Fabrication and optical characterizations of gold nanoshell opal
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We fabricated three-dimensional photonic crystals by self-assembling gold nanoshells via forced sedimentation method. Gold nanoshells with a diameter of 458 nm (418-nm silica core and 20-nm gold shell) were synthesized and self-assembled into a 5-m-thick opal structure. We observed reflection peaks at 710 and 1240 nm, which are believed to be from the complete three-dimensional photonic band gap and the (111) directional gap, respectively. Theses results were in good agreement with the photonic band-structure calculations done by the finite-difference time-domain method. Angle-resolved reflectivity measurements were also performed to investigate the existence of the complete three-dimensional photonic band gap.
I. INTRODUCTION
Photonic band gap materials hold a high promise of enabling many novel optical applications such as highly integrated optical waveguides,1 high Q cavity,2 and thresholdless laser,3 and have naturally been the subject of extensive research during the past decade or so. Although there has been much progress in two-dimensional photonic crystal (PC) structures, achieving the complete three-dimensional photonic band gap, with which the fullest effect of photonic band formation is expected, still remains a formidable challenge. Also at issue is the difficulty of achieving the photonic band gap in the visible spectrum. The visible photonic band gap can enable many useful applications such as lighting devices. However, it remains a difficult task primarily because of the lack of suitable materials. Several approaches, including layer-by-layer lithography, holographic lithography, and self-assembly, have been reported to achieve the complete three-dimensional photonic band gap in microwave and optical regimes.4–8 Among them, the self-assembly method using colloidal particles is an attractive approach because it provides a simple, fast, and cost-effective way to produce threedimensional PC structures. The self-assembly process uses highly monodispersed nanoparticles such as silica (SiO2) and polystyrene and assembles them into the facecentered cubic (fcc) structure to form the threedimensional PC structure often called artificial opal. To
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Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2006.0398 J. Mater. Res., Vol. 21, No. 12, Dec 2006
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realize a complete three-dimensional photonic band gap, the opal structure must be inverted by infiltrating with high-index dielectric material. Inverted opals exhibiting complete photonic band gap in the near-infrared region have been demonstrated using silicon.6,7 However, this approach cannot be easily extended to the visible range because it is difficult to find a high-index material transparent in the visible. Alternatively, PC structures containing metal could generate the complete photonic band gap regardless of the crystal symmetry.9 To apply this scheme for operation in the visible or near infrared region, a few hundred nanometer-size metal particles with
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