Discovery may lead to terahertz technology for quantum sensing

Newswise — HOUSTON – (July 20, 2023) –

The electromagnetic spectrum consists of various wavelengths, with visible light being just a small part of it. The manipulation of light waves beyond what the human eye can perceive has led to significant technological advancements, such as cell phones and CT scans.

Researchers from Rice University have identified an untapped segment of the spectrum that they plan to exploit. This portion lies in the mid- and far-infrared light, specifically ranging from 5 to 15 terahertz, with wavelengths between 20 and 60 micrometers. Unlike the higher optical frequencies and lower radio frequencies, this region lacks well-established commercial products.

Rui Xu, a third-year doctoral student at Rice and the lead author of an article published in Advanced Materials, stated the potential significance of this underexplored region. Referred to as "the new terahertz gap," it is much less accessible compared to the rest of the 0.3-30 terahertz range. Nonetheless, it holds promise for essential applications, such as the study and development of quantum materials for quantum electronics closer to room temperature. Moreover, it could facilitate the sensing of functional groups in biomolecules, thereby aiding medical diagnosis.

This research was conducted at the Emerging Quantum and Ultrafast Materials Laboratory, led by co-author Hanyu Zhu, who holds the William Marsh Rice Chair and serves as an assistant professor of materials science and nanoengineering at Rice University.

Researchers have faced a significant challenge in finding suitable materials to manipulate light within the "new terahertz gap." This type of light strongly interacts with the atomic structures of most materials, leading to rapid absorption. However, a breakthrough has been made by Zhu's research group using strontium titanate, an oxide of strontium and titanium.

Strontium titanate has a unique property that allows its atoms to couple remarkably well with terahertz light, giving rise to new particles known as phonon-polaritons. These phonon-polaritons are confined to the material's surface, preventing their loss within the material itself.

What sets strontium titanate apart from other materials is its capability to support phonon-polaritons across the entire 5-15 terahertz range, thanks to its quantum paraelectricity property. The atoms in strontium titanate experience substantial quantum fluctuations and random vibrations, efficiently capturing light without becoming trapped by it, even at temperatures as low as absolute zero.

To demonstrate the potential of strontium titanate phonon-polariton devices, the researchers designed and fabricated ultrafast field concentrators within the frequency range of 7-13 terahertz. These devices effectively compress the light pulse into a volume smaller than the wavelength of light while maintaining its short duration. As a result, they achieve an immensely strong transient electric field, reaching nearly a gigavolt per meter. This breakthrough opens up new possibilities for harnessing and utilizing light in the challenging terahertz frequency range.

The remarkable strength of the electric field generated by the strontium titanate phonon-polariton devices opens up exciting possibilities. This potent electric field can be harnessed to induce changes in the structure of materials, giving rise to novel electronic properties. Additionally, it can create a new nonlinear optical response from even small quantities of specific molecules, which can be easily detected using a standard optical microscope.

Zhu emphasized that the design and fabrication techniques developed by his research group are versatile and can be applied to numerous commercially available materials. This breakthrough holds the potential to pave the way for the development of photonic devices operating within the 3-19 terahertz range. Such advancements could lead to significant progress in various fields that rely on terahertz technology.

 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).