Control of the oscillator strength of the exciton in a single InGaN-GaN quantum dot.
ABSTRACT We report direct evidence for the control of the oscillator strength of the exciton state in a single quantum dot by the application of a vertical electric field. This is achieved through the study of the radiative lifetime of a single InGaN-GaN quantum dot in a p-i-n diode structure. Our results are in good quantitative agreement with theoretical predictions from an atomistic tight-binding model. Furthermore, the increase of the overlap between the electron and hole wave functions due to the applied field is shown experimentally to increase the attractive Coulomb interaction leading to a change in the sign of the biexcitonic binding energy.
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ABSTRACT: Low threshold lasers realized within compact, high quality optical cavities enable a variety of nanophotonics applications. Gallium nitride (GaN) materials containing indium gallium nitride (InGaN) quantum dots and quantum wells offer an outstanding platform to study light matter interactions and realize practical devices such as efficient light emitting diodes and nanolasers. Despite progress in the growth and characterization of InGaN quantum dots, their advantages as the gain medium in low threshold lasers have not been clearly demonstrated. This work seeks to better understand the reasons for these limitations by focusing on the simpler, limited-mode microdisk cavities, and by carrying out comparisons of lasing dynamics in those cavities using varying gain media including InGaN quantum wells, fragmented quantum wells, and a combination of fragmented quantum wells with quantum dots. For each gain medium, we utilize the distinctive, high quality (Q~5500) modes of the cavities, and the change in the highest intensity mode as a function of pump power to better understand the dominant radiative processes. The variations of threshold power and lasing wavelength as a function of gain medium help us identify the possible limitations to lower-threshold lasing with quantum dot active medium. In addition, we have identified a distinctive lasing signature for quantum dot materials, which consistently lase at wavelengths shorter than the peak of the room temperature gain emission. These findings not only provide better understanding of lasing in nitride-based quantum dot cavity systems, but also shed insight into the more fundamental issues of light matter coupling in such systems.Proceedings of the National Academy of Sciences 07/2014; 111(39). DOI:10.1073/pnas.1415464111 · 9.81 Impact Factor
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ABSTRACT: The quantum confined Stark effect is investigated for the first time in bovine serum albumin (BSA) protected Au8 and Au25 nanoclusters. We observed a red-shift of 63 meV in Au8 nanoclusters upon an increase in pH from 2.14 to 12.0. Such behavior could be well explained in terms of the presence of a linear polar component and a quadratic polarizable component. In contrast, Au25 nanoclusters exhibit more complicated Stark shifts due to their specific core/semiring structure. A plateau of the Stark shift was observed in both absorption and fluorescence, showing an offset of 30 meV. The lifetime measurements confirm that the plateau is due to the screening effect of the semirings in Au25@BSA. Moreover, the dual fluorescent bands of Au25 nanoclusters exhibit two different Stark shifts of 79 and 52 meV, respectively. The experimental data indicate that the Stark shift in both Au8@BSA and Au25@BSA has a significant linear polar component due to their asymmetric structure. This study suggests that gold nanoclusters can become a potentially useful candidate in probing local electric fields and also in pH-sensing in nanoscale environment of biological systems.The Journal of Physical Chemistry C 02/2013; 117(7):3621–3626. DOI:10.1021/jp3116676 · 4.84 Impact Factor
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ABSTRACT: We present a theoretical investigation on the electronic properties of alloyed InxGa1−xN ultrathin single-quantum wells (SQWs) embedded in GaN matrix. The empirical tight-binding method with sp3s∗ basis set, including spin–orbit interaction and nearest-neighbor two-center overlap integrals, is used to study the number of bound states, quantum confinement (QC) energy and the band-gap energy of (InxGa1−xN)Nw/GaN SQWs versus the well composition and parameters; namely width (Nw) and depth (via valence band offset, VBO). The results show strong correlation between the bound states (number and QC energy) and the well’s composition and parameters. Furthermore, the results were used to model experimental photoluminescence (PL) data of three samples containing Nw = 1, 3 and 5 monolayers (MLs), which were fabricated by A. Yoshikawa and coworkers using rate-flow plasma molecular-beam epitaxy (rf-MBE). The results have revealed that in all these three samples, the indium mole fraction would not exceed 25% and, consequently, the three wells are shown to contain at maximum 1, 2 and 3 electronic bound states, respectively. It is deduced that the maintaining of low indium content (x < 0.25) is the secret for the achievement of high structural and optical qualities of the produced samples with free of misfit dislocations.Journal of Alloys and Compounds 03/2015; 626. DOI:10.1016/j.jallcom.2014.11.146 · 2.73 Impact Factor