Ph.D. Dissertation Defense

High Power Mid-infrared Quantum Cascade Laser Array
for Standoff Photoacoustic Chemical Detection

Xing Chen

1:00-3:00pm Friday, 17 May 17 2013, TRC CASPR Conference Room

Quantum cascade lasers (QCLs) are compact, portable, powerful semiconductor laser sources with emission wavelengths from mid-infrared (mid-IR) to terahertz (THz) regions of the electromagnetic spectrum. Mid-IR (i.e. wavelengths from 3 to 20 µm) QCLs are of great importance in a wide range of applications such as trace gas sensing, environmental monitoring, free space communication, medical diagnosis and so on. High power QCLs are particularly important to applications such as infrared counter measure (IRCM) and standoff chemical detections. In such applications, the system performances critically depend on the amount of power a QCL can produce. This dissertation includes two major studies: the first part of the dissertation includes design, fabrication and characterization of high power mid-IR QCL arrays; the second part involves standoff chemical detection using QCLs as laser sources and photoacoustic effect as sensing technologies.

In the first part of the dissertation, we design, fabricate and characterize multi-emitter QCL arrays consisting of multiple narrow laser stripes. Simulation results indicate that the proposed multi-emitter laser arrays present much better thermal performance than a broad area laser device, while having the same thermal management ability as a single narrow stripe device. We have successfully fabricated edge emitting and surface emitting QCL arrays with 5 and 16 emitters. Experimental results show that, with the same laser cavity length, a QCL array with 5 emitters produces over 3 times more power than a single emitter laser device. QCL array with 16 emitters generates about 4 W output peak power at wavelength ~7.9 µm. We have also fabricated surface emitting QCL arrays and demonstrated single mode emission.

The second part of the dissertation involves using high power mid-IR QCLs to perform standoff chemical detections based on photoacoustic sensing technologies. Photoacoustic effect is a light-matter interaction effect that involves generation of acoustic waves when a medium absorbs electromagnetic energy from light. It has been known as a sensitive spectroscopic technique for chemical sensing.

Standoff photoacoustic chemical detection with distance more than 41 feet using quantum cascade laser operated at relatively low power, less than 40 mW, is demonstrated. A simplified theoretical model is developed for pulsed laser photoacoustic effect in open-air environment. The standoff photoacoustic signal can be calibrated as a function of different parameters such as laser pulse energy, gas vapor concentration and detection distance. The results yield good agreements with theoretical model. Standoff detection of solid phase explosive chemicals has also been demonstrated by the use of an ultra-sensitive microphone and acoustic reflector. More than 8 feet detection distance is obtained for standoff photoacoustic sensing of explosives.

Committee: Drs. Fow-Sen Choa (Chair), Brian Cullum, Yordan Kostov, Ryan Robucci, Chen-Chia Wang and Li Yan

Google announced that it will establish a Quantum Artificial Intelligence Lab with NASA to study the use of quantum computing for machne lerning in support of problems including web search, speech recognition, planning and scheduling, search for exoplanets, and support operations in mission control centers.

"We believe quantum computing may help solve some of the most challenging computer science problems, particularly in machine learning. Machine learning is all about building better models of the world to make more accurate predictions. If we want to cure diseases, we need better models of how they develop. If we want to create effective environmental policies, we need better models of what’s happening to our climate. And if we want to build a more useful search engine, we need to better understand spoken questions and what’s on the web so you get the best answer."

The lab will use a D-Wave Systems computer and share it with external researchers via NASA's USRA. According to D-wave Systems, "Installation has already begun at NASA's Ames Research Center in Moffett Field, California, and the system is expected to be available to researchers during Q3."

"As part of the selection process, Google, NASA and USRA created a series of benchmark and acceptance tests that the new D-Wave 512-qubit system was required to pass before the installation at NASA Ames could proceed. In all cases, the D-Wave Two system met or exceeded the required performance specifications, in some cases by a large margin."

If you are interested in finding out more about quantum computing, you might talk to two members of the CSEE faculty who work on the topic: Professors Sam Lomonaco and Joel Morris.

Update! Liz's Reflections Post-Graduation:

The following full-time positions have been posted within the past week to UMBCworks. Login to your account (via the link in the Jobs & Internships topic in myUMBC) and find details and application instructions as well as hundreds of other job postings!

9258226 - Assistant Director - Kumon of Elkridge
9258211 - Fitness Center Instructor I - YMCA of Central Maryland (Parkville)
9258206 - Proposal Coordinator - NewWave Telecom and Technologies
9258191 - Development Fellow (Paid) - Cultural Tourism DC
9258156 - Employment Consultant - EBP - Humanim
9258144 - Lab Engineer - STX LLC

The following part-time positions have been posted within the past week to UMBCworks. Login to your account (via the link in the Jobs & Internships topic in myUMBC) and find details and application instructions as well as hundreds of other job postings!

9258210 - Lifeguard - YMCA of Central Maryland (Randallstown)
9258181 - Upper School Science Teacher - The Park School of Baltimore
9258185 - ABA Therapist - Private Centreville Home
9258178 - Chess Coach or Aide - Silver Knights Chess
9258155 - AutoCAD Drafting Position - Roomtag, LLC