Non-Linear and Non-Local Light-Matter Interaction in Quantum Materials

The optical response of quantum materials under illumination is interesting. The fine balance between many microscopic processes in quantum materials can be tipped by low-intensity light leading to a nonlinear response. Further, tipping such equilibrium also leads to an incommensuracy of the quantum order resulting in domains with different electronic properties. This coexistence of multiple quantum many-body phases or electronic inhomogeneity is fundamental to quantum materials and gives rise to a nonlocal optical response. This optical nonlocality in quantum materials captures the competition between different electronic phases and serves as a useful material parameter. Here, we present a model to describe the optical non-linearity and nonlocality in quantum materials and validate it by optical experiments carried out on a charge density wave (CDW) material, 1T-TaS2. 1T-TaS2, a quasi-2D material, supports nearly commensurate CDWs at room temperature. CDW domain walls exist in every layer of 1T-TaS2. Each CDW domain can now stack on top of the others in neighboring layers differently giving rise to a stacking order. This stacking order is sensitive to low-intensity incoherent light. Light tips the competition between center-to-corner and center-to-corner stacking types. As a result, the reflectance of TM polarized light changes with illumination intensity manifesting in an optical nonlinearity.<br/><br/>Apart from nonlinearity arising from the underlying competition between different orders, the optical response of quantum materials is also nonlocal. The nonlocality arises from spatial inhomogeneity. The existence of domains of different orders results in optical nonlocality. In 1T-TaS2, the CDW domains in each layer corresponding to a particular stacking type are about 50 nm wide in any layer<br/>at room temperature. This level of spatial inhomogeneity at visible frequencies would result in appreciable nonlocality. Employing a single-shot energy-momentum microscopy, we capture the non-linear and non-local optical response of 1T-TaS2 and infer the quantum order in this material. This work not only sheds light on light-matter interaction at low illumination intensities in quantum materials, but also demonstrates a quick and non-invasive technique to design, discover, and optimize quantum materials.