J. M. Hvam

Publications (4)3.69 Total impact

• Source

  Available from: ArXiv

  Article: Spontaneous emission from large quantum dots in nanostructures: Exciton-photon interaction beyond the dipole approximation

  S. Stobbe, P. T. Kristensen, J. E. Mortensen, J. M. Hvam, J. Mørk, P. Lodahl
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  ABSTRACT: We derive a rigorous theory of the interaction between photons and spatially extended excitons confined in quantum dots in inhomogeneous photonic materials. We show that beyond the dipole approximation, the radiative decay rate is proportional to a nonlocal interaction function, which describes the interaction between light and spatially extended excitons. In this regime, light and matter degrees of freedom cannot be separated and a complex interplay between the nanostructured optical environment and the exciton envelope function emerges. We illustrate this by specific examples and derive a series of important analytical relations, which are useful for applying the formalism to practical problems. In the dipole limit, the decay rate is proportional to the projected local density of optical states, and we obtain the strong and weak confinement regimes as special cases.
  Phys. Rev. B. 12/2011; 86(8).
• Source

  Available from: Sven Höfling

  Article: Large quantum dots with small oscillator strength

  S. Stobbe, T.W. Schlereth, S Höfling, A. Forchel, J. M. Hvam, P. Lodahl
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  ABSTRACT: We have measured the oscillator strength and quantum efficiency of excitons confined in large InGaAs quantum dots by recording the spontaneous emission decay rate while systematically varying the distance between the quantum dots and a semiconductor-air interface. The size of the quantum dots is measured by in-plane transmission electron microscopy and we find average in-plane diameters of 40 nm. We have calculated the oscillator strength of excitons of that size and predict a very large oscillator strength due to Coulomb effects. This is in stark contrast to the measured oscillator strength, which turns out to be much below the upper limit imposed by the strong confinement model. We attribute these findings to exciton localization in local potential minima arising from alloy intermixing inside the quantum dots. Comment: 4 pages, 3 figures, submitted
  Physical Review B 06/2010; 80:155307. · 3.69 Impact Factor
• Source

  Available from: dtu.dk

  Conference Proceeding: Dark-bright exciton spin-flip rates of quantum dots determined by a modified local density of optical states

  P. Lodahl, J. Johansen, B. Julsgaard, J.M. Hvam
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  ABSTRACT: This work investigates the influence of dark excitons on the radiative dynamics of semiconductor quantum dots (QDs). Dark excitons have total angular momentum of 2 and contribute to the fine structure of the exciton ground state. As opposed to bright excitons that have total angular momentum 1, dark excitons cannot recombine directly via electric dipole transitions. However, slow recombination does take place since the dark exciton can undergo a spin-flip process thereby transferring it into a bright exciton. A technique is demonstrated to extract the dark-bright exciton spin-flip rate based on time-resolved detection of spontaneous emission. Using the modified local density of optical states of a GaAs-air interface and its known dependence on distance (z), it was recently shown that the bright exciton radiative and non-radiative processes could be fully unravelled. Hence, the spin-flip rate can be extracted. The spontaneous emission decay curves are found to be bi-exponential and the dark excitons contribute to the slow component with a rate dominated by non-radiative processes. The spin-flip rate can be extracted reliably from the ratio of the amplitudes of the slow and fast components.
  Lasers and Electro-Optics 2009 and the European Quantum Electronics Conference. CLEO Europe - EQEC 2009. European Conference on; 07/2009
• Source

  Available from: ArXiv

  Article: Size-Dependence of the Wavefunction of Self-Assembled Quantum Dots

  J. Johansen, S. Stobbe, I. S. Nikolaev, T. Lund-Hansen, P. T. Kristensen, J. M. Hvam, W. L. Vos, P. Lodahl
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  ABSTRACT: The radiative and non-radiative decay rates of InAs quantum dots are measured by controlling the local density of optical states near an interface. From time-resolved measurements we extract the oscillator strength and the quantum efficiency and their dependence on emission energy. From our results and a theoretical model we determine the striking dependence of the overlap of the electron and hole wavefunctions on the quantum dot size. We conclude that the optical quality is best for large quantum dots, which is important in order to optimally tailor quantum dot emitters for, e.g., quantum electrodynamics experiments.
  08/2007;