J. T. Olesberg

Auger recombination in narrow-gap semiconductor superlattices incorporating antimony

A comparison is performed between measured and calculated Auger recombination rates for four different narrow-gap superlattices based on the InAs/GaSb/AlSb material system. The structures are designed for optical or electrical injection for mid-infrared laser applications, with wavelengths ranging from 3.4 to 4.1 μm. The electronic band structures are computed employing an accurate 14-band restricted basis set (superlattice K⋅p) methodology that utilizes experimental information about the low-energy electronic structure of the bulk constituents. The superlattice band structures and their associated matrix elements are directly employed to compute Auger recombination rates. Varying amounts of Auger recombination suppression are displayed by the various superlattices as compared to bulk mid-infrared systems. The greatest disagreement between theory and experiment is shown for the structure predicted to have the most Auger suppression, suggesting the suppression is sensitive either to theoretical or growth u...

Tunable Laser Diode System for Noninvasive Blood Glucose Measurements

Optical sensing of glucose would allow more frequent monitoring and tighter glucose control for people with diabetes. The key to a successful optical noninvasive measurement of glucose is the collection of an optical spectrum with a very high signal-to-noise ratio in a spectral region with significant glucose absorption. Unfortunately, the optical throughput of skin is low due to absorption and scattering. To overcome these difficulties, we have developed a high-brightness tunable laser system for measurements in the 2.0–2.5 μm wavelength range. The system is based on a 2.3 μm wavelength, strained quantum-well laser diode incorporating GaInAsSb wells and AlGaAsSb barrier and cladding layers. Wavelength control is provided by coupling the laser diode to an external cavity that includes an acousto-optic tunable filter. Tuning ranges of greater than 110 nm have been obtained. Because the tunable filter has no moving parts, scans can be completed very quickly, typically in less than 10 ms. We describe the performance of the present laser system and avenues for extending the tuning range beyond 400 nm.

Electron-spin decoherence in bulk and quantum-well zinc-blende semiconductors

A theory for longitudinal (T1) and transverse (T2) electron spin coherence times in zincblende semiconductor quantum wells is developed based on a non-perturbative nanostructure model solved in a fourteen-band restricted basis set. Distinctly different dependences of coherence times on mobility, quantization energy, and temperature are found from previous calculations. Quantitative agreement between our calculations and measurements is found for GaAs/AlGaAs, InGaAs/InP, and GaSb/AlSb quantum wells.

Room-temperature electron spin relaxation in bulk InAs

Polarization-resolved, subpicosecond pump–probe measurements at a wavelength of 3.43 μm are used to determine the electron spin relaxation time T1 in bulk InAs at room temperature. The measured T1 of 19±4 ps is in excellent agreement with the theoretical value of 21 ps, which is obtained from a nonperturbative calculation based on the D’yakonov–Perel’ mechanism of precessional spin relaxation [M. I. D’yakonov and V. I. Perel’, Sov. Phys. JETP 38, 177 (1974)].

Selectivity assessment of noninvasive glucose measurements based on analysis of multivariate calibration vectors.

Background: Selectivity is paramount for the successful implementation of noninvasive spectroscopic sensing for the painless measurement of blood glucose concentrations in people with diabetes. Selectivity issues are explored for different multivariate calibration models based on noninvasive near-infrared spectra collected from an animal model. Methods: Noninvasive near-infrared spectra are collected through a fiber-optic interface attached to a thin fold of skin on the back of an anesthetized laboratory rat while glucose levels are varied in a controlled manner. Results and Discussion: Partial least-squares (PLS) calibration models are generated from noninvasive spectra collected during a single, 2-hour blood glucose transient. Calibration vectors are compared for optimized PLS calibration models created with correct and incorrect assignments of glucose concentrations to each noninvasive spectrum. Although both PLS models appear functional and seem capable of predicting glucose concentrations accurately during this transient, only the model generated from correct glucose assignments gives a credible calibration vector. When correct glucose assignments are used, the PLS calibration vector matches the corresponding net analyte signal calibration vector. No similarity in these calibration vectors is evident when incorrect glucose assignments are used. Conclusions: Glucose-specific spectral information is present within noninvasive near-infrared spectra collected from a rat model using a transmission geometry. Apparently functional, yet incorrect, calibration models can be generated, and the propensity to create such false PLS calibration models calls into question the validity of past reports. An analysis of calibration vectors can provide valuable insight into the chemical basis of selectivity for multivariate calibration models of complex systems.

Theoretical performance of mid-infrared broken-gap multilayer superlattice lasers

We present calculations of the differential gain and threshold current densities for a 3.7 μm multiple quantum well structure consisting of a “well” composed of several periods of an InAs/InGaSb superlattice alternating with a quinternary alloy “barrier.” We find serious limitations to the optical properties of active regions composed of these multiple quantum wells, and propose a four-layer superlattice structure which corrects these problems.

III-V interband 5.2 μm laser operating at 185 K

We report the operation of a III-V interband laser at a wavelength beyond 5 μm and temperatures above 90 K. The active region consists of a strain compensated broken gap four layer superlattice of InAs/Ga0.6In0.4Sb/InAs/Al0.3Ga0.42In0.28As0.5Sb0.5 grown by molecular beam epitaxy. The maximum operating temperature under 2.01 μm pulsed optical excitation was 185 K at a wavelength of 5.2 μm. The peak pump intensity at the 80 K threshold was 62 kW/cm2, and the characteristic temperature (T0) of the threshold intensity was 37 K. This T0 is comparable to the best observed values for 3–4.5 μm lasers based on the InAs/GaInSb material system.

Differential gain, differential index, and linewidth enhancement factor for a 4 μm superlattice laser active layer

We describe temporally and spectrally resolved measurements of the material differential gain, differential refractive index, and linewidth enhancement factor for a multilayer superlattice intended for use in midwave-infrared semiconductor lasers. We find good agreement between measured quantities and theoretical predictions based on a superlattice K⋅p formalism. The superlattice was designed for suppression of Auger recombination and intersubband absorption, and we find that the strategies employed in this process result in other characteristics that are desirable in a semiconductor laser gain medium. Specifically, for carrier densities and wavelengths appropriate to threshold in an optimized cavity configuration, this structure has a differential gain of approximately 1.5×10−15 cm2, a value comparable to that reported for near-infrared strained quantum wells. The peak gain and peak differential gain are nearly spectrally coincident, leading to a small value for the differential index. The large differen...

MBE-grown high-efficiency GaInAsSb mid-infrared detectors operating under back illumination

This paper describes molecular beam epitaxial growth, processing and room temperature characterization of lattice-matched GaInAsSb mid-infrared detectors on GaSb substrates for room temperature operation. For the first time, we demonstrate GaInAsSb detectors operating under back-illumination, a critically important geometry for flip-chip-mounted focal plane arrays, and achieve performance equal or superior to front-illuminated detectors. Very high quantum efficiency and flat spectral response are achieved for the back-illuminated detectors due to improved carrier collection efficiency, photon recycling and reduced carrier recombination. In situ RHEED intensity oscillations and post-growth XRD are used for coarse and fine tuning of GaInAsSb lattice matching, respectively.

Advanced near‐infrared monitor for stable real‐time measurement and control of Pichia pastoris bioprocesses

Near‐infrared spectroscopy is considered to be one of the most promising spectroscopic techniques for upstream bioprocess monitoring and control. Traditionally the nature of near‐infrared spectroscopy has demanded multivariate calibration models to relate spectral variance to analyte concentrations. The resulting analytical measurements have proven unreliable for the measurement of metabolic substrates for bioprocess batches performed outside the calibration process. This paper presents results of an innovative near‐infrared spectroscopic monitor designed to follow the concentrations of glycerol and methanol, as well as biomass, in real time and continuously during the production of a monoclonal antibody by a Pichia pastoris high cell density process. A solid state instrumental design overcomes the ruggedness limitations of conventional interferometer‐based spectrometers. Accurate monitoring of glycerol, methanol, and biomass is demonstrated over 274 days postcalibration. In addition, the first example of feedback control to maintain constant methanol concentrations, as low as 1 g/L, is presented. Postcalibration measurements over a 9‐month period illustrate a level of reliability and robustness that promises its adoption for online bioprocess monitoring throughout product development, from early laboratory research and development to pilot and manufacturing scale operation.