Measurement and control of the 3-dimensional dipole orientation of semiconductor nanocrystals

Abstract

When light is confined to a length-scale smaller than the diffraction limit by coupling to surface plasmons (SPs) in metal nanostructures, many new physical phenomena occur that can lead to enhanced light-matter interactions. Often, these SPs are generated via coupling of semiconductor nanocrystals (SNCs) to the plasmon modes in the structure. Due to the anisotropic nature of the emission dipole in many of these SNCs, the efficiency of the coupling is generally said to depend on the orientation of the emission dipole. Because experimental evidence for an orientation-dependent coupling efficiency is lacking behind theory, methodologies were developed to enable experimental verification of such an effect. Firstly, polarization-resolved fuorescence microscopy was used to determine the emission dipole properties and orientations of various types of SNCs. It was found that the particles either have emission resembling that of a 2D degenerate dipole or a mixed 1D-2D state with predominantly 1D dipole character, but in all cases the emission properties exhibited some degree of polydispersity. As a result the in-plane orientations of the SNCs could be accurately determined, but accurate measurement of the out-of-plane angles was not achieved. Secondly, a novel method was developed for simultaneously controlling the positions and orientations of rod-like SNCs on a variety of substrates using electron beam lithography and a lift-off procedure. With scanning electron microscopy (SEM) and polarization resolved fluorescence microscopy it was determined that ordered ensembles of SNCs had a high degree of alignment (S = 0.75 − 0.85) with optical properties approaching those of the individual particles. Orientational control was possible over centimeter distances and down to the single particle level without influence on the degree of alignment. Lastly, a proof-of-principle was given that the orientation of rod-like SNCs can be controlled in 3 dimensions using 3D-structured substrates and thermal scanning probe lithography.