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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