Photonic time-crystalline behaviour mediated by phonon squeezing in Ta₂NiSe₅

Journal Article · 29 April 2024 · Nature Communications

DOI: https://doi.org/10.1038/s41467-024-47855-8 · OSTI ID:2405178

Michael, Marios H.  ^[1]; Haque, Sheikh Ul  ^[2]; Windgaetter, Lukas ^[3]; Latini, Simone ^[3]; Zhang, Yuan ^[4]; Rubio, Angel  ^[5]; Averitt, Richard D.  ^[4]; Demler, Eugene ^[6]

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1. Harvard Univ., Cambridge, MA (United States); Max Planck Institute for the Structure and Dynamics of Matter, Hamburg (Germany); SLAC
2. Univ. of California, San Diego, La Jolla, CA (United States); SLAC National Accelerator Laboratory (SLAC), Menlo Park, CA (United States)
3. Max Planck Institute for the Structure and Dynamics of Matter, Hamburg (Germany)
4. Univ. of California, San Diego, La Jolla, CA (United States)
5. Max Planck Institute for the Structure and Dynamics of Matter, Hamburg (Germany); Flatiron Institute, New York, NY (United States)
6. Harvard Univ., Cambridge, MA (United States); Eidgenoessische Technische Hochschule (ETH), Zurich (Switzerland)

Photonic time crystals refer to materials whose dielectric properties are periodic in time, analogous to a photonic crystal whose dielectric properties is periodic in space. Here, we theoretically investigate photonic time-crystalline behaviour initiated by optical excitation above the electronic gap of the excitonic insulator candidate Ta₂NiSe₅. We show that after electron photoexcitation, electron-phonon coupling leads to an unconventional squeezed phonon state, characterised by periodic oscillations of phonon fluctuations. Squeezing oscillations lead to photonic time crystalline behaviour. The key signature of the photonic time crystalline behaviour is terahertz (THz) amplification of reflectivity in a narrow frequency band. The theory is supported by experimental results on Ta₂NiSe₅ where photoexcitation with short pulses leads to enhanced THz reflectivity with the predicted features. We explain the key mechanism leading to THz amplification in terms of a simplified electron-phonon Hamiltonian motivated by ab-initio DFT calculations. Our theory suggests that the pumped Ta₂NiSe₅ is a gain medium, demonstrating that squeezed phonon noise may be used to create THz amplifiers in THz communication applications.

• Research Organization: SLAC National Accelerator Laboratory (SLAC), Menlo Park, CA (United States)
• Sponsoring Organization: USDOE Office of Science (SC)
• Grant/Contract Number: AC02-76SF00515
• OSTI ID: 2405178
• Journal Information: Nature Communications, Journal Name: Nature Communications Journal Issue: 1 Vol. 15; ISSN 2041-1723
• Publisher: Nature Publishing GroupCopyright Statement
• Country of Publication: United States
• Language: English