Turning tantalum nickel selenide into a photonic time crystal to amplify THz laser light

A theory team from the MPSD has shown jointly with researchers in the United States and Switzerland that excitation with laser light can produce a particular quantum ion state in the excitonic insulator candidate Ta2NiSe5, turning it into a photonic time crystal. Their work, explaining the key mechanism leading to terahertz (THz) amplification in the material, has been published in Nature Communications.

The researchers were studying tantalum nickel selenide (Ta2NiSe5) in order to understand its electron behaviour when the material is exposed to laser light. They uncovered an intriguing aspect: The movements of its ions can be used for the amplification of laser light in the THz frequency. In essence, the initial laser pulses force the ions in the material into a particular swinging state which can then be used to boost the intensity of a second laser beam. The amplification of THz waves is potentially crucial for future information technologies, since they can transport large amounts of data.

The theory team demonstrated that the ions in the Ta2NiSe5 probe become entangled and oscillate when stimulated by laser pulses, leading to a “squeezed state” in the THz frequency region. The energy stored in these oscillations can then be extracted by a second “probe” laser beam, also in the THz range. That beam is amplified by the ions’ swinging motions, similar to a child on a swing whose leg movements lead to an increase in the swing amplitude. The entangled ion oscillations slow down and eventually stop, while transferring their energy to the THz probe beam.

The results of the study represent a proof of principle of how strong electron-ion coupling can lead to THz amplifiers. On fast timescales, the response of the material to light is dominated by fast electrons, while interaction with light in the THz region is dominated by slow-moving ion oscillations. Strong electron-ion coupling in quantum materials could therefore allow for an efficient conversion of readily accessible high frequency radiation to THz radiation where lasers with higher amplitude and narrow band amplifiers are needed.

“Parametric amplification and lasing in the THz region has become a hot topic due to its potential for use in THz communication. THz frequencies increase the bandwidth for the wireless transmission of information by a 1,000 times compared to low-frequency Gigaherz communications. Such speeds would be necessary for implementing wireless virtual and augmented reality,” explains lead author Marios Michael. “This area of research naturally intersects with condensed matter theory, which studies the collective properties of many-body systems, because collective oscillations such as ion vibrations of materials are typically found in the THz region.”

The collaboration involved scientists from the MPSD’s Theory Department, Harvard University, the University of California San Diego and the Stanford Institute for Materials and Energy Sciences in the United States, as well as ETH Zurich in Switzerland.

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