Circular Phononics: Driving Matter by Phonon Angular Momentum

原子的结构排列及其对称性决定了物质的物理性质和平衡状态。如果这种空间结构可以被超快激光脉冲任意调制或改变，我们就可以按需实现具有材料特征的物质的新瞬态。非平衡态及其相互关联的转变从根本上是由能量、线性动量和角动量的交换和守恒决定的。尽管晶格激发（声子）和其他自由度之间的能量和线性动量交换是固态物理的基石，声子角动量通常被认为是角动量守恒的原因，其主动控制仍然难以捉摸。在这个项目中，我和我的团队将准备和相干控制非零角动量的声子态，研究和主动操纵耦合的电子自由度和自旋自由度。因此，我们将建立声子角动量作为一种新型的超快调谐旋钮，用于综合材料控制。我们将首先证明在原型材料中定义制备和检测圆偏振相干声子的原理。然后，我们将继续研究具有固有手性晶格模式的范德华层状半导体和Weyl半金属。在这些时间反转对称性破缺系统中，圆偏振平面内声子将为自旋电子学和谷电子学提供一种新的非侵入性处理方法，而圆平面间剪切模式将产生或调制莫尔<s:1>电势，从而为“动态涡旋电子学”铺平道路。最终，我们的方法可以扭转在超快退磁过程中长期存在的角动量损失问题：我们将通过主动将角动量从晶格转移到磁绝缘体的自旋有序系统来寻求逆超快爱因斯坦-德哈斯效应，从而加入声子和磁学领域。提议的项目需要桌面设计和实现一个相位稳定、强场、太赫兹和中红外（MIR）光源，该光源具有可切换的螺旋度。这项技术的发展还将为广泛的光谱学和场驱动现象提供工具，如太赫兹分子圆二色性、螺旋分辨振动光谱学和太赫兹到MIR体制中的霍尔效应。总的来说，通过这个研究项目，我们将把角动量引入到超快材料控制的结构动力学库中。

The structural arrangement of atoms and its symmetries dictate the physical properties and equilibrium states of matter. If this spatial structure can be modulated or changed at will by an ultrafast laser pulse, we can realize new transient states of matter with material features on demand. Non-equilibrium states and their interconnecting transitions are fundamentally determined by exchange and conservation of energy, linear momentum, and angular momentum. Even though exchange of energy and linear momentum between lattice excitations (phonons) and other degrees of freedom is a cornerstone of solid-state physics, phonon angular momentum is commonly just assumed to account for angular momentum conservation and its active control remains elusive.In this project, my team and I will prepare and coherently control phonon states with nonzero angular momentum to study and actively manipulate coupled electronic and spin degrees of freedoms. Thereby, we will establish phonon angular momentum as a novel ultrafast tuning knob for comprehensive material control. We will begin with the first proof of principle of defined preparation and detection of circular polarized coherent phonons in prototypical materials. Then we will proceed to van der Waals layered semiconductors and Weyl semimetals with inherently chiral lattice modes. In these time reversal symmetry-broken systems, circularly polarized in-plane phonons will provide a new non-invasive handle for spin- and valleytronics, whereas circular inter-plane shear modes will create or modulate Moiré potentials, thus paving the way for “dynamic twistronics”. Eventually, our approach allows inverting the long-standing problem of angular momentum loss during ultrafast demagnetization: We will seek an inverse ultrafast Einstein – de Haas effect by actively transferring angular momentum from the lattice to the spin-ordered system of a magnetic insulator, thereby joining the fields of phononics and magnonics.The proposed project requires the tabletop design and implementation of a phase-stable, strong-field, THz and mid-infrared (MIR) light source with switchable helicity on a single-shot basis. This technological development will additionally provide tools for a wide range of spectroscopies and field-driven phenomena, such as THz molecular circular dichroism, helicity-resolved vibrational spectroscopy, and Hall effects in the THz to MIR regime. Overall, with this research project, we will introduce angular momentum to the arsenal of structural dynamics for ultrafast material control.