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2007-2008 Seminar Series in Electrical Engineering
Spring 2008 (Physical and Wave Electronics Area)

Mid-Infrared Quantum Cascade Lasers

Claire Gmachl
Princeton University

Monday, May 12, 2008 at 1:00PM

54-134 Engineering IV Building
Refreshments Served

Abstract: Quantum Cascade (QC) lasers are a rapidly evolving mid-infrared, semiconductor laser technology based on intersubband transitions in multiple coupled quantum wells. The lasers’ strengths are their wavelength tailorability, high performance and fascinating design potential. We will first give an overview of QC lasers followed by a discussion of several recent highlights, such as the quest for high performance operation and the implementation of unconventional laser schemes. For high-performance, we examine room temperature continuous wave operated QC lasers in both atmospheric windows around 5 mm and 8 – 10 mm wavelength, respectively. The quest for high power and high efficiency QC lasers requires QC lasers that have a low intrinsic threshold, a high characteristic temperature, a low voltage defect, and superior heat sinking. QC lasers with a few percent wall-plug efficiency at room temperature, and few 10 % efficiency at low temperatures are possible. To this effect, we study not only high performance, conventional QC lasers, but also innovative designs with undoped and “ballistic” injectors. Important aspects in the development of high performance QC lasers are a thorough understanding of the modal gain and loss. We have conducted a study of these quantities as a function of temperature, and for different active region design strategies, especially in the 8 – 10 mm wavelength range. While the gain is well understood, the loss measurements indicate several sources of as yet not well understood, and likely not fundamental origins of waveguide loss, which shows the way to the design of even better QC lasers. In the area of novel QC laser designs, we focus on lasers with multiple optical transitions in each active region. While examining the potential for “cascaded” laser emission in QC lasers, we discovered a dual wavelength (~ 9.6 m and ~ 8.2 m) QC laser with anti-correlated light power characteristics. This is explained by two consecutive laser transitions between three subbands, with the second transition occurring high in k-space. This is against the common wisdom of semiconductor physics, that all significant (optical device) processes happen at the band-minima at k = 0. Time permitting, we will briefly review recent work on QC structures in II-VI materials. We will conclude with a short review of QC laser applications and an outlook at the challenges and promises ahead in QC lasers. This work is supported in part by MIRTHE (NSF-ERC) and DARPA-EMIL, and is conducted together with many valued colleagues, incl. K.J. Franz, A.J. Hoffman, S.S. Howard, Z.J. Liu, S. Menzel, S. Schartner, W.O. Charles, F.-S. Choa, J.W. Cockburn, M.C. Tamargo, F.J. Towner, and X. Wang.

Biography: Claire Gmachl received the Ph.D. degree (sub auspicies praesidentis) in electrical engineering from the Technical University of Vienna, Austria, in 1995. In 1996, she joined Bell Laboratories, Lucent Technologies, Murray Hill, NJ, as Post-Doctoral Member of Technical to work on Quantum Cascade laser devices and microcavity lasers. In March 1998 she became a Member of Technical Staff in the Semiconductor Physics Research Department and a Distinguished Member of Staff in 2002. In September 2003, Gmachl joined Princeton University as an Associate Professor in the Department of Electrical Engineering and adjunct faculty to PRISM; since July 2007 she is Full Professor at Princeton University. Gmachl is the Director of MIRTHE, the NSF Engineering Research Center on Mid-InfraRed Technologies for Health and the Environment. Prof. Gmachl has authored and co-authored more than 170 publications, has given more than 100 presentations at conferences and seminars, and holds 26 patents. She is an Associate Editor for Optics Express and a member of the IEEE/LEOS Board of Governors. Dr. Gmachl is a 2005 MacArthur Fellow. She has won various awards and is a member of several professional societies.