Research on terahertz radiation
By controlling the ultra-fast pulses of a tabletop laser in their laboratory, a team of Los Alamos scientists has created high-frequency terahertz radiation, the kind that could revolutionize high-resolution microscopy and spectroscopy in the lab and may one day have applications in medical imaging and security scanners.
In the past, scientists have been able to coax this high-frequency, short-wavelength generation only with large laser accelerators -- massive machines that speed up electrons often housed in huge complexes. Because only a handful of these devices exist, researchers who want to observe and use nonlinear terahertz effects -- including spectroscopy to glean precise information about the compounds they were trying to identify—have been limited by access to these laser sources powerful enough to produce the energy required to eek out terahertz waves.
“We used a laser you can have in your lab, rather than having to go to a large user facility,” said Laboratory physicist Toni Taylor, leader of Los Alamos’s Center for Integrated Nanotechnologies (MPA-CINT). “It’s a very practical source for a research tool.”
Taylor and her colleagues published their work in the current edition of Nature Photonics.
Their technique would allow researchers the opportunity to produce this type of radiation using a laser small enough that it could be housed in the laboratory of a single investigator. To accomplish this, the team focused speedy laser pulses through a lens and into a cylinder filled with gas. The energy from the laser knocked some of the gas’s electrons free in a process called ionization to create plasma. The liberation of the electrons in turn released radiation—in this case, with a frequency of up to 75 terahertz.
The researchers experimented with several gases, including air, nitrogen, and helium, but in the end, krypton proved to be the best source of the terahertz generation. Krypton’s outer-most electrons are farther from the nucleus than those in helium, for example, and the element is therefore more easily ionized.
Scientists studying light and other electromagnetic waves often use what’s called a beta barium borate crystal in the gas cell. In effect, this crystal shortens the wavelength of the radiation and doubles its frequency. The team found that by changing the location of the crystal inside the cell, they could control the balance between terahertz and high-harmonic radiation.
“Before, people never thought about the correlation between high harmonics and terahertz generation,” said Ki-Yong Kim, lead author on the paper and formerly a physicist at Los Alamos (now at the University of Maryland).
High harmonics also are used in microscopic imaging, and scientists know that terahertz and high-harmonic radiation are generated from antipodal processes. While terahertz generation arises from electrons floating away from the gas atoms, high harmonics are generated when the liberated electrons slam back into the newly formed ions.
But no one had proposed a mechanism that related the two types of radiation. Kim and his colleagues show that they are anti-correlated—that is, as one type increases, the other decreases.
Beyond the lab bench, the technique the team developed may have applications in larger facilities with lasers capable of generating even higher energy and hence higher frequency radiation.
“The question is: What if you use a really big system?” Kim said. “Will this technique be scalable?”
Researchers James Glownia and George Rodriguez of CINT are coauthors of the paper. Glownia now works at the Department of Energy’s Office of Basic Energy Sciences in Germantown, Maryland.
