Apertureless Scanning Nearfield Optical Microscopy with Ultra-high Temporal Resolution

By combining an apertureless scanning nearfield optical microscope (aSNOM) with a pump probe scheme, we create a novel experimental tool called pump probe apertureless scanning near field optical microscope (ppaSNOM), that combines a temporal resolution of 1ps with a spatial resolution of 20nm. This alloptical technique far below the diffraction limit of light allows to study ultrafast processes on the nano scale. As a proof of principle system we choose the mechanical oscillations exhibited by gold nano discs that are impulsively heated through a short pump pulse. First we provide the theoretical foundation needed to understand the optical and mechanical properties of gold nano particles. In particular we use a FEM solver to predict the mechanical properties as well as the field distributions of gold nano discs promising high signal contrast for the experiment operating at 800nm. Furthermore the absorption and scattering cross section calculated with the T-Matrix are used to derive the ideal sample structure. Before aSNOM and farfield pump probe scheme are combined, we characterize them separately. The aSNOM is an interferometric technique that collects light scattered of a dielectric AFM tip. It allows for the simultaneous acquistion of the sample topography, magnitude and phase of the z-component of the electrical nearfield with a spatial resolution of 20nm. The aSNOM measurements of a gold disc with 100nm radius and 50nm height reveal a dipolar plasmon resonance which agrees very well with FEM simulations. In a next step we apply the pump probe measurement scheme to gold nano discs. The impulsive heating of a gold nano disc through a short pump pulse starts mechanical oscillations in the disc. As the volume changes periodically, the optical properties are modulated by the mechanical mode. In this experiment we measure the transient transmission signal of an individual gold nano disc. Changing the delay between pump and probe reveals an oscillatory delay trace as expected. The data analyzation reveals a mechanical oscillation frequency of 10GHz which we can attribute to the first order breathing mode in agreement with FEM simulations. In a last step we combine the aSNOM with the pump probe scheme in order to create a tool with 1ps temporal and 20nm spatial resolution. As a proof of principle measurement we are looking for a pump induced perturbation of the nearfield signal. We use FEM calculations to simulate the 2D distribution of the nearfield perturbation which reveals a dipolar shape. The measurements of several gold discs show no differential nearfield response. Instead we see a ring like structure in the differential nearfield signal that overlaps with the AFM topography. We conclude that the ring structure is an AFM artefact and that our signal is buried in the noise floor. We use the measurement data to estimate an upper limit for the relative pump induced perturbation. The results agree with T-Matrix simulations which suggest that an increase in relative sensitivity by a factor of 10¯² is needed. By reaching the shot noise limit with our ppaSNOM and modifying certain aspects, the detection of timeresolved nearfield signals seems feasible. Some ideas for possible modifications to the ppaSNOM, such as an increase in collection efficiency of the objective or the replacement of the dielectric AFM tip by a metallic tip, are given in the end. Due to time constraints an implementation of the modifications was not possible.

Level thesis:degree_level

thesis.doctoral

Grantor dc:publisher

Universität Bayreuth

Year

2015

Author dc:creator

• Brandstetter, Matthias

Contributors dc:contributor

• Lippitz, Markus

Repository record source_url

https://epub.uni-bayreuth.de/id/eprint/2721/

OAI identifier oai:identifier

oai:epub.uni-bayreuth.de:2721