受驱YBa$_2$Cu$_3$O$_{6+x}$中的压缩约瑟夫森等离激元
Squeezed Josephson plasmons in driven YBa$_2$Cu$_3$O$_{6+x}$
AI总结：
本研究针对YBa₂Cu₃O₆₊ₓ中多模式相干耦合难以观测的问题，采用双中红外泵浦结合非线性光学测量的二维光谱方法，发现区中心声子可放大相反动量约瑟夫森等离子体激元对并产生层间相位涨落压缩态，为光诱导超导性的微观机制提供了新认识。
摘要：
量子材料中受驱集体模式的物理机制是多种显著非平衡功能响应的基础，包括增强的磁性、铁电性和超导性。然而，多种模式之间的同时相干耦合难以通过单泵浦-探测（一维）光谱技术捕捉，且目前人们对其的理解往往仍较为匮乏。一个典型例子是YBa$_2$Cu$_3$O$_{6+x}$中约瑟夫森等离激元的声子介导放大效应，该体系中至少有三种固体简正模式发生相干混合，作为超导性增强的来源。本研究突破了该体系以往的泵浦-探测实验局限，采用一对相互延迟、载波包络相位稳定的中红外泵浦脉冲，结合时间调制的二阶非线性光学极化率测量，获取了二维频率图谱。研究发现，受驱的区中心声子会放大量子化的相反动量约瑟夫森等离子体激元对，产生层间相位涨落的压缩态。这种压缩态是该材料及其他材料中光诱导超导性微观物理机制的潜在重要组成部分。
英文摘要：
The physics of driven collective modes in quantum materials underpin a number of striking non-equilibrium functional responses, which include enhanced magnetism, ferroelectricity and superconductivity. However, the coherent coupling between multiple modes at once are difficult to capture by single-pump probe (one-dimensional) spectroscopy, and often remain poorly understood. One example is phonon-mediated amplification of Josephson plasmons in YBa$_2$Cu$_3$O$_{6+x}$, in which at least three normal modes of the solid are coherently mixed as a source of enhanced superconductivity. Here, we go beyond previous pump-probe experiments in this system and acquire two-dimensional frequency maps using pairs of mutually delayed, carrier envelope phase stable mid-infrared pump pulses, combined with measurements of the time-modulated second-order nonlinear optical susceptibility. We find that the driven zone-center phonons amplify coherent pairs of opposite-momentum Josephson plasma polaritons, generating a squeezed state of interlayer phase fluctuations. The squeezed state is a potentially important ingredient in the microscopic physics of photo-induced superconductivity in this and other materials.
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