van der Waals (vdW) interfaces provide exciting new paradigms for creating and manipulating spins in solid state systems. Many outstanding questions and opportunities lie in understanding the microscopic spin and charge generation and transfer mechanisms across vdW interfaces. Ultrafast optics possesses unique advantages over conventional magnetotransport measurements. For example, time-resolved Kerr rotation microscopy uses sub-micron laser beam at terahertz speed (femtosecond pulse duration) to measure spin, valley, and magnon dynamics at unparallel spatial and temporal resolutions.
Previous work highlights:
Ultrafast imaging of spin-valley dynamics in monolayer transition metal dichalcogenides (TMDs) and their heterostructures
We develop and employ ultrafast time-resolved Kerr rotation microscopy with ~150 fs temporal and sub-micron spatial resolutions to image spin-valley polarizations in monolayer TMDs and their heterostructures. In monolayer CVD-grown WS2, we reveal a highly-complex spatial dependence that we attribute to competing roles of dark trions, strains, and localized states. We also show that long-lived (> 5 ns) spin-valley polarization is robust to an in-plane magnetic field up to 700 mT, indicative of spins and valleys that are stabilized with a strong intrinsic spin-orbit field. In monolayer p-type WSe2/graphene heterostructures, we discover a strong reduction of spin-valley lifetime (~ 5 ps) due to spin transfer into graphene. A key piece of evidence is a non-zero photocurrent detected in the heterostructure, which suggests that 5 ps is the rate at which spin-polarized holes leaving WSe2, as oppose to dephasing in WSe2.
– E. J. McCormick, M. J. Newburger, Y. K. Luo, K. M. McCreary, S. Singh, I. B. Martin, E. J. Cichewicz, B. T. Jonker and R. K. Kawakami, “Imaging spin dynamics in monolayer WS2 by time-resolved Kerr rotation“, 2D Materials 5, 011010 (2018).
– M. Newburger, Y. K. Luo, K. M. McCreary, I. B. Martin, E. J. McCormick, B. T. Jonker, R. K. Kawakami, Ultrafast spin and charge dynamics in monolayer WSe2-graphene heterostructure devices, in preparation.
Future directions:
High-resolution, accurate space-time spin-orbit torque readout using magneto-optical techniques
Optical readout using the magneto-optical Kerr (MOKE) effect provides a unique advantage as it can avoid artifacts from transport techniques such as thermoelectric voltages, nonlinear-in-current Hall resistances, and modification of transport coefficients by magnons or heating. I will continue to advance the resolutions of magneto-optical microscopy in the Kerr sensitivity, temporal, and spatial domains by developing near-field MOKE microscopy and ultrafast Sagnac interferometry. These pioneering techniques will be uniquely suitable to image the atomic-scale domains and disorders in low dimensional magnetic systems. In addition, ultrafast optics such as TRKR with THz resolution is a natural and powerful platform to investigate antiferromagnetic dynamics, where much faster magnetic resonance and switching rates are expected that are beyond the capacity of any transport measurement.