Inhalt des Dokuments
- © Grosse, TUB
A surface plasmon wave is a hybrid of a charge density wave with an electromagnetic wave that propagates along a metal-dielectric interface. Surface plasmons (SP) offer some unique properties, such as field confinement and enhancement. We study the nonlinear frequency conversion that occurs when femto-second pulses of laser light excite surface plasmons in metal nanofilms and structures. In addition, the effects of local field enhancements on the emission from nanocrystals and molecules are studied in metallic nanocavities. We seek to uncover the microscopic origins of the plasmon-photon nonlinear interaction and to apply that knowledge to new optical devices.
Fig.: A surface-plasmon mode frozen in time. Calculated electric field.
Three-wave mixing and second-harmonic generation
- © Grosse, TUB
Nonlinear effects occur when the strength of the surface plasmon field becomes comparable to those experienced by the conduction electrons in their local environment. Second-harmonic generation (SHG) is the most elementary of nonlinear processes that has been associated with the excitation of surface plasmons. Knowing the mechanism of the nonlinear interaction is of great benifit for designing new optical devices involving metals, especially on the nano-scale.
Fig.: Surface-plasmon second-harmonic generation (SHG) experimentally obtained from a 57nm silver film in the Kretschmann geometry. SHG emission at 400nm enhanced by surface plasmons (bottom left), along with residual fundamental light at 800nm (top).
Nonlinear k-space spectroscopy
- © Grosse, TUB
We have used k-space spectroscopy to characterize the nonlinear phase-matching conditions in three-wave mixing and SHG involving surface plasmons and photons. We found that when a metal film is driven near to SP resonance, the nonlinear process that dominates is where two fundamental SP annihilate to generate a second-harmonic photon. This occurs because compared to a free-space beam of light of the same optical power, a surface plasmon (SP) mode that propagates along a silver-air interface can have an intensity that is greater by a couple orders of magnitude. Identiftying which nonlinear interaction takes place has implications for demonstrating SP down-conversion in the near-field.
Fig.: Experimental setup for nonlinear k-space spectrometry.
The Kretschmann-Raether geometry is the most widely used for surface plasmon resonance (SPR) sensing. Depositing a uniform 50nm gold film onto a glass prism is a relatively simple operation. Furthermore, the photon-plasmon coupling efficiency can approach unity, which leads to a sensitivity in parts-per-million for reading out the refractive index of the outer medium. The alternative is the Otto geomtery, where the SP propagating along the flat interface of bulk metal is coupled to photons via an air gap and a glass prism. Despite the difficulty of mainting a constant air gap of a few hundred nanometers, the Otto geometry allows one to study novel materials that can't be vapour deposition. The coupling efficiency can also be easily controlled via the air gap. In our lab, we have successfully demonstrated this geometry for measuring nonlinear SP processes.
Dr. Nicolai Grosse
(1) N. B. Grosse, et al., Phys. Rev. Lett. 108, 136802
(2) J. Heckmann, et al., Optics Express, 21, 28856 (2013).