Fabrication of Hollow Metal “Nanocaps” and Their Red‐Shifted Optical Absorption Spectra

Abstract

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 ...

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