Ultrafast dynamics and nonlinear behavior of surface -plasmon polaritons in optical microcavities
We have created microscopic Fabry-Perot optical resonator cavities with mode volumes on the order of ten cubic wavelengths between the flattened end of a tunneling microscope tip and a semi-transparent metal film. An attenuated total reflection geometry is employed to transfer power from a laser beam to the cavity via surface-plasmon polariton generation on the cavity mirror surfaces and subsequent radiative decay into optical cavity modes. We compare the dependence of the axial emission from these modes on cavity length and angle of incidence with the predictions of a stratified-medium model based on Maxwell's Equations. The mode structure has a finesse which approaches the theoretical limit.
Under intense illumination by a pulsed laser, second-harmonic radiation produced by weakly localized surface-plasmon polaritons is also coupled into the cavity and detected in axial emission. The mode structure is similar to that of the fundamental as regards finesse and dependence on cavity length, but with the expected halving of the spatial period. Utilizing the second-harmonic emission we performed time-resolved second-order autocorrelation measurements to investigate the temporal relaxation of surface-plasmon polaritons that are confined to the cavity structure.
We also demonstrate interferometric, first-order autocorrelation measurements under femtosecond laser pulse illumination. Interference is detected in the speckle of the emission cone resulting from the radiative decay of directionally scattered surface plasmon polaritons. This method is used to study the lifetime of freely propagating surface-plasmon polaritons along a silver/air interface.