Ultrafast terahertz emission from emerging materials with symmetry breaking

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Ultrafast optical field incident on material with broken spatial symmetry (blue), time reversal symmetry (yellow), or both (green) to generate THz radiation. The inserts illustrate various mechanisms leading to high-frequency optical fields with rectified THz currents. (Image courtesy of Los Alamos National Laboratory). Credit: Hou-Tong Chen

Terahertz (THz) emission spectroscopy has emerged as a valuable technique for studying the static physical properties and ultrafast dynamics that occur in novel material systems that may remain hidden from other probes.

In this review article, the authors review a broad selection of recent THz emission studies involving low-dimensional and quantum materials, emphasizing unifying symmetry considerations and opportunities to exploit the interplay between intrinsic and extrinsic (artificial nanomicroscale) structure for property of the designer.

In a new article published in the magazine Light: science and applications, a team of scientists, led by Hou-Tong Chen of the Center for Integrated Nanotechnologies at Los Alamos National Laboratory, reviews a selection of recent studies that have used terahertz (THz) 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 metallic nanostructures.

“Although a variety of nonlinear optical spectroscopy exists, terahertz emission allows for the probing of material properties and dynamics that may remain hidden to other techniques,” said one of the paper’s lead authors, Jacob Pettine. “This method has therefore become very important for interrogating new materials.”

The central concept of THz emission spectroscopy is the rectification of high-frequency optical fields into low-frequency fields, similar to the rectification needed to convert alternating currents (AC) from the wall into directional currents (DC) that can power devices or charge batteries . Underlying any rectification process is a broken symmetry, often a space mirror/reversal symmetry, although time reversal symmetry breaking becomes fundamental in magnetic systems.

“At the most basic level, the emission of terahertz radiation requires some kind of directionality in your material, in space and/or time,” noted co-lead author Nicholas Sirica. “So if you get terahertz light, it immediately tells you something about the symmetry of the system.”

Co-lead author Prashant Padmanabhan added: ‘Detailed information on the material structure, electronic and magnetic properties, and light-matter interactions can therefore be obtained by measuring the THz field emitted in response to different polarisations, frequencies or amplitudes of the incident light. “.

A complementary theme explored in the review is the interplay between intrinsic (i.e. atomic lattice) and extrinsic (artificial/nanoscale) structuring, where artificial structuring can introduce new symmetries and improve THz current responses that might otherwise be weak or prohibited in the intrinsic / bulk material.

So far, the emphasis has mainly been on exploring (i) complex bulk properties of emerging quantum materials, or (ii) intricate behaviors that can occur in low-dimensional/nanostructured forms of relatively simple metals, semimetals, or semiconductors. One of the goals of this review is to highlight the opportunities at the intersection of these ideas.

“In this review paper, we aim to provide an overview of the essential systems and underlying mechanisms explored so far via THz emission,” Chen noted. “We also seek to highlight opportunities for engineering such material and light-matter interaction symmetries into artificially structured systems, such as plasmonic metasurfaces.”

The interplay between intrinsic, extrinsic and hybrid material structuring can stimulate the discovery of exotic properties and phenomena beyond existing material paradigms, the paper notes.

More information:
Jacob Pettine et al, Ultrafast terahertz emission from emerging materials with symmetry breaking, Light: science and applications (2023). DOI: 10.1038/s41377-023-01163-w

About the magazine:
Light: science and applications

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