Generation of broadband longitudinal fields for applications to ultrafast tip-enhanced near-field microscopy

We report on the generation of broadband longitudinal fields within a tightly focused spot by using a segmented wave plate combined with a phase tailored broadband laser pulse. Their field distribution is characterized by observing the scattered light from a gold-coated glass fiber tip as it is scanned across the focused beam spot. It is observed that efficient coupling to the tip-enhanced field can be achieved over a broad bandwidth of more than 100nm, resulting in a positive contrast at the centre of the focus in the spectrally resolved Rayleigh scattering image. Temporal characteristics of the nonlinear excitation at the tip apex observed by using the fringe resolved autocorrelation technique indicate the possibilities of ultrafast spectroscopy by utilizing the tip-enhanced longitudinal fields.