Reports: G648268-G6: Theoretical Investigation for the Enhanced Absorption of Nanostructured Semi-Conductor Materials

Shengli Zou , University of Central Florida

Objective and summary of achievements: The objective of the proposal is to understand the nano-structure dependence of metal and semi-conductor film using electrodynamics theory. We designed a structure which may focus light on the bottom of nano-wells embedded in a silver film. We also found incident polarization dependence of light propagation through a film composed of periodic silver prisms.

1. Impact to the PI's research

The award helps the early career development of the PI and members in the PI's group. Seven peer-reviewed papers were published as a result of the award in the year 2010-2011. The achievements from the projects had been used as preliminary results for external funding applications from agencies, including NSF and DOE.

2. Educational impact

Two graduate students and one visiting scholar were involved in the project. Graduate student Haining Wang and Jennifer Reed calculated light propagation through a film composed of periodically arranged silver prism. Haining Wang also calculated the vibrational oscillation of hollow gold nanospheres in collaboration with Dr. Zhang at the University of California at Santa Cruz.

3. Research achievements

3.1 Surface-plasmon-assisted electromagnetic wave propagation.

Using electrodynamics tools, we investigated the effect of surface plasmons on the propagation direction of electromagnetic waves around a spherical silver nanoparticle and nano-structured silver film. The studies showed that the calculated effective index of refraction of a spherical silver nanoparticle from the Kramers-Kronig transformation method may not represent the index of refraction of the system but is consistent with the Poynting vector (the energy flow) direction at the microscopic scale. Using a silver film composed of periodic triangular prisms, we numerically demonstrated that electromagnetic waves may propagate along different directions depending on the incident polarization direction. When the incident polarization is in the plane of incidence and the surface plasmons are excited, the refracted light ray propagates along the same side of the surface normal as the incident wave. When the incident polarization is perpendicular to the plane of incidence, the refracted light ray always propagates on the opposite side of the surface normal. The results show that a silver film composed of periodic nano-sized triangular prisms may be used as a filter to simultaneously generate two polarized light rays of orthogonal polarizations from one light source.

3.2 Light trapping at bottom of wells embedded in a silver film.  

The electromagnetic contribution to the surface enhanced Raman scattering (SERS) derives from the distribution of electromagnetic “hot spots” around nanostructured metal surfaces. For an optimized SERS signal, analyte molecules need to be placed within the intense electric fields localized at these hot spots. Using numerical simulations, we demonstrate the possibility of creating controllable hot spots through proper engineering of the plasmonic modes supported by periodic arrays of nanoscale cavities in thin silver films. We investigate the tunability of surface plasmon resonance wavelength and local field enhancement by systematically varying the sample thickness, periodicity, and the nanocavity morphology. The gradual evolution of the absorption spectrum with these parameters helps reveal the relative contributions from surface plasmon polaritons propagating or localized at the metal surfaces. The calculations suggest that when the nanocavities are deep relative to the film thickness, there is strong confinement of surface plasmons which produces several orders of magnitude enhanced electric fields across the cavity bottom surfaces. Enhanced local electric field at the bottom of the cavity provides an efficient optical trap and the system can be used for reproducible SERS detection, especially for big nanometer sized biomolecules.

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