Visible light is only a small part of the electromagnetic spectrum, and the manipulation of light waves at frequencies outside the range of human vision has made technologies such as mobile phones and CT scans possible. Researchers at Rice University have developed a plan to harness previously unused portions of the spectrum.
The picture shows three ultrafast terahertz field concentrators made by Xu Rui, a graduate student at the Laboratory of Emerging Quantum and Ultrafast Materials at Rice University. The bottom layer (visible in white squares) is made of strontium titanate, and its surface pattern is a concentrator structure - a microscopic array of concentric circles that concentrate infrared light of terahertz frequencies. These arrays are clearly visible under the microscope (inset), but when viewed with the naked eye, they resemble fine-grained dotted patterns. Photo by Gustavo Raskosky/Rui Xu/Rice University added illustration
Identify gaps in the spectrum
Rui Xu, a third-year doctoral student at Rice University and first author of a recent article published in the journal Advanced Materials, said: "There is a clear gap between mid-infrared and far-infrared light, in the wavelength range of about 5-15 terahertz and 20-60 microns, compared with higher optical frequencies and lower radio frequencies, there are currently no good commercial products." "
The research was conducted at the Laboratory of Emerging Quantum and Ultrafast Materials at Hanyu Zhu, the William Marsh-Rice Chair Professor and Assistant Professor of Materials Science and Nanoengineering.
Quantum quasi-electric lensing (cross-sectional view) that focuses pulses of light at frequencies of 5-15 terahertz. The incoming terahertz light pulse (red, upper left) is converted into a surface phonon-polaron (yellow triangle) by a toroidal polymer grating on a strontium titanate (blue) substrate and a disc resonator (gray). The width of the yellow triangle indicates the increase in the electric field as the phonon-polaron propagates through each grating interval before reaching the disc resonator used to focus and enhance the outgoing light (red in the upper right corner). The strontium titanate molecular atomic structure model in the lower left depicts the motion of titanium (blue), oxygen (red), and strontium (green) atoms in the phonon-polaron oscillation mode. Image credit: Zhu Labs/Courtesy of Rice University
The importance and challenges of terahertz gaps
"Optical techniques in this frequency region — sometimes referred to as the 'new terahertz gap' because it is far more inaccessible than other frequency regions in the 0.3-30 terahertz 'gap' — could be very useful for researching and developing quantum materials for quantum electronics close to room temperature, as well as sensing functional groups in biomolecules for medical diagnosis," Zhu said. "
The challenge for researchers has always been to find the right material to carry and process light in the "new terahertz gap." This light interacts strongly with the atomic structure of most materials and is quickly absorbed by them.
Rui Xu, a materials science and nanoengineering student at Rice University, is the first author of a study showing that strontium titanate has the potential to enable efficient photonic devices at frequencies of 3-19 terahertz. Photo by Gustavo Raskosky / Rice University
Strontium titanate and quantum paraelectrics
Zhu's team used strontium titanate, an oxide of strontium and titanium, to turn strong interactions into advantages.
"Its atoms couple so strongly to terahertz light that new particles called phonon-polarons are formed, which are confined to the surface of the material and do not disappear inside the material," Xu said. "
Unlike other materials that support higher frequencies of phonon-polaron and generally have a narrow range, strontium titanate supports phonon-polaron throughout the 5-15 terahertz gap because strontium titanate has a property called quantum cisoelectric. Strontium titanate atoms exhibit huge quantum fluctuations and random vibrations, so they can effectively capture light without being captured by the captured light itself, even at zero Kelvin.
"We demonstrated the concept of strontium titanate phonon-polaron devices in the 7-13 terahertz frequency range by designing and manufacturing ultrafast field concentrators," Xu said. "This device can squeeze light pulses into a volume smaller than the wavelength of light and maintain a short duration. As a result, we achieve a strong transient electric field of nearly gigavolts per meter.
Hanyu Zhu is the Wilhelm-Marsh-Rice Chair Professor and Assistant Professor of Materials Science and Nanoengineering at Rice University. Photo by Jeff Fitlow Photography/Rice University
Future impact and application
The electric field is so strong that it can be used to change the structure of a material, resulting in new electronic properties, or to create a new nonlinear optical response from a tiny amount of a specific molecule that can be detected with an ordinary light microscope. Zhu said the design and fabrication methods developed by his research group are applicable to many commercially available materials and can enable photonic devices in the 3-19 terahertz range.