The use of fs lasers to directly write phonic structures <i>inside</i> a glass has great potential as a fabrication method for three-dimensional all-optical integrated components. The ability to use this technique with different glass compositions --specifically tailored for a specific photonics application -- is critical to its successful exploitation. Consequently, it is important to understand how glass composition effects waveguide fabrication with fs laser pulses and how different glasses are structurally modified after exposure to fs laser pulses. We have used confocal laser spectroscopy to monitor the changes in glass structure that are associated with waveguide fabrication. Using a low power continuous wave (cw) Ar laser as excitation source we have measured both Raman and fluorescence spectra of the modified regions. Raman spectroscopy provides us with information on the network structure, whereas fluorescence measurements reveal the presence of optically active point defects in the glass. In this paper we review our work on fs-laser fabrication and characterization of photonic structures in glass and discuss the effect of glass composition on processing parameters and structural modification.
This paper is an overview of research in my group over the past 10 to 15 years. Our work has explored the synthesis, assembly, nanostructure characterization, and optical properties of a wide variety of semiconductor quantum dots in II-VI, III-V, and other systems. Our early work was aimed at applications in photonics and fiber optics but more recently we have worked on biomolecular engineering with quantum dots for immunoassays and related interests. The chosen hosts for the quantum dots are glasses, polymers and sol-gel prepared xerogels. The synthesized quantum dot nanocomposites have been most commonly characterized by X-ray diffraction (XRD), atomic force microscopy (AFM), and high resolution transmission electron microscopy (HRTEM). Absorption and photoluminescence (PL) spectroscopy data are also reported on selected quantum dot samples. A short summary of ongoing research in our laboratory on magnetic iron oxide nanocrystals for biological applications is also presented.
New fluorophores that can be excited using visible or near-infrared radiation are of considerable interest for application in environmental and complex bioassays, where background fluorescence is exacerbated by ultra-violet or blue excitation. Useful labels for biomolecules include infrared emitting semiconductor nanoparticles that can be blue-shifted into the near-infrared and visible through quantum confinement effects, oxides of iron, and rare earth oxides. In this work, the synthesis of 6 nm average diameter lead selenide nanocrystals (well below the Bohr exciton diameter of 92 nm) through a reverse micelle technique; and the synthesis of iron and europium oxides with particles less than 5 nm in diameter by pulsed laser ablation is reported. The europium oxide nanoparticles' emission showed a large Stokes shift (144 nm or 216 nm, depending on excitation wavelength); a narrow, symmetric emission line at 610 nm (FWHM of 8 nm); and long lifetime (300 μs). The Eu<sub>2</sub>O<sub>3</sub> nanoparticles, which were coated with silica for functionalization, displayed a greatly enhanced sensitivity over a conventional ELISA (0.025 ng ml<sup>-1</sup> vs. 0.1 ng ml<sup>-1</sup>) when run in an atrazine immunoassay.
Refractive index changes have been induced inside bulk fused silica by using femtosecond (fs) laser pulses tightly focused inside the material. Waveguides have been fabricated inside the glass by scanning the glass with respect to the focal point of the laser beam. The refractive index change is estimated to be ~ 10<sup>-4</sup>. Other more complex three-dimensional structures have also been fabricated (curved waveguides, splitters, and interferometers). We also report on fluorescence spectroscopy of the fs-modified fused silica using a confocal microscopy setup. Using a 488 nm excitation source, a fluorescence at 630 nm is observed from the modified glass, which is attributed to the presence of non-bridging oxygen hole center (NBOHC) defects created by the fs pulses. The fluorescence decays with prolonged exposure to the 488 nm light, indicating that the defects are being photobleached by the excitation light.
Dye TPB has been successfully introduced into polymeric multilayer films by means of LB technique without any chemical modification. X-ray diffraction and optical absorption data indicate that the films have ordered structure with a period of about 5.8 nm that is similar to a superlattice. The TPB doped polymeric LB films have also been used to fabricate an electroluminenscence (EL) device that emits in the blue region at room temperature. Compared with cast films, the photoluminescence and electroluminescence spectra of the TPB-doped LB films show that the exciton shifts to higher energy and that the full width at half maximum of the emission peak becomes narrower.
GeS2 is known to be a good chalcogenide glass former with a transmission cut off at 1 1 . tm and has been studied for its application in the mid infrared region. The rare earth sulfides (La-Er) form reasonably good and stable glasses when mixed with other chalcogenides such as Ga2S3. In this work glass formation was studied in the GeS2- La2S3 system. Two compositions containing 60 mol and 92. 5 mol GeS2 respectively were studied and the effect of composition on the microstructure and thermal stability of these glasses were investigated. Microstructural analyses were performed on the as prepared and heat treated glasses using TEM and SEM/EDXA. Glasses rich in GeS2 exhibited primary (6-80 nm) and secondary (3-13 nm) phase seperation at the molecular level. Differential thermal analysis performed on these glasses indicated glass transition temperatures (Tg) of 590C and 420C for the two compositions studied. The glasses were stable and the (Tg) was observed to decrease with increasing contents of GeS2 in these glasses. 1.