My research group is working on the development of new quantum devices, especially lasers, and their optimization for sensor systems and their applications in environment and health. Our special focus is currently on Quantum Cascade (QC) lasers, a novel type of semiconductor injection laser based on electronic intersubband transitions in the conduction band of a coupled quantum well heterostructure. Quantum wells are slivers of one type of semiconductor material, just a few atomic layers thick, interleaved with another type of semiconductor (the barriers). Many performance features of the lasers, such as their power, modulation capabilities, or emission wavelength in the mid- to far-infrared, are designed into the device by choice of the thicknesses of the quantum wells and barriers. With some 500 to 1000 quantum wells and barriers in a single laser, there are endless opportunities for creativity! Our current projects include the development of high-temperature, high-power, high-efficiency QC lasers. Widely tunable, monolithic, and external cavity QC lasers are being developed for optical sensors in environmental, medical, and security applications. While my group is focused mainly on the development of the lasers, we maintain strong collaborations with many expert spectroscopists who are building sensor systems in academia, government, and industrial labs. My group's working style is collaborative and open. Group members expect to work hard and to have a lot of fun, too! Device modeling on the computer, device fabrication in the PRISM cleanroom facilities, and optical device characterization in our labs each take up about 30 percent of the work week. The remaining 10 percent or so is reserved for tasks such as scientific writing or attending conferences. I am also the director of MIRTHE, the NSF-sponsored Engineering Research Center for Mid-InfraRed Technologies for Health and the Environment. A group of 6 universities and approximately 40 faculty, 120 students, and 40 industry and practitioner organizations, MIRTHE members collaborate to develop the next-generation sensor platforms for chemical trace gas sensing using mid-infrared spectroscopy. For more information check out our web-page: www.mirthecenter.org.
Y. Yao, X.J. Wang, J.Y. Fan, and C.F. Gmachl, “High performance "continuum-to-continuum" quantum cascade lasers with a broad gain bandwidth of over 400 cm 1”, Appl. Phys. Lett. 97(8) 081115 (2010)
W.O. Charles, Y. Yao, K.J. Franz, Q. Zhang, A. Shen, C. Gmachl, and M.C. Tamargo, “Growth of Znx,Cd(1-x ')Se/ZnxCdyMg(1-x-y)Se-InP quantum cascade structures for emission in the 3-5 mm range”, J. Vac. Sci. B 28(3) (2010)
Y. Yao, W.O. Charles, T. Tsai, J.X. Chen, G. Wysocki, and C.F. Gmachl, ”Broadband quantum cascade laser gain medium based on a "continuum-to-bound" active region design”, Appl. Phys. Lett. 96(21) 211106 (2010)
P.Q. Liu, A.J. Hoffman, M.D. Escarra, K.J. Franz, J.B. Khurgin, Y. Dikmelik, X.J. Wang, J.Y. Fan, and C.F. Gmachl, “Highly power-efficient quantum cascade lasers”, Nature Photonics 4(2) 95 – 96 (2010)
K.J. Franz, P.Q. Liu, J.J.J. Raftery, M.D. Escarra, A.J. Hoffman, S.S. Howard, Y. Yao, Y., Dikmelik, X.J. Wang, J.Y. Fan, J.B. Khurgin, and C. Gmachl, “Short Injector Quantum Cascade Lasers”, IEEE J. Quantum Electron. 46(5) 591 – 600 (2010)
Y. Yao, K. J. Franz, X.J. Wang, J.Y. Fan, and C. Gmachl, "A widely voltage-tunable quantum cascade laser based on "two-step" coupling" , Applied Physics Letters 95(2) 021105 (2009)
F. Toor, S. S. Howard, D.L. Sivco, and C.F. Gmachl, "A Compact Four-Wavelength Quantum-Cascade Laser Source", IEEE Journal of Quantum Electronics 45(8): 904-909 (2009)
M.D. Escarra, A. J. Hoffman, K.J. Franz, S.S. Howard, R. Cendejas, X.J. Wang, J.Y. Fan, and C. Gmachl, "Quantum cascade lasers with voltage defect of less than one longitudinal optical phonon energy" , Applied Physics Letters 94(25) 251114 (2009)
A.O. Dirisu, D. G. Revin, Z.J. Liu, K. Kennedy, J.W. Cockburn, and C.F. Gmachl, "Characterization of Quantum-Cascade Lasers Using Single-Pass Transmission Spectroscopy" , IEEE Journal of Quantum Electronics 45(5-6): 586-593 (2009).
E.N. Bentil, F. Toor, A.J. Hoffman, M.D. Escarra, and C.F. Gmachl, "Rapid and Minimally Invasive Quantum Cascade Wafer Testing" , IEEE Photonics Technology Letters 21(8): 531-533 (2009)
A.J. Hoffman, P. X. Braun, M.D. Escarra, S.S. Howard, K.J. Franz, X.J. Wang, J.Y. Fan, and C. Gmachl, "Lasing-induced reduction in core heating in high wall plug efficiency quantum cascade lasers" , Applied Physics Letters 94(4) 041101 (2009)
K.J. Franz, S. Menzel, A.J. Hoffman, D. Wasserman, J.W. Cockburn, and C. Gmachl, "High k-space lasing in a dual-wavelength quantum cascade laser", Nature Photonics 3(1): 50-54 (2009)
S.S. Howard, Z. Liu, D. Wasserman, A.J. Hoffman, T.S. Ko, and C Gmachl, “High-Performance Quantum Cascade Lasers: Optimized Design Through Waveguide and Thermal Modeling” IEEE J. Sel. Top. Quant. Electron, 13, 1054 (2007)
Z.J. Liu, C.F. Gmachl, L.W. Cheng, F.S. Choa, F.J. Towner, X.J. Wang, and J.Y. Fan “Temperature dependence of optical gain and loss in lambda approximate to 8.2-10.2 mu m quantum-cascade lasers” , IEEE J. Quantum Electron. 44 (5-6) 485–492 (2008)
A.J. Hoffman, L. Alekseyev, S.S. Howard, K.J. Franz, D. Wasserman, V.A. Podolskiy, E.E. Narimanov, D.L. Sivco, C. Gmachl, “Negative refraction in semiconductor metamaterials,” Nature Materials, advance online 14 Oct, (2007); Nature Materials 6, 946 - 950 (2007)