Two-dimensional terahertz spectroscopy (2DTS), a terahertz analog of nuclear magnetic resonance, is a new technique poised to address many open questions in complex condensed matter systems. The conventional theoretical framework used ubiquitously for interpreting multidimensional spectra of discrete quantum level systems is, however, insufficient for the continua of collective excitations in strongly correlated materials. Here, we develop a theory for 2DTS of a model collective excitation, the Josephson plasma resonance in layered superconductors. Starting from a mean-field approach at temperatures well below the superconducting phase transition, we obtain expressions for the multidimensional nonlinear responses that are amenable to intuition derived from the conventional single-mode scenario. We then consider temperatures near the superconducting critical temperature $T_c$, where dynamics beyond mean-field become important and conventional intuition fails. As fluctuations proliferate near $T_c$, the dominant contribution to nonlinear response comes from an optical parametric drive of counterpropagating Josephson plasmons, which gives rise to 2D spectra that are qualitatively different from the mean-field predictions. As such, and in contrast to one-dimensional spectroscopy techniques, such as third harmonic generation, 2DTS can be used to directly probe thermally excited finite-momentum plasmons and their interactions. Our theory can readily be tested in cuprates, and we discuss implications beyond the present context of Josephson plasmons.

中文翻译：

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集体激发的二维太赫兹光谱原理：层状超导体中约瑟夫森等离子体的情况


二维太赫兹光谱（2DTS）是核磁共振的太赫兹模拟，是一种有望解决复杂凝聚态物质系统中许多悬而未决的问题的新技术。然而，普遍用于解释离散量子能级系统的多维光谱的传统理论框架不足以解释强相关材料中集体激发的连续体。在这里，我们开发了一种集体激发模型的 2DTS 理论，即层状超导体中的约瑟夫森等离子体共振。从远低于超导相变的温度下的平均场方法开始，我们获得了多维非线性响应的表达式，这些表达式符合从传统单模场景导出的直觉。然后我们考虑超导临界温度附近的温度 $T_c$ ，超出平均场的动力学变得重要，而传统的直觉则失效了。随着波动扩散到附近 $T_c$ ，非线性响应的主要贡献来自反向传播约瑟夫森等离子体激元的光学参数驱动，这产生了与平均场预测在质量上不同的二维光谱。因此，与一维光谱技术（例如三次谐波产生）相比，2DTS 可用于直接探测热激发的有限动量等离子体激元及其相互作用。我们的理论可以很容易地在铜酸盐中进行测试，并且我们讨论了超出约瑟夫森等离子体激元当前背景的影响。