Quantum Matter/Materials Science Research Lecture

***Refreshments at 3:45pm in 112 East Bridge

Abstract:

Symmetry broken materials trigger new responses that can elucidate their novel properties that can also be useful for many applications. We will discuss new types of optical wavevector engineered nonlocal photogalvanic techniques that we are developing by shaping optical beams with control over their intensity and phase profiles to study symmetry broken quantum topological materials. These techniques utilize nonlocal light-matter interactions that go beyond the electric-dipole approximation and are sensitive probes of materials' symmetry, quantum geometry and topology, especially for systems with large electronic coherence lengths.

We will first discuss our early efforts to explore the properties of MoxW1-xTe2, which are type-II Weyl semimetals, by using optical beams with spatially inhomogeneous intensity profiles. We will describe how spatially inhomogeneous optical excitation coupled with unique symmetry, band structure and inversion, large Berry curvature and topology of Weyl semimetals produces a strong photogalvanic response from which microscopic insights can be obtained. We will then discuss the orbital photogalvanic effect (OPGE) driven by the helical phase gradient of optical beam, that is characterized by a current winding around the optical beam axis with a magnitude proportional to its quantized OAM mode number. OPGE accesses different properties of the material via a more complex carrier excitation mechanism and symmetry characteristics. We will finally discuss a new type of a nonlinear opto-twistronic Hall effect in supertwisted WS2 moire system formed by a screw-dislocation-driven mechanism. The optical Hall current reflects the structural handedness of the supertwisted system originating from the system's noncommutative geometry, along with and an unusual photon-momentum dependence of the optical response from the moire potential. Our response function theory can explain the origin of the photon momentum dependent nonlinear photocurrents, revealing new observables of the system going beyond Berry curvature and other conventional band geometrical quantities. These studies seamlessly connect 2D and 3D twistronics and provides a bridge connecting the electrons and photons by overcoming their significant length scale differences in conventional systems, which can be important in realizing extreme large optical nonlinearities in materials for many applications. The talk will conclude by demonstrating our recent efforts to create an integrated on-chip photonics platform that utilize the complex vectorial states of light enabled by symmetry broken materials.

More about the Speaker:

Ritesh Agarwal is a Srinivasa Ramanujan Distinguished Scholar & Professor in the Department of Materials Science and Engineering at the University of Pennsylvania. He obtained his BS/MS from IIT Kanpur, MS from Chicago, PhD from UC Berkeley followed by a postdoctoral fellowship at Harvard. His research interests include investigating structural, optical, and electronic properties of low-dimensional systems and the development of new probes to study complex phases of matter. Recently, his group has focused on studying the role of quantum geometry and topology in electronic and optical systems and to engineer light-matter interactions to fabricate on-chip chiral photonic devices. Ritesh is the recipient of the NSF CAREER award, NIH Director's New Innovator Award and the SPIE Nanoengineering Pioneer Award. In 2022 he received the George H. Heilmeier Award for Excellence in Faculty Research at Penn Engineering. He is an elected fellow of the Optical Society of America (now Optica) and the American Physical Society.