Visualizing surface plasmon polariton wavepackets in space and time

Abstract

A novel near-field optical microscope is developed to visualize surface plasmon polaritons (SPP) in space and time. We present the first phasesensitive and time-resolved SPP measurements and determination of the SPP group velocity.The use of surface plasmon polaritons (SPPs) in combination with (nano) structured metal films, allows for the manipulation of optical fields on a scale much smaller than the wavelength of light. For this reason SPP guides have been studied extensively, both experimentally and theoretically, see ref.[1] and references therein. Especially near-field studies [2,3] give a detailed insight on SPP behavior inside such guides.Time-and phase-resolved Photon Scanning Tunneling Microscopy (PSTM) has proven to be a powerful tool in analyzing optical phenomena inside nanostructures [4]. We applied this technique for the first time on SPPs inside a metal SPP guide. Combining classical near-field microscopy with an interferometer, allows us to measure the phase of a propagating SPP. The propagation of ultrafast SPP wavepackets is followed by tuning an optical delay line. Fig 1a shows the topography of a typical guide under investigation, obtained with shear-force feedback [5]. The dimensions of the guide are 6 μm wide and 80 μm long. The metal is gold and has a thickness of 55 nm, deposited on BK7 glass. The SPPs are excited at the entrance of the guide, using a Kretschmann-Raether configuration [6]. A femtosecond Optical Parametric Oscillator, at a wavelength (in air) of 1500nm, is used to excite femtosecond SPP pulses. The SPPs travel along the guide and are scattered out at the end. Part of the SPP field is reflected, resulting in a standing wavepattern at the end of the waveguide.Our measurement technique simultaneously measures the time-resolved SPP pulse amplitude (Fig 1b), and its phase (Fig 1c). Taking more time frames will result in the visualization of traveling SPP pulses. Group-and phase velocity follow directly from these measurements. The new microscope will enable new insight into SPP propagation in metallic nanostructures. For suitably chosen nanostructures, slow SPPs are anticipated; we will present our latest results. a1795_1.pdf QMC5.pdf 1-55752-813-6/06/$25.00 ©2006 IEEE