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G. J. Schinner,
J. Repp,
K. Kowalik-Seidl,
E. Schubert,
M. P. Stallhofer,
A. K. Rai,
D. Reuter,
A. D. Wieck,
A. O. Govorov, A. W. Holleitner,
and J. P. Kotthaus
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ABSTRACT: We explore the photoluminescence of spatially indirect, dipolar Mahan excitons in a gated double quantum well diode containing a mesoscopic electrostatic trap for neutral dipolar excitons at low temperatures down to 250 mK and in quantizing magnetic fields. Mahan excitons in the surrounding of the trap, consisting of individual holes interacting with a degenerate two-dimensional electron system confined in one of the quantum wells, exhibit strong quantum Hall signatures at integer filling factors and related anomalies around filling factor ν=2/3,3/5, and 1/2, reflecting the formation of composite fermions. Interactions across the trap perimeter are found to influence the energy of the confined neutral dipolar excitons by the presence of the quantum Hall effects in the two-dimensional electron system surrounding the trap.
Physical Review B 01/2013; 87(4):041303(R). · 3.69 Impact Factor
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ABSTRACT: With gate-defined electrostatic traps fabricated on a double quantum well we
are able to realize an optically active and voltage-tunable quantum dot
confining individual, long-living, spatially indirect excitons. We study the
transition from multi excitons down to a single indirect exciton. In the few
exciton regime, we observe discrete emission lines reflecting the interplay of
dipolar interexcitonic repulsion and spatial quantization. The quantum dot
states are tunable by gate voltage and employing a magnetic field results in a
diamagnetic shift. The scheme introduces a new gate-defined platform for
creating and controlling optically active quantum dots and opens the route to
lithographically defined coupled quantum dot arrays with tunable in-plane
coupling and voltage-controlled optical properties of single charge and spin
states.
04/2012;
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A. Laucht,
S. Pütz,
T. Günthner,
N. Hauke,
R. Saive,
S. Frédérick,
M. Bichler,
M. -C. Amann, A. W. Holleitner,
M. Kaniber,
J. J. Finley
[show abstract]
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ABSTRACT: We investigate single photon generation from individual self-assembled InGaAs
quantum dots coupled to the guided optical mode of a GaAs photonic crystal
waveguide. By performing confocal microscopy measurements on single dots
positioned within the waveguide, we locate their positions with a precision
better than 0.5 \mum. Time-resolved photoluminescence and photon
autocorrelation measurements are used to prove the single photon character of
the emission into the propagating waveguide mode. The results obtained
demonstrate that such nanostructures can be used to realize an on-chip, highly
directed single photon source with single mode spontaneous emision coupling
efficiencies in excess of beta~85 % and the potential to reach maximum emission
rates >1 GHz.
01/2012;
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Nano Letters 01/2012; · 13.20 Impact Factor
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ABSTRACT: We study the photoluminescence (PL) of a two-dimensional liquid of oriented
dipolar excitons in In_{x}Ga_{1-x}As coupled double quantum wells confined to a
microtrap. Generating excitons outside the trap and transferring them at
lattice temperatures down to T = 240 mK into the trap we create cold
quasi-equilibrium bosonic ensembles of some 1000 excitons with thermal de
Broglie wavelengths exceeding the excitonic separation. With decreasing
temperature and increasing density n <= 5*10^10 cm^{-2} we find an increasingly
asymmetric PL lineshape with a sharpening blue edge and a broad red tail which
we interpret to reflect correlated behavior mediated by dipolar interactions.
From the PL intensity I(E) below the PL maximum at E_{0} we extract at T < 5 K
a distinct power law I(E) \sim (E_{0}-E)^-|\alpha| with -|\alpha|\sim -0.8 in
the range E_{0}-E of 1.5-4 meV, comparable to the dipolar interaction energy.
11/2011;
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K. Müller,
A. Bechtold,
C. Ruppert,
M. Zecherle,
G. Reithmaier,
M. Bichler,
H. J. Krenner,
G. Abstreiter, A. W. Holleitner,
J. M. Villas-Bôas,
J. J. Finley
[show abstract]
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ABSTRACT: We employ ultrafast pump-probe spectroscopy to directly monitor electron
tunneling between discrete orbital states in a pair of spatially separated
quantum dots. Immediately after excitation, several peaks are observed in the
pump-probe spectrum due to Coulomb interactions between the photo-generated
charge carriers. By tuning the relative energy of the orbital states in the two
dots and monitoring the temporal evolution of the pump-probe spectra the
electron and hole tunneling times are separately measured and resonant
tunneling between the two dots is shown to be mediated both by elastic and
inelastic processes. Ultrafast (< 5 ps) inter-dot tunneling is shown to occur
over a surprisingly wide bandwidth, up to ~8 meV, reflecting the spectrum of
exciton-acoustic phonon coupling in the system.
11/2011;
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ABSTRACT: We show that a magnetic field perpendicular to an AlGaAs/GaAs coupled quantum
well efficiently traps dipolar excitons and leads to the stabilization of the
excitonic formation and confinement in the illumination area. Hereby, the
density of dipolar excitons is remarkably enhanced up to $\sim 10^{11}
cm^{-2}$. By means of Landau level spectroscopy we study the density of excess
holes in the illuminated region. Depending on the excitation power and the
applied electric field, the hole density can be tuned over one order of
magnitude up to $\sim 2.5$ $10^{11} cm^{-2}$ - a value comparable with typical
carrier densities in modulation-doped structures.
Physical Review B 01/2011; 83(83):081307(R). · 3.69 Impact Factor
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ABSTRACT: We apply a pump- and probe-scheme to coplanar stripline circuits to investigate the photocurrent response of GaAs photoswitches in time and space. We find a displacement current pulse, as has been reported earlier. A time-delayed second pulse is interpreted by a transport current. A time-of-flight analysis allows us to determine the velocity of the photogenerated charge carriers in the transport current. It exceeds the Fermi and the single-particle quantum velocities. This suggests that the excitation of a collective electron-hole plasma and not single charge-carriers dominates the ultrafast transport current in GaAs photoswitches. Comment: 10 pages, 3 figures
05/2010;
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ABSTRACT: A phase-stable superposition of femtosecond pulses from a compact erbium-doped fiber source and their second harmonic is shown to induce ultrashort approximately microA current bursts in single unbiased GaAs nanowires. Current injection relies on a quantum interference of one- and two-photon absorption pathways. The vector direction of the current is solely dictated by the polarization and relative phase of the harmonically related light components while its power dependence is consistent with a third order optical nonlinearity.
Nano Letters 04/2010; 10(5):1799-804. · 13.20 Impact Factor
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ABSTRACT: A phase-stable superposition of femtosecond pulses and their second harmonic induces ultrashort microampere current bursts in single unbiased GaAs nanowires. Current injection relies on quantum interference of one- and two-photon absorption pathways. Comment: 2 pages, 1 figure
02/2010;
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Applied Physics Letters 01/2010; 97:011104. · 3.84 Impact Factor
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ABSTRACT: We report on photocurrent and photoconductance processes in a freely suspended p -doped single GaAs nanowire. The nanowires are grown by molecular beam epitaxy, and they are electrically contacted by a focused ion beam deposition technique. The observed photocurrent is generated at the Schottky contacts between the nanowire and metal source-drain electrodes, while the observed photoconductance signal can be explained by a photogating effect induced by optically generated charge carriers located at the surface of the nanowire. Both optoelectronic effects are sensitive to the polarization of the exciting laser field, enabling polarization dependent photodetectors.
Applied Physics Letters 09/2009; · 3.84 Impact Factor
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ABSTRACT: We experimentally investigate the lateral diffusion of dipolar excitons in coupled quantum wells in two (2D) and one (1D) dimensions. In 2D, the exciton expansion obeys nonlinear temporal dynamics due to the repulsive dipole pressure at a high exciton density, in accordance with recent reports. In contrast, the observed 1D expansion behaves linearly in time even at high exciton densities. The corresponding 1D diffusion coefficient exceeds the one in 2D by far and depends linearly on the exciton density. We attribute the findings to screening of quantum well disorder by the dipolar excitons.
Physical Review Letters 09/2009; 103(12):126402. · 7.37 Impact Factor
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ABSTRACT: The photocurrent properties of freely suspended single-walled carbon nanotubes (CNTs) are investigated as a function of uniaxial strain. We observe that at low strain, the photocurrent signal of the CNTs increases for increasing strain, while for large strain, the signal decreases, respectively. We interpret the nonmonotonous behavior by a superposition of the influence of the uniaxial strain on the resistivity of the CNTs and the effects caused by Schottky contacts between the CNTs and the metal contacts.
Applied Physics Letters 07/2009; · 3.84 Impact Factor
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ABSTRACT: We present a chemical route to covalently couple the photosystem I (PS I) to carbon nanotubes (CNTs). Small linker molecules are used to connect the PS I to the CNTs. Hybrid systems, consisting of CNTs and the PS I, promise new photo-induced transport phenomena due to the outstanding optoelectronic properties of the robust cyanobacteria membrane protein PS I.
03/2009;
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ABSTRACT: We report on a photoconductive gain in semiconductor quantum wires which are lithographically defined in an AlGaAs/GaAs quantum well via a shallow-etch technique. The effect allows resolving the one-dimensional subbands of the quantum wires as maxima in the photoresponse across the quantum wires. We interpret the results by optically induced holes in the valence band of the quantum well which shift the one-dimensional subbands of the quantum wire. Here we demonstrate that the effect persists up to a temperature of about 17 Kelvin.
11/2008;
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A. W. Holleitner
[show abstract]
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ABSTRACT: We report on a dynamic photoconductive gain effect in quantum wires which are lithographically fabricated in an AlGaAs/GaAs quantum well via a shallow-etch technique. The effect allows resolving the one-dimensional subbands of the quantum wires as maxima in the photoresponse across the quantum wires. We interpret the results by optically induced holes in the valence band of the quantum well which shift the chemical potential of the quantum wire. The nonlinear current-voltage characteristics of the quantum wires also allow detecting the photoresponse effect of excess charge carriers in the conduction band of the quantum well. The dynamics of the photoconductive gain are limited by the recombination time of both electrons and holes.
Phys. Rev. B. 09/2008; 78(11).
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ABSTRACT: We investigate the photoresponse of individual single-walled carbon nanotube-nanocrystal-hybrids. The nanocrystals are bound to the nanotubes via molecular recognition. We find that the photoresponse of the hybrids can be adjusted by the absorption characteristics of the nanocrystals. Surprisingly, the photoresponse of the hybrids exhibits a slow time constant of about 1 ms after excitation of the nanocrystals, and a fast time constant, if only the nanotubes are excited. The data suggest a bolometrically induced current increase in the nanotubes caused by photon absorption in the nanocrystals.
08/2008;
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[show abstract]
[hide abstract]
ABSTRACT: We report on a dynamic photoconductive gain effect in quantum wires which are lithographically fabricated in an AlGaAs/GaAs quantum well via a shallow-etch technique. The effect allows resolving the one-dimensional subbands of the quantum wires as maxima in the photoresponse across the quantum wires. We interpret the results by optically induced holes in the valence band of the quantum well which shift the chemical potential of the quantum wire. The non-linear current-voltage characteristics of the quantum wires also allow detecting the photoresponse effect of excess charge carriers in the conduction band of the quantum well. The dynamics of the photoconductive gain are limited by the recombination time of both electrons and holes.
06/2008;
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[show abstract]
[hide abstract]
ABSTRACT: We report on optically induced transport phenomena in freely suspended
channels containing a two-dimensional electron gas (2DEG). The submicron
devices are fabricated in AlGaAs/GaAs heterostructures by etching techniques.
The photoresponse of the devices can be understood in terms of the combination
of photogating and a photodoping effect. The hereby enhanced electronic
conductance exhibits a time constant in the range of one to ten milliseconds.
05/2008;
