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TRANSCRIPT
Terahertz Imaging: From Quantum Cascade Lasers to
Wind Turbine Defect Detection
Christopher S. BairdUniversity of Massachusetts Lowell
Research PresentationWest Texas A&M University
April 15, 2016
Teaching Background
● Graduate Electromagnetics I: 2008-2016
● Graduate Electromagnetics II: 2009-2016
● Undergraduate Physics II Lab: 2004
● Undergraduate Physics I Lab: 2003
● Exploring the Universe Lab: 2002-2004
Adjunct Faculty at theUniversity of Massachusetts Lowell
Research Background
● Leader of the computational electromagnetics team
● Co-authored dozens of academic publications
● Helped win $47 million in research funding over 10 years ● Collaborated with:
– The Photonics Center at UMass Lowell
– The U.S. Army National Ground Intelligence Center
– The University of Texas at Dallas and WindSTAR
Senior Scientist at theSubmillimeter-Wave Technology Lab (STL) 2007-2015
Research Background
● High School Seniors:Duncan Pettengill, Chris Emma, Tom Socorelis, Betty Makovoz, Alex Petrosillo, Sam O'Brien
● Undergraduate Students: Adam Boudreau, Bryan Crompton, Jareth Arnold, Chris Evans
● Graduate Students: Bob Martin, Karen Uttecht, Phil Slingerland
Mentor of original student research
What are Terahertz Waves?
Electromagnetic waves with a frequencybetween radio waves and infrared
Why Terahertz?
● Able to penetrate many materials
● Non-ionizing, unlike x-rays
● Higher resolution images than radio waves
● Less thermal noise than infrared
● The most under-developed frequency range
Terahertz Research Projects at STL
● Military stealth technology
● CO2 lasers
● Atmospheric spectroscopy
● Imaging of military vehicles
● Materials characterization
BOLD = projects that I worked on directlyImages from: http://www.uml.edu/Research/STL/publications
Terahertz Research Projects at STL
● Concealed weapons detection
● Automatic target recognition
● Cancer detection
● Quantum cascade lasers
● Wind turbine defect detection
BOLD = projects that I worked on directlyImages from: http://www.uml.edu/Research/STL/publications
Research Plan
● Small start-up costs – only 2 or 3 desktop workstations needed (a few thousand dollars)
● New undergraduate students become engaged and productive right away
● Research hours are flexible and can work around students' classwork
● Continue computational research on terahertz quantum cascade lasers and wind turbine defect detection
Benefits of This Research
Quantum Cascade Lasers (QCL's)
● First compact source of coherent terahertz waves
● Usability of QCL's is currently limited by need for cryogenic temperatures (< 80 K) and lack of high power (mW)
● Improvements can be found by understanding the physics of QCL's through computational modeling
What is a QCL?● A stack of alternating
nanoscale semiconductor layers
● This forms a series of quantum wells in the conduction band
● Electrons assume quantum states in the wells
● Applying a voltage slants the quantum wells, causing electrons to cascade down through the layers
Stimulated Emission
● Varying the thickness of the layers allows the electron wave states to be controlled
● If designed properly, one transition possibility will have a higher probability of stimulated photon emission than other transition modes
● If the lower laser state is quickly depopulated, a population inversion results and lasing occurs
Computational Modeling
● Solving the physics equations inside QCL's allows us to investigate the physics and optimize designs
● The equations must be solved numerically and iteratively
● My team developed computer code that solves the equations and accurately predicts the laser frequency, laser power, and electron temperatures of any QCL
Results: Scaling Study● The 2.83 THz Vitiello QCL
structure was used*
● All semiconductor layers were uniformly scaled in size by a scaling factor and the lasing frequency was calculated
● Our computational results (line) matched our experimental results (dots)**
*M. S. Vitiello et al, Appl. Phys. Lett. 90(19), 191115
**X. Qian, C. Baird et al, Terahertz Technology and Applications V, SPIE 8261 (82610K)
**P. Slingerland, Ph.D. dissertation supervised by C. Baird, University of Massachusetts Lowell
Results: Barrier Shift Study● The 1.8 THz
Kumar design was used*
● One barrier was shifted slightly in position
● The computed electron temperature and laser power were improved**
*S. Kumar et al, Nature Physics, 7 (166)
**P. Slingerland, Ph.D. dissertation supervised by C. Baird, University of Massachusetts Lowell
Terahertz QCL Conclusions
● My team has developed a computational code that accurately calculates the physical behavior of terahertz quantum cascade lasers
● The lasing frequency varies non-trivially with the layer width scale factor
● The laser power is improved by reducing the upper laser level's electron temperature
Terahertz QCL Future Work
● Use the code to further investigate how thermal effects restrict lasing power
● Invesitagate novel QCL approaches such as graded barriers
● Develop a method to calculate individual electron level temperatures away from equilibrium
Wind Turbine Defect Detection● Wind turbine blades that are used
in commerical power generation contain subsurface manufacturing defects that lead to failure
● Using coherent terahertz waves and ISAR imaging, defects can be detected
● My team developed and tested new imaging techniques in order to optimize defect detection
Images from: R. Martin, C. Niezrecki, R. Giles, C. Baird, WindSTAR IAB Conference, June 2015
Coherent Terahertz ISAR Imaging● ISAR images are formed by
acquiring coherent continous-wave scattering measurements at several frequencies and object rotations
● The data is then 2D Fourier transformed to yield spatial images
● ISAR images are acquired in four polarization channels: HH, HV, VH, and VV
ISAR Image CompositingThe individual ISAR images of the wind turbine samples did not capture the defects well. Therefore, my team developed an image compositing algorithm that combines many ISAR images taken at different angles
single image 8 images composited 360 images composited
Images from: R. Martin, C. Baird, R. Giles, A. Schoenberg, C. Niezrecki, Smart Materials and Nondestructive Evaluation for Energy Systems, SPIE Vol. 9806 (980618)
Optimized Euler Transform● Traditionally, the four
polarizations make up a scattering matrix:
● My team developed an optimized Euler tranform that diagonalizes the scattering matrix and converts the data to more meaningful parameters*:
*C. Baird, W. T. Kersey, R. Giles, W. E. Nixon, Radar Sensor Technology X, SPIE 6210 (62100A)
Optimized Euler Transform
HH Image Euler m Image
Images from: R. Martin, C. Baird, R. Giles, A. Schoenberg, C. Niezrecki, Smart Materials and Nondestructive Evaluation for Energy Systems, SPIE 9806 (980618)
Wind Turbine Project Conclusions
● My team found that terahertz ISAR imaging can indeed detect subsurface defects in wind turbine blades
● The Euler m parameter leads to better detection of defects than traditional H/V imaging
● Compositing images significantly improves target detection
Wind Turbine Future Work
● Statistically analyze the other Euler paramater images of wind turbine samples to find out why they did not perform as well as the Euler m parameter images
● Develop a more intelligent image compositing algorithm (only include statistically reliable images)
● Investigate other polarization transformation algorthms
Questions?
Publications
For a complete listing of my publications, go to:
http://faculty.uml.edu/cbaird/CV.html
For a complete listing of my lab's publications, go to:
http://www.uml.edu/Research/STL/publications
Wavegiude Effects
Solve the waveguide equations using a transfer matrix method to find the light's mode profile as a function of frequency
Image from: C. Baird, B. Crompton, P. Slingerland, R. Giles, W. E. Nixon, Terahertz Physics, Devices, and Systems IV, SPIE 7671 (767109)
Schrödinger-Poisson Equation● Solve the Poisson equation using
the RK4 method to find the built-in voltage due to the space charge
● Solve the Schrödinger equation using the RK4 method to find the electron wave states
● Find the space charge as the sum of the electron wave states
Find Transition RatesUse Fermi's golden rule to find all transition rates
Find State Densities● Use the rate equations and assume
equilibrium to find the amount of electrons and photons in each wave state
Find Electron Temperatures, Repeat● Solve the energy balance equations
to find the electron temperatures*
● Repeat all steps until the solution converges
● From the photon populations, calculate the laser frequency and laser power
*P. Slingerland, C. Baird, R. Giles, Semiconductor Science and Technology 27 (6)
Results: Lasing Power Study
● The 2.83 THz Vitiello design was again used
● The layer width scale factor and applied bias were varied and the lasing power was computed
● Our computational results matched the expected trends*
*P. Slingerland, Ph.D. dissertation supervised by C. Baird, University of Massachusetts Lowell
Scoring Algorithm for Wind Turbine Defect Imaging
Quantitative Evaluation ofImaging Methods
● My team developed an algorithm that numerically scores how well defects show up in an image in order to evaluate the Euler parameters and compositing algorithm
● The results were averaged over many defects in many different wind turbine samples
● Lower scores represent better imaging of defects
Results
Image from: B. Martin, M.S. thesis supervised by C. Baird, University of Massachusetts Lowell
Another Wind Turbine Sample