The optical response from metal nanoparticles and nanostructures is dominated by surface plasmon generation and is critically dependent on the local structure and geometry. Electron energy-loss spectroscopy (EELS), combined with recent developments in spectrum imaging and data processing, has been used to observe the energy and distribution of surface plasmons excited by fast electrons. The energy of the plasmon responses is consistent with the optical response and with calculations. For gold and silver rods and ellipsoids, longitudinal, transverse and distinct cluster modes were readily identified and mapped. The spatial resolution of the presented maps is one order of magnitude better than that achievable with scanning near-field optical microscopy (SNOM)-based techniques.

interferometry. [9] Recently, Hickmann and co-workers [20] pointed out that the phase of light, u, which is directly related to the dispersion relation and therefore to the group delay s = ¶u/ ¶x, could be calculated from transmission spectra via Kramers±Kronig relations. We verified their observations for the case of 3D colloidal photonic crystals through careful comparison of calculations with measurements in our whitelight interferometry experiment. For the region of the fundamental stop gap, measured and calculated group delays differ only by a constant offset. In our case, absolute numbers are not required to show an increase in group delay for slow photon modes at the band-edge with respect to normal propagating modes inside the band. Therefore, neglecting the constant offset, we calculate the relative group delay Ds from the transmission spectra shown in Figure 1. As displayed in Figure 4, the group delayÐespecially of the air mode at the edge of the fundamental stop gapÐincreases with increasing plasma exposure time. This result is even more striking because a decreasing group delay is expected from theory for decreasing filling fractions (not shown). Gradient and surface smoothing together outbalance this behavior for plasma exposure times of up to 15 min. Afterwards, the ongoing reduction of the filling fraction and the onset of the collapse of the photonic crystal leads to an overall reduction of the group delay. Our results demonstrate for the first time that a properly chosen post-fabrication technique is able to improve the optical quality of photonic crystals and reduce losses in slow photon modesÐa milestone on the way to an all-optical transistor.[21]We are currently developing similar techniques, which might be directly applicable to high-refractive-index materials such as silicon.In conclusion, we have presented enhanced coupling to slow photon modes, based on graded 3D colloidal photonic crystals with reduced surface roughness. This is achieved by a convenient and precise post-fabrication method to incorporate filling fraction gradients into 3D polymer colloidal photonic crystals. Plasma etching allows for the formation of a gradient, which can be tailored by sample pretreatment and which facilitates coupling to low group velocity modes. Furthermore, surface roughness is reduced, resulting in higher structural and optical quality colloidal photonic crystals. Finally, it provides a reliable method to modify the position of the fundamental stop band in a precise manner. In this paper we describe the interesting optical and IR absorption spectra of randomly oriented suspensions of discrete cap-shaped metal nanoparticles. The present ªnanocapsº have a more complex and versatile geometry than related core± shell particles recently reported by others. [1,2] Nanocaps of Au, Ag, Cu, Al, and Cr were produced and the effect of variations in deposition angle and thickness investigated. We show that the absorption peaks of these shapes are strongly redshifted relative to those of solid nanospheres, and ...

The far-field plasmonic behaviour of nanoporous gold films with void densities, f, ranging from 60% to 90% has been investigated and modelled. These layers have good dc conductivity and quite different nanostructure to traditional porous layers in which the metal percolates. Our gold films with f above 70% have high thermal emittance for a conductor at their thicknesses, and their flat spectral response at visible and near infrared wavelengths is not metal-like. We derive effective optical constants which become plasmonic at wavelengths between 1.8 m and 4 m for f from 72% to 87%. This onset is at much longer wavelengths than that in bulk gold. For f below 70% the onset of plasmonic behaviour is much closer to the dense material. A simple test is implemented to test for surface plasmon polaritons (SPP) under illumination. The more porous films show no evidence of SPP, while the less porous films display weak evidence. Thus by tailoring f in these nanostructures we can tailor the onset of effective plasmonic response across a wide range and emittance from around 0.9 down to low values. An effective uniform metal response is thus found in the presence of surface nanostructure without the interface absorption found in dense gold layers with structured surfaces.

2D arrays of silver semi-shells of 100 and 200 nm diameter display complex reflection and transmission spectra in the visible and near-IR. Here these spectral features are deconstructed and it is demonstrated that they result from the coupling of incident light into a delocalized Bragg plasmon, and the latter's induction of localized Mie plasmons in the arrays. These phenomena permit the excitation of transverse dipolar plasmon resonances in the semi-shells despite an ostensibly unfavorable orientation with respect to normally incident light. The resulting spectral feature in the mid-visible is strong and tunable.

The surfaces of nanoscale gold particles and components are oxide-free under normal ambient conditions. This unusual attribute permits the exploration of microstructures and functionalities that would not be feasible for less noble metals. Here we consider the electrochemical properties of mesoporous gold sponges, prepared by de-alloying an AuAl2 precursor. The sponges have a high specific surface area, with an average pore diameter of 12 nm, but are prone to sinter. They may be prepared in bulk, or, more usefully, as coatings. Their electrochemical capacitance divided by their nominal surface area is high and, at a cell voltage of 0.6 V, reaches 100 mF/cm 2 for bulk samples and 2 mF/cm 2 for coatings. This is up to a thousand times greater than the 50 to 100 μF/cm 2 exhibited by a planar gold surface.