Ultrafast Microscopy of Spin-Momentum Locked Surface Plasmon Polaritons

Nano-femto control of SPP fields at plasmonic metal/vacuum interfaces is of interest for photocatalysis, molecular sensing, interconversion of optical and electronic signals etc. Through two-correlated field interferometry, it is possible to capture the plasmon field evolution in time and space. A very efficient and nonperturbative way of imaging SPP generation, propagation, and decay with unsurpassed temporal and spatial resolution is the interferometric time-resolved multiphoton photoemission electron microscopy (ITR-mPEEM). ITR-mPEEM records the spatiotemporal distribution of photoelectrons contributed by the interference cross-term between the incident field and SPPs it launches, through the nonlinear (usually) two-photon photoemission (2PP). Phase-correlated excitation pulses can be used to image SPP’s interferences at a specific instance in time, which is defined by scanning of the interpulse delay with attosecond accuracy. Previous ITR-mPEEM experiments were used to image mostly quasi-1D propagation and focusing of SPP wave packets at noble metal surfaces and localized electronic responses of nanoscale metal particles and 1D wire antennas, but the role of SAM (spin angular momenta) in the SPP coupling or propagation was not addressed.

In this paper, authors have explored the effect of the optical electric field polarization, and particularly SAM, on the SPP generation and propagation at an Ag-vacuum interface, which preserves time reversal symmetry. They have performed 3D (x, y, and t) nano-femto ITR-mPEEM imaging of SPP excitation and propagation with differently polarized light on a micrometer-scale truncated equilateral triangle-shaped single-crystalline Ag(111) island grown on a Si(111) substrate. They have studied both experimentally and computationally 2P-PEEM images formed by spin-momentum locked SPPs on single crystalline Ag (111) islands, with specific focus on the coupling efficiency and phase for excitations with various polarization states.

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