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SUMMARY:IEM Distinguished Lecturers Seminar: The Search for Efficient Visi
 ble Light Emitters: Past\, Present\, Future
DTSTART:20231006T131500
DTEND:20231006T140000
DTSTAMP:20260924T075015Z
UID:d3ca73c9a43c0b065619aec20b215619db6fcffc042170dbd2d5519c
CATEGORIES:Conferences - Seminars
DESCRIPTION:Prof. Claude Weisbuch\nMaterials Department\, University of Ca
 lifornia\, USA & Laboratoire de Physique de la Matière Condensée\, Écol
 e Polytechnique\, CNRS\, Institut Polytechnique de Paris\, France\nThe sem
 inar will take place in ELA 2 and will be simultaneously broadcasted in t
 he main auditorium in Neuchâtel Campus (MC A1 272).\n\nCoffee and cookie
 s will be served at 13:00 before the seminar\, in front of the two auditor
 iums. \n\nAbstract\nWe first recall the evolution of lighting technologie
 s throughout the ages\, and how the demand for improvements was such that 
 any new technology giving better and cheaper lighting was immediately impl
 emented. Thus\, every revolution in energy sources – gas\, petrol electr
 icity – was first put to large-scale use in lighting. However\, “ancie
 nt” techniques based on improvements of thermal light emission have phys
 ical limits around a few percent.\nLight emitting diodes\, based on the di
 rect conversion of electricity into light in semiconductors took a long ti
 me to emerge as a universal high efficiency replacement solution. In its p
 re-history\, defined as the period before the appearance of high-quality m
 aterials\, the quantum efficiency\, the efficiency of transformation of el
 ectricity -electron-hole pairs- into light\, was in the percent range. Wit
 h the availability of high purity GaAs\, efficiencies shot up immediately 
 to 50%+ and allowed laser action. However\, this was in the infrared spect
 ral region\, and the emergence of high efficiency LEDs throughout the visi
 ble range took another 30 years to develop\, culminating with the discover
 y of the high-efficiency nitride LEDs in the late 80’ies - early 90’ie
 s\, leading to the 2014 physics Nobel prize attribution to Akasaki\, Amano
  and Nakamura. Before that breakthrough\, it was long thoughtthat LEDs wou
 ld only be of use for indicators\, flat panel displays\, and to traffic li
 ghts. 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 t
 o a revolution in lighting efficiency\, still in progress\, aiming at 60%+
  reduction in electricity consumption for lighting.\nInterestingly\, contr
 ary to the expected impact of the stimulated emission process\, laser effi
 ciency remains below that of modern LEDs.\nA long-standing challenge is th
 e tantalizing possibility to obtain electrical efficiencies above 100% by 
 scavenging thermal energy from the environment\, similarly to the principl
 e 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 supp
 lied by the electrical source to the recombination species. The transforma
 tion 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 la
 tter type of energy expected to have a lower entropy than heat\, thus cont
 radicting the second principle.\n\nBio\nClaude Weisbuch is a French semico
 nductor physicist. He held a number of positions in academia\, industry\, 
 government.\nHe is now an emeritus “Directeur de Recherche” at the Cen
 tre National de la Recherche Scientifique (CNRS) at Ecole Polytechnique\, 
 France and a Distinguished Professor in the materials department of Univer
 sity of California at Santa Barbara.  He has been at Bell Laboratories (1
 979-1981)\, then at Saint Gobain and Thomson-CSF (now Thales). He was “d
 irecteur scientifique” (chief scientist) of Délégation Générale 
 pour l’Armement (procurement)\, ministry of defense\, France\, from 1992
  to 1998.\nHe founded in 2002 a high-tech company\, Genewave\, Paris\, dev
 oted to fluorescence based molecular diagnostics systems\, based on his wo
 rk on light extraction in LEDs\, which merged in 2014 with Mobidiag and wa
 s acquired by Hologic in 2021.\nHe has authored or co-authored more than 2
 60 papers and 35 patents. Since 1993\, he worked on light extraction in LE
 Ds through microcavities and photonic crystals\, and since 2013 on fundame
 ntal processes in nitride LEDs.\n 
LOCATION:ELA 2 https://plan.epfl.ch/?room==ELA%202 https://epfl.zoom.us/j/
 66144079474
STATUS:CONFIRMED
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