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Enhanced Coherent Terahertz Emission from Critical Superconducting Fluctuations in YBa2Cu3O6.6

D. Nicoletti1,*, M. Rosenberg1, M. Buzzi1, M. Fechner1, Y. Liu2, S. Nakata2, B. Keimer2, R. A. Vitalone3, D. N. Basov3 et al.

P. E. Dolgirev4, E. Demler5, M. H. Michael1, and A. Cavalleri1,6

  • *Contact author: daniele.nicoletti@mpsd.mpg.de
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Phys. Rev. X 15, 041036 – Published 24 November, 2025

DOI: https://doi.org/10.1103/lbph-97sg

Abstract

Coherent terahertz (THz) emission is emerging as a powerful new tool to probe symmetry breakings in quantum materials. This method relies on second order optical nonlinearities and is complementary to second harmonic generation spectroscopy. Here, we report coherent THz emission from Josephson plasmons in underdoped YBa2Cu3O6+x, and find that the amplitude of the emitted field increases dramatically close to the superconducting transition temperature, TC. We show theoretically how emission is enhanced by critical superconducting fluctuations, a nonlinear analog of critical opalescence. This observation is expected to be of general importance for the study of many thermal and quantum phase transitions.

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Coherent terahertz emission (CTE) spectroscopy analyzes terahertz waves emitted by materials as a result of photoexcitation with ultrafast optical pulses. Normally, this emission cannot occur in solids with inversion symmetry, but it becomes possible when that symmetry is disrupted—for example, by specific patterns of charge density waves that periodically rearrange electrons. In this study, we observe CTE in the high-temperature superconductor YBa2Cu3O6+x (YBCO) and find that the emitted terahertz signal grows dramatically near the superconducting transition temperature. This enhancement arises from critical fluctuations of the superconducting state through a nonlinear optical effect similar to the light scattering seen in fluids near a critical point.

In our experiment, we find that the frequency of the terahertz emission closely matches the natural oscillation frequency of superconducting charge pairs moving across atomic layers in the crystal—a mode known as a Josephson plasmon. As the temperature approaches the superconducting transition, the emitted field intensifies and a second, higher-frequency component appears at twice the main frequency. This new signal indicates that pairs of these interlayer oscillations are excited and strengthened by the growing superconducting fluctuations.

Our results demonstrate that CTE spectroscopy can sensitively track the fluctuations of superconductivity close to the transition temperature, offering a new window into critical behavior near phase changes. Because the emission likely arises from surface oscillations rather than from the bulk, future terahertz near-field microscopy could help visualize where and how these surface modes arise.

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References (68)

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Phys. Rev. X 15, 041036– Published 24 November, 2025
  • Received 3 April 2025
  • Revised 4 July 2025
  • Accepted 17 October 2025
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