Higgs-mediated optical amplification in a non-equilibrium superconductor

TL;DR

This paper proposes a theoretical mechanism where a non-equilibrium superconductor can amplify incident light via Higgs mode oscillations, supported by experimental evidence in optically driven K$_3$C$_{60}$.

Contribution

It introduces a novel non-equilibrium optical amplification mechanism mediated by Higgs mode oscillations in superconductors, supported by experimental validation.

Findings

Large Higgs mode oscillations can induce parametric amplification.

The amplification effect disappears with slower excitation onset.

Experimental results in K$_3$C$_{60}$ support the theory.

Abstract

The quest for new functionalities in quantum materials has recently been extended to non-equilibrium states, which are interesting both because they exhibit new physical phenomena and because of their potential for high-speed device applications. Notable advances have been made in the creation of metastable phases and in Floquet engineering under external periodic driving. In the context of non-equilibrium superconductivity, examples have included the generation of transient superconductivity above the thermodynamic transition temperature, the excitation of coherent Higgs mode oscillations, and the optical control of the interlayer phase in cuprates. Here, we propose theoretically a novel non-equilibrium phenomenon, through which a prompt quench from a metal to a transient superconducting state could induce large oscillations of the order parameter amplitude. We argue that this oscillating mode could act as a source of parametric amplification of the incident radiation. We report experimental results on optically driven K$_3$C$_{60}$ that are consistent with these predictions. The effect is found to disappear when the onset of the excitation becomes slower than the Higgs mode period, consistent with the theory proposed here. These results open new possibilities for the use of collective modes in many-body systems to induce non-linear optical effects.