NANO-OPTICS Research Topics
Surface Plasmon Photonics
Surface plasmons are hybrid modes of a light field and a coherent electron oscillation at the interface of a metal and a dielectric material. The electromagnetic fields of these surface modes are maximum in the interface and decay exponentially along the perpendicular directions. This property enables the realization of 'flat optics', which makes surface plasmons promising candidates for a future photonics technology. Furthermore, surface plasmon waveguides can beat the diffraction limit that constricts the miniaturization of conventional devices, rendering a true 'nano-optics' possible. Finally, the optical fields of surface plasmons can be strongly enhanced with respect to the exciting light, which is an appealing property for a variety of sensor applications.
We work on the development of functional plasmonic elements as nanostructured lenses, mirrors or beamsplitters and combinations thereof, as surface plasmon interferometers. Furthermore we investigate dynamic and active systems as plasmon sources or detectors based on organic optoelectronics. Besides electron beam lithography for sample fabrication we mainly apply near field optical and leakage radiation microscopy.
Near Field Optics of Metal Nanostructures
Metal nanoparticles can sustain local surface plasmon excitations, particle plasmons. These femtosecond-lifetime excitations depend strongly on particle geometry and give rise to a variety of effects, as frequency-dependent absorption and scattering and near field enhancement. Particle plasmons enable the concentration of light fields to nanoscale volumes and play a key role in surface enhanced spectroscopy.
Our research deals with the fundamental properties of particle plasmons which we investigate by optical spectroscopy and near field optical microscopy on lithographically tailored or chemically synthesited noble metal nanoparticles and ensembles thereof.
Surface Enhanced Spectroscopy
The properties of molecules can be strongly modified upon their electromagnetic interaction with particle plasmons. On one hand, resonant plasmon modes can expose molecules to very strong exciting fields. On the other hand, particle plasmons can strongly alter the local photonic density of states controlling the radiative and non-radiative decay channels of molecules.
We investigate the fluorescence and Raman scattering properties of molecules interacting with plasmonic nanoparticles with steady state and time-resolved techniques. Special emphasis is laid on the engineering of molecular de-excitation.
Porous silicon is obtained by dry or wet etching of a doped silicon wafer. Dependent on the doping density of the silicon and on the etching parameters various pore diameters in the so-called micro (2 – 4 nm), meso (5 – 50 nm) and macroporous regime (50 nm – 20 µm) can be achieved.
We use the chemical route to fabricate samples with quasi regular pore-arrangements in the meso/macroporous regime. These nanostructured silicon samples are electrochemically filled with a metal, especially ferromagnets to achieve a semiconducting/ferromagnetic nanocomposite material. Furthermore nanoparticles are incorporated into the pores. The physical properties of these hybrid materials are investigated by SQUID magnetometry, FTIR- and Raman-spectroscopy and by magnetotransport measurements. The morphology of the system is investigated by scanning and transmission electron microscopy.
Femtosecond Dynamics of Particle Plasmons (currently not active)
To understand the decay mechanism of particle plasmons and to gain deepened insight into the physics of plasmons in general a study of the temporal behaviour of particle plasmons is of fundamental importance.
Therefore we address the dynamic aspects of particle plasmons by fs time-resolved interferometric autocorrelation measurements. Second or third harmonic generation at the particle surface serves as a noninvasive probe of the particle plasmon oscillation. Comparing the measured interferometric autocorrelation function of the plasmon field with simulations based on a harmonic oscillator model we extract the temporal characteristics of the plasmon oscillation. Our experiments reveal that the decay of particle plasmons which can be described as the loss of coherence of the oscillating electrons (dephasing) occurs on a sub-10 fs time scale.
Nanoparticles for Sensor Applications
Nanoparticles as Photodiode Filters