Day 2 :
University of Minnesota Duluth, USA
Keynote: Comprehensive study of dynamic behavior of nonlinear dispersive quantum-cascade lasing medium under different cavity configurations
Time : 9:30-10:15
Jing Bai is a tenured Associate Professor and the Director of Graduate Studies in Department of Electrical Engineering at the University of Minnesota Duluth (UMD), where she started as an Assistant Professor in August 2007. She has received her PhD Degree in Electrical and Computer Engineering at Georgia Institute of Technology in 2007. Her current research activities focus on nanoscale optoelectronic and photovoltaic devices, biomedical devices and nonlinear optics. Her research is supported by grants from the National Science Foundation (NSF), the Whiteside Institute for Clinical Research, the Graduate School of University of Minnesota, etc. She received the SCSE Young Teacher Award in UMD in 2012. She is a member of the Institute of Electrical and Electronics Engineers (IEEE), IEEE Women in Engineering (WIE), the Optical Society of America (OSA), the American Physics Society (APS) and the International Society for Optical Engineering (SPIE).
Since the first demonstration in 1994, quantum-cascade lasers (QCLs) have become one of the most important solid-state mid-infrared (MIR) coherent light sources for various applications in environment sensing, medical diagnosis and free-space communication. The dynamic analysis of MIR QCLs is crucial for QCLs to have reliable performance in these applications. An explicit description of the dynamics of QCLs is inevitably complicated when compared to conventional lasers because of the unique combination of ultrafast carrier scatterings and gain recovery, significant nonlinearities and dispersion effect in a QCL medium. However, the group-velocity dispersion (GVD) has not been explicitly addressed in the study of dynamic behaviors in QCLs. In our study, we carefully examined the effect of GVD on the pulse progression in both time and frequency domains as well as the interplay between GVD and self-phase modulation (SPM) in the cavity. Moreover, we carried out the study for QCLs with both ring and Fabry-Perot (FP) cavities. Comparisons of QCLs’ behaviors in the two types of cavities manifest the influence of spatial-hole burning (SHB) which is only supported in a FP cavity but not a ring cavity. We found out from our simulation that the SPM and GVD have cancellation effects in the time domain. In the frequency domain, they affect the spectrum in different aspects. The anomalous GVD effect excites the symmetric side modes around the central mode. The SPM broadens the line width of each mode, but it does not change the spectral spacing among exited modes. When co-existing in the lasing medium, both GVD and SHB induce side modes, though, through two different mechanisms, i.e., the lasing instability of the former and the gain saturation of standing waves of the later. The pair of modes due to SHB has much closer spectral separation and higher peak intensity than those by GVD.
Hebei University of Technology, China
Keynote: Studies on improving the external quantum efficiency of deep ultraviolet light emitting diodes
Time : 10:15-11:00
Wengang Bi is an elected Fellow of the Optical Society of America (OSA). He has received his PhD from University of California, San Diego, Department of Electrical and Computer Engineering in 1997. After devoting his career to forefront research and development at Hewlett Packard Laboratories, Agilent Technologies Inc. and Philips-Lumileds, he is currently working at Hebei University of Technology, Tianjin, China as a distinguished professor and chief scientist in the State Key Laboratory of Reliability and Intelligence of Electrical Equipment. His research interests include GaN-based semiconductor materials and devices, colloidal quantum dots and their applications to lighting and display. He is the Editor of the book - Handbook of GaN Semiconductor Materials and Devices, has published more than 80 papers and holds more than 25 patents. He has served as a member of the technical committee or organizing committee for a number of international conferences.
III-nitride based deep ultraviolet light emitting diodes (DUV LEDs) are promising candidates for replacing conventional mercury DUV light sources due to being environment-friendly. However, the external quantum efficiency (EQE) of the DUV LEDs is pretty low at the current stage, primarily attributed to the low internal quantum efficiency (IQE) and low light extraction efficiency (LEE). Therefore, solving the issues hindering the IQE and the LEE is of importance for advancing the DUV LEDs towards various applications such as in medical, air and water purification, etc. In this talk, we will review the current status and key factors affecting the IQE and LEE of DUV LEDs and present our research on improving both. Examples include proposing a charge inverter made of an electrode-insulator-semiconductor. We will demonstrate the effectiveness of the charge inverter in improving the whole transport and injection from the p-electrode into the p+-GaN layer/LEDs, which in turn will enhance the IQE and the output power. To improve the LEE for DUV LEDs, we propose an inclined sidewall scattering structure imbedded with air cavities that is formed by a metal bottom and a flat parallel top (Bottom-metal) and studied its light extraction properties using three-dimensional finite difference time domain (3D FDTD) simulations. We find that the imbedded air cavity helps the Bottom-metal structured DUV LEDs to scatter the light into the escape cone via total internal reflection and Fresnel scattering, thus avoiding the light absorption from the sidewall metal mirror in the reported inclined sidewall metal structure (Sidewall-metal). In addition, the unique air cavity having a bottom metal also increases the scattering ability of the Bottom-metal structured DUV LEDs owing to the fact that light within the air cavity directing downwards will be reflected back towards the parallel top interface of the air cavity/AlGaN and will not be subject to total internal reflection.
Indian Institute of Technology Delhi, India
Time : 11:20- 12:05
Dalip Singh Mehta is currently a Professor at the Department of Physics, Indian Institute of Technology Delhi. Previously, he worked as Associate Professor and Assistant Professor (June 2002 - Dec. 2012) at Indian Institute of Technology Delhi. Before Joining the Institute he was JSPS Post-Doctoral Fellow, in Japan, Post-Doctoral Fellow National Dong Hwa University, Taiwan, Research Associate, NPL, New Delhi, STA Post-Doctoral Fellow NIRE, Tsukuba, Japan and UNESCO Research Fellow Tokyo Institute of Technology Tokyo, Japan. He has contributed more than 110 research papers in International Refereed Journals, and more than 150 in International and National Conferences. He has delivered more than 45 Invited Talks/Lectures in various International and National Conferences and Universities. He has supervised 13 Ph. D. students and currently supervising 9 Ph. D. students. He has also supervised about 40 M. Tech./B. Tech. students major projects. He received Teaching Excellence Award 2013 from the Indian Institute of Technology Delhi, India.