A. W. Holleitner

Technische Universität München, München, Bavaria, Germany

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Publications (93)443.87 Total impact

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    ABSTRACT: Arrays of metal nanoparticles in an organic matrix have attracted a lot of interest due to their diverse electronic and optoelectronic properties. Recent work demonstrates that nanoparticle arrays can be utilized as a template structure to incorporate single molecules. In this arrangement, the nanoparticles act as electronic contacts to the molecules. By varying parameters such as the nanoparticle material, the matrix material, the nanoparticle size, and the interparticle distance, the electronic behavior of the nanoparticle arrays can be substantially tuned and controlled. Furthermore, via the excitation of surface plasmon polaritons, the nanoparticles can be optically excited and electronically read-out. The versatility and possible applications of well-ordered nanoparticle arrays has been demonstrated by the realization of switching devices triggered optically or chemically and by the demonstration of chemical and mechanical sensing. Interestingly, hexagonal nanoparticle arrays may also become a useful platform to study the physics of collective plasmon resonances that can be described as Dirac-like bosonic excitations.
    Chemical Society Reviews 01/2015; · 24.89 Impact Factor
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    ABSTRACT: We introduce a confocal shift-interferometer based on optical fibers. The presented spectroscopy allows measuring coherence maps of luminescent samples with a high spatial resolution even at cryogenic temperatures. We apply the spectroscopy onto electrostatically trapped, dipolar excitons in a semiconductor double quantum well. We find that the measured spatial coherence length of the excitonic emission coincides with the point spread function of the confocal setup. The results are consistent with a temporal coherence of the excitonic emission down to temperatures of 250 mK.
    11/2014;
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    Nadine Erhard, Alexander Holleitner
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    ABSTRACT: Photocurrent spectroscopy is a versatile technique to identify and understand the optoelectronic dynamics occurring in semiconductor nanowires. Conventional photocurrent spectroscopy allows to explore the morphology and material properties of nanowires as well as their contact interfaces. Using time-resolved photocurrent spectroscopy one gets additional information on the multiple photocurrent generation mechanisms and their respective timescales. This chapter discusses various aspects of the photocurrent spectroscopy and it summarizes the physical mechanisms behind the photocurrent and photoconductance effects in semiconductor nanowires.
    09/2014;
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    ABSTRACT: Nonradiative transfer processes are often regarded as loss channels for an optical emitter1, since they are inherently difficult to be experimentally accessed. Recently, it has been shown that emitters, such as fluorophores and nitrogen vacancy centers in diamond, can exhibit a strong nonradiative energy transfer to graphene. So far, the energy of the transferred electronic excitations has been considered to be lost within the electron bath of the graphene. Here, we demonstrate that the trans-ferred excitations can be read-out by detecting corresponding currents with picosecond time resolution. We electrically detect the spin of nitrogen vacancy centers in diamond electronically and con-trol the nonradiative transfer to graphene by electron spin resonance. Our results open the avenue for incorporating nitrogen vacancy centers as spin qubits into ultrafast electronic circuits and for harvesting non-radiative transfer processes electronically.
    08/2014;
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    ABSTRACT: We report on the photoconductance in two-dimensional arrays of gold nanorods which is strongly enhanced at the frequency of the longitudinal surface plasmon of the nanorods. The arrays are formed by a combination of droplet deposition and stamping of gold nanorod solutions on SiO2 substrates. We find that the plasmon induced photoconductance is sensitive to the linear polarization of the exciting photons. We interpret the occurrence of the photoconductance as a bolometric enhancement of the arrays' conductance upon excitation of the longitudinal surface plasmon resonance of the nanorods.
    physica status solidi (RRL) - Rapid Research Letters 11/2013; · 2.39 Impact Factor
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    ABSTRACT: Undoped diamond, a remarkable bulk electrical insulator, exhibits a high surface conductivity in air when the surface is hydrogen-terminated. Although theoretical models have claimed that a two-dimensional hole gas is established as a result of surface energy band bending, no definitive experimental demonstration has been reported so far. Here, we prove the two-dimensional character of the surface conductivity by low temperature characterization of diamond in-plane gated field-effect transistors that enable the lateral confinement of the transistor's drain-source channel to nanometer dimensions. In these devices, we observe Coulomb blockade effects of multiple quantum islands varying in size with the gate voltage. The charging energy and thus the size of these zero-dimensional islands exhibits a gate voltage dependence which is the direct result of the two-dimensional character of the conductive channel formed at hydrogen-terminated diamond surfaces.
    Physical Review B 10/2013; 89(11). · 3.66 Impact Factor
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    ABSTRACT: There's the rub: Friction of single polymers on solid bodies in a liquid environment was investigated. Apart from expected mechanisms, such as slip and stick, a third nanoscale friction mechanism exists that is independent of normal force, velocity, and adsorbed polymer length. A model is proposed for this mechanism that is based on measurements with various polymers on topographically and chemically nanostructured surfaces.
    Angewandte Chemie International Edition 05/2013; · 11.34 Impact Factor
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    ABSTRACT: Voltage-tunable quantum traps confining individual spatially indirect and long-living excitons are realized by providing a coupled double quantum well with nanoscale gates. This enables us to study the transition from confined multiexcitons down to a single, electrostatically trapped indirect exciton. In the few exciton regime, we observe discrete emission lines identified as resulting from a single dipolar exciton, a biexciton, and a triexciton, respectively. Their energetic splitting is well described by Wigner-like molecular structures reflecting the interplay of dipolar interexcitonic repulsion and spatial quantization.
    Physical Review Letters 03/2013; 110(12). · 7.73 Impact Factor
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    ABSTRACT: Conventional scanning photocurrent microscopy experiments on semiconductor nanowires are typically limited to timescales exceeding 10 ps. Yet, it is known from optical experiments that carrier relaxation and transport processes can occur on much faster timescales in such wires. We therefore apply a recently developed pump-probe photocurrent spectroscopy based on coplanar striplines [1] to investigate the photocurrent dynamics of single GaAs- and InAs-nanowires with a picosecond time-resolution [2]. The ultrafast photocurrent response of the nanowires is sampled in the time-domain with the help of Auston switches. We discuss data on single InAs-nanowires which are interpreted in terms of a photo-thermoelectric current and the transport of photogenerated holes to the electrodes as the dominating ultrafast photocurrent contributions. Moreover, we show that THz radiation is generated in the optically excited InAs-nanowires, which we interpret in terms of a dominating photo-Dember effect [3]. The results are relevant for nanowire-based optoelectronic and photovoltaic applications as well as for the design of nanowire-based THz sources. [1] L. Prechtel, et al. Nature Communications 3, 646 (2012). [2] L. Prechtel, et al. Nano Letters . 12, 2337 (2012). [3] N. Erhard, et al. (2013).
    03/2013;
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    ABSTRACT: To clarify the ultrafast temporal interplay of the different photocurrent mechanisms occurring in single InAs-nanowire-based circuits, an on-chip photocurrent pump-probe spectroscopy based on coplanar striplines was utilized. The data are interpreted in terms of a photo-thermoelectric current and the transport of photogenerated holes to the electrodes as the dominating ultrafast photocurrent contributions. Moreover, it is shown that THz radiation is generated in the optically excited InAs-nanowires, which is interpreted in terms of a dominating photo-Dember effect. The results are relevant for nanowire-based optoelectronic and photovoltaic applications as well as for the design of nanowire-based THz sources.
    Annalen der Physik 02/2013; 525(1-2). · 1.51 Impact Factor
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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). · 3.66 Impact Factor
  • Angewandte Chemie 01/2013;
  • Sebastian Thunich, Claudia Ruppert, Alexander W. Holleitner, Markus Betz
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    ABSTRACT: Femtosecond ω/2ω pulse pairs derived from a compact Er:fibre source induce coherently controlled currents in low temperature grown GaAs. They are characterized by analyzing charge accumulation at contacts closeby. We focus on the photoresponse of bowtie optical antennas integrated into such metal-semiconductor-metal structures. Antennas are designed to enhance the ω field and to confine it into the 50 nm antenna gap. The coherently controlled current is markedly enhanced by the plasmonic nanostructure. However, we find an only unpronounced dependence on the antenna length which is probably related to the large refractive index of GaAs and intricate resonance conditions for ultrabroadband excitation light.
    Applied Physics Letters 12/2012; 101(25). · 3.52 Impact Factor
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    C. Kastl, T. Guan, X. Y. He, K. H. Wu, Y. Q. Li, A. W. Holleitner
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    ABSTRACT: We report on the optoelectronic properties of thin films of Bi2Se3 grown by molecular beam epitaxy. The films are patterned into circuits with typical extensions of tens of microns. In spatially resolved experiments, we observe submicron photocurrent patterns with positive and negative amplitude. The patterns are independent of the applied bias voltage, but they depend on the width of the circuits. We interpret the patterns to originate from a local photocurrent generation due to potential fluctuations.
    Applied Physics Letters 10/2012; 101:251110. · 3.52 Impact Factor
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    ABSTRACT: Photosynthesis is used by plants, algae and bacteria to convert solar energy into stable chemical energy. The initial stages of this process-where light is absorbed and energy and electrons are transferred-are mediated by reaction centres composed of chlorophyll and carotenoid complexes. It has been previously shown that single small molecules can be used as functional components in electric and optoelectronic circuits, but it has proved difficult to control and probe individual molecules for photovoltaic and photoelectrochemical applications. Here, we show that the photocurrent generated by a single photosynthetic protein-photosystem I-can be measured using a scanning near-field optical microscope set-up. One side of the protein is anchored to a gold surface that acts as an electrode, and the other is contacted by a gold-covered glass tip. The tip functions as both counter electrode and light source. A photocurrent of ∼10 pA is recorded from the covalently bound single-protein junctions, which is in agreement with the internal electron transfer times of photosystem I.
    Nature Nanotechnology 09/2012; 7(10):673-6. · 31.17 Impact Factor
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    ABSTRACT: We use GaAs-based quantum point contacts as mesoscopic detectors to analyze the flow of photogenerated electrons in a two-dimensional electron gas at a perpendicular magnetic field. Whereas charge transport experiments always measure the classical cyclotron radius, we show that this changes dramatically when detecting the photoinduced nonequilibrium current in magnetic fields. The radius of the photocurrent flow patterns surprisingly exceeds the classical cyclotron value by far, both in experiment and Monte Carlo simulations. We identify electron-electron scattering as the underlying reason.
    Physical review. B, Condensed matter 09/2012; 86(11). · 3.77 Impact Factor
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    ABSTRACT: We investigate the average propagation length of photogenerated nonequilibrium electrons in a two-dimensional electron gas using a quantum point contact as a local photocurrent detector. To this end, electrons are photogenerated both quasiresonantly and nonresonantly to the optical interband transition in the quantum well comprising the two-dimensional electron gas. The photocurrent is analyzed as a function of the distance between the excitation spot in the two-dimensional electron gas and the detector. We find that the determined propagation length depends nonmonotonically on the laser intensity. We interpret the observation by an interplay of an enlarged scattering phase space of the photogenerated electrons and the screening of sample specific scatterers.
    Physical review. B, Condensed matter 09/2012; 86(11). · 3.77 Impact Factor
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    ABSTRACT: The authors investigate the spontaneous emission dynamics of self-assembled InGaAs quantum dots embedded in GaAs photonic crystal waveguides. For an ensemble of dots coupled to guided modes in the waveguide we report spatially, spectrally, and time-resolved photoluminescence measurements, detecting normal to the plane of the photonic crystal. For quantum dots emitting in resonance with the waveguide mode, a ~21x enhancement of photoluminescence intensity is observed as compared to dots in the unprocessed region of the wafer. This enhancement can be traced back to the Purcell enhanced emission of quantum dots into leaky and guided modes of the waveguide with moderate Purcell factors up to ~4x. Emission into guided modes is shown to be efficiently scattered out of the waveguide within a few microns, contributing to the out-of-plane emission and allowing the use of photonic crystal waveguides as broadband, efficiency-enhancing structures for surface-emitting diodes or single photon sources.
    Journal of Applied Physics 05/2012; 112(9). · 2.21 Impact Factor
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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 photogenerated 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) interdot 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.
    Physical Review Letters 05/2012; 108(19). · 7.73 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;

Publication Stats

1k Citations
443.87 Total Impact Points

Institutions

  • 2008–2014
    • Technische Universität München
      • Walter Schottky Institut (WSI)
      München, Bavaria, Germany
  • 2010
    • Philipps University of Marburg
      Marburg, Hesse, Germany
  • 2001–2010
    • Ludwig-Maximilians-University of Munich
      • Center for Nanoscience (CeNS)
      München, Bavaria, Germany
  • 2004–2006
    • University of California, Santa Barbara
      • Department of Physics
      Santa Barbara, CA, United States
    • University of Wisconsin, Madison
      • Department of Electrical and Computer Engineering
      Madison, MS, United States