Sunita Kedia

Laser emission from self-assembled active photonic crystal matrix

Three-dimensionally ordered photonic crystals were grown using self-assembly technique from Rhodamine-B dye doped polystyrene micro-spheres resulting in a stop band at 611 nm overlapping the emission spectrum of the dye. When excited at a wavelength away from the stop band, using a frequency-doubled Nd:YAG laser, the crystal showed angle- dependent suppression of spontaneous emission of the dye in the wavelength range of the photonic stop band and enhancement at the band edge, in reflection and transmission geometries. Spectral narrowing, a sharp threshold and a highly directional emission, all indicative of stimulated emission, were observed from the active photonic crystal matrix.

Analysis of Lasing in Dye-Doped Photonic Crystals

Recently, we experimentally demonstrated room-temperature lasing of self-assembled opal photonic crystal (PhC) made of rhodamine-B-doped polystyrene colloids. Here, we explain this experimental observations by analyzing the phenomenon of light amplification in dye-activated PhCs via a complex-valued permittivity of the colloids. We show that the lasing is facilitated by the enhanced distributed feedback due to the reduced group velocity in the vicinity of the photonic band edge. This simple approach to the analysis of PhC lasing behavior allows us to calculate the lasing wavelength in close agreement with the experimental value. It also enables the estimation of gain coefficient required for lasing and may prove useful in design of compact PhC-based lasers.

Emission studies in double-layered and triple-layered photonic crystal microcavities

Three-dimensional pseudo-bandgap polystyrene photonic crystals (PhCs) fabricated by the self-assembly technique are used to design two models of microcavities, namely the double-layered and the triple-layered PhC heterostructures, to study the laser-induced emission from the embedded Rhodamine-B dye. The dye molecules in the colloids of these PhC matrices are excited with a Q-switched frequency-doubled Nd:YAG laser at 532 nm, and the emission is recorded in different directions for different input powers. The spontaneous emission of the dye gets modified due to the inhomogeneous distribution of density of states in the PhC. The Bragg crystal planes serve as reflecting cavity mirrors for the embedded gain medium, and at higher pump powers, direction-dependent spectral narrowing of the emission spectrum is clearly observed.

Surface treatment of 45S5 Bioglass using femtosecond laser to achieve superior growth of hydroxyapatite

45S5 Hench bioglass (BG) has gained interest in research because of its potential clinical applications. Several studies in-vivo and in-vitro have been in progress to improve biointegration efficiency of this glass. In present contribution, surface modification of Hench BG has been done employing a femtosecond (fs) laser beam, resulting in increased effective surface area of the sample. These surface modified samples were subsequently immersed in simulated body fluid for varying number of days and characterized using scanning electron microscope, energy dispersive X-ray analysis, X-ray diffraction, and micro-Raman spectroscopy. In-vitro studies indicated superior growth of hydroxyapatite (HAP) layer on the laser treated samples in comparison to the untreated samples. The presence of strong X-ray diffraction peaks confirmed faster growth of HAP on laser treated samples. Raman peaks, five times more intense and relatively narrower represented higher crystallinity of hydroxyapatite layer on laser treated BG.

PHOTOLUMINESCENCE OF ZINC OXIDE INVERSE PHOTONIC CRYSTAL

Three-dimensional photonic crystals prepared by self-assembly method from polymethyl methacrylate colloids are infiltrated with zinc oxide (ZnO) prepared using sol–gel technique. The polymer template is removed by chemical method and heat treatment to obtain inverse photonic crystals of ZnO. The inverse crystal fabricated by the chemical method is further heated at high temperature and the X-ray diffraction establishes the presence of single-crystalline ZnO. The photoluminescence is recorded from the inverse photonic crystals by exciting them with He–Cd laser at 325 nm. The as-prepared inverse crystals show only UV emission while the inverse crystal obtained by the chemical route and treated at high temperature shows the visible emission due to oxygen vacancy defects.

Waveguide patterning on thin film and self-assembled photonic crystals

Waveguide (WG) structures are patterned on polymeric thin films and on three-dimensional colloidal photonic crystals (PhC). Different techniques such as direct laser writing, electron beam lithography (EBL), optical lithography and femto-second laser writing are used to pattern the WG structures on PPR, polymethyl methacrylate (PMMA) and SU-8 photoresists. All these techniques are found to be successful in writing the WG structures on thin films. Air channel WGs are formed by EBL and direct laser writing techniques on PMMA and PPR, respectively. The air channel is further infiltrated with a higher index zinc oxide by sol-gel chemistry for guidance of light by total internal reflection. The structure written on SU-8 by optical lithography and on PMMA thin film by femto second laser writing resulted in the ridged WG structures where the guidance is possible by total internal reflection. The WG writing is also successfully carried out by electron beam lithography and femto second laser writing on PhCs fabricated from PMMA and polystyrene colloidal particles using inward growing self-assembly method. Unlike the earlier WG structures, the light guidance is possible due to photonic band gap effect in PhC WG, only for the wavelengths that lie within the stop band of the PhC. The quality of the written structures is characterized using images from scanning electron microscope, atomic force microscope and optical microscope. For optical characterization, a diode laser beam is successfully guided through the WG structure fabricated on PMMA PhC by EBL and on SU-8 thin film by optical lithography method.