We show theoretically and experimentally that photonic band gaps can be realized using metal or metal-coated spheres as building blocks. Robust photonic gaps exist in any periodic structure built from such spheres when the filling ratio of the spheres exceeds a threshold. The frequency and the size of the gaps depend on the local order rather than on the symmetry or the global long range order. Good agreement between theory and experiment is obtained in the microwave regime. Calculations show that the approach can be scaled up to optical frequencies even in the presence of absorption.PACS numbers: 42.70.Qs Photonic band gap (PBG) is a spectral gap in which electromagnetic waves cannot propagate in any direction [1]. Recently, two promising routes have been discovered that may lead to PBG in the IR͞optical frequencies: (i) microfabrication [2] and (ii) inverse-opal and related techniques [3]. Both methods seek to create some predefined artificial structure with an interconnected array of high dielectrics. Here we propose an alternate route. Instead of emphasizing the structure, we focus on the building blocks. The building blocks we propose are spheres with a dielectric core, a metal coating, and an outer insulating layer. With multiple coatings of variable thicknesses, these coated spheres have continuously tunable scattering cross sections and resonances. In analogy with semiconductor physics, we have designable "photonic atoms" which have continuously tunable properties. Depending on how we assemble these spheres together, we can choose the crystal structure which in turn can be changed by external fields [4]. In this paper, we show by physical argument and by explicit calculation and experimentation that any periodic structure formed from such spheres can exhibit photonic band gaps. This contrasts with the conventional PBG systems where the global symmetry and the structure factors are equally important, which in turn lead to added difficulties in their fabrication.In order to handle the calculation involving spherical scatterers with metallic coating, we developed a band structure code based on the multiple scattering technique (MST) [5]. We checked our results against photonic band structures calculated using the finite-difference time domain (FDTD) method, where the convergence has been carefully monitored [6]. The test case is the photonic band structure of ideal metal spheres arranged in the diamond structure with a filling ratio f 0.31, embedded in a medium with e 2.1. This is a demanding test case since the metal spheres touch at f 0.34. With our code, we obtain a gap͞midgap frequency of 0.56 (with angular momentum up to l 7), which is in excellent agreement with that of FDTD [6]. Our result lies between their finest grid value of 0.53 and the extrapolated value of 0.56. The transmission spectra reported below are computed with the layer-MST formalism of Stefanou-Yannopapas-Modinos [7]. The agreement between the band structure code and the transmission code is excellent.Since metallic elements are invo...

We study theoretically the optical properties of a three-dimensional self-assembly of spherical-metal nanoparticles. Our band-structure calculations show that photonic band gaps in near infrared and optical frequencies can be realized in a fcc lattice of metal particles with radii of approximately 160 nm. When absorption is taken into account, we found that the gap is preserved in good metals like silver. The metal nanoparticles can be replaced by dielectric spheres coated with a layer of good metal. DOI: 10.1103/PhysRevB.64.113108 PACS number͑s͒: 42.70.Qs, 78.67.Bf Photonic crystals are three-dimensional periodic dielectric structures, with lattice constants of the order of the desired electromagnetic ͑EM͒ wavelength of operation. The free-EM wave dispersion in such crystals is greatly modified due to the presence of periodicity and refractive-index contrast. Under favorable circumstance, a photonic band gap ͑PBG͒ in the dispersion relation can open up, in which light cannot propagate in any direction. 1 Such a structure is believed to have a deep impact on a wide range of photonic applications. [2][3][4][5] Recently, considerable progress has been made in constructing three-dimensional photonic crystals with submicrometer periodicity, using the sophisticated ''top-down'' fabrication techniques, 6 the self-assembly of monodispersed microspheres and related infiltration method, 7 and the holographic lithography. 8 Compared to the micromachining methods for the production of PBG crystal for visible wavelengths, the self-organization process is a very cheap and relatively easy method too.On the other hand, the interaction of light with nanoscale metal particles has been the subject of extensive research. 9 Researchers have used metal nanoparticles as building blocks for novel materials. [10][11][12][13][14] We will show in this paper that metallic nanoparticles can be used as building blocks for photonic crystals operating at near IR and optical frequencies. It is not intuitively obvious that metallic nanoparticles can serve the purpose. First, metals are dispersive and can be rather absorbing at IR and optical frequencies. Second, it is very difficult to fabricate metal nanoparticles that are monodispersed spheres with radii of the order of 150 nm, the scale length needed to realize photonic gaps at optical frequencies. We are going to show that both of these problems can be solved. First, our calculations will show that the absorption problem can be alleviated by a careful choice of the metal component. In order to build a convincing case, we will use experimentally measured dielectric constants for the frequency range of interest ͑in contract to Drude or Drude-like models that are used frequently in the literature͒ 15 in all of our calculations. We will also suggest a simple and practical way ͑using metal coated silica spheres͒ to bypass the need of using metal nanoparticles of large ͑150 nm͒ radius.We will draw our conclusions from results that are based on band-structure calculations employing periodic bo...

A high VFA value is associated with postoperative recurrence of Crohn's disease and has clinical implications with respect to optimizing prophylaxis for each individual. However, further studies are needed to confirm the predictive role of this biomarker in a different data set.