太赫兹参量放大作为激子凝聚体动力学的报告者
Terahertz parametric amplification as a reporter of exciton condensate dynamics
AI总结：
本文利用时间分辨太赫兹光谱研究Ta2NiSe5中激子绝缘体凝聚动力学，发现光激发后太赫兹反射率显著增强且呈BEC-like温度依赖，揭示参量反射率增强源于声子压缩，为监测激子凝聚体动力学和纠缠太赫兹光子产生提供了新途径。
摘要：
凝聚体是量子材料中涌现性的标志，超导体和电荷密度波是突出的例子。激子绝缘体（EI）是这一体系中的一个引人注目的新成员，表现出电子-空穴对的自发凝聚。然而，凝聚体可观测量可能因与晶格的寄生耦合而被掩盖。时间分辨太赫兹（THz）光谱可以通过测量量子动力学来消除这些遮蔽因素。我们以Ta2NiSe5为目标，这是一种假定的室温激子绝缘体，其中电子-晶格耦合主导结构相变（Tc=326 K），阻碍了激子关联的识别。光激发后，太赫兹反射率出现显著增强，并表现出类似玻色-爱因斯坦凝聚（BEC）的温度依赖性。这发生在远低于Tc的温度下，表明了一种监测激子凝聚体动力学的新方法。尽管如此，动态凝聚体-声子耦合仍然存在，增强反射率光谱在选定的红外活性声子频率处出现峰值。这表明参量反射率增强源于声子压缩，并通过Fresnel-Floquet理论和密度泛函计算得到验证。我们的结果强调，相干动力学可以驱动参量受激发射，并带来包括纠缠太赫兹光子产生在内的伴随可能性。
英文摘要：
Condensates are a hallmark of emergence in quantum materials with superconductors and charge density wave as prominent examples. An excitonic insulator (EI) is an intriguing addition to this library, exhibiting spontaneous condensation of electron-hole pairs. However, condensate observables can be obscured through parasitic coupling to the lattice. Time-resolved terahertz (THz) spectroscopy can disentangle such obscurants through measurement of the quantum dynamics. We target $Ta_{2}NiSe_{5}$, a putative room-temperature EI where electron-lattice coupling dominates the structural transition ($T_{c}$=326 K), hindering identification of excitonic correlations. A pronounced increase in the THz reflectivity manifests following photoexcitation and exhibits a BEC-like temperature dependence. This occurs well below the $T_{c}$, suggesting a novel approach to monitor exciton condensate dynamics. Nonetheless, dynamic condensate-phonon coupling remains as evidenced by peaks in the enhanced reflectivity spectrum at select infrared-active phonon frequencies. This indicates that parametric reflectivity enhancement arises from phonon squeezing, validated using Fresnel-Floquet theory and density functional calculations. Our results highlight that coherent dynamics can drive parametric stimulated emission with concomitant possibilities, including entangled THz photon generation.
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