Complex Dielectric Media
With complex dielectrics, we intend dielectric structures with an index of refraction that has variations on a length scale that is roughly comparable to the incident wavelength. Such structures strongly scatter light. An homogeneous, non-absorbing medium is transparent and ordinary refraction and reflections can be observed. A sample with random dielectric constant is opaque, and typically white, when absorption is absent, while a periodically modulated one is iridescent and exhibits wavelength dependent reflection. A glass of milk, white paint or the iridescent wings of some butterflies are examples of complex dielectrics.
The playground of disorder materials is composed basically by oxides or semiconductor powders, random solid distributions of polydisperse spheres, nanostructured semiconductors such as porous GaP or by colloidal suspensions of polymer spheres8. A single dielectric sphere with size comparable to the wavelength of light can sustain electromagnetic resonances, called Mie modes. In ordinary disordered materials like a semiconductor powder, the individual modes of each building block are usually neglected and a homogeneous distribution of light modes, in wave-vector and frequency, is assumed.4 A great advantage could be to take profit from the monodispersity of the constituents of a disordered material.
Photonic Glasses versus conventional disordered media
We have presented a new material for light. This new three dimensional system is composed by monodisperse polymer spheres arranged in a completely disordered (random) way. We dubbed this material “photonic glass” in opposition as “photonic crystals” which are ordered arrangements of the same building blocks. Due to the resonant behaviour of the spheres, discrete light states exist, and therefore every sphere acts as a meta-atom for light.
Figure 3- Up: picture shows an opal-based photonic crystal (squared) which shows visible iridescences due to Bragg reflections and a photonic glass (round) without any trace of Bragg features. Low: (left) SEM image from a photonic crystal cleaved edge (scale bar is 10 µm). (Right) SEM image from a photonic glass surface (scale bar is 10 µm). Both samples are made of PS spheres (2% of polidispersity measured by TEM) and have centimetres squared areas and millimetres thick.
From photonic crystals to photonic glasses. Fig.2 a) SEM images from a photonic glass surface. The inset shows a cleaved edge of a photonic glass. b) Thin disordered film grown by vertical deposition with PS and PMMA spheres with different diameter (457 nm and 260 nm respectively). c) Thin disorder film grown by vertical deposition with PS and PMMA spheres with the same diameter (260 nm). The vacancies are the original positions of the PS spheres before selectively etching them with Cyclohexane.