Comet Calendar, The Official Event Calendar for UT Dallas en-us This week's events for Engineering and Computer Science at UT Dallas Additively Manufactured MicroChannels for Heat Exchange by. Dr. Karen Thole Friday, Oct 19
(11 a.m. - 12 p.m.)


Recent technological advances in the field of additive manufacturing (AM), particularly with direct metal laser sintering (DMLS), have increased the potential for creating novel heat exchange components. Using DMLS broadens the design space and allows for increasingly small and complex geometries to be fabricated with little increase in time or cost.  Challenges arise when attempting to evaluate the advantages of DMLS for specific applications, particularly because of how little is known regarding the effects of surface roughness, which is inherent in the AM process.  This presentation shows resulting pressure drop and heat transfer measurements of flow through as produced microchannels that have been manufactured using DMLS in an effort to better understand resulting roughness effects.  Results presented also show the effect of build direction and channel shape on the roughness as well as build tolerances.  Results showed significant augmentation of these parameters compared to smooth channels, particularly with the friction factor for microchannels with small hydraulic diameters. However, augmentation of Nusselt number did not increase proportionally.


Dr. Karen A. Thole is a Distinguished Professor and Head of the Department of Mechanical and Nuclear Engineering at The Pennsylvania State University.  She is the founder of the Steady Thermal Aero Research Turbine Laboratory (START) lab, which focuses on gas turbine heat transfer and is a center of excellence for a major turbine engine manufacturer.  She has published over 200 archival papers and advised 70 dissertations and theses. She currently serves on the ASME Board of Governors.  Dr. Thole co-founded the Engineering Ambassadors, which is a professional development program with an outreach mission.  She was recently recognized as SWE’s 2014 Distinguished Engineering Educator and in 2015 with ASME’s George Westinghouse Gold Medal and the Edwin F. Church Medal. She holds two degrees in Mechanical Engineering from the University of Illinois, and a PhD from the University of Texas at Austin.

Double Tunneling-Injection Lasers and Lasers with Asymmetric Barrier Layers: Optimizing Performance by Suppressing Recombination Outside the Active Region, presented by Prof. Levon Asryan as part of the Materials Science and Engineering Colloquium Series, hosted by Dr. William Vandenberghe Friday, Oct 19
(11 a.m. - 12 p.m.)

ABSTRACT: In conventional diode lasers with a low-dimensional active region, the electrons and holes are first injected from the cladding layers into a bulk reservoir, which also serves as the waveguiding layer [optical confinement layer (OCL)]. The carriers are then transported to the active region and, finally, captured into the latter. Due to bipolar (i.e., both electron and hole) population in the OCL, a certain fraction of the injection current goes into electron-hole recombination there. This parasitic recombination outside the active region is a major source of the temperature dependence of the threshold current. In addition, the carrier capture from the OCL into the active region is not instantaneous. For this reason, the carrier density in the OCL, and hence the parasitic recombination rate, rise, even above the lasing threshold, with injection current. This leads to sublinearity of the light-current characteristic (LCC) and limits the output power, especially at high pump currents.

To suppress the recombination outside the quantum-confined active region, two design approaches were proposed. One of the approaches exploits double tunneling-injection (DTI), i.e., tunneling injection of both electrons and holes into the active region from two separate quantum wells (QWs). In a DTI laser, the quantum-confined active region, located in the central part of the OCL, is clad on each side by a thin barrier and a QW. Electrons and holes are injected into the active region by tunneling from the corresponding QWs. Ideally, there should be no second tunneling step, i.e. out-tunneling from the active region into the ‘foreign’ QWs (electron-injecting QW for holes and hole-injecting QW for electrons).

The other approach is based on independent tailoring of the conduction and valence bandedges by means of the use of two asymmetric barrier layers (ABLs) – one on each side of the active region. The ABL in the electron-injecting side of the structure should ideally prevent holes from entering that side while not hindering the electron-injection into the active region. The ABL in the hole-injecting side should prevent electrons from entering that side while not hindering the hole-injection into the active region. A laser utilizing ABLs was termed a bandedge-engineered laser or an ABL laser.

In both DTI and ABL lasers, there will be no electrons (holes) in the hole- (electron-) injecting side of the structure, i.e., the bipolar population will be suppressed outside the active region.

As discussed in this presentation, the total suppression of bipolar population and, hence, of recombination outside the active region would result in virtually temperature-insensitive threshold current, close-to-one internal quantum efficiency, and linear LCC. The dynamic characteristics would also be improved in these lasers.

BIO: Levon Asryan received the PH.D. and Doctor of Science degrees from Ioffe Institute, Saint Petersburg, Russia, in 1998 and 2003, respectively, in both physics and mathematics.  He is a Professor with Virginia Tech.  Before joining Virginia Tech, he was with the Ioffe Institute of Physics and Technology, Saint Petersburg, Russia, and State University of New York at Stonybrook.  His research interests include the physics of semiconductors, nano and optoelectronics, photonics, and theory of semiconductor lasers with a low-dimensional active region.  Dr. Asryan was a recipient of the Highest Award of the Russian Federation (State Prize) in Science and Technology for 2001 for his contribution to “fundamental investigations of heterostructures with quantum dots and development of quantum dot lasers” and the first IEEE Journal of Quantum Electronics Best Paper Award for his work on quantum dot lasers.