Browsing by Subject "Optical Gain"
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Item Open Access Attractive versus repulsive excitonic interactions of colloidal quantum dots control blue-to red-shifting (and non-shifting) amplified spontaneous emission(American Chemical Society, 2013-11-21) Cihan, A. F.; Kelestemur, Y.; Guzelturk, B.; Yerli, O.; Kurum, U.; Yaglioglu, H. G.; Elmali, A.; Demir, Hilmi VolkanTunable, high-performance, two-photon absorption (TPA)-based amplified spontaneous emission (ASE) from near-unity quantum efficiency colloidal quantum dots (CQDs) is reported. Besides the absolute spectral tuning of ASE, the relative spectral tuning of ASE peak with respect to spontaneous emission was shown through engineering excitonic interactions in quasi-type-II CdSe/CdS core/shell CQDs. With core shell size adjustments, it was revealed that Coulombic exciton-exciton interactions can be tuned to be attractive (type-I-like) or repulsive (type-II-like) leading to red- or blue-shifted ASE peak, respectively, and that nonshifting ASE can be achieved with the right core shell combinations. The possibility of obtaining ASE at a specific wavelength from both type-I-like and type-II-like CQDs was also demonstrated. The experimental observations were supported by parametric quantum-mechanical modeling, shedding light on the type-tunability. These excitonically engineered CQD-solids exhibited TPA-based ASE threshold as low as 6.5 mJ/cm(2) under 800 nm excitation, displaying one of the highest values of TPA cross-section of 44 660 GM.Item Open Access Synthesis and characterization of colloidal quantum wells: from simple size-tuned core to complex multi-crown structures(Bilkent University, 2018-08) Dede, DidemAs a new class of semiconductor nanocrystals, colloidal quantum wells (CQWs), also commonly known as nanoplatelets (NPLs), exhibit remarkable electronic and optical properties that will potentially nd a wide range of use from nanophotonics to optoelectronics. NPLs feature step-like absorption pro les and discrete emission spectra with giant oscillator strength resulting in high recombination rates. All these features make these atomically- at structures highly attractive for light-harvesting and -generating applications. In this thesis, to understand the size-tuned properties of their two-dimensional architecture, we conducted a systematic study on the core-only NPLs by using a set of 4 monolayer (ML) CdSe cores as our working model and carefully altered their aspect ratio while keeping their lateral area constant. In such a core-only NPL structure, electron and hole are both con ned in the core resulting in type-I electronic band alignment. By decreasing the width of these NPLs to a value comparable to or less than their exciton Bohr radius, we observe additional con nement e ects emerge. Subsequently, by growing CdSe1xTex alloyed crown around these starting 4 ML CdSe cores, we nd type-II electronic band alignment is obtained. Thanks to their spatially indirect excitons, these core crown NPLs show extraordinarily long radiative lifetimes. Moreover, with the increased absorption cross-section owing to their added crown, high-performance optical gain is achieved via their core/crown heterostructure. However, in this form, their usage is limited since they are unstable in solution forming gels and they exhibit strong tendency to form stacks in lms. To address this problem, here we proposed and developed a multi-crown architecture by additionally growing a CdS crown around the periphery of the type- II heterostructure, enabling excellent optical gain media with enhanced stability. The structural and optical characterizations of the synthesized multi-crown NPLs indicate that this complex architecture holds great promise for making devices in colloidal nanophotonics and optoelectronics.