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VERSION:2.0
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BEGIN:VEVENT
UID:69d289d7cc3c9
DTSTART:20230321T090000Z
SEQUENCE:0
TRANSP:OPAQUE
LOCATION:ICFO Auditorium and Online (Teams)
SUMMARY:ICFO | NIMA TAGHIPOUR 
CLASS:PUBLIC
DESCRIPTION:Colloidal semiconductors quantum dots (CQDs) have emerged as a 
 promising solutionprocessed gain material that can be engineered via low-c
 ost and scalable chemical techniques. Owing to quantum confinement\, their
  emission wavelengths and optical properties can be tuned from the visible
  to the infrared. Despite these possible advantages\, the realization of l
 asing action in CQDs is complicated and fundamentally stems from the non-u
 nity degeneracy of the band-edge state. This results in high optical gain 
 thresholds\, demanding multiexcitons for achieving lasing action. This\, i
 n turn\, leads to a very short optical gain lifetime which is caused by Au
 ger recombination.\nFollowing the first demonstration of lasing action in 
 CQDs\, this field has thus far experienced remarkable development with mat
 erials offering emission in the visible showing limited application potent
 ial. However\, the possibility of lasing in the infrared\nregion would ope
 n a new realm of applications for this material platform in optical teleco
 mmunications\, photonic integrated circuits\, and LIDAR applications. To u
 nleash those applications\, the demonstration of solution-processed infrar
 ed lasers in the eye-safe window between 1.5-1.6 &mu\;m operating robustly
  at room temperature is a prerequisite.\nMidgap trap states in CQDs limit 
 the performance of optoelectronics devices. In particular\, PbS CQDs suffe
 r from a very fast trap-assisted Auger process leading to high lasing thre
 sholds. To suppress this type of Auger process\, in this work\, we use a b
 inary nanocomposite of PbS CQDs and ZnO nanocrsystals (NCs) where the form
 er serves as the infrared gain medium and the latter as a remote passivant
  for midgap traps in PbS CQDs. This binary heterostructure drastically sup
 presses the Auger process and lowers the lasing thresholds.\nLow threshold
  infrared CQD-laser has been thought to be not possible because of 8-fold 
 degeneracy of the band-edge state in the infrared-emitting Pb-chalcogenide
  CQDs. In this Thesis\, we demonstrate that using core-alloyed shell heter
 ostructured\nCQD comprising PbS as core and PbSSe as shell allows suppress
 ing Auger process. Furthermore\, by applying doping to specially engineere
 d CQDs\, we demonstrate a substantial reduction in lasing threshold down t
 o sub-single exciton level per-dot thanks to the blocking of the ground st
 ate absorption. Employing these CQDs has drastically improved the net moda
 l coefficient of the medium and brought it on par with a gain coefficient 
 of epitaxially grown III-V infrared semiconductors.\nThe realization of CQ
 D infrared laser-diodes will have a profound impact in many disciplines. H
 ere\, by engineering the electric field distribution in our devices\, we s
 how stimulated emission in a record ultra-thin gain media which is beyond 
 the slab\nwaveguide theoretical limit by introducing scatterers implemente
 d by ZnO NCs. We employ this thin gain media as the active layer in a full
 -stack light emitting diode (LED) device. Also\, to overcome the existing 
 challenge underpinned by the optical losses of the metal contacts that hav
 e prevented the realization of stimulated emission in a LED\, we use an en
 gineered transparent conductive oxide and graphene as anode and cathodeof 
 the LED\, respectively. Finally\, our proposed LED structure leads us to r
 ealize a dual function device showing strong infrared spontaneous- and sti
 mulated-emission under\nelectrical- and optical-pumping\, respectively.\nI
 n summary\, we have demonstrated that CQDs can emerge as a robust technolo
 gy for the realization of infrared lasers. Our proposed CQD systems lead u
 s to achieve high performance laser devices under optical excitation and u
 sing CQD heterostructures asan active medium in the proposed LED structure
  paves the way towards the future development of infrared CQD-laser diodes
 .\nThesis Director: Prof Dr. Gerasimos Konstantatos
DTSTAMP:20260405T161207Z
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