Chris Coretsopoulos

CONSUMPTION OF THE MOLECULAR OXYGEN IN POLYMERIZATION SYSTEMS USING PHOTOSENSITIZED OXIDATION OF DIMETHYLANTHRACENE

ABSTRACT It is well known that the presence of oxygen in free radical polymerization systems leads to an inhibition period and a lowered ultimate conversion. In this contribution, we report a method for consuming molecular oxygen photochemically before the polymerization takes place, thereby allowing the reaction to proceed in an oxygen-free environment. The method is based on the generation of singlet oxygen by reaction of the ground state oxygen with the excited triplet state of the singlet oxygen generator (a porphyrin, Znttp). The singlet oxygen is then consumed by reaction with a second compound (the singlet oxygen trapper, dimethylanthracene). The possible factors that might affect the efficiency of the singlet oxygen generation/trapping processes were discussed and the effectiveness of a dimethylanthracene/Znttp combination for consumption of oxygen was investigated in two acrylate systems of different viscosity.

Cascaded active regions in 2.4μm GaInAsSb light-emitting diodes for improved current efficiency

By cascading multiple GaInAsSb active regions, the authors have fabricated 2.4μm light-emitting diodes that, for a given light output, operate at reduced current and higher voltage, which can be advantageous for battery-powered sensor applications. Tunnel heterojunctions separating emission regions add no measurable series resistance. Devices are demonstrated at room temperature with continuous wave output.

Optical microsensor for continuous glucose measurements in interstitial fluid

Tight control of blood glucose levels has been shown to dramatically reduce the long-term complications of diabetes. Current invasive technology for monitoring glucose levels is effective but underutilized by people with diabetes because of the pain of repeated finger-sticks, the inconvenience of handling samples of blood, and the cost of reagent strips. A continuous glucose sensor coupled with an insulin delivery system could provide closed-loop glucose control without the need for discrete sampling or user intervention. We describe an optical glucose microsensor based on absorption spectroscopy in interstitial fluid that can potentially be implanted to provide continuous glucose readings. Light from a GaInAsSb LED in the 2.2-2.4 μm wavelength range is passed through a sample of interstitial fluid and a linear variable filter before being detected by an uncooled, 32-element GaInAsSb detector array. Spectral resolution is provided by the linear variable filter, which has a 10 nm band pass and a center wavelength that varies from 2.18-2.38 μm (4600-4200 cm-1) over the length of the detector array. The sensor assembly is a monolithic design requiring no coupling optics. In the present system, the LED running with 100 mA of drive current delivers 20 nW of power to each of the detector pixels, which have a noise-equivalent-power of 3 pW/Hz1/2. This is sufficient to provide a signal-to-noise ratio of 4500 Hz1/2 under detector-noise limited conditions. This signal-to-noise ratio corresponds to a spectral noise level less than 10 μAU for a five minute integration, which should be sufficient for sub-millimolar glucose detection.

Controller for a Continuous Near Infrared Glucose Sensor

The design and development of a controller for a near infrared glucose sensor is described. The sensing technology involves the sampling of interstitial fluid in a micro fabricated chamber and measurement of the absorbance of the fluid in a non-destructive and reagent free manner. The glucose levels are estimated based on the absorbance data. This new technology relies on the unique optical characteristics of glucose in a near infrared spectrum and will be used as the sensing technology in a feedback controlled insulin delivery system for the in situ treatment of diabetes. The sensor element will be implanted in the subcutaneous tissues of the human body and the controller will enable the operation of the sensor for months with minimal user intervention. First generation prototype of the controller has been developed and implemented. Currently, thorough testing of the first generation prototype system is in progress and work on a second generation prototype based on ZigBee wireless technology is underway

PIN versus PN homojunctions in GaInAsSb 2.0-2.5 micron mesa photodiodes

The performances of a pin versus a pn structure from GaInAsSb materials operating at room temperature are compared both from a theoretical point of view and experimentally. Theoretically, it is found in materials limited by generation-recombination currents, pn junctions have a higher D* than pin junctions. The thinner depletion region of pn junctions results in a lower responsivity but a higher dynamic resistance, giving an overall higher D* compared to a pin structure. A series of five p+pn+ Ga0.80In0.20As0.18Sb0.82 detector structures latticed matched to GaSb substrates and with 2.37 μm cut off wavelength were grown by molecular beam epitaxy and processed into variable size mesa photodiodes. Only the doping of the absorbing (p) region was varied from sample to sample, starting with nominally undoped (~1x1016 cm-3 pbackground doping due to native defects) and increasing the doping until a p+n+ structure was attained. Room temperature dynamic resistance-area product R0A was measured for each sample. A simple method is presented and used to disentangle perimeter from areal leakage currents. All five samples had comparable R0As. Maximum measured R0A was 30 Ω-cm2 in the largest mesas. Extracted R0As in the zero perimeter/area limit were about ~50 Ω-cm2 (20-100 Ω-cm2) for all samples. Within uncertainty, no clear trend was seen. Tentative explanations are proposed.