Introduction

As outlined in the previous chapters, optical antennas concentrate incident light within a small spatial volume. As shown throughout this book, these nanostructures may lead to strong local field enhancements depending on their size and shape. Because of that connection, one often encounters figures plotting the near-field as if it were a purely intrinsic property of an optical antenna. However, this viewpoint does not provide a complete description, because the incident radiation must also have an influence. In this chapter, we deal with the question of how one can make use of the degrees of freedom present in the external field in order to manipulate the spatial and temporal properties of the excited near-field. Specifically, we will discuss the usage of shaped femtosecond laser pulses as they contain a broad bandwidth of different frequencies that can be modulated. It will turn out that amplitude, phase and polarization properties are relevant for controlling nano-optical excitations coherently.

It is intuitively clear that the external field must be relevant for the properties of antenna fields. For example, using monochromatic incident light, the local oscillation frequency is the same as that of the external field in the limit of linear response. Upon changing the frequency, however, the amplitude of the local field changes even when the external spectral field amplitude is kept constant, because the field enhancement factor in general varies while moving into or out of material resonances. Furthermore, a phase difference can exist between the external and local field, i.e. their oscillation maxima need not occur at the same time.