Rice University researchers have identified a way to utilize the “new terahertz gap” using strontium titanate, enabling the development of innovative optical technologies in the 3-19 terahertz range. This discovery could lead to advancements in quantum materials and medical diagnostics.
Metal oxide’s properties could enable a wide range of terahertz frequency photonics.
Visible light is a mere fraction of the electromagnetic spectrum, and the manipulation of light waves at frequencies beyond human vision has enabled such technologies as cell phones and CT scans.
Rice University researchers have a plan for leveraging a previously unused portion of the spectrum.
Pictured are three samples of ultrafast terahertz field concentrators fabricated by graduate student Rui Xu in Rice University’s Emerging Quantum and Ultrafast Materials Laboratory. The bottom layers (visible as white squares) are made of strontium titanate with concentrator structures — microscopic arrays of concentric rings that concentrate terahertz frequencies of infrared light — patterned on their surfaces. The arrays are visible with a microscope (inset) but have the appearance of a fine-grained pattern of dots when viewed with the naked eye. Credit: Photo by Gustavo Raskosky/added inset by Rui Xu/Rice University
Identifying the Gap in the Spectrum
“There is a notable gap in mid- and far-infrared light, roughly the frequencies of 5-15 terahertz and wavelengths ranging from 20-60 micrometers, for which there are no good commercial products compared with higher optical frequencies and lower radio frequencies,” said Rui Xu, a third-year doctoral student at Rice and lead author on an article published recently in the journal Advanced Materials.
The research was performed in the Emerging Quantum and Ultrafast Materials Laboratory of co-author Hanyu Zhu, William Marsh Rice Chair and assistant professor of materials science and nanoengineering.
Illustration of a quantum paraelectric lens (cross-section) that focuses light pulses with frequencies from 5-15 terahertz. Incoming terahertz light pulses (red, top left) are converted into surface phonon-polaritons (yellow triangles) by ring-shaped polymer gratings and disk resonators (grey) atop a substrate of strontium titanate (blue). The width of the yellow triangles represents the increasing electric field of the phonon-polaritons as they propagate through each grating interval prior to reaching the disk resonator that focuses and enhances outgoing light (red, top right). A model of the atomic structure of a strontium titanate molecule at bottom left depicts the movement of titanium (blue), oxygen (red) and strontium (green) atoms in the phonon-polariton oscillation mode. Credit: Image courtesy of Zhu lab/Rice University
The Importance and Challenges of the Terahertz Gap
“Optical technologies in this frequency region ⎯ sometimes called ‘the new terahertz gap’ because it is far less accessible than the rest of the 0.3-30 terahertz ‘gap’ ⎯ could be very useful for studying and developing quantum materials for quantum electronics closer to room temperature, as well as sensing functional groups in biomolecules for medical diagnosis,” Zhu said.
The challenge faced by researchers has been identifying the proper materials to carry and process light in the “new terahertz gap.” Such light strongly interacts with the atomic structures of most materials and is quickly absorbed by them.
Rui Xu, a Rice University materials science and nanoengineering student, is a lead author on a study that shows strontium titanate has the potential to enable efficient photonic devices at frequencies from 3-19 terahertz. Credit: Photo by Gustavo Raskosky/Rice University
Strontium Titanate and Quantum Paraelectricity
Zhu’s group has turned the strong interaction to its advantage with strontium titanate, an oxide of strontium and titanium.
“Its atoms couple with terahertz light so strongly that they form new particles called phonon-polaritons, which are confined to the surface of the material and are not lost inside of it,” Xu said.
Unlike other materials that support phonon-polaritons in higher frequencies and usually in a narrow range, strontium titanate works for the entire 5-15 terahertz gap because of a property called quantum paraelectricity. Its atoms exhibit large quantum fluctuations and vibrate randomly, thus capturing light effectively without being self-trapped by the captured light, even at zero degrees Kelvin.
“We proved the concept of strontium titanate phonon-polariton devices in the frequency range of 7-13 terahertz by designing and fabricating ultrafast field concentrators,” Xu said. “The devices squeeze the light pulse into a volume smaller than the wavelength of light and maintain the short duration. Thus, we achieve a strong transient electric field of nearly a gigavolt per meter.”
Hanyu Zhu is the William Marsh Rice Chair and assistant professor of materials science and nanoengineering at Rice University. Credit: Photo by Jeff Fitlow/Rice University
Future Implications and Applications
The electric field is so strong that it can be used to change the materials’ structure to create new electronic properties, or to create a new nonlinear optical response from trace amounts of specific molecules which can be detected by a common optical microscope. Zhu said the design and fabrication methodology developed by his group are applicable to many commercially available materials and could enable photonic devices in the 3-19 terahertz range.
Reference: “Phonon Polaritonics in Broad Terahertz Frequency Range with Quantum Paraelectric SrTiO3” by Rui Xu, Tong Lin, Jiaming Luo, Xiaotong Chen, Elizabeth R. Blackert, Alyssa R. Moon, Khalil M. JeBailey and Hanyu Zhu, 19 June 2023, Advanced Materials.
DOI: 10.1002/adma.202302974
Other co-authors of the paper are Xiaotong Chen, a postdoctoral researcher in materials science and nanoengineering; Elizabeth Blackert and Tong Lin, doctoral students in materials science and nanoengineering; Jiaming Luo, a third-year doctoral student in applied physics; Alyssa Moon, now at Texas A&M University and formerly enrolled at Rice in the Nanotechnology Research Experience for Undergraduates Program; and Khalil JeBailey, a senior in materials science and nanoengineering at Rice.
The research was supported by the National Science Foundation (2005096, 1842494, 1757967) and the Welch Foundation (C-2128).
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