Using Light to Expose Hidden Characteristics in Materials

Certain materials have desired features that are hidden; scientists can use light to reveal these properties, just like using a flashlight to see in the dark.

Using Light to Expose Hidden Characteristics in Materials

When certain quantum materials are excited by light, they may reveal properties that would otherwise remain hidden. Image Credit: iStock / RomoloTavani

Utilizing a sophisticated optical method, scientists at the University of California, San Diego, have expanded their understanding of Ta2NiSe5 (TNS), a quantum material. The study was published in the journal Nature Materials.

Since light is the quickest object in the universe, materials can be agitated by various external stimuli, most notably changes in temperature or pressure. However, materials respond very quickly to optical stimuli, exposing features that would otherwise remain concealed.

In essence, we shine a laser on a material, and it is like stop-action photography where we can incrementally follow a certain property of that material by looking at how constituent particles move around in that system; we can tease out these properties that are really tricky to find otherwise.

Richard Averitt, Study Author and Professor, Department of Physics, UC San Diego

Lead author Sheikh Rubaiat Ul Haque, a 2023 graduate of UC San Diego who is currently a Postdoctoral Scholar at Stanford University, carried out the experiment. He and another Graduate Student in Averitt's group, Yuan Zhang, enhanced a method known as terahertz time-domain spectroscopy. With the help of Haque's advancements, scientists were able to assess a material's properties across a wider frequency range using this technique.

The work was based on a hypothesis developed by ETH Zürich Professor Eugene Demler, one of the paper's other authors. Demler and his Graduate Student Marios Michael came up with the theory that some quantum materials could become a medium that amplifies light at terahertz frequencies when activated by light. This prompted a thorough investigation of TNS's optical characteristics by Haque and associates.

A hole is left behind when a photon excites an electron to a higher level. An exciton is produced if the electron and hole are bonded together. Excitons can also result in the formation of a condensate, which is a state in which particles unite and function as one.

The team was able to observe anomalous terahertz light amplification, thanks to Haque's technique, which was supported by Demler's theory and used density functional calculations by Angel Rubio's group at the Max Planck Institute for the Structure and Dynamics of Matter. This revealed some of the TNS exciton condensate's hidden properties.

Since condensates are a well-defined quantum state, it may be possible to imprint some quantum characteristics on light by employing this spectroscopic method. This could have consequences for the developing field of quantum materials-based entangled light sources, which are light sources with interconnected features.

I think it is a wide-open area. Demler’s theory can be applied to a suite of other materials with nonlinear optical properties. With this technique, we can discover new light-induced phenomena that haven’t been explored before.

Sheikh Rubaiat Ul Haque, Lead Author and Postdoc Scholar, Stanford University

The research was funded by the DARPA DRINQS Program, the Swiss National Science Foundation, the Army Research Office, the European Research Council, the Cluster of Excellence ‘Advanced Imaging of Matter’ (AIM), Grupos Consolidados, Deutsche Forschungsgemeinschaft, and the Flatiron Institute.

Journal Reference

Haque, S. R. U.,, (2024). Terahertz parametric amplification as a reporter of exciton condensate dynamics. Nature Materials.

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