Nano-optics
Imaging beyond the Diffraction Limit
Fluorescence and Lifetime Modification
Surface Enhanced Raman Scattering
Advances in recent years in nano- and bio-technology have led to an urgent need for optical imaging tools able to resolve features with sizes in the range of 1 nm to 1 µm. Our research is aimed at developing a variety of highly sensitive optical techniques which allow high resolution spectroscopy of biological systems. By using specifically designed nanostructures we are able to modify the emission intensity and lifetime of nearby fluorophores, as well as enhancing the Raman scattering cross-section by many orders of magnitude.
SNOM
In scanning near-field optical microscopy (SNOM), a nanoscopic optical probe is raster scanned in close proximity to the surface of a sample of interest. The near-field optical interaction between the probe and sample is exploited to provide resolutions beyond the diffraction limit that are inaccessible by traditional far-field optical microscopy, while simultaneously enhancing the fluorescence intensity. Research is concerned with the development of "apertureless-SNOM" (A-SNOM) for tip-enhanced Raman microscopy and single nanoparticle imaging, as well as theoretical simulations using the finite difference time domain (FDTD) method.
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(a) Calculated fluorescence enhancement (solid line) for a molecule a distance d from a gold tip. (b) Schematic of experimental arrangement for A-SNOM |
1 mm x 1 mm images of a CdSe/ZnS quantum dot cluster: (a) Fluorescence confocal image (no tip) (b) Fluorescence ASNOM image with gold tip (c) & (d) Line profiles showing simultaneous resolution and intensity enhancement in the presence of a tip. |
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Plasmonics
We are also investigating plasmonic fluorescence enhancement effects for arrays of metal nanostructures. The presence of a metallic surface dramatically alters the emission properties of a locally situated fluorophore: the resonant excitation of plasmons on silver and gold nanoparticles can produce a large localised enhancement of the electric field, resulting in a strong enhancement in fluorescence emission. We have synthesised highly ordered, periodic Au and Ag nanoparticle arrays using nanosphere lithography; the plasmon resonance of these structures can be tuned over the visible spectrum via changes in particle size and the refractive index of the surrounding medium. Using scanning confocal imaging we have observed highly localised enhancements in the emission of fluorescent molecules in the vicinity of these nanoparticles. Additional radiative decay channels are also created which lead to an order of magnitude increase in decay rate. The strong distance dependence of these enhancement effects suggests that these structures may offer a means of selectively mapping the location of specific target molecules within a larger excitation volume, such as fluorophore-tagged proteins in cell membranes.
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(a) AFM image of Ag nanoparticle array. Scale bar = 500 nm. (b) Confocal fluorescence intensity map of R6G deposited on a nanoparticle region similar to (a). (c) Raman spectrum from R6G on nanoparticles. (d) Raman intensity map of the 1500 cm -1 line, from a region similar to (a). |
SERS
Raman spectroscopy provides an enticing alternative to fluorescence, as it has many potential advantages and it is able to provide high-resolution information with chemical specificity. As such, the technique is finding increasing application in biological systems as it allows rapid diagnosis using spectral deconvolution techniques such as principal component analysis. However, the very weak cross-section of the Raman scattering process (some 14 orders of magnitude less than fluorescence cross-sections) makes it impractical for many biological applications that require low laser powers and short integration times. We are investigating ways of boosting the Raman signal via surface enhanced Raman scattering (SERS); this involves the use of noble metal nanoparticles to enhance the Raman signal of nearby molecules by many orders of magnitude (see Fig. d). Devices that utilise this effect can act as ultra-sensitive biosensors, with diverse applications in all areas of pathogen detection.
Recent Selected Publications
T. Ritman-Meer, N. I. Cade, and D. Richards, Appl. Phys. Lett. 91, 123122 (2007).
Spatial imaging of modifications to fluorescence lifetime and intensity by individual Ag nanoparticles
D. Richards, ‘Near-field microscopy – throwing light on the nanoworld’ in ‘Advances in Nanoengineering', eds. A. G. Davies and J. M. T. Thompson (Imperial College Press, London, 2006), in press
F. M. Huang and D. Richards, J. Opt. A: Pure Appl. Opt. 8, S234-S238 (2006)
Fluorescence enhancement and energy transfer in apertureless scanning near-field optical microscopy
F. M. Huang, F. Festy, and D. Richards, Appl. Phys. Lett. 87, 183101 (2005).
Tip-enhanced fluorescence imaging of quantum dots.
A. L. Demming, F. Festy, and D. Richards, J. Chem. Phys. 122, 184716 (2005).
Plasmon resonances on metal tips: Understanding tip-enhanced Raman scattering.
F. Festy, A. Demming, and D. Richards. Ultramicroscopy 100, 437-441 (2004)
Resonant Excitation of tip plasmons for tip-enhanced Raman SNOM.
D. Richards and A.Z. Zayats (eds.). Theme Issue of Phil. Trans. Royal Soc. A vol. 362, Issue 1817 (2004)
Nano-optics and near-field microscopy
D. Richards, Phil. Trans. Royal Soc. A 361, 2843-2857 (2003).
Near-field microscopy – Throwing light on the nanoworld.
Key Personnel
Tom Ritman-Meer
Ferhat Culfaz