Jennifer Steele

Resonant and non-resonant generation and focusing of surface plasmons with circular gratings

We report the generation and focusing of surface plasmon polariton (SPP) waves from normally incident light on a planar circular grating milled into a silver film. The focusing mechanism is explained by using a simple coherent interference model of SPP generation on the circular grating by the incident field. Experimental results concur well with theoretical predictions and highlight the requirement for the phase matching of SPP sources in the grating to achieve the maximum enhancement of the SPP wave at the focal point. NSOM measurements show that the plasmonic lens achieves more than a 10-fold intensity enhancement over the intensity of a single ring of the in-plane field components at the focus when the grating design is tuned to the SPP wavelength. We discuss the techniques adaptability for surface enhanced nano-scale spectroscopy.

Propagation of surface plasmons on Ag and Cu extended one-dimensional arrays on silicon substrates

Propagating surface plasmon waves can be supported by Cu and Ag periodic array, or grating, structures on silicon substrates. The plasmon dispersion characteristics, such as group velocity and bandgap associated with these structures are measured experimentally. In the infrared region of the spectrum (1.3–1.6μm) the properties of surface plasmons supported by these Ag and Cu periodic structures are virtually indistinguishable. The plasmon dispersion can be modified by varying either the grating period or the plasmon order. The plasmonic bandgap for this array geometry increases with increasing plasmon order.

Utilizing higher order surface plasmon modes on wire gratings for metal enhanced fluorescence

Metal enhanced fluorescence (MEF) has received much attention because of possible biomedical and sensing applications. MEF includes two mechanisms for fluorescence enhancement: (1) the enhanced electromagnetic field associated with surface plasmons increasing the excitation of fluorophores and (2) excited fluorophores radiating via induced surface plasmons. The second mechanism results in enhanced directional emission when fluorophores are located near a metal film or grating. This work focuses on gold wire gratings fabricated on a silica substrate coated with a layer of fluorophores. Previous studies on corrugated film gratings show that coupling to higher order as well as substrate side plasmon modes occurs with lower efficiency. We find for wire gratings, fluorophores couple to higher order plasmon modes on both the active and substrate side of the gold wires with uniform efficiency. We also measure directional enhanced fluorescence on both the active (reflection) and substrate (transmission) side of the gratings. Utilizing higher order modes allows gratings with micron and larger sized features to enhance fluorescence wavelengths in the visible range, greatly loosening fabrication requirements for potential applications. The ability to measure enhanced fluorescence in transmission also makes wire gratings appropriate for applications favoring a linear optical set up.