Tunable surface plasmon polaritons and ultrafast dynamics in 2D nanohole arrays.

High-quality and unique surface plasmon resonance (SPR) with a narrow linewidth and controllable resonance energy plays a key role in wide applications including ultrahigh-resolution spectroscopy, on-chip sensing, optical modulation, and solar cell technology. In this work, the response of surface plasmon polariton (SPP) modes in Au nanohole arrays has been effectively tuned by properly adjusting the sample orientation without changing the geometrical parameters, and a very narrow linewidth down to 8 nm is achieved via the strong interference of two (0, -1) and (-1, 0) SPP modes in the Γ-M direction under transverse magnetic polarization. These results have been validated excellently by finite-element-method numerical simulations. More importantly, we have quantitatively investigated the contribution of conduction-band electron distribution to the SPP intensity of the array within a 20 ps timescale with ultrahigh sensitivity by utilizing home-built femtosecond transient absorption spectroscopy, and observed the minimum SPP intensity at ∼700 fs following excitation with a 0.2 μJ pulse. This study may help enhance the understanding toward the intrinsic micromechanism of SPR, thus offering opportunities for potential applications in strong coupling and new-style optical wave manipulations.