Papers

AVS2001 Session PH-ThM: Photonic Materials: Studies on the Nano Scale

Thursday, November 1, 2001 8:20 AM in Room 120

Thursday Morning

Start	Invited?	Item
8:20 AM		PH-ThM-1 Chalcogenide Glasses as Nonlinear Optical Materials and Their Application in Optical Communication Systems H.Y. Hwang (Bell Laboratories, Lucent Technologies) Chalcogenide glasses exhibit an enhanced Kerr nonlinear refractive index n2, which can be used in a number of applications such as optical switching, pulse shaping, etc. We have explored a range of bulk chalcogenide glasses for optimization at communications wavelengths (~1.55 ?m), motivating current experiments in thin film planar waveguides as well as fiber devices. Low-loss single mode waveguides have been fabricated in a number of ways, and their linear and nonlinear optical properties will be presented. Strong Bragg gratings have also been fabricated using replica molding from holographically generated templates. Finally, the use of chalcogenide films as lateral waveguide cladding of semiconductor optical devices will be described.
9:00 AM		PH-ThM-3 Crystallization Kinetics in Chalcogenide Glasses S. Ziegler (Aachen University Technology, Germany) Chalcogenide alloys are frequently used for rewritable optical data storage where submicron sized regions of the film are switched between an amorphous and a crystalline state. Since the kinetics of this process are crucial for the success of the technology employ a combintion of different techniques to study the transformation. The activation energy for crystallization is determined by measurements of the electrical resistance upon heating. The structural changes upon heating is derived from x-ray diffraction while x-ray reflectometry is employed to measure the density change upon crystallization. For a number of different alloys density and thickness changes upon crystallization between 5 and 10% are observed. These density changes are accompanied by irreversible stress changes which could possibly limit the lifetime of the films. Nevertheless the measured stress change is much smaller than the stress change expected for a purely elastic deformation. This can be explained by a viscous flow of the amorphous pulse which can also account for the changes of film topography upon crystallization is observed by atomic force microscopy. Microscopic measurements of the crystallization kinetics reveal of correlation with the film stoichiometry, in particular the relative abundance of Ge-Te bonds in GeSbTe alloys. Concepts of combinatorial synthesis are employed to identify phase change materials with fast transformation kinetics.
9:20 AM		PH-ThM-4 Single Quantum Dots as Tunable Artificial Atoms D. Hessman, M. Holm, J. Persson, M.-E. Pistol, C. Pryor, L. Samuelson (Lund University, Sweden) We present micro-photoluminescence studies of self-assembled InP quantum dots (QDs) embedded in GaInP. The QDs are pyramid shaped and usually about 15 nm high, with a slightly elongated base of about 40 by 50 nm. There are however also smaller QDs with similar lateral extension but with a considerably smaller height. The change in size is, as expected, accompanied by a change in quantum confinement with a corresponding change in emission energy. In addition, there is a transition from a single sharp emission peak for the smallest dots to several 1 meV broad emission lines emitted over a 50 meV energy range for the largest dots. The reason for this behaviour is unintentional doping in the barrier material, resulting in electron accumulation in the QDs. This gives rise to emission in an energy range corresponding to the energy range occupied by these electrons. Larger QDs accumulate more electrons and thus emit over a larger energy range. For the largest dots, the number of electrons is large enough that Coulomb-induced dephasing sets in, resulting in a dramatic line-width broadening. By depositing a semi-transparent Schottky gate on top of the sample, photoluminescence spectra of single QDs can be obtained as a function of bias. Varying the bias, the number of electrons in a large QD is tuned in the range 0-15. For biases such that only a few electrons are present in the QD, the Coulomb-induced dephasing is reduced and the originally 1 meV broad lines split up into sharp lines. We conclude that InP/GaInP is a very interesting system, with QDs acting as tunable artificial atoms, controllable both by size and external bias.
10:20 AM		PH-ThM-7 Dynamic Response of the Electro-optic Effect in Epitaxial Ferroelectric Thin Films B.W. Wessels (Northwestern University) Ferroelectric thin films are of considerable promise for use as electro-optic, and non-linear optical materials. Electro-optic (EO) waveguide devices fabricated from thin films offer several advantages over bulk material including lower driving voltages, smaller size, higher modulation speeds and the potential for monolithic integration. Recently we reported ferroelectric thin film electro-optic modulators that operate at frequencies up to 20 GHz. We have also investigated the dynamic response of the electro-optic effect in thin film BaTiO₃ and KNbO₃ . The dynamic response has a temporal dependence given by the expression t^-mexp(-Bt)ⁿ. Measurements of the film birefringence, polarization and dielectric transients show qualitative agreement over 11 orders of magnitude in time. The observed dependence is attributed to the dynamic response of ferroelectric domains.
11:00 AM		PH-ThM-9 Two-stage Growth of Patterned Epitaxial Lithium Niobate for Photonic Application V. Joshkin (University of Wisconsin-Madison); K. Dovidenko, S. Oktyabrsky (NYS Center for Advanced Thin Film Technoogy); D. Saulys, T.F. Kuech, L. McCaughan (University of Wisconsin-Madison) LiNbO3 is an ideal material for linear and nonlinear photonic crystals. Potential commercial applications have long been frustrated by the chemical stability of this material. We present a new two-stage growth method for fabricating patterned crystalline LiNbO3 structures for photonic applications. The method is based on physical and chemical properties of amorphous and polycrystal LiNbO3 films grown by high pressure chemical vapor deposition (CVD) from metal alkoxide precursors. In the first stage, the CVD technique is used to deposit amorphous or polycrystalline LiNbO3 films on a crystalline substrate at high deposition rates (~2micron/hr). Patterned structure can now be formed after this first stage using a rapid wet or dry etching of amorphous LiNbO3 (up to 6micron/min depending on etching regimes). In the second stage, a post-growth anneal at high temperature(900°C- 1100°C) converts the film to single crystal LiNbO3. Under the proper annealing conditions, the LiNbO3 bulk self-diffusivity dominates the surface mobility, allowing epitaxial films that maintain the shape of micron-size pattern. These patterned structures are characterized by AES, SEM, HRTEM and DXRD. The effect of substrate on film quality is investigated. Lift-off processing on films grown by two-stage growth technique is demonstrated. Comparison of high vacuum chemical beam epitaxy with high pressure CVD from alkoxides is performed.