Probing amplified Josephson plasmons in YBa₂Cu₃O_6+x by multidimensional spectroscopy

Area of Science:

• Quantum Materials Science
• Nonlinear Spectroscopy
• Condensed Matter Physics

Background:

• Nonlinear driving of collective modes in quantum materials causes unique non-equilibrium responses.
• One-dimensional pump-probe spectroscopy struggles to capture complex, multi-mode couplings.
• Phonon-mediated amplification of Josephson plasmons in YBa2Cu3O6+x is linked to its optical response.

Purpose of the Study:

• To investigate the nonlinear dynamics of collective modes in YBa2Cu3O6+x.
• To explore phonon-mediated amplification of Josephson plasmons using advanced spectroscopic techniques.
• To understand the underlying mechanisms of the superconducting-like optical response.

Main Methods:

• Utilized two-dimensional nonlinear spectroscopy.
• Excited apical oxygen phonons using pairs of phase-stable mid-infrared pump pulses.
• Detected time-modulated second-order nonlinear optical susceptibility.

Main Results:

• Observed parametric amplification of Josephson plasma polaritons by driven phonons.
• Identified amplification of coherent pairs of opposite-momentum Josephson plasma polaritons.
• Characterized the resulting state as a squeezed state of the Josephson plasma.

Conclusions:

• Two-dimensional nonlinear spectroscopy effectively probes complex mode couplings in quantum materials.
• Phonon-driven amplification plays a key role in the exotic optical properties of YBa2Cu3O6+x.
• The findings provide new insights into non-equilibrium dynamics and quantum phenomena in superconductors.