Christopher Baird

Application of multi-subband self-consistent energy balance method to terahertz quantum cascade lasers

We present simulation results for two resonant phonon terahertz quantum cascade lasers using a self-consistent energy balance model, which determines the electron temperature for each conduction subband. These temperatures, along with the electron populations and scattering rates, are determined in a manner similar to previously published models. However, the presented model is able to converge through the use of an algorithm that appears to be robust. The predicted individual subband electron temperatures, population densities and scattering rates are compared to previously published Monte Carlo and experimental studies for both lasers, where subband temperature variations were observed. These quantities were chosen since they provided the only comparison to modeling results from other studies.

Classification of targets using optimized ISAR Euler imagery

Various approaches exist to enable target classification through a decomposition of the polarimetric scattering matrix. Specifically, the Euler decomposition attempts to express the target scattering properties through more physically relevant parameters. Target classification in general has been limited by signature variability and the saturation of images by non-persistent scatterers.1 The Euler decomposition is sensitive to additional parameter ambiguities.2 It will be demonstrated how undesirable ambiguities may be identified and mitigated. Through the analysis of polarimetric ISAR signatures obtained in compact radar ranges at the University of Massachusetts Lowell Submillimeter Technology Laboratory (STL)3,4,5,6 and the U.S. Army National Ground Intelligence Center (NGIC), the cause of non-persistent scatters will be investigated. A proper characterization of non-persistence should lead to better optimization of the Euler decomposition, and thus improve target classification.

Development and assessment of a complete ATR algorithm based on ISAR Euler imagery

The Euler decomposition, when applied to the polarization scattering matrix, attempts to extract phenomenological information about the scattering target. Because the Euler parameters constitute a more physically relevant set of parameters than the traditional HH-VV ISAR representations, they have potential to improve ATR performance. The Euler parameters usefulness in target recognition, however, is effected by several layers of signature variability. Unfortunately, many of the variability layers are often omitted in a typical ATR study. A complete ATR algorithm was therefore developed that allows for all layers of variability and requires no previous knowledge of the targets position, orientation, or average reflectivity. The complete ATR algorithm was then used to assess the effectiveness of Euler ISAR imagery in target recognition when all layers of variability are considered. The general approach and sub-methods used to construct the complete ATR system will be presented, including the methods to determine the targets orientation, registration, and to compare it to a library of pre-rendered target images. Finally, the performance of the Euler parameters in target recognition using the complete ATR algorithm will be presented.

Exploitation of ISAR imagery in Euler parameter space

Efforts are being made to exploit the full-polarimetric radar scattering nature of ground targets to extract maximum information, enabling target identification and classification. These efforts have taken varied approaches to decomposing the polarimetric scattering matrix into more meaningful, phenomenological parameter spaces. The Euler parameters have potential value in target classification but have historically met with limited success due to ambiguities that arise in the decomposition as well as the parameters sensitivity to noise and target movement. Using polarimetric ISAR signatures obtained from stationary targets in compact radar ranges at the University of Massachusetts Lowell Submillimeter Technology Laboratory (STL)1,2,3,4 and the U.S. Army National Ground Intelligence Center (NGIC), correlation studies were performed in the Euler parameter space to assess to its impact on improving target classification. Methods for deriving explicit transform equations that minimize ambiguities will be presented, as well as the results of the correlation studies.

Optimization of semi-insulating surface-plasmon waveguides within terahertz QCL's using computational models

The possibility of a compact source of coherent terahertz radiation is being realized through the development of quantum cascade lasers (QCLs). These lasers consist of a semiconducting heterostructure active region and an internal waveguide that make intraband lasing transitions possible. The use of terahertz QCLs in promising applications such as medical imaging, defense, and security is currently limited by low output laser power. Systematic optimization of the QCLs waveguide reduces mode losses, improves confinement, and increases output power. Waveguide optimization is especially important for lasers operating at low terahertz frequencies where semi-insulating surface-plasmon waveguide performance degrades significantly. Prediction codes have been developed that systematically optimize semi-insulating surface-plasmon waveguides. The methods and results of these optimizations will be presented for a full suite of terahertz QCL waveguides at different frequencies. The use of the optimization code to investigate graded-doping waveguide structures will also be presented.

One-half milliwatt 2.31 THz continuous-wave QCL operating at 77K

THz semi-insulating surface plasmon waveguide QCLs based on the bound-to-continuum design have been developed with 2.31 THz output powers of ~0.5 milliwatt from a single facet, input powers of ~5 watts, and threshold current densities of 117 A/cm2 operating continuous wave at 77K. These results were achieved by depositing alloy metal on both contact layers, only annealing the bottom metal layers, and thinning the substrate thickness to ~170 μm to assure good heat dissipation. The structure was based on a previously published 2.83 THz design that was scaled to emit at 2.31 THz. The demonstration of this high temperature, high power laser with low input power enables its use in compact, coherent THz transceivers for heterodyne detection with liquid nitrogen cooling.

The effects of various approximations on electron-electron scattering calculations in QCLs

Quantum cascade lasers (QCLs) employ the mid- and far-infrared intersubband radiative transitions available in semiconducting heterostructures. Through the precise design and construction of these heterostructures the laser characteristics and output frequencies can be controlled. When fabricated, QCLs offer a lightweight and portable alternative to traditional laser systems which emit in this frequency range. The successful operation of these devices strongly depends on the effects of electron transport. Electron-electron scattering is an essential mechanism involved in electron transport and approximations are often made in finding the electron-electron scattering rate in order to simplify calculations. Results will be presented characterizing various effects which are sometimes ignored in calculating electron-electron scattering rates. These effects include state-blocking, electron screening, temperature dependence, as well as the inclusion of all possible transitions that can occur in three periods of the QCL active region. These effects will be presented in the context of several QCL active region designs, including those grown and fabricated at the University of Massachusetts Lowell Photonics Center.

The effects of electron temperature in terahertz quantum cascade laser predictions

Quantum cascade lasers (QCLs) employ the mid- and far-infrared intersubband radiative transitions available in semiconducting heterostructures. Through the precise design and construction of these heterostructues the laser characteristics and output frequencies can be controlled. When fabricated, QCLs offer a lightweight and portable alternative to traditional laser systems which emit in this frequency range. The successful operation of these devices strongly depends on the effects of electron transport. Studies have been conducted on the mechanisms involved in electron transport and a prediction code for QCL simulation and design has been completed. The implemented approach utilized a three period simulation of the laser active region. All of the wavefunctions within the simulation were included in a self-consistent rate equation model. This model employed all relevant types of scattering mechanisms within three periods. Additionally, an energy balance equation was studied to determine the temperature of electron distributions separately from the lattice temperature. This equation included the influence of both electron-LO phonon and electron-electron scattering. The effect of different modelling parameters within QCL electron temperature predictions will be presented along with a description of the complete QCL prediction code.

The effects of a common approximation used in the polarizability calculations of electron-electron subband screening in quantum well devices

Quantum well devices can be investigated through the use of computational predictions of the electron-electron subband scattering rates. A high-accuracy prediction requires the calculation of the quantum well electron polarizability. An approximation is sometimes made that renders the integral in the polarizability equation analytically solvable. This study comprehensively quantifies the error and the limitations introduced through the use of this approximation. The approximate polarizability equation is found to introduce significant error for certain scenarios. Furthermore, the approximate polarizability equation is found to fail to give any numeric answer at all for relatively high temperatures and low electron densities.

Terahertz ISAR and x-ray imaging of wind turbine blade structures

During the manufacture of wind turbine blades, internal defects can form which negatively affect their structural integrity and may lead to premature failure. The purpose of this research was to conduct preliminary testing of nondestructive evaluation techniques that have the potential to scale up to larger areas. The techniques investigated were: Terahertz frequency fully-polarimetric inverse synthetic aperture radar (ISAR), and x-ray imaging. The terahertz ISAR technique employed standard polarimetric radar cross-section processing, and additionally applied an optimized polarimetry transformation known as the Euler transformation. Also, image back-rotation and compositing algorithms were used to combine multiple ISAR images into a single image to aid in defect detection. ISAR data were collected using a frequency modulated continuous wave 100 GHz radar system. The x-ray technique utilized a commercial airport cargo x-ray scanner. Multiple fiberglass samples with defects representative of manufacturing wind turbine blade defects were investigated using each of the techniques. Out-of-plane defects and resin dry patches were the primary defects of interest in these samples. Images were created of each sample using each of the techniques. Comparing these images with defect diagrams of the samples indicated that these techniques could effectively indicate the presence of certain defects.