Exotic Electron Liquid for Terahertz Detection

In conventional electronic devices, electricity requires the movement of electrons (blue spheres) and their positive counterparts, holes (red spheres), which behave much like the gas molecules in our atmosphere. Although they move rapidly and collide infrequently in the gas phase, electrons and holes can condense into liquid droplets akin to liquid water in devices composed of ultrathin materials. (Source: QMO Lab, UC Riverside)

By bombarding an ultrathin semi­conductor sandwich with powerful laser pulses, physicists at the Uni­versity of Cali­fornia, Riverside, have created the first “electron liquid” at room temperature. The achieve­ment opens a pathway for development of the first practical and efficient devices to generate and detect light at terahertz wave­lengths. Such devices could be used in appli­cations as diverse as communi­cations in outer space, cancer detection, and scanning for concealed weapons.

The research could also enable explora­tion of the basic physics of matter at infini­tesimally small scales and help usher in an era of quantum meta­materials, whose structures are engineered at atomic dimensions. The physicists were led by Associate Professor of Physics Nathaniel Gabor, who directs the UCR Quantum Materials Opto­electronics Lab. In their experiments, the scientists constructed an ultrathin sandwich of the semi­conductor molybdenum ditelluride between layers of carbon graphene. The layered structure was just slightly thicker than the width of a single DNA molecule. They then bombarded the material with superfast laser pulses, measured in qua­drillionths of a second.

“Normally, with such semi­conductors as silicon, laser excitation creates electrons and their positively charged holes that diffuse and drift around in the material, which is how you define a gas,” Gabor said. However, in their experi­ments, the researchers detected evidence of conden­sation into the equivalent of a liquid. Such a liquid would have properties resembling common liquids such as water, except that it would consist, not of molecules, but of electrons and holes within the semi­conductor.

“We were turning up the amount of energy being dumped into the system, and we saw nothing, nothing, nothing – then suddenly we saw the formation of what we called an “anomalous photo­current ring” in the material,” Gabor said. “We realized it was a liquid because it grew like a droplet, rather than behaving like a gas. What really surprised us, though, was that it happened at room tempera­ture,” he said. “Previously, researchers who had created such electron-hole liquids had only been able to do so at tempera­tures colder than even in deep space.”

The electronic properties of such droplets would enable development of opto­electronic devices that operate with unprece­dented effi­ciency in the terahertz region of the spectrum, Gabor said. Terahertz wavelengths are longer than infrared waves but shorter than microwaves, and there has existed a terahertz gap in the tech­nology for utilizing such waves. Terahertz waves could be used to detect skin cancers and dental cavities because of their limited pene­tration and ability to resolve density dif­ferences. Similarly, the waves could be used to detect defects in products such as drug tablets and to discover weapons concealed beneath clothing.

Terahertz trans­mitters and receivers could also be used for faster communi­cation systems in outer space. And, the electron-hole liquid could be the basis for quantum computers, which offer the potential to be far smaller than silicon-based circuitry now in use, Gabor said. More generally, Gabor said, the tech­nology used in his labora­tory could be the basis for engineering “quantum meta­materials,” with atom-scale dimensions that enable precise mani­pulation of electrons to cause them to behave in new ways.

In further studies of the electron-hole “nano­puddles,” the scientists will explore their liquid pro­perties such as surface tension. “Right now, we don’t have any idea how liquidy this liquid is, and it would be important to find out,” Gabor said. He also plans to use the tech­nology to explore basic physical phenomena. For example, cooling the electron-hole liquid to ultra-low tempera­tures could cause it to transform into a quantum fluid with exotic physical properties that could reveal new funda­mental principles of matter.

In their experi­ments, the researchers used two key tech­nologies. To construct the ultra­thin sandwiches of molybdenum ditel­luride and carbon graphene, they used elastic stamping. In this method, a sticky polymer film is used to pick up and stack atom-thick layers of graphene and semiconductor. And to both pump energy into the semi­conductor sandwich and image the effects, they used multi-parameter dynamic photo­response micro­scopy developed by Gabor and Arp. In this technique, beams of ultrafast laser pulses are mani­pulated to scan a sample to optically map the current generated. (Source: UC Riverside)

Reference: T. B. Arp et al.: Electron–hole liquid in a van der Waals heterostructure photocell at room temperature, Nat. Phot., online 4 February 2019; DOI: 10.1038/s41566-019-0349-y

Link: Laboratory of Quantum Materials Optoelectronics (N. Gabor), University of California, Riverside, USA

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