Tracking light-induced states in materials through ultrafast spectroscopy

The advent of time- and angle-resolved photoemission spectroscopy (trARPES) has revolutionized research into nonequilibrium processes and ultrafast material design. A key goal in this field is to engineer electronic properties on demand through intense coherent pumping, known as Floquet engineering. Although the first experimental observation of Floquet-Bloch states occurred over a decade ago, detecting clear evidence of light-matter hybrid states or other distinctive signatures of Floquet-Bloch states has proven challenging. However, in the past two years, a new generation of experiments has brought Floquet engineering closer to reality. This progress has been supported by precise theoretical models that account for experimental conditions. In this talk, we will discuss the theory of trARPES within the framework of nonequilibrium Green's functions (NEGF), combined with first-principles modeling of light-matter interactions. We demonstrate how the NEGF framework has provided additional evidence and complementary insights into the nature of Floquet-Bloch states in recent experiments. Specifically, we will explore the sub-cycle limit of Floquet states [1], light-matter hybrid states in black phosphorus [2], and the interferometric observation of Floquet-Bloch states in graphene [3] and transition metal dichalcogenides [4].