Tailored quantum dots for entangled photon pair creation

We compare the asymmetry-induced exchange splitting delta_1 of the bright-exciton ground-state doublet in self-assembled (In,Ga)As/GaAs quantum dots, determined by Faraday rotation, with its homogeneous linewidth gamma, obtained from the radiative decay in time-resolved photoluminescence. Post-growth thermal annealing of the dot structures leads to a considerable increase of the homogeneous linewidth, while a strong reduction of the exchange splitting is simultaneously observed. The annealing can be tailored such that delta_1 and gamma become comparable, whereupon the carriers are still well confined. This opens the possibility to observe polarization entangled photon pairs through the biexciton decay cascade.Comment: 4 pages, 4 figure

Systematic study of carrier correlations in the electron-hole recombination dynamics of quantum dots

The ground state carrier dynamics in self-assembled (In,Ga)As/GaAs quantum dots has been studied using time-resolved photoluminescence and transmission. By varying the dot design with respect to confinement and doping, the dynamics is shown to follow in general a non-exponential decay. Only for specific conditions in regard to optical excitation and carrier population, for example, the decay can be well described by a mono-exponential form. For resonant excitation of the ground state transition a strong shortening of the luminescence decay time is observed as compared to the non-resonant case. The results are consistent with a microscopic theory that accounts for deviations from a simple two-level picture.Comment: 8 pages, 7 figure

High Purcell factor generation of indistinguishable on-chip single photons

On-chip single-photon sources are key components for integrated photonic quantum technologies. Semiconductor quantum dots can exhibit near-ideal single-photon emission, but this can be significantly degraded in on-chip geometries owing to nearby etched surfaces. A long-proposed solution to improve the indistinguishablility is to use the Purcell effect to reduce the radiative lifetime. However, until now only modest Purcell enhancements have been observed. Here we use pulsed resonant excitation to eliminate slow relaxation paths, revealing a highly Purcell-shortened radiative lifetime (22.7 ps) in a waveguide-coupled quantum dot–photonic crystal cavity system. This leads to near-lifetime-limited single-photon emission that retains high indistinguishablility (93.9%) on a timescale in which 20 photons may be emitted. Nearly background-free pulsed resonance fluorescence is achieved under π-pulse excitation, enabling demonstration of an on-chip, on-demand single-photon source with very high potential repetition rates

Formation of allenyl ketones, 3-ethynylcoumarins, and arylfurans, furylfurans, and furylthiophenes by flash vacuum thermolysis of 3-methylidenefuran-2(3 H)-ones

Flash vacuum thermolysis (FVT) of 3-methylidenefuran-2(3H)-ones 3 causes cheletropic extrusion of CO with formation of allenyl ketones 4. o-Chloro- and o-bromophenylmethylidenefuranones also afford allenyl ketones upon flash vacuum thermolysis, but in addition, 3-ethynylcoumarins 6 are formed via E/Z isomerization of the methylidenefuranones, cyclization, halogen atom migration, and HCl (HBr) elimination. The presence of strongly electron-withdrawing groups (nitroaryl or acetyl) on the acylallene moiety causes rearrangement to give 2-arylfurans 10 and 13 as well as 2-furylfurans and 2-furylthiophenes 16 by cyclization of the allenyl ketones. The reaction mechanisms are supported by calculations at the M06-2X/6-311+G(d,p) level of theory

Laser mode feeding by shaking quantum dots in a planar microcavity

Semiconductor light emission can be changed considerably in an optical resonator(1). Prerequisite is that the electronic transitions involved in light generation are in resonance with a cavity mode. Although resonance can be arranged through dedicated fabrication, there are cases where this is virtually impossible. As an example, we study a planar microcavity containing an inhomogeneous quantum dot ensemble with a spectral broadening much larger than the optical mode width, so that resonance is achieved for a tiny dot fraction only. Still, the laser threshold can be crossed at moderate optical pumping. We demonstrate that strain pulses generated by ultrafast acoustics techniques can be used to modulate the transition energies so that resonance with the optical mode is dynamically induced for a much larger dot fraction. As a result, the emission output can be enhanced by more than two orders of magnitude, which is potentially useful for modulating light sources.</p

Direct observation of correlations between individual photon emission events of a microcavity laser

Lasers are recognized for coherent light emission, the onset of which is reflected in a change in the photon statistics(1). For many years, attempts have been made to directly measure correlations in the individual photon emission events of semiconductor lasers(2,3). Previously, the temporal decay of these correlations below or at the lasing threshold was considerably faster than could be measured with the time resolution provided by the Hanbury Brown/Twiss measurement set-up(4) used. Here we demonstrate a measurement technique using a streak camera that overcomes this limitation and provides a record of the arrival times of individual photons. This allows us to investigate the dynamical evolution of correlations between the individual photon emission events. We apply our studies to micropillar lasers(5) with semiconductor quantum dots(2,3,6-8) as the active material, operating in the regime of cavity quantum electrodynamics(9). For laser resonators with a low cavity quality factor, Q, a smooth transition from photon bunching to uncorrelated emission with increasing pumping is observed; for high-Q resonators, we see a non-monotonic dependence around the threshold where quantum light emission can occur. We identify regimes of dynamical anti-bunching of photons in agreement with the predictions of a microscopic theory that includes semiconductor-specific effects.</p