IEM Distinguished Lecturers Seminar: The Search for Efficient Visible Light Emitters: Past, Present, Future


Event details

Date 06.10.2023
Hour 13:1514:00
Speaker Prof. Claude Weisbuch
Materials Department, University of California, USA & Laboratoire de Physique de la Matière Condensée, École Polytechnique, CNRS, Institut Polytechnique de Paris, France
Location Online
Category Conferences - Seminars
Event Language English
The seminar will take place in ELA 2 and will be simultaneously broadcasted in the main auditorium in Neuchâtel Campus (MC A1 272).

Coffee and cookies will be served at 13:00 before the seminar, in front of the two auditoriums. 

We first recall the evolution of lighting technologies throughout the ages, and how the demand for improvements was such that any new technology giving better and cheaper lighting was immediately implemented. Thus, every revolution in energy sources – gas, petrol electricity – was first put to large-scale use in lighting. However, “ancient” techniques based on improvements of thermal light emission have physical limits around a few percent.
Light emitting diodes, based on the direct conversion of electricity into light in semiconductors took a long time to emerge as a universal high efficiency replacement solution. In its pre-history, defined as the period before the appearance of high-quality materials, the quantum efficiency, the efficiency of transformation of electricity -electron-hole pairs- into light, was in the percent range. With the availability of high purity GaAs, efficiencies shot up immediately to 50%+ and allowed laser action. However, this was in the infrared spectral region, and the emergence of high efficiency LEDs throughout the visible range took another 30 years to develop, culminating with the discovery of the high-efficiency nitride LEDs in the late 80’ies - early 90’ies, leading to the 2014 physics Nobel prize attribution to Akasaki, Amano and Nakamura. Before that breakthrough, it was long thoughtthat LEDs would only be of use for indicators, flat panel displays, and to traffic lights. LED lamps for general lighting only emerged recently as the dominant application of LEDs thanks to dramatic decrease in cost, and continuous improvements of color quality and energy conversion efficiency. This led to a revolution in lighting efficiency, still in progress, aiming at 60%+ reduction in electricity consumption for lighting.
Interestingly, contrary to the expected impact of the stimulated emission process, laser efficiency remains below that of modern LEDs.
A long-standing challenge is the tantalizing possibility to obtain electrical efficiencies above 100% by scavenging thermal energy from the environment, similarly to the principle of operation of heat pumps. It was observed very early, in the 50’ies, that the energy of emitted photons could be larger than the energy supplied by the electrical source to the recombination species. The transformation of this single event into a useable, everyday light source requires advances in emission efficiency at low injection currents, still an open problem given the physics of spontaneous light emission. It also raises a surprising contradiction, at first glance, about transforming the energy supplied by the heat reservoir of the LED environment into light, the latter type of energy expected to have a lower entropy than heat, thus contradicting the second principle.

Claude Weisbuch is a French semiconductor physicist. He held a number of positions in academia, industry, government.
He is now an emeritus “Directeur de Recherche” at the Centre National de la Recherche Scientifique (CNRS) at Ecole Polytechnique, France and a Distinguished Professor in the materials department of University of California at Santa Barbara.  He has been at Bell Laboratories (1979-1981), then at Saint Gobain and Thomson-CSF (now Thales). He was “directeur scientifique” (chief scientist) of Délégation Générale pour l’Armement (procurement), ministry of defense, France, from 1992 to 1998.
He founded in 2002 a high-tech company, Genewave, Paris, devoted to fluorescence based molecular diagnostics systems, based on his work on light extraction in LEDs, which merged in 2014 with Mobidiag and was acquired by Hologic in 2021.
He has authored or co-authored more than 260 papers and 35 patents. Since 1993, he worked on light extraction in LEDs through microcavities and photonic crystals, and since 2013 on fundamental processes in nitride LEDs.