| 1. |
- Boţan, Vitalie, 1979-
(författare)
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Bose-Einstein Condensation of Magnetic Excitons in Semiconductor Quantum Wells
- 2006
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- In this thesis regimes of quantum degeneracy of electrons and holes in semiconductor quantum wells in a strong magnetic field are studied theoretically. The coherent pairing of electrons and holes results in the formation of Bose-Einstein condensate of magnetic excitons in a single-particle state with wave vector K. We show that correlation effects due to coherent excitations drastically change the properties of excitonic gas, making possible the formation of a novel metastable state of dielectric liquid phase with positive compressibility consisting of condensed magnetoexcitons with finite momentum. On the other hand, virtual transitions to excited Landau levels cause a repulsive interaction between excitons with zero momentum, and the ground state of the system in this case is a Bose condensed gas of weakly repulsive excitons. We introduce explicitly the damping rate of the exciton level and show that three different phases can be realized in a single quantum well depending on the exciton density: excitonic dielectric liquid surrounded by weakly interacting gas of condensed excitons versus metallic electron-hole liquid. In the double quantum well system the phase transition from the excitonic dielectric liquid phase to the crystalline state of electrons and holes is predicted with the increase of the interwell separation and damping rate.We used a framework of Green's function to investigate the collective elementary excitations of the system in the presence of Bose-Einstein condensate, introducing "anomalous" two-particle Green's functions and symmetry breaking terms into the Hamiltonian. The analytical solution of secular equation was obtained in the Hartree-Fock approximation and energy spectra were calculated. The Coulomb interactions in the system results in a multiple-branch structure of the collective excitations energy spectrum. Systematic classification of the branches is proposed, and the condition of the stability of the condensed excitonic phase is discussed.
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| 2. |
- Grivas, Christos, et al.
(författare)
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Generation of Multi-Gigahertz Trains of Phase-Coherent Femtosecond Laser Pulses in Ti:Sapphire Waveguides
- 2018
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Ingår i: Laser & Photonics reviews. - : WILEY-V C H VERLAG GMBH. - 1863-8880 .- 1863-8899. ; 12:11
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Tidskriftsartikel (refereegranskat)abstract
- Miniature lasers producing ultrashort phase-coherent pulses at high repetition rates by stable mode-locking in ambient conditions can offer unique capabilities in various applications, spanning from microwave photonics to telecom and biological imaging techniques. Here, the operation of graphene mode-locked lasers based on channel waveguides written by femtosecond and picosecond laser pulses in Ti:sapphire crystals is demonstrated. Trains of pulses of 41.4 fs duration at a 21.25 GHz repetition rate are generated by capitalizing on the formation of solitons in their monolithic resonators through Gires-Tournois interferometers. The latter, allow for effective pulse shaping via tuning of the intracavity group delay dispersion while simultaneously enabling ultralow laser operating thresholds. A number of features of these sources, including their high-brightness and broad bandwidth, are essential ingredients for achieving high longitudinal resolution and sensitivity, which are the primary performance metrics of the Fourier domain/spectral domain variant of optical coherence tomography systems. A further doubling of the laser repetition rate to 42.5GHz is achieved by coherent pulse interleaving in optical fiber technology, thereby underlining the potential of the Ti:sapphire waveguide lasers to produce highly stable, wide-spaced combs of phase-coherent optical frequency lines.
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| 3. |
- Junis Rindermann, Jan, et al.
(författare)
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Dependence of Resonance Energy Transfer on Exciton Dimensionality
- 2011
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Ingår i: Physical Review Letters. - : American Physical Society. - 0031-9007 .- 1079-7114. ; 107:23, s. 236805-
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Tidskriftsartikel (refereegranskat)abstract
- We investigate the dependence of resonance energy transfer from Wannier-Mott excitons to an organic overlayer on exciton dimensionality. We exploit the excitonic potential disorder in a single quantum well to tune the balance between localized and free excitons by scaling the Boltzmann distribution of excitons through temperature. Theoretical calculations predict the experimentally observed temperature dependence of resonance energy transfer and allow us to quantify the contribution of localized and free excitons. We show that free excitons can undergo resonance energy transfer with an order of magnitude higher rate compared to localized excitons, emphasizing the potential of hybrid optoelectronic devices utilizing resonance energy transfer as a means to overcome charge transfer related limitations.
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| 4. |
- Rindermann, Jan Junis, et al.
(författare)
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The effect of exciton dimensionality on resonance energytransfer: advances for organic color converters in hybridinorganic/organic LEDs
- 2012
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Ingår i: Proceedings of SPIE. - : SPIE. - 9780819488985 ; , s. 82550I-01-82550I-10
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Konferensbidrag (refereegranskat)abstract
- The dependence of resonance energy transfer from Wannier-Mott excitons to an organic overlayer on excitondimensionality is studied experimentally and by means of supporting simulations. The variation of temperatureeffectively tunes the balance between localized and free excitons, and allows to investigate the effect of theexcitonic potential disorder on resonance energy transfer. Our theoretical calculations give insight into theexperimentally observed temperature dependence of resonance energy transfer, and allow us to quantify thecontribution from localized and free excitons. It is shown that free excitons can undergo resonance energytransfer at a rate that is an order of magnitude higher compared to localized excitons. In planar geometriesnonradiative resonance energy transfer is dominating over radiative energy transfer and hence we propose hybridinorganic-organic LEDs which are optimized for resonance energy transfer to an organic or QD-based colorconverter.
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