CPM Seminar

THz quantum cascade lasers - back to the future

Zbig Wasilewski

Department of Electrical and Computer Engineering, Department of Physics and Astronomy and Waterloo Institute for Nanotechnology
University of Waterloo

Since their initial demonstration in 2002, (Kohler, 2002 #24291 [1]) THz QCLs have achieved a remarkable progress and can now operate in 1.2-4.5THz spectral range with maximum operating temperatures for the best devices of ~200K[2]. Increasing their operating temperatures to commercially available thermoelectric coolers range (~240K) will make THz QCLs very attractive to a broad range of potential applications in areas such as biological sensing, pharmaceutical sciences, THz wave imaging, security screening and ICT, to mention just a few. In 2007 we have proposed[3] the simplest at the time resonant phonon depopulation (RP)-type THz QCL design based on a three quantum well GaAs/Al_0.15Ga_0.85As active module, which proved to be particularly effective in increasing the population inversion and hence the gain at higher temperatures. By combining unique to molecular beam epitaxy (MBE) capabilities with theoretical modeling of the THz QCLs we pushed the maximum lasing temperature for these devices to the new world record of 199.5K[2], while the insight gained on the way spurred vigorous activities which led to new promising laser designs[4,5]. Even though the achieved operating temperatures have already surpassed early expectations, there are no obvious fundamental limits which would prevent THz QCLs from operating right up to room temperature[6]. Nevertheless, the present record temperature of 199.5K has remained unchallenged for over two years now, despite intense work by the leading groups in North America, Europe and Japan. It also took that long to establish a new MBE laboratory at the University of Waterloo, after my move there from the National Research Council in 2012. The research program aimed at improving THz QCLs performance targeting two different material systems - arsenides and antimonides - is presently ramping up in our lab. Perhaps the most controversial and the least understood is the role of interfacial roughness on scattering of the tunneling electrons, which can be an important gain limiting factor in GaAs/AlGaAs material system as well as other material systems presently considered for THz QCL devices. Thus further optimization of MBE growth may well be the key to promote progress in this area.

In this talk, after general introduction, I will give an overview of the state of the art in the field of THz QCL devices, indicate the key roadblocks to achieving higher operating temperatures and discuss possible paths to further improvements.

References:
[1] R. Kohler, A. Tredicucci, F. Beltram, H. E. Beere, E. H. Linfield, A. G. Davies, D. A. Ritchie, R. C. Iotti and F. Rossi, Nature 417 (2002) 156.
[2] S. Fathololoumi, E. Dupont, C. W. I. Chan, Z. R. Wasilewski, S. R. Laframboise, D. Ban, A. M�ty�s, C. Jirauschek, Q. Hu and H. C. Liu, Opt. Express 20 (2012) 3866.
[3] H. Luo, S. R. Laframboise, Z. R. Wasilewski, G. C. Aers, H. C. Liu and J. C. Cao, Appl. Phys. Lett. 90 (2007) 041112.
[4] E. Dupont, S. Fathololoumi, Z. R. Wasilewski, G. Aers, S. R. Laframboise, M. Lindskog, S. G. Razavipour, A. Wacker, D. Ban and H. C. Liu, J. Appl. Phys. 111 (2012) 073111.
[5] A. Matyas, R. Chashmahcharagh, I. Kovacs, P. Lugli, K. Vijayraghavan, M. A. Belkin and C. Jirauschek, J. Appl. Phys. 111 (2012) 103106.
[6] Y. Chassagneux, Q. J. Wang, S. P. Khanna, E. Strupiechonski, J.-R. Coudevylle, E. H. Linfield, A. G. Davies, F. Capasso, M. A. Belkin and R. Colombelli, IEEE Trans. Terahertz Sci.Techn. 2 (2012) 9.