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Experimental Resonant Two Photon Study Of The Lowest Confined Exciton Fine Structure In Cucl Quantum Dots
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Book Synopsis Experimental Resonant Two-Photon Study of the Lowest Confined Exciton Fine Structure in CuCl Quantum Dots by : A. V. Baranov
Download or read book Experimental Resonant Two-Photon Study of the Lowest Confined Exciton Fine Structure in CuCl Quantum Dots written by A. V. Baranov and published by . This book was released on 1998 with total page 4 pages. Available in PDF, EPUB and Kindle. Book excerpt: Fine structure of low-energy confined exciton in CuCl spherical nanocrystals has been studied by a resonant size-selective two-photon-excited luminescence (TPL) spectroscopy with high spectral resolution. A band of resonance luminescence (RL) arising due to annihilation of the lowest-energy exciton and its LO-phonon replica were observed to be doublets. Analysis has shown that the components of the doublets connect wfth the pair of exciton states which have been attributed to low-energy confined states (1S) of two transversal excitons (T1 and T2) split by the size-dependent spin-orbit interaction. The physical reason of the splitting is similar to removing of Kramers degeneration of electron states in the bulk crystals of T(sub d) symmetry.
Book Synopsis Towards On-demand Generation of Entangled Photons with Quantum Dots by : Arash Ahmadi
Download or read book Towards On-demand Generation of Entangled Photons with Quantum Dots written by Arash Ahmadi and published by . This book was released on 2020 with total page pages. Available in PDF, EPUB and Kindle. Book excerpt: The biexciton-exciton cascade in quantum dots (QDs) yields entangled photon pairs, and recent developments in engineering photonic structures around the dot for efficient light extraction and proper control of the fine-structure splitting, has led to significant breakthroughs in achieving an ideal entangled photon source -- exhibiting properties such as high extraction efficiency, low multi-photon emission and high entanglement fidelity. This thesis presents our development in enhancing the performance of entangled photon generation of QDs towards near-unity efficiency and fidelity. We study InAsP quantum dots in photonic nanowires for efficient light extraction and to reveal the effect of a quantum dot with high nuclear spin on the entanglement fidelity. Revealing the ultimate limit of entanglement in QDs is still a challenge, and it is generally believed that a sample made of atoms possessing high nuclear spins exhibits limited entanglement due to the dephasing caused by spin-spin interactions. Moreover, it is assumed that the fine-structure splitting of QDs degrades the entanglement fidelity, as it introduces a which-path information in the biexciton-exciton cascade. We have performed two-photon state tomography on InAsP QDs in InP nanowires, comprising of nuclear spin 9/2, and have realized that such a source is capable of generating entangled photon pairs with negligible dephasing during the emission lifetime, with a peak concurrence C = 77% and a count-weighted average concurrence 62%. This result was obtained by comparing the outcome of our measurements with a theoretical model that assumes no dephasing, however, takes into account the details of the detection system, i.e., dark-count rates and timing resolution, as well as, specific features of the QD, e.g., emission lifetime, fine-structure splitting, multi-photon emission probability, etc. Proper engineering of tapered nanowires allows for extraction of the emitted photons with high efficiency, which makes it possible to perform a complete two-photon state tomography and monitoring the evolution of the exciton state, as well as, the entanglement fidelity of the photon-pair, during the emission lifetime. This enabled us to reveal the effect of an imperfect detection system, as well as, multi-photon emission in recording low values for entanglement fidelity. Moreover, our calculations show that proper analysis of the time-energy uncertainty relation provides the necessary means to measure a high entanglement fidelity even in the presence of fine-structure splitting; since, a detection system with high timing resolution can overlook the which-path information, as the uncertainty in measuring the energy of the photons will be larger than the fine-structure splitting. Therefore, realizing near-unity entanglement fidelity using QDs is merely a technical challenge, i.e., resonant excitation of the QD, in order to suppress multi-photon emission, and use of a detection system with low timing jitter and dark-count rates. As a next step to achieve this goal, we have performed the resonant two-photon excitation scheme for the first time on a QD in a photonic nanowire, which resulted in an enhancement of the performance of our source. We managed to improve the single-photon purity of the source, g^(2)(0), by two orders of magnitude; moreover, analysis of the emission spectrum reveals that this scheme increases the pair-production efficiency to values above 93% for the biexciton-exciton cascade. Taking the efficiency of our experimental setup into consideration, the results indicate an approximately eight-fold enhancement of the pair-extraction efficiency as compared to the quasi-resonant excitation scheme (12.55% vs. 1.6%). Based on these results, two-photon state tomography on our source, under resonant two-photon excitation and performed by a detection system with high timing resolution and low dark-count rates, yields near-unity entanglement fidelity. By considering the enhancement in pair-extraction efficiency that we obtained, QDs in photonic nanowires can surpass the performance of spontaneous parametric down conversion sources in terms of entanglement fidelity and pair-extraction efficiency. Even though we have shown a finite fine-structure splitting does not degrade the entanglement fidelity in QDs, for various application in quantum information removing the fine-structure splitting is beneficial. Therefore, we have proposed a universal all-optical approach for removing the fine-structure splitting. In this scheme, the energy of the photons are shifted by using a pair of quarter-wave plates and fast-rotating half-wave plates after they have been emitted; and as a result, the energy difference between the two decay paths in the biexciton-exciton cascade is erased. This method is applicable to any QD source and can be easily implemented using the currently available technology; moreover, no further sample processing will be required in order to achieve zero fine-structure splitting.
Download or read book Technical Digest written by and published by . This book was released on 1997 with total page 340 pages. Available in PDF, EPUB and Kindle. Book excerpt:
Book Synopsis Spins in Optically Active Quantum Dots by : Oliver Gywat
Download or read book Spins in Optically Active Quantum Dots written by Oliver Gywat and published by John Wiley & Sons. This book was released on 2009-11-13 with total page 219 pages. Available in PDF, EPUB and Kindle. Book excerpt: Filling a gap in the literature, this up-to-date introduction to the field provides an overview of current experimental techniques, basic theoretical concepts, and sample fabrication methods. Following an introduction, this monograph deals with optically active quantum dots and their integration into electro-optical devices, before looking at the theory of quantum confined states and quantum dots interacting with the radiation field. Final chapters cover spin-spin interaction in quantum dots as well as spin and charge states, showing how to use single spins for break-through quantum computation. A conclusion and outlook round off the volume. The result is a primer providing the essential basic knowledge necessary for young researchers entering the field, as well as semiconductor and theoretical physicists, PhD students in physics and material sciences, electrical engineers and materials scientists.
Book Synopsis Functionalization of Resonant Interactions in Coupled Quantum Dots by :
Download or read book Functionalization of Resonant Interactions in Coupled Quantum Dots written by and published by . This book was released on 2017 with total page 304 pages. Available in PDF, EPUB and Kindle. Book excerpt: The focus of this dissertation is discovering and finding functional uses of resonant interactions that occur in coupled quantum dots (i.e. quantum dot molecules, QDMs). Coupled quantum dots compared to individual quantum dots offer a more versatile platform to study interactions between photons, charges, and phonons. Applying an electric field in a QDM system allows tuning of the electronic energy states and controls particle tunneling between quantum dots, thus coupling the quantum dots. For the past two decades, extensive research into coupled quantum dot systems has led to a deeper understanding and control of the charge, spin and photonic properties of single quantum states. This dissertation continues the pursuit to further understand these QDM systems with a particular emphasis on exploring the physics of resonant interactions between electronic states, phononic states, and optical transitions. This work consists of three components, the first is a novel experimental method that is capable of achieving high resolution spectra. The second addresses the resonant interaction between discrete neutral exciton and optical phonons that generates a molecular polaron, which can be tuned to render QDM emissions transparent or amplify weak transitions. The final component addresses ultrafast optical charging into specific charge states of a QDM. In the first research chapter we report on an optical spectroscopy method which is capable of resolving spectral features beyond that of the spin fine structure and homogeneous linewidth of single quantum dots using a standard, easy to use spectrometer setup. This method incorporates both laser and photoluminescence spectroscopy, combining the advantage of laser linewidth limited resolution with multi-channel photoluminescence detection. Using this method allows the ability of greatly improving the resolution beyond that of a common single stage spectrometer. The method uses phonons to assist in the measurement of the photoluminescence of a single quantum dot after resonant excitation of its ground state transition. The phonons allow us to separate and filter out the elastically scattered excitation laser light. An advantageous feature of this method is that it is easily incorporated into standard spectroscopy set-ups, being accessible to a number of researchers. In the second research chapter we report on the coherent interaction between photons and phonons mediated by quantum dot molecular excitons. Fano resonances occur between an indirect discrete state and the optical phonon band's continuum of states. This quantum interference is highly tunable with excitation energy and laser power and allows for the phonons to behave in a coherent and non-dissipative manner. This feature has led to rendering QDM optical transitions transparent and can alternately be used to amplify weak coupling channels. This finding, using phonons in a coherent manner, can lead to new technologies in the emerging field of phononics. In the third research chapter we report on optical charging of quantum dot molecules. The excited state spectra of the neutral and singly charged excitons in QDMs are being studied via photoluminescence excitation spectroscopy (PLE). We find an anti-correlated behavior of the resonances in the PLE spectra of different charge states, allowing for selective optical charging of the QDMs. The PLE spectra are analyzed across the regions of resonances between the indirect exciton and excited direct transitions of the low energy dot of the dot pair. The charging process seen in the excited state spectra of the trion and neutral exciton is explained by the competition between various transition rates at the resonance between the two charge states. These distinct resonances are examples of optical charging and de-charging process within the QDM. We present the experimental results and mathematical model describing this charging process.
Book Synopsis Japanese Journal of Applied Physics by :
Download or read book Japanese Journal of Applied Physics written by and published by . This book was released on 1997 with total page 924 pages. Available in PDF, EPUB and Kindle. Book excerpt:
Book Synopsis Experimental and Theoretical Study of the Optical Properties of Semiconductor Quantum Dots by : Gero Nootz
Download or read book Experimental and Theoretical Study of the Optical Properties of Semiconductor Quantum Dots written by Gero Nootz and published by . This book was released on 2010 with total page 113 pages. Available in PDF, EPUB and Kindle. Book excerpt: The aim of this dissertation is to gain a better understanding of the unique electronic structure of lead salt quantum dots (QDs) and its influences on the nonlinear optical (NLO) properties as well as the time dynamics of the photogenerated charge carriers. A variety of optical techniques such as Z-scan, two-photon excited fluorescence and time-resolved pump probe spectroscopy are used to measure these properties. The one-photon as well as the degenerate and nondegenerate two-photon absorption (2PA) spectra are measured and the electronic wave functions from a four-band envelope function formalism are used to model the results. We observe local maxima in the 2PA spectra for QD samples of many different sizes at energies where only 1PA is predicted by the model. This is similar to the previously measured transitions in the 1PA spectra which are not predicted by the model but accrue at the energies of the two-photon allowed transitions. Most importantly we observe 2PA peaks for all samples at the energy of the first one-photon allowed transition. This result can only be understood in terms of symmetry breaking and therefore is strong evidence that other transitions, not predicted by the model if the selection rules are left intact, also have the origin in the lifted spatial symmetry of the wave functions. On the other hand, the uniquely symmetric eigenenergies of these quantum-confined energy states in the conduction and valance bands explain the observed trend toward larger two-photon cross-sections as the quantum confinement is increased in smaller QDs. Moreover, this unique feature is shown to reduce the possible relaxation channels for photoexcited carriers, which is confirmed experimentally by the reduced carrier relaxation rate as compared to CdSe QDs which lack this symmetry. Carrier multiplication (CM), a process in which several electrons are excited by the absorption of a single photon is studied in PbS QDs. We show that for PbS QDs with radius smaller than 2.5 nm the parameters of CM get very close to the theoretical optimum. Next-generation solar cells operating under these ideal conditions could potentially have conversion efficiency of up to 42%. This compares favorably to the 30% efficiency limit of a single junction silicon solar cell.
Book Synopsis Semiconductor Quantum Dots II: Volume 642 by : Rosa Leon
Download or read book Semiconductor Quantum Dots II: Volume 642 written by Rosa Leon and published by . This book was released on 2001-11-09 with total page 408 pages. Available in PDF, EPUB and Kindle. Book excerpt: The MRS Symposium Proceeding series is an internationally recognised reference suitable for researchers and practitioners.
Download or read book Physics Briefs written by and published by . This book was released on 1993 with total page 1074 pages. Available in PDF, EPUB and Kindle. Book excerpt:
Book Synopsis Quantum Confined States and Room Temperature Spin Coherence in Semiconductor Nanocrystal Quantum Dots by : Ahmad Khastehdel Fumani
Download or read book Quantum Confined States and Room Temperature Spin Coherence in Semiconductor Nanocrystal Quantum Dots written by Ahmad Khastehdel Fumani and published by . This book was released on 2015 with total page 103 pages. Available in PDF, EPUB and Kindle. Book excerpt: In this thesis we studied the signature of quantum confined electronic structure of CdSe nanocrystal quantum dots (NCQDs) in the electron spin behavior via time resolved Faraday rotation (TRFR) measurement at room temperature. We found that there is a distinction between the optical spin pumping efficiency(SPE) spectrum and the optical absorbance. Although the first peak in the SPE coincides with the band edge absorption peak, the rest of the two spectra show significant distinctions. There is a second peak in the signal from the larger NCQDs 200~meV higher in energy. The SPE diminishes afterwards. In smaller NCQDs there is only one peak in the signal. We model this behavior following a 6-band effective mass approximation spin dependent absorbance calculation. We also investigated the dynamics of the spin signal and the presence of two precessing components in the TRFR signal. We find that the inhomogeneous dephasing is not sufficient to explain the dynamics of both components. We associate one of the components to the spin signal from neutral excitons and the other one to positively charged trions. We describe the data using a magnetic field dependent spin decoherence mechanism that occurs via fluctuation of the exciton energy levels in between the available fine structure states. This model captures the TRFR experiment data on both short and long timescales. Additionally, we developed a NCQD-polymer composite optical waveguide and investigated the optical spin measurement on these structures.
Book Synopsis Resonant Photon-exciton Interaction in Semiconductor Quantum Dots by : Martin Kroner
Download or read book Resonant Photon-exciton Interaction in Semiconductor Quantum Dots written by Martin Kroner and published by . This book was released on 2008 with total page 184 pages. Available in PDF, EPUB and Kindle. Book excerpt:
Book Synopsis Band Edge Spectroscopy of CdSe Quantum Dots by : Masaru Kenneth Kuno
Download or read book Band Edge Spectroscopy of CdSe Quantum Dots written by Masaru Kenneth Kuno and published by . This book was released on 1998 with total page 386 pages. Available in PDF, EPUB and Kindle. Book excerpt:
Book Synopsis Control of the Quantum Dot Energy by a Photon: Observation of Two-Exciton and Three-Exciton States in Quantum Dots by : Yasuaki Masumoto
Download or read book Control of the Quantum Dot Energy by a Photon: Observation of Two-Exciton and Three-Exciton States in Quantum Dots written by Yasuaki Masumoto and published by . This book was released on 1998 with total page 4 pages. Available in PDF, EPUB and Kindle. Book excerpt: Antibonding two-exciton and three-exciton states were observed in CuCl quartum dots. This observation demonstrates the controlability of the quartum dot energy by a photon.
Book Synopsis Complex Dynamics in Planar Two-electron Quantum Dots by : Sebastian Schröter
Download or read book Complex Dynamics in Planar Two-electron Quantum Dots written by Sebastian Schröter and published by Sudwestdeutscher Verlag Fur Hochschulschriften AG. This book was released on 2014-10-07 with total page 160 pages. Available in PDF, EPUB and Kindle. Book excerpt: Quantum dots play an important role in a wide range of recent experimental and technological developments. In particular they are promising candidates for realisations of quantum bits and further applications in quantum information theory. The harmonically confined Hooke's atom model is experimentally verified. In order to study effects of an anharmonic confinement potential on spectral properties of planar two-electron quantum dots a sophisticated numerical approach is developed. Classical and quantum features of complexity and chaos are investigated. The concept of offset entanglement, the entanglement of harmonic models in the non-interacting limit, is introduced. It shows that only in the groundstate the electrons are not entangled in the fermionic sense. The applicability, validity, and origin of Hund's first rule in general quantum dot models is further addressed. In fact Hund's first rule is only applicable, and in this case also valid, for one pair of singlet and triplet states in Hooke's atom. For more realistic models of two-electron quantum dots an extension of Hund's first rule for unnatural parity states, the alternating rule, is found to be valid.
Book Synopsis Time-Resolved Electronic Relaxation Processes in Self-Organized Quantum Dots by :
Download or read book Time-Resolved Electronic Relaxation Processes in Self-Organized Quantum Dots written by and published by . This book was released on 2005 with total page 34 pages. Available in PDF, EPUB and Kindle. Book excerpt: The authors have performed a comprehensive set of experiments on the dynamics of electrons and holes in semiconductor quantum dots, and a complete picture of the dynamics as a function of carrier density and temperature has emerged. Specifically, they have used two- and three-pulse femtosecond differential transmission spectroscopy to study the dependence of quantum dot carrier dynamics on temperature. At low temperatures and densities, the rates for relaxation between the quantum dot confined states and for capture from the barrier region into the various dot levels could be directly determined. For electron-hole pairs generated directly in the quantum dot excited state, relaxation is dominated by electron-hole scattering, and occurs on a 5-ps time scale. Capture times from the barrier into the quantum dot are on the order of 2 ps (into the excited state) and 10 ps (into the ground state). The phonon bottleneck was clearly observed in low-density capture experiments, and the conditions for its observation (namely, the suppression of electron-hole scattering for non-geminately captured electrons) were determined. As temperature increases beyond about 100 K, the dynamics become dominated by the reemission of carriers from the lower dot levels due to the large density of states in the wetting layer and barrier region. Measurements of the gain dynamics show fast (130-fs) gain recovery due to intradot carrier-carrier scattering, and picosecond-scale capture. Direct measurement of the transparency density versus temperature shows the dramatic effect of carrier reemission for the quantum dots to thermally activated scattering. The carrier dynamics at elevated temperatures are thus strongly dominated by the high density of the high-energy continuum states relative to the dot-confined levels. Deleterious hot carrier effects can be suppressed in quantum dot lasers by resonant tunneling injection.
Book Synopsis Semiconductor Quantum Dots in Glasses by : Li-Chi John Liu
Download or read book Semiconductor Quantum Dots in Glasses written by Li-Chi John Liu and published by . This book was released on 1993 with total page 386 pages. Available in PDF, EPUB and Kindle. Book excerpt:
Book Synopsis Self-assembled Quantum Dots in Advanced Structures by : Megan Elizabeth Creasey
Download or read book Self-assembled Quantum Dots in Advanced Structures written by Megan Elizabeth Creasey and published by . This book was released on 2012 with total page 254 pages. Available in PDF, EPUB and Kindle. Book excerpt: Advances in nanofabrication have bolstered the development of new optical devices with potential uses ranging from conventional optoelectronics, such as lasers and solar cells, to novel devices, like single photon or entangled photon sources. Quantum encryption of optical communications, in particular, requires devices that couple efficiently to an optical fiber and emit, on demand, indistinguishable photons. With these goals in mind, ultrafast spectroscopy is used to study the electron dynamics in epitaxially grown InAs/GaAs quantum dots (QDs). Quantifying the behavior of these systems is critical to the development of more efficient devices. Studies of two newly developed InGaAs QD structures, quantum dot clusters (QDCs) and QDs embedded in photonic wires, are presented herein. GaAs photonic wires with diameters in the range of 200 to 250 nm support only the fundamental HE11 guided mode. To fully quantify these new systems, the emission dynamics of QDs contained within wires in a large range of diameters are studied. Time correlated single photon counting measurements of the ground state exciton lifetimes are in very good agreement with predicted theoretical values for the spontaneous emission rates. For diameters smaller than 200 nm, QD emission into the HE11 mode is strongly inhibited and non-radiative processes dominate the decay rate. The best small diameter wires exhibit inhibition factors as high as 16, on par with the current state of the art for photonic crystals. The QDCs are the product of a hybrid growth technique that combines droplet heteroepitaxy with standard Stranski-Krastanov growth to create many different geometries of QDs. The work presented in this dissertation concentrates specifically on hexa-QDCs consisting of six InAs QDs around a GaAs nanomound. The first ever spectral and temporal properties of QDs within individual hexa-QDCs are presented. The QDs exhibit narrow exciton resonances with good temperature stability, indicating that excitons are well confined within individual QDs. A distinct biexponential decay is observed even at the single QD level. This behavior suggests that non-radiative decay mechanisms and exciton occupation of dark states play a significant role in the recombination dynamics in the QDCs.
