Coupling Mediated Coherent Control of Localized Surface Plasmon Polaritons.

We investigate the phase-dependent excitation of localized surface plasmon polaritons in coupled nanorods by using nonlinear spectroscopy. Our design of a coupled three-nanorod structure allows independent excitation with cross-polarized light. Here, we show that the excitation of a particular plasmon mode can be coherently controlled by changing the relative phase of two orthogonally polarized light fields. Furthermore, we observe a phase relation for the excitation that is dominantly caused by damping effects.

Electro-optic tuning of split ring resonators embedded in a liquid crystal.

Two-dimensional arrays of split ring resonators for near-infrared frequencies are embedded in a liquid crystal (LC) and the influences of LC alignment, temperature, and electric fields on the resonance frequencies are studied. The results show that tunability can not only be achieved by influencing the state of polarization of the incident radiation, but also by direct interaction of the evanescent field of the resonant modes with the LC. Depending on the LC alignment, the field-induced shift of the resonance frequency is found to vary for different excited modes. Some guidelines for the design of tunable frequency selective metasurfaces can be deduced from these experimental results.

Effect of alignment on a liquid crystal/split-ring resonator metasurface.

A metasurface comprising a two-dimensional array of split-ring resonators with resonance frequencies in the near-infrared region is fabricated and embedded in a uniformly aligned liquid crystal. The change of the dielectric permittivity in proximity to the plasmonic structure by the replacement of air with the liquid crystal results in a decrease in resonance frequencies. The resonance shift can be attributed to the interaction of the evanescent field of the excited resonant plasmon modes with the liquid crystal. This shift in resonance frequency is found to depend on the liquid-crystal alignment and to vary for different modes. Also, the resulting effects of changes in temperature or applied external electric field on the metasurface depend on the liquid-crystal alignment and may differ from mode to mode. These observations indicate that the characteristic frequencies of the resonant split-ring resonator modes may depend on different evanescent field components interacting with the liquid crystal. Consequently, certain design rules should be taken into account for the development of tunable metasurfaces based on liquid crystals.

Observation of nano-dewetting in colloidal crystal drying.

The drying of colloidal crystals is connected with a continuous shrinkage process. However, several minutes after starting the drying, the system seems to take a breath before it shrinks monotonously until its final state after about one day. This short period we call "v"-event because of the shape of the curve characterizing the lattice constant: a decrease followed by a counter-intuitive increase which ends after one hour. This event is found in time-dependent optical spectra. It is assigned to the start of a nano-dewetting process occurring at the colloidal particles.

Structure and Optical Properties of Opal Films Made by an Out-of-Plane Electric Field-Assisted Capillary Deposition Method.

Self-assembled opals that are considered as a promising candidate for three-dimensional photonic crystals often suffer from the existence of internal defects. Defects influence optical properties and limit the applicability of opal films. Directed assembly using external fields may offer a certain degree of tunability in the opal formation process. We investigate the effect of an out-of-plane electric field on the formation and optical properties of opal films deposited using the capillary deposition method. The application of an electric field of intermediate strength (20-30 V/cm) can improve opal quality. The quality of opal films was found to depend on the polarity of the bottom substrate resulting from the beneficial influence of an asymmetry between the growths and the interplay with gravity. The negatively charged bottom substrate results in slightly better opal quality. This finding shows the potential of electric fields to tune opal formation in order to reduce the defect content.