Photonic band gaps

The manufacture of a photonic crystal requires extreme process control because a deviation from perfect periodicity in the order of a few percent of the wavelength worsens the optical performance. Macroporous silicon is a potential candidate for the realization of such structures because of its photolithographic patterning. The precision of the macroporous structures is reflected in the transmission measurements along the T-M and T-K directions, which exhibit a photonic band-gap centered at 5 pm, as shown in Fig. 10.16. For measurement the macroporous... 

R. W. ZioUcowski and T. Liang, Design and characterization of a grating-assisted coupler enhanced by a photonic-band-gap structure for effective wavelength-division demultiplexing. Opt. Lett. 22, 1033-1035 (1997). 

However, to make these photonic devices some method of controlling light is required so that it can be manipnlated for a particular application. In other words there is a need to be able to trap a photon of a particular wavelength, and then release it only as reqnired. This is the photonic equivalent of the semi-conductor which controls the flow of electrical cnrrent in electronic devices such as transistors. These light manipnlating materials wonld have a photonic band gap that performs an equivalent role for photons as do electronic band gap semi-conductors for electrons. This new class of materials, known as photonic band gap crystals, was first proposed in 1987, and the constrnction of these artihcial crystals has been an area for intensive research since the mid-1990s. ... 

Photonic band gap crystals can be dehned as long-range ordered structures whose relative permittivity varies as a spatially periodic function. These three-dimensional periodic structnres have a feature size comparable to or shorter than the wavelength of visible light. 

It is a very difficult task to construct such materials and so far three major methods have been utilised in attempts to produce useful photonic band-gap materials. 

Self-assembly of highly charged colloidal spheres can, under the correct conditions, lead to 3D crystalline structures. The highly charged spheres used are either polystyrene beads or silica spheres, which are laid down to give the ordered structures by evaporation from a solvent, by sedimentation or by electrostatic repulsion (Figure 5.34). The structures created with these materials do not show full photonic band gap, due to their comparitively low relative permittivity, although the voids can be in-filled with other materials to modify the relative permittivity. 

A better way of achieving a complete 3D photonic band gap is considered to be by constructing a lattice of air balls surrounded by an interconnecting matrix of material with a higher refractive index. The successful construction of such crystals by growth techniques is unlikely and likewise the build up by deposition techniques is no simple procedure. 

Figure 5.34 Outline of self-assembly route to photonic, filled photonic and macroporous photonic band gap crystals.

In one example, the colloidal structure, is made by sedimentation of polystyrene beads, giving voids in the range 120-1000 mn, and the voids are filled with TiO generated from titanium tetrapropoxide. The polystyrene bead lattice is then removed by calcining to give an iridescent material, but not with a full photonic band gap. In this case one of the controlling factors is the refractive index of the matrix, which needs to be greater than 2.8. 

However, recently, material constructed using silica sphere photonic crystals as the template has given a complete 3D-photonic band gap centred on 1.46 pm, which is a favoured wavelength for hbre-optic communications. Sintering of the silica array causes each sphere to be joined by short necks. Silicon is then grown in the... 

Whilst trae 3D photonic band gap materials operating in the microwave and millimetre range have been produced, those operating in the visible region of the spectrum are still awaited. When this eventually happens the optical computer will no longer be a pipe dream. 

C.M. Soukoulis (Ed.), Photonic Band Gap Materials, Kluwer, Boston, MA, 1996. 

Three-dimensional (3D) structuring of materials allows miniaturization of photonic devices, micro-(nano-)electromechanical systems (MEMS and NEMS), micro-total analysis systems (yu,-TAS), and other systems functioning on the micro- and nanoscale. Miniature photonic structures enable practical implementation of near-held manipulation, plasmonics, and photonic band-gap (PEG) materials, also known as photonic crystals (PhC) [1,2]. In micromechanics, fast response times are possible due to the small dimensions of moving parts. Femtoliter-level sensitivity of /x-TAS devices has been achieved due to minute volumes and cross-sections of channels and reaction chambers, in combination with high resolution and sensitivity of optical con-focal microscopy. Progress in all these areas relies on the 3D structuring of bulk and thin-fllm dielectrics, metals, and organic photosensitive materials. 

Red-orange opals contain larger spheres than blue-green opals. How do the wavelengths of the photonic band gap vary with the colour ... 

The photonic band gap wavelength increases as the size of the spheres increase, so that it will be larger for the longer wavelength orange-red colours. 

Soukoulis. C.M. Photonic Band Gaps and Localization, Klriwer Academic Publishers. Norwell. MA. 1993. 

---