Coherent control and angular momentum transfer in semiconductor and plasmonic nanostructures

Semiconductor and plasmonic nanostructures have attracted considerable attention during the last decade. Semiconductors are the basis of today´s information technology because of the possibility of tailoring electrical and optical properties on a detailed level. An elementary optical excitation in semiconductors is the electron-hole pair (exciton) with large oscillator strength which potentially enables ultrafast information processing. However, for quantum information applications excitons have been scarcely considered because of their limited optical coherence time due to complex many body interactions and their short radiative lifetime (below 1 ns) being the downside of the large oscillator strength. In nanostructures such as quantum dots (QDs) the optical decoherence is weak but still limited by radiative decay. Therefore approaches to involve the long-lasting coherence of resident electron spins have been pursued recently. The key excitation in plasmonics is the surface plasmon polariton (SPP), which is a coupled oscillation of the electromagnetic field and electron plasma in a metal. SPPs allow electromagnetic energy to be concentrated in nanoscale volumes at the interface between a metal and a dielectric, leading to an enhancement of linear and nonlinear optical effects. Current state of the art in nanotechnology allows fabrication of sophisticated metallic structures thus making possible light engineering at the nanoscale. Plasmonic structures may be shaped to manipulate the polarization of the light into novel “orbital” angular momentum states. Therefore, placing semiconductor nanostructures in the vicinity of plasmonic structures may establish transfer of angular momentum to a QD spin as well as efficient coherent control of electronic states. In order to achieve these goals detailed knowledge of near-field distribution of the electromagnetic fields and their propagation, optical response of the system under excitation with optical pulses- which is governed by dynamics of electronic states in each of the constituents (semiconductor and plasmonic nanostructures) and combined hybrid structures are required. First, we present the results on ultrafast optical response and polarization resolved near-field studies from pure plasmonic structures. We demonstrate that SPPs in one-dimensional plasmonic structures can be efficiently used by probing with light to access a series of dynamics of the conduction electrons in the metal from a femtosecond to a nanosecond timescale [1]. Furthermore excitation of SPP leads to non-uniform temperature distribution in plasmonic structures which is manifested in non-trivial signal transients. Near-field scanning optical microscopy has been used in order to explore magnetic field component of light in plasmonics by measuring how a sub-wavelength hole in metal generates magnetic fields which interfere with the incoming field [2]. Controlling direction was also an important feature of this work: we elaborated a design for a double-period grating that allows control of the direction of light by controlling phase between the two periods [3]. Apart from nano-holes, plasmonic gratings and antennas we considered nanostructures based on Tamm plasmon states which are generated at the interface between a metal and a distributed Bragg reflector. By introducing an additional grating we present an evidence of optical Tamm states to SPPs coupling, which can be used for beaming and polarization control applications [4]. Second, we report on optical studies of active structures where combination of both semiconductor and plasmonic constituents leads to enhancement of light-matter interaction which is manifested in amplified emission, polarization control and coherent optical response. In particular, it is demonstrated that variations of the optical antenna length give rise to periodic amplification of the integral emission intensity from QDs heterostructures surrounded by metallic antennas.