UP1/1 Near-Field focusing of surface plasmons

C. Ropers

Nano Lett. 7 (2007) 2784-2788 Grating-coupling of surface plasmons onto metallic tips: A nanoconfined light source

C. Ropers, C. C. Neacsu, T. Elsaesser, M. Albrecht, M. B. Raschke and C. Lienau

Measurement and manipulation of light confinement on the nanoscale are key challenges in nano-optics. The exceptional optical properties of metals can be utilized to create nanostructures with strongly enhanced local fields. A prominent example is the field enhancement at the apex of sharp metallic needles, which has led to the development of apertureless near-field optical microscopy with a spatial resolution of the order of ten nanometers. Unfortunately, this method is often complicated by a large background and interferences between the wanted signal originating from the tip apex and unwanted light scattered off the tip shaft.
We have recently developed a novel strategy towards optically exciting the apex of such nanometric metal tips [RNE07]. The new approach is based on the resonant excitation of evanescent surface waves, so-called surface plasmon polaritons (SPPs), in a grating on the tip shaft (see Fig.1). From the groove pattern, SPPs couple onto the smooth part of the shaft and propagate towards the tip apex. The narrowing of the conical taper leads to an ever-increasing spatial concentration of the surface plasmon wave.

Fig.1. a): Scanning electron micrograph of a tip prepared with a grating on its shaft. The apex excitation scheme is superimposed. b): Scattered light image from the tip illuminated at the grating, demonstrating a strong nonlocal excitation of the apex mediated by SPPs.

As a result of this efficient spatial excitation transfer, the size of the excitation spot is reduced from a few microns in and near the grating to only few tens of nanometers. The large spatial separation of the far-field excitation from the apex and the resulting suppression of background signals make this new local light source a very promising candidate for applications in near-field optical microscopy and spectroscopy.

UP1/2 Spectroscopy of molecular nanostructures

M. Breusing

Optical studies of molecular switches in solution have been performed successfully for several years. Our aim is to investigate switching molecules attached to a surface. To achieve a well ordered sample, self assembled monolayers (SAM) of alkyl-chains on Si(111) were prepared in cooperation with the chemistry department of the TU Berlin. Due to our characterization of these SAMs, by means of AFM and FTIR, the preparation method was improved in order to obtain the optimum degree of coverage. In the next step these chains will act as anchor for the molecular switches.

Fig.2. Luminescence rise of CdZnSe core-shell quantum dots dissolved in toluene (black); the fit of this curve is plotted in green taking into account the system response (red) of excitation and the gating pulse.

Exciton dynamics in quantum dots have been studied extensively by time-resolved luminescence spectroscopy. Using a newly implemented setup for femtosecond luminescence up-conversion, we measured the emission rise-time of CdZnSe core-shell quantum dots (QD) with a time resolution in the range of 100 fs. We observe a non-instantaneous rise with time constants of ~500 fs. Measurements on modified QDs provide insight into the dynamics of charge separation in the core-shell structure.

UP1/3 : Localized multiphoton emission of femtosecond electron pulses from metal nanotips

C. Ropers

Phys. Rev. Lett. 98 (2007) 043907/1-4

C. Ropers, D. R. Solli, C. P. Schulz, C. Lienau and T. Elsaesser

Femtosecond electron and X-ray diffraction are among the most important topics in ultrafast science, allowing for probing structural dynamics of molecular and solid state systems in real-time. These techniques are still in an early stage, and large efforts are currently put into the development of sophisticated femtosecond electron or X-ray sources suitable for experiments with high temporal resolution. In electron diffraction, overcoming temporal smearing due to spatial propagation effects and to Coulomb repulsion of electron bunches produced at kHz repetition rates is particularly challenging. Ultimately, therefore, a point-like source of single electrons with temporal resolution in the regime of few femtoseconds or even below would be highly desirable.
We have developed and demonstrate a novel approach towards realizing such a point-like ultrafast electron source [RSS07]. By illuminating ultrasharp gold tips with 7-fs pulses from an 80 MHz Ti:sapphire oscillator, we induce emission of an intense flux of up to 10⁷ electrons per second. Due to the local field enhancement this emission is strongly localized at the apex of the metallic tip with a radius of curvature of only few tens of nanometers. Our experiments show that electrons are generated from a short-lived nonequilibrium carrier distribution and that, depending on the bias voltage, different parts of this distribution function are emitted. The generation mechanism changes from one-photon-assisted tunneling at high bias voltages to four-photon-induced emission at zero bias. Since, under our excitation conditions, we achieve a strong electron emission even in the absence of a bias, we can directly use this electron source for spatial imaging of nanostructures. A first image of a nanometer sized slit in a metal film recorded with this new tip-enhanced electron microscope is shown in Fig. 1. Due to the strong optical nonlinearity of the electron emission, the image mainly probes the local optical field near the metallic nanostructure with a spatial resolution of a few tens of nanometers. Effortsto use this novel electron source for applications in ultrafast electron diffraction are currently underway.

Fig. 1. (top) Schematic of the experiment with metal nanotip (gray) and surface groove (yellow structure). In the experiments, the metal tip is illuminated with a 7 fs light pulse and raster scanned across the nanostructure. The locally varying electron yield is monitored. (bottom) Electron image of the nano groove. The colored part of the image displays the local electron generation rate (bright: high yield), which allows for a determination of the profile and the local electromagnetic field strength of the nano groove.