Introduction to the subject:
       In this emerging area, Photonic band gap (PBG) materials (also called as photonic crystals) are an interesting class of materials which are constituted of a periodically varying dielectric constant (refractive index) and thus show a band gap for light transmission. The range of wavelengths that are absent in transmission will be present in reflection, when the constituent materials are non-absorbing in that range. The band gap region is decided by the modified Bragg?s law, which relates the high reflection wavelength to the periodicity and the effective index. The periodicity can be in one-, two or three dimensions. The standard methods employed for this are based on semiconductor technology, such as lithography. Self-assembly is a simpler and cheaper alternative where spherical colloids, monodispersed in an appropriate solvent, assemble under thermodynamic equilibrium into 3D-ordered crystals. The main drawback of self-assembly is the number of unwanted defects in the crystal during the process of ordering. The structural characterization with a resolution of 1 mm will show the close-packed arrangement of spheres and the optical characterization will show the peak reflectance wavelength. It is established that the self-assembled samples of colloidal spheres usually arrange themselves in an fcc structure with the [111] plane parallel to the substrate. In such samples, the wavelength of minimum transmission (or peak reflection) is given by l = 2 d111(neff2 ? sin2q)1/2, where d111 is the interplanar spacing for [111] planes, neff is the effective refractive index of the spheres and the voids taken along with their fill fractions and q is the angle of incidence on the [111] plane. The period d111 is related to the sphere size. Hence, the approximate position of the band is known for the chosen material and sphere size. It is also clear that to achieve a photonic stop band in the visible, one requires the period to be less than a micron. The photonic crystals have many interesting applications in optoelectronics and optical communications such as filters, low-threshold lasers, waveguides and photonic crystal fibers. When a defect is introduced in a photonic crystal, it forms a resonant cavity so that light is localized at the defect. If the ?defect? contains an active species that emits light under excitation, then the cavity effect is capable of modifying the spontaneous and stimulated emission characteristics. High Q cavities are required for the fabrication of low-threshold lasers from such active species.
       On-going work:
       In the Physics department at IIT Bombay, Prof. R.Vijaya initiated experimental work in this area in 2004 with funding from DST. The bulk sedimented photonic crystal samples showed good ordering but were not suitable for optical studies. The thin film photonic crystals, synthesized by the convective self-assembly method and horizontal inward growing self-assembly method have shown very good optical and structural characteristics apart from being mechanically stable. 3-D ordered photonic crystals have been prepared from sub-micron size colloidal spheres of inorganic and polymeric materials. Their stop band in the UV, Vis and near-IR wavelengths, with reflectance of 40-60%, have been achieved by the appropriate choice of the sphere size and the refractive index of the material of the sphere. Their tuning characteristics in terms of the angle of incidence of light, temperature variation and infiltrated materials have been studied. The Transfer Matrix Method is used to calculate the transmission coefficient for these materials.
       Since May 2006, horizontal inward growing self-assembly has resulted in samples with reflectance exceeding 60%. Experiments on novel methods of infiltration, characterization of the infiltrated materials, inversion of the direct template and heterostructure preparation are in progress.
       Text Box: SEM and AFM images of photonic crystals
      
Projected activities:
       It is planned to (a) study the emission characteristics of the active species infiltrated in the photonic crystals through PL measurements, (b) characterize the samples for nonlinear optical device performance related to frequency conversion and all-optical switching and (c) calculate the vacuum field fluctuations and local density of states of emission of active media embedded in different environments