Tracking the temporal and spectral evolution of femtosecond pulses on plasmonic nanowires

Summary form only given. Plasmonic structures offer a promising platform for all-optical signal processing and ultrafast switching devices due to the achievable field enhancement [1]. In this regard, plasmonic nanowires (NWs), which are thin and narrow gold stripes of varying width, are especially interesting since they can guide surface plasmon polaritons (SPPs) and confine the electro-magnetic field in two-dimensions [2]. To date pulse propagation on these structures has been only characterized by input-output measurements [3]. Here, we present a local investigation of ultrashort pulse propagation on these plasmonic NWs using a near-field scanning optical microscope that allows us to measure the temporal as well as the spectral evolution of the pulses. We determine the group velocity of the guided mode and find strong indications of spectral reshaping due to nonlinear processes. We measure the temporal dynamics of SPP pulses on plasmonic NWs with widths ranging from 40 to 200 nm to investigate the dependency of the group index ng and the propagation length of the guided mode on the geometrical cross-section. As Fig. 1a shows the SPP is slowed down for decreasing NW widths. The experimentally determined group index ng drops from a value of 2.2 at a width of 40 nm to 1.8 at a width of 180 nm which is in good agreement with numerical calculations. At the same time the measured propagation length decreases from 25 to 3 μm also following the trend of the numerical calculations. By spectrally analyzing the field picked up by the aperture near-field probe at different positions along the plasmonic NW, we visualize the evolution of the local power spectral density (PSD) of the SPP pulse as it propagates. One exemplary measurement is shown in Fig. 1b, presenting 4 spectra taken at different locations along a 40 nm wide plasmonic NW for an input power of 5 mW. We observe that the PSD broadens as the SPP pulse propagates further along the nanowire. We will also pres- nt the power dependence of the observed broadening to elucidate the nonlinear processes occurring en route. In conclusion, we locally investigated the dynamics of ultrashort pulse propagation along plasmonic NWs. Local measurements in the time domain allowed us to unambiguously extract the group index and propagation length for different nanowire widths. In addition, we are able to locally determine the PSD, which indicates nonlinear processes in pulse propagation on plasmonic nanowires.