 100 billionths of one billionth of one second.
This effect has been widely studied in gases, where atoms are spaced far enough apart to be considered as individuals. Ghimire and Reis wanted to see what it might look like when they introduced an intense laser field to a solid, specifically a crystal, where the atoms are neatly arranged in repeating, evenly spaced patterns.
"We weren't 100 percent sure what the answer would be, but we thought it would look different than the atomic case," Reis said.
For example, because the atoms in a crystal are packed so closely together, the researchers expected that an electron might crash into an atom other than the one it left. They also knew that fleeing electrons couldn't travel far before hitting another atom, and so would be scattered before gaining the maximum possible amount of energy.
But because there are many electrons simultaneously being torn from and returning to many atoms in the crystal, the researchers found that the emitted photons are combined coherently, that is, added together to form a burst of high-energy photons in a laser-like beam emanating from the crystal.
"Because of this coherence, we wondered if we could produce much more intense light with solids than we can with atomic gases," Ghimire said. So the group sought the best means to study the behavior in crystals.
"In principle, the experiment was really, really simple," Reis said. "The problem was finding the right laser." They needed a laser with high intensity, long wavelengths and short pulses. This magic combination existed in the lab of Louis DiMauro and Pierre Agostini at Ohio State. (Critical contributions to the experiment also came from Anthony DiChiara and Emily Sistrunk, also at Ohio State.)
High-intensity lasers by nature have high electric fields, which can turn a crystal into a hot soup of ions called plasma—affecting electron behavior and photon emissions and destroying the crystal in the process. "This is probably the main reason that high harmonic generation in solids has never been seen before," Reis said. The researchers got around this problem by using lots of photons in the mid-infrared region of the spectrum, which have long wavelengths and low energy.
Atoms in the zinc oxide crystal must absorb several of these low-energy photons in order for an electron to escape. Whether the electron returns to the parent atom or another, it emits light in the ultraviolet region of the spectrum.
"By measuring the light that comes out of the crystal, we can tell what the electrons are doing inside it," Ghimire said.
Ghimire and Reis are now studying how the presence of incoming laser light affects the electronic and optical properties of the crystal—for example, whether the material acts as a semiconductor or an insulator, and how light is transmitted or reflected.
"A key thing to remember is that all of these effects are reversible," Reis said. "We shine light onto this crystal and make all sorts of things happen, but they only happen when the light is there. As soon as we take it away, things go back to normal."