Kai Kratzer

Nanoantenna-enhanced ultrafast nonlinear spectroscopy of a single gold nanoparticle

Optical nanoantennas, just like their radio-frequency equivalents, enhance the light-matter interaction in their feed gap. Antenna enhancement of small signals promises to open a new regime in linear and nonlinear spectroscopy on the nanoscale. Without antennas especially the nonlinear spectroscopy of single nanoobjects is very demanding. Here we present the first antenna-enhanced ultrafast nonlinear optical spectroscopy. In particular, we use the antenna to determine the nonlinear transient absorption signal of a single gold nanoparticle caused by mechanical breathing oscillations. We increase the signal amplitu-de by an order of magnitude, which is in good agreement with our analytical and numerical models. Our method will find applications in linear and nonlinear spectroscopy of single nanoobjects, especially in simplifying such challenging experiments as transient absorption or multiphoton excitation.

Automatic, optimized interface placement in forward flux sampling simulations

Forward flux sampling (FFS) provides a convenient and efficient way to simulate rare events in equilibrium or non-equilibrium systems. FFS ratchets the system from an initial state to a final state via a series of interfaces in phase space. The efficiency of FFS depends sensitively on the positions of the interfaces. We present two alternative methods for placing interfaces automatically and adaptively in their optimal locations, on-the-fly as an FFS simulation progresses, without prior knowledge or user intervention. These methods allow the FFS simulation to advance efficiently through bottlenecks in phase space by placing more interfaces where the probability of advancement is lower. The methods are demonstrated both for a single-particle test problem and for the crystallization of Yukawa particles. By removing the need for manual interface placement, our methods both facilitate the setting up of FFS simulations and improve their performance, especially for rare events which involve complex trajectories through phase space, with many bottlenecks.

Homogeneous and Heterogeneous Crystallization of Charged Colloidal Particles

Crystallization of macroions happens in many applications like protein purification, photonic crystals or structure determination. However, the mechanism of crystallization in these systems is only poorly understood. We thus study homogeneous nucleation and crystallization of charged spherical particles using molecular dynamics (MD) computer simulations with the software package ESPResSo.

Heterogeneous and Homogeneous Crystallization of Soft Spheres in Suspension

Nucleation, i.e., the onset of a phase transition like crystal growth, is a rare event with waiting times in the order of days. Yet, it is an event on the molecular scale, and therefore difficult to study, both experimentally and by computer simulations. Our interest is in the role of long range interactions in nucleation, in particular electrostatic and hydrodynamic interactions mediated by solvent molecules. In order to model the solvent, we use a lattice fluid that is propagated by the fluctuating Lattice Boltzmann (LB) method. Our implementation uses a graphics card (GPU) to propagate the solvent and is coupled to the Molecular Dynamics (MD) simulation package ESPResSo. Using this code, we study the heterogeneous crystallization in Yukawa-like colloidal systems. Our simulations allow to observe the growth of a crystal in a channel with and without hydrodynamic interactions, and indicate that hydrodynamic interactions slow down the crystallization. Additionally, we present results on the homogeneous crystallization of Yukawa particles. While heterogeneous nucleation can be observed directly in simulations, homogeneous nucleation requires special sampling techniques. We use our own Forward Flux Sampling implementation, the Flexible Rare Event Sampling Harness Systems (FRESHS). FRESHS can control popular MD simulation packages as back-end, making it a versatile tool to study rare events. Our simulations confirm previous results at higher supersaturations, which show that the nucleation mechanism involves two steps, namely the formation of a metastable bcc phase and the transformation to a stable fcc phase.

Nanoantenna-enhanced ultrafast nonlinear spectroscopy of a single plasmonic nanodisc

An optical nanoantenna increases the nonlinear transient absorption signal of a single 40 nm gold nanodisc by an order of magnitude. In agreement with our calculations the signal is spectrally shifted to the antenna resonance.

Plasmon hybridization enhances the transient absorption signal of a single nanoparticle

A tiny variation of a single metal nanoparticles dielectric properties has only a weak influence on the light field. We demonstrate, using antenna concepts, how plasmon hybridization helps to increase the particles influence.