Terahertz-emission spectroscopy has emerged as a valuable technique for investigating static physical properties as well as ultrafast dynamics occurring in novel material systems, which may remain hidden to other probes. In a new article in the journal Light: Science & Applications, a team of Los Alamos National Laboratory scientists led by Hou-Tong Chen of the Lab's Center for Integrated Nanotechnologies reviews a selection of recent studies that have used terahertz-emission spectroscopy to uncover basic properties and complex dynamical behaviors of emerging materials. These include quantum materials such as superconductors and magnets, as well as low-dimensional materials, such as graphene and metal nanostructures.
"Although a variety of nonlinear optical spectroscopies exists, terahertz emission allows you to probe material properties and dynamics that can remain hidden to other techniques," said Jacob Pettine, a materials scientist at Los Alamos and a co-author of the paper. "This method has therefore become quite important for interrogating novel materials."
The AC/DC angle
The central concept of terahertz-emission spectroscopy is the rectification, or conversion, of high-frequency optical fields into low-frequency currents, similar to the rectification required to convert alternating currents from the wall into direct currents that can power household devices or charge batteries. Underlying any rectification process is a broken symmetry - often a spatial mirror/inversion symmetry, though time-reversal symmetry breaking becomes key in magnetic systems.
"At the most basic level, the emission of terahertz radiation requires some sort of directionality in your material, in space and/or time," co-lead author Nicholas Sirica, also of Los Alamos, noted. "So, if you get any terahertz light out, it immediately tells you something about the symmetry of the system."
Co-lead author Prashant Padmanabhan added, "You can then gain detailed insights into material structure, electronic and magnetic properties and light-matter interactions by measuring the emitted terahertz field in response to different incident light polarization, frequency or amplitude."
The advantage of artificial structuring
A complementary theme explored in the review is the interplay between intrinsic (that is, atomic lattice) and extrinsic (artificial/nanoscale) structuring, where artificial structuring can introduce new symmetries and enhance terahertz-current responses that might be otherwise weak or forbidden in the intrinsic/bulk material. So far, the emphasis has primarily been on exploring either complex bulk properties of emerging quantum materials or intricate behaviors that can occur in low-dimensional/nanostructured forms of relatively simple metals, semimetals or semiconductors. One effort of this review is to highlight opportunities at the intersection of these ideas.
"In this review paper, we aim to provide an overview of the essential systems and basic mechanisms explored thus far via terahertz emission," noted Chen. "We also try to highlight opportunities for designing such material and light-matter interaction symmetries in artificially structured systems."
The interplay between intrinsic, extrinsic and hybrid material structuring may stimulate the discovery of exotic properties and phenomena beyond existing material paradigms, the paper notes.
Funding: Los Alamos National Laboratory's Laboratory Directed Research and Development program. This work was performed, in part, at the Center for Integrated Nanotechnologies, an Office of Science User Facility operated for the U.S. Department of Energy Office of Science.
Paper: Ultrafast Terahertz Emission from Emerging Symmetry-Broken Materials, Light: Science & Application. Jacob Pettine, Prashant Padmanabhan, Nicholas Sirica, Rohit P. Prasankumar, Antoinette J. Taylor and Hou-Tong Chen
