CAREER: Probing Quantum Materials Modified by Terahertz Quantum Fluctuations

非技术描述真空并不是真正的空无一物，而是包含根据量子力学不断波动的光波。同样，冷却到接近绝对零度的材料并不是真正的冻结，而是包含不断波动的原子运动。虽然这些波动在日常生活中通常可以忽略不计，但理论上，当驻波被压缩到一个非常小的体积（称为空腔）中时，它们就会变得很大，而空腔的尺寸与波的波长相比。根据最近的一些理论和实验研究，在这种情况下，波动波的强度可能足以改变浸入波中的材料的性质，包括原子结构、导电性和磁性。这项研究试图回答基于特定系统的空腔修饰材料新范式中的几个主要开放问题：电磁波和原子振动的混合，自然发生在许多离子晶体中。在太赫兹频率的振动共振附近，混合波的波长缩小，因此量子涨落有望增强。研究小组计划直接测量小腔内光和物质的量子涨落，并跟踪改变后的能量演化和材料性质。该结果有望通过免费收集量子力来优化材料。该项目还支持向不同教育水平的受众传播量子材料的基本概念。首席研究员计划制定一个模块化的研究生课程，以弥合现有材料教育与建立量子基础设施的社会需求之间的知识差距，为社区大学中代表性不足的群体提供本科研究机会，并与休斯顿独立学区的高中教师合作，设计一门关于量子技术材料的课程。这些努力是美国国家科学基金会量子飞跃大构想和国家量子计划的一部分，旨在为未来建立量子劳动力。新出现的实验证据和理论模型为玻色子模式（包括光子和声子）的腔增强量子涨落提供了可能性，可以显著改变激发态和基态下材料的结构、输运和磁性行为。因此，量子涨落可以作为一个控制旋钮，在不需要主动输入的情况下变换量子材料。然而，关于太赫兹频率放大波动的可能性或在腔耦合的亮极化子和更多的腔解耦局域态之间的性质转移的实验验证很少。该研究提供了亚波长腔中超强耦合声子极化子量子涨落幅度和后果的实验知识，这可能指导未来通过腔工程实现各种功能量子材料，可能包括超导体、铁磁体、铁电体和拓扑电子材料。首席研究员和研究小组计划在太赫兹频率下开发敏感的电光显微镜，以量化波动电场和原子位移。这些测量结果可以为实际有耗量子材料中腔量子电动力学的理论分析提供更坚实的基础。量子显微镜结合时间分辨光谱揭示了声子-极化激子激发的相干性和居群动力学，揭示了可能的腔诱导结构跃迁。最后，所获得的知识有助于设计和实现混合太赫兹腔-材料系统，以修改二维层状材料和异质结构中的自旋和电荷态，其中电子相互作用强且外部可调谐。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

Nontechnical descriptionVacuum is not truly empty but contains light waves that are constantly fluctuating according to quantum mechanics. Similarly, materials cooled to near-absolute zero temperature are not truly frozen but contain constantly fluctuating atomic motions. Although these fluctuations are usually negligible in daily life, theoretically they can grow large when a standing wave is compressed into a very small volume, called a cavity, with dimensions compared with the wave’s wavelength. Under such circumstances, according to some recent theoretical and experimental research, the fluctuating wave may be strong enough to change the properties of materials immersed in the wave, including the atomic structure, electrical conductivity, and magnetic properties. This research attempts to answer a few major open questions in this new paradigm of cavity modified materials based on a specific system: the mixing of electromagnetic waves and atomic vibrations that naturally happens in many ionic crystals. Near the vibrational resonance, which is in the terahertz frequencies, the wavelength of the mixed wave shrinks, so the quantum fluctuation is expected to enhance. The research team plans to directly measure the quantum fluctuation of the light and matter inside small cavities, and track the modified energy evolution and materials properties. The results promise insights into optimizing materials by harvesting quantum forces for free. The project also supports the dissemination of the basic concepts of quantum materials to audiences at different levels of education. The principal investigator plans to formulate a modular graduate course to bridge the knowledge gap between existing materials education and the societal need for building quantum infrastructure, to offer an undergraduate research opportunity targeting underrepresented groups in community colleges, and to work with high school teachers in the Houston Independent School District to design a lesson about materials in quantum technology. These efforts are part of NSF’s Big Idea of Quantum Leap and the National Quantum Initiative to build a quantum workforce for the future.Technical descriptionEmerging experimental evidence and theoretical models open a possibility for cavity-enhanced quantum fluctuations of bosonic modes, including photons and phonons, to significantly alter materials’ structural, transport, and magnetic behaviors in both the excited states and the ground state. The quantum fluctuations thus may serve as a control knob to transform quantum materials without the need for active input. However, experimental validation is scarce about a possibility of the amplified fluctuations in terahertz frequencies or the transfer of properties between cavity-coupled bright polaritons and much more numerous cavity-decoupled localized states. This research provides the much-needed experimental knowledge on the magnitude and consequence of the quantum fluctuations of ultra-strongly coupled phonon-polaritons in sub-wavelength cavities, which may guide future efforts to realize various functional quantum materials through cavity engineering, possibly including superconductors, ferromagnets, ferroelectrics, and topological electronic materials. The principal investigator and research team plans to develop sensitive electro-optic microscopy in the terahertz frequencies to quantify the fluctuating electric fields and atomic displacements. The measurements can provide a firmer ground to the theoretical analysis of cavity quantum electrodynamics in realistic, lossy quantum materials. The quantum microscopy combined with time-resolved spectroscopy reveal the coherence and population dynamics of phonon-polariton excitations, which shed light on possible cavity-induced structural transitions. Finally, the acquired knowledge facilitates the design and realization of hybrid terahertz cavity-materials systems to modify the spin and charge states in two-dimensional layered materials and heterostructures, where the electronic interactions are strong and externally tunable.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.