Surface plasmons created at the interface of metals and dielectrics due to the optical coupling of light have applications in areas such as sub-wavelength optics, data storage, optoelectronic circuits, diffraction-limited microscopy and bio-photonics. In particular, these surface localized fields can be used to sense ultra-low concentrations of materials. For example, the enhanced electromagnetic field produced by plasmonic nanostructures can be used to couple light to fluorescent dye molecules in the immediate environment and thereby provide the extreme signal amplification necessary for detecting trace quantities of biomolecules tagged with fluorophores.

Nano-gaps are important

In the study, the scientists used a simple PDMS-based microcontact printing/replication process to reproduce the surface features of an HD-DVD-R disc (dissected into two parts to reveal the grating pattern on the inner side of the polycarbonate substrate) on conventional glass substrates. An important consequence of the fabrication process was the generation of defects in the form of nano-gaps that cut across the printed gratings.

The presence of nanogaps within the grating structures led to substantial field localization and amplification – propagating surface plasmon polaritons (SPPs) travel as surface waves with high field intensity towards the metallic nanogap where the sudden field discontinuity causes “extreme crowding” of the surface charges, leading to very high field intensities.

Gratings with embedded nanogaps, when studied with spin-coated rhodamine-590 dye, gave fluorescence enhancements as high as 36-fold on the gratings and 118-fold on nanogaps when compared with glass slides typically used in the lab.

Cost-effective approach

Imaging single molecule fluorescence often requires sophisticated and expensive optical set-ups. A direct consequence of this research is the possibility of extending this work and fine-tuning the structures to enable visualization of single molecule fluorescence and Forster Resonance Energy Transfer (FRET) between single dye pairs. This will enable fundamental understanding of protein folding and conformational changes that were typically limited to laboratories having specialized equipment.

Full details can be found in the journal Nanotechnology.