An optical fiber-taper probe for wafer-scale microphotonic device characterization

A small depression is created in a straight optical fiber taper to form a local probe suitable for studying closely spaced, planar microphotonic devices. The tension of the "dimpled" taper controls the probe-sample interaction length and the level of noise present during coupling measurements. Practical demonstrations with high-Q silicon microcavities include testing a dense array of undercut microdisks (maximum Q = 3.3x10^6) and a planar microring (Q = 4.8x10^6).Comment: 8 pages, 5 figures, for high-res version see http://copilot.caltech.edu/publications/index.ht

Travelling waves in hyperbolic chemotaxis equations

Mathematical models of bacterial populations are often written as systems of partial differential equations for the densities of bacteria and concentrations of extracellular (signal) chemicals. This approach has been employed since the seminal work of Keller and Segel in the 1970s [Keller and Segel, J. Theor. Biol., 1971]. The system has been shown to permit travelling wave solutions which correspond to travelling band formation in bacterial colonies, yet only under specific criteria, such as a singularity in the chemotactic sensitivity function as the signal approaches zero. Such a singularity generates infinite macroscopic velocities which are biologically unrealistic. In this paper, we formulate a model that takes into consideration relevant details of the intracellular processes while avoiding the singularity in the chemotactic sensitivity. We prove the global existence of solutions and then show the existence of travelling wave solutions both numerically and analytically

Adiabatic self-tuning in a silicon microdisk optical resonator

We demonstrate a method for adiabatically self-tuning a silicon microdisk resonator. This mechanism is not only able to sensitively probe the fast nonlinear cavity dynamics, but also provides various optical functionalities like pulse compression, shaping, and tunable time delay

Lasers incorporating 2D photonic bandgap mirrors

Semiconductor lasers incorporating a 2D photonic lattice as a one end mirror in a Fabry-Perot cavity are demonstrated. The photonic lattice is a 2D hexagonal close-packed array with a lattice constant of 220 nm. Pulsed threshold currents of 110 mA were observed from a 180 μm laser

Two-dimensional photonic band-gap mirrors at 850 and 980 nm

Summary form only given. Photonic band-gap (PBG) crystals can be fabricated in semiconductor devices through the etching of patterns of holes in the device, resulting in a periodic dielectric structure. One of the more practical uses of photonic crystals in optoelectronic devices is for thin, high-reflectivity mirrors. The use of hexagonal arrays of etched circular holes results in a 2-D photonic band-gap mirror that can be tuned to a specific wavelength by varying the hole radius and the lattice spacing. 2-D mirror characterization is performed by evaluating the light emission from an active waveguide

Single quantum dot spectroscopy using a fiber taper waveguide near-field optic

Photoluminescence spectroscopy of single InAs quantum dots at cryogenic temperatures (~14 K) is performed using a micron-scale optical fiber taper waveguide as a near-field optic. The measured collection efficiency of quantum dot spontaneous emission into the fundamental guided mode of the fiber taper is estimated at 0.1%, and spatially-resolved measurements with ~600 nm resolution are obtained by varying the taper position with respect to the sample and using the fiber taper for both the pump and collection channels.Comment: 4 pages, 3 figure

Lasers incorporating two-dimensional photonic crystal mirrors

Photonic bandgap crystals are expected to be of use in defining microcavities for modifying spontaneous emission and as highly reflective mirrors. There are several reports of microfabricating one-dimensional structure. Here, we describe the incorporation of a microfabricated two-dimensional photonic lattice in an edge-emitting semiconductor laser structure. We demonstrate laser operation in a cavity formed between a cleaved facet and a microfabricated periodic lattice

Social Relationships and Self-Directed Behavior in Hamadryas Baboons (Papio hamadryas hamadryas)

Self-directed behavior, such as self-scratching and self-grooming, is a behavioral indicator of anxiety in nonhuman primates. Patterns of self-directed behavior are used to identify social and environmental factors related to primate anxiety. This study explored the social context in which individuals in a captive group of hamadryas baboons (Papio hamadryas hamadryas) exhibited self-directed behavior. Self-directed behavior in a partner’s presence was predicted to increase with relationship insecurity. More than 130 hours of behavioral observations were conducted on 12 baboons. Self-directed and social behavior were recorded with focal sampling to determine each animal’s self-directed behavior rate in the presence of each other group member. These data were also used to calculate variation in response to approach over time, a newly proposed measure of relationship insecurity. Aggressive and submissive behavior were recorded ad libitum to construct a dominance hierarchy. High-ranking animals were found to exhibit significantly higher rates of self-directed behavior than low-ranking animals. Adults also exhibited higher rates of self-directed behavior than juveniles. Self-directed behavior rate increased with the relative dominance rank of the social partner in close proximity. Self-directed behavior rate also increased with the overall amount of aggression the social partner exhibited over the course of the study. No relationship was found between self-directed behavior rate in a partner’s presence and relationship insecurity. Results suggest that baboons in this group experienced anxiety related to their own dominance rank and that of their social partners. Captivity and the steeply linear nature of the group’s dominance hierarchy may have prevented any possible relationship insecurity effects from emerging. Variation in response to approach over time did not positively correlate with other relationship variables, suggesting it may serve as an independent and viable measure of relationship security

Surface Encapsulation for Low-Loss Silicon Photonics

Encapsulation layers are explored for passivating the surfaces of silicon to reduce optical absorption in the 1500-nm wavelength band. Surface-sensitive test structures consisting of microdisk resonators are fabricated for this purpose. Based on previous work in silicon photovoltaics, coatings of SiNx and SiO2 are applied under varying deposition and annealing conditions. A short dry thermal oxidation followed by a long high-temperature N2 anneal is found to be most effective at long-term encapsulation and reduction of interface absorption. Minimization of the optical loss is attributed to simultaneous reduction in sub-bandgap silicon surface states and hydrogen in the capping material.Comment: 4 pages, 3 figure