Discoveries could lead to terahertz technology for quantum sensing

(Nanowerk News) Visible light is only a small part of the electromagnetic spectrum, and the manipulation of light waves at frequencies beyond human vision has made technologies such as cell phones and CT scans possible.

Rice University researchers have plans to tap into previously unused portions of the spectrum.

“There is an important gap in mid and far infrared light, roughly the frequency of 5-15 terahertz and the wavelengths ranging from 20-60 micrometers, where there is no good commercial product compared to the higher optical frequencies and lower radio frequencies,” said Rui Xu, third year doctoral student at Rice and lead author of the article published in Advanced Materials (“Phonon Polaritonics in the Broad Terahertz Frequency Range with Quantum Paraelectric SrTiO3“).

The research was conducted at the Emerging Quantum and Ultrafast Materials Laboratory with co-author Hanyu Zhu, William Marsh Rice Chair and assistant professor of materials science and nanoengineering. Illustration of a quantum paraelectric lens (cross-section) focusing pulses of light with frequencies from 5-15 terahertz. The incoming terahertz pulse of light (red, top left) is converted to surface phonon-polaritons (yellow triangles) by a ring-shaped polymer lattice and disc resonator (gray) on a strontium titanate substrate (blue). The width of the yellow triangle represents the increase in the phonon-polariton electric field as it propagates through each lattice interval before reaching the disk resonator which focuses and enhances the outgoing light (red, top right). The model of the atomic structure of the strontium titanate molecule at lower left depicts the motion of titanium (blue), oxygen (red) and strontium (green) atoms in the phonon-polariton oscillation mode. (Image: Rice University)

“Optical technology in this frequency region ⎯ is sometimes called the ‘new terahertz gap’ because it is much more difficult to access than other 0.3-30 terahertz ‘gaps’ ⎯ can 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,” said Zhu.

The challenge the researchers face is identifying the right materials to carry and process the light in the new terahertz gap. Such light strongly interacts with the atomic structure of most materials and is quickly absorbed by it. The Zhu group has turned the strong interaction to its advantage with strontium titanate, an oxide of strontium and titanium.

“The atoms coupled with terahertz light so strongly that they formed new particles called phonon-polaritons, which are confined to the surface of the material and are not lost within it,” Xu said.

Unlike other materials which support phonon-polaritons at higher frequencies and usually over a narrow range, strontium titanate acts over the entire 5-15 terahertz gap due to a property called quantum paraelectricity. Its atoms exhibit large quantum fluctuations and vibrate randomly, thus capturing light effectively without being trapped by the captured light themselves, even at zero degrees Kelvin. Pictured are three samples of a terahertz ultrafast field concentrator fabricated by graduate student Rui Xu at Rice University’s Emerging Quantum and Ultrafast Materials Laboratory. The bottom layer (shown as a white box) is made of strontium titanate with a concentrator structure ⎯ a microscopic arrangement of concentric rings that concentrate terahertz frequencies of infrared light ⎯ patterned on its surface. The array is visible with a microscope (inset) but has the appearance of a fine grained dot pattern when viewed with the naked eye. (Image: Gustavo Raskosky; insert added by Rui Xu, Rice University)

“We proved the concept of a phonon-polariton strontium titanate device in the 7-13 terahertz frequency range by designing and manufacturing an ultrafast field concentrator,” said Xu. “The device squeezes light pulses into a volume that is smaller than the wavelength of light and maintains a short duration. In doing so, we achieve a strong transient electric field of nearly one gigavolt per meter.”

The electric field is so strong that it can be used to change the structure of materials to create new electronic properties, or to create new nonlinear optical responses from certain trace amounts of molecules that can be detected by an ordinary optical microscope. Zhu said the design and fabrication methodology developed by his group is applicable to many commercially available materials and can enable photonic devices in the 3-19 terahertz range.