(VIDEO) CASPR spearheads femtosecond research on QC laser structures
July 10, 2009--At the University of Maryland, Baltimore County (UMBC; Baltimore, MD), Anthony Johnson and his research team are spearheading an effort to better understand gain mechanisms in quantum-cascade (QC) lasers. As director of the Center for Advanced Studies in Photonics Research (CASPR) at UMBC, Johnson is putting the finishing touches on a tunable femtosecond mid-infrared source using National Science Foundation (NSF) funding as part of a project under the MIRTHE umbrella.
July 10, 2009--At the University of Maryland, Baltimore County (UMBC; Baltimore, MD), Anthony Johnson and his research team are spearheading an effort to better understand gain mechanisms in quantum-cascade (QC) lasers. As director of the Center for Advanced Studies in Photonics Research (CASPR) at UMBC, Johnson is putting the finishing touches on a tunable femtosecond mid-infrared source using National Science Foundation (NSF) funding as part of a project under the MIRTHE umbrella (see " MIRTHE exhibits mid-IR leadership").
The Mid-Infrared Technologies for Health and the Environment (MIRTHE; Princeton, NJ) Center is an NSF Engineering Research Center headquartered at Princeton University and launched in 2006. In addition to Johns Hopkins University, City College of New York, Rice University, and Texas A&M University, UMBC is also a MIRTHE member, and Anthony Johnson is both a Deputy Director and Materials Research Thrust Leader for MIRTHE. With the help of CASPR research associate Dr. Elaine Lalanne, PhD students Sheng Liu, Victor Torres, Robinson Kuis, Raymond Edziah and MS student Shelley Watts, Johnson will use his 3-12 µm, 150 fs, 250 kHz femtosecond laser for resonant intersubband pump-probe spectroscopy of an active QCL supplied by MIRTHE Director Claire Gmachl's group at Princeton University. This ultrafast characterization tool will permit us to understand the operation and limitations of QCLs and provide feedback to our academic and industrial collaborators and partners to optimize their composition, structure, and manufacturing techniques.
A typical QCL can consist of hundreds of nanoscale layers of semiconductor material. We propose to use ultrafast techniques in the mid-IR to investigate the gain recovery dynamics of an operating QCL below and above threshold and as a function of temperature. Johnson hopes to time resolve the injection of electrons and their transit across the various active regions of the QCL. This electron transit time is ultimately related to the efficiency of the QCL and depends upon the device design, material composition, and fabrication process. At the fundamental level this electron transit time should depend upon interface roughness and compositional non-uniformity. We work closely with our MIRTHE collaborator Dr. Michael Weimer (Texas A&M) who performs high-resolution X-ray diffraction (HRXRD) and cross-sectional STM (scanning tunneling microscopy) measurements. Using cross-sectional STM, the Weimer group has recently shown that high performance QCLs can be produced with varying degrees of interface roughness. At UMBC we hope to measure the electron transit times of these various samples and correlate these measurements to QCL quantum efficiencies.
Dr. Lalanne explains that CASPR's tunable mid-IR femtosecond laser was challenging to build--especially considering that mid-IR optics for beam-steering and wavelength conversion are not easy to obtain in the commercial realm. Much of the optics used in the CASPR setup are zinc-selenide- and calcium-fluoride-based, or gold-coated optics. To generate a 4 µm output, for example, the difference frequency generation (DFG) technique is used by mixing the 1.3 µm signal and 2 µm idler beams from the OPA (optical parametric amplifier) in a nonlinear crystal such as AgGaS2 to produce mid-IR laser radiation. Thus far, tunable femtosecond mid-IR pulses have been achieved between 3-10 µm for QCL research by tuning the signal and idler beams from the OPA using AgGaS2 and AgGaSe2 nonlinear crystals.