Physics Colloquium: Terahertz nonlinear photocurrents - Unraveling many-body dynamics in topological materials

"Nonlinear transport and optical phenomena, such as charge and spin photogalvanic effects and high-harmonic generations, are of significant interest for their practical application in photo-detection, opto-spintronics, and nanoscale imaging of quantum materials. In the low-frequency regime, such as in infrared and terahertz (THz) ranges, the period of the driving field's oscillation aligns closely with the electron relaxation lifetime, implying the importance of many-body scatterings in nonlinear transport. 

Understanding low-frequency nonlinear current provides a pathway to probe and leverage quantum geometry and many-body properties of electrons in crystalline materials. In this talk, I will discuss light-induced nonlinear transport within charge, spin, and thermal channels in topological quantum materials like graphene, WTe2, and Bi2Te3. 

After briefly reviewing the inter-band nonlinear optics in 2D materials, I will explore the contributions to THz third-harmonic generation of Dirac fermions in graphene, highlighting the crucial role of nonlinear-light matter coupling driven by inter-particle interaction. Additionally, I will examine the nonlinear Hall effects in WTe2, focusing on resonant enhancement and an exotic sub-gap light-induced spin Hall current. 

Lastly, I will introduce a nonlinear thermoelectric effect for probing the long-lived dynamics of odd-parity Fermi liquid modes at topological insulator surfaces. These photo-thermoelectric currents, characterized by unique heat capacitance terms tied to electron topology, show significant potential for ultrafast optical switching in thermoelectric transport, particularly in opto-thermal sensors."

Bio:

Rostami completed his PhD in Physics at IPM in Tehran, Iran, in 2015 and worked as a researcher in Italy (at Scuola Normale Superiore di Pisa and IIT in Genoa) and Sweden (at Nordita/Stockholm University) from 2015 to 2022. Later in 2022, I joined the Department of Physics at the University of Bath in the UK as an Assistant Professor. Currently, his research activities and interests cover various areas of theoretical condensed matter physics, including nonlinear optical spectroscopy, nonlinear quantum transport, strain physics/engineering, and nonlinear phononics in two-dimensional materials such as graphene and transition metal dichalcogenides (TMDs), along with their heterostructures. 

Welcome!