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Physical Review B 02/2013; 87:075315. · 3.69 Impact Factor
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S Winnerl,
F Göttfert,
M Mittendorff, H Schneider,
M Helm,
T Winzer,
E Malic,
A Knorr,
M Orlita,
M Potemski,
M Sprinkle,
C Berger,
W A de Heer
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ABSTRACT: We present the results of pump-probe experiments on multilayer graphene samples performed in a wide spectral range, namely from the near infrared (photon energy 1.5 eV) to the terahertz (photon energy 8 meV) spectral range. In the near infrared, exciting carriers and probing at higher photon energies provides direct evidence for a hot carrier distribution. Furthermore, spectroscopic signatures of the highly doped graphene layers at the interface to SiC are observed in the near-infrared range. In the mid-infrared range, the various relaxation mechanisms, in particular scattering via optical phonons and Auger-type processes, are identified by comparing the experimental results to microscopic modeling. Changes from induced transmission to induced absorption are attributed to probing above or below the Fermi edge of the graphene layers. This effect occurs for certain photon energies in the near-infrared range, where it is related to highly doped graphene layers at the interface to SiC, and in the far-infrared range for the quasi-intrinsic graphene layers. In addition to the relaxation dynamics, the saturation of pump-induced bleaching of graphene is studied. Here a quadratic dependence of the saturation fluence on the pump photon energy in the infrared spectral range is revealed.
Journal of Physics Condensed Matter 01/2013; 25(5):054202. · 2.55 Impact Factor
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S Winnerl,
M Orlita,
P Plochocka,
P Kossacki,
M Potemski,
T Winzer,
E Malic,
A Knorr,
M Sprinkle,
C Berger,
W A de Heer, H Schneider,
M Helm
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ABSTRACT: We study the carrier dynamics in epitaxially grown graphene in the range of photon energies from 10 to 250 meV. The experiments complemented by microscopic modeling reveal that the carrier relaxation is significantly slowed down as the photon energy is tuned to values below the optical-phonon frequency; however, owing to the presence of hot carriers, optical-phonon emission is still the predominant relaxation process. For photon energies about twice the value of the Fermi energy, a transition from pump-induced transmission to pump-induced absorption occurs due to the interplay of interband and intraband processes.
Physical Review Letters 12/2011; 107(23):237401. · 7.37 Impact Factor
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ABSTRACT: Time and wavelength resolved spectroscopy requires optical sources emitting very short pulses and a fast detection mechanism capable of measuring the evolution of the output spectrum as a function of time. We use table-top Ti:sapphire lasers and a free-electron laser (FEL) emitting ps pulses as excitation sources and a streak camera coupled to a spectrometer for detection. One of the major aspects of this setup is the synchronization of pulses from the two lasers which we describe in detail. Optical properties of the FEL pulses are studied by autocorrelation and electro-optic sampling measurements. We discuss the advantages of using this setup to perform photoluminescence quenching in semiconductor quantum wells and quantum dots. Carrier redistribution due to pulsed excitation in these heterostructures can be investigated directly. Sideband generation in quantum wells is also studied where the intense FEL pulses facilitate the detection of the otherwise weak nonlinear effect.
The Review of scientific instruments 10/2011; 82(10):103107. · 1.52 Impact Factor
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ABSTRACT: In this paper we present a portable quantum cascade laser (QCL) based infrared magnetospectrometer covering the spectral range from 5 to 120 μm. The variation of the excitation wavelength is enabled by an easy change of the QCL plug-in modules, while the use of any other external source is also possible. The performance of the setup is illustrated via cyclotron-resonance studies under pulsed magnetic fields up to 60 T.
The Review of scientific instruments 03/2011; 82(3):033108. · 1.52 Impact Factor
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ABSTRACT: In this paper, we report a correction to the model potential of the Ga acceptor in germanium, evidenced by high-magnetic-field photoconductivity measurements. We found that under high magnetic fields the chemical shift of the binding energy of Ga acceptors vanishes, contrary to the results given by the generally accepted theory. To fit our data, we found that the central-cell correction should contain a repulsive part (i.e., it must be bipolar), in contrast to the purely attractive screened point-charge potential widely used in the literature.
Physical Review B 01/2011; · 3.69 Impact Factor
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ABSTRACT: We report on phase matched THz emission from GaAs using the anomalous dispersion introduced by optical phonon absorption at the reststrahlenband in GaAs. For this system tunability of the emitted THz frequencies by changing the near infrared excitation wavelength is predicted. We investigate this phenomenon for an oversized double metallized GaAs waveguide. A shift in the THz spectra is observed when the near-infrared wavelength is varied. Enhanced emission is found when phase matching is achieved at 1.4 µm.
Optics Express 09/2010; 18(19):19574-80. · 3.59 Impact Factor
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ABSTRACT: A two-photon detector based on intersubband transitions in GaAs/AlGaAs quantum wells operating in the Terahertz regime below the Reststrahlenband is reported. Resonantly enhanced optical nonlinearities enables sensitive quadratic detection at pJ pulse energies. We demonstrate its use in a quadratic autocorrelator for far-infrared picosecond pulses at around 7 THz.
Optics Express 08/2009; 17(15):12279-84. · 3.59 Impact Factor
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ABSTRACT: We report on emission and detection of pulsed terahertz radiation of radial and azimuthal polarization by microstructured photoconductive antennas. To this end the electrode geometry of the emitter is inverse to the desired THz field pattern and a second periodic structure prevents destructive interference effects. Beam profiles of freely propagating THz waves are studied for divergent and refocused beams. They can be well described as the lowest order Bessel-Gauss modes with a divergence comparable to linearly polarized Gaussian beams. Additionally, mode sensitive detection is demonstrated for radially polarized radiation.
Optics Express 03/2009; 17(3):1571-6. · 3.59 Impact Factor
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O Drachenko,
D V Kozlov,
V Ya,
Aleshkin,
V I Gavrilenko,
K V Maremyanin,
A V Ikonnikov,
B N Zvonkov,
M Goiran,
J Leotin,
G Fasching,
S Winnerl, H Schneider,
J Wosnitza,
M Helm
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ABSTRACT: We report a systematic study of the cyclotron resonance CR absorption of two-dimensional holes in strained InGaAs/GaAs quantum wells QWs in the quantum limit. The energies of the CR transitions are traced as a function of magnetic field up to 55 T. A remarkable CR line splitting was evidenced when the resonant field exceeds 20 T. We analyze our data with a 4 4 Luttinger Hamiltonian including strain and QW potentials using two different methods to calculate Luttinger parameters for ternary alloys. We found excellent agreement with the experiment when linear interpolation of the Luttinger parameters is used. Nowadays strained-layer InGaAs/GaAs heterostructures remain potentially interesting for many applications, such as high-frequency electronics, solar cells, and infrared lasers see for review Ref. 1. Additionally, during the last few years, the rapidly growing area of spintronics has increased the interest in these structures due to demonstration of effi-cient spin injection 2,3 and circular-polarized electrolumines-cence in InGaAs/GaAs Schottky diodes, as well as the dis-covery of the anomalous Hall effect in Mn -doped InGaAs/
Physical Review B 02/2009; 79(79):073301. · 3.69 Impact Factor
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ABSTRACT: We report on scalable photoconductive antennas for both emission and detection of terahertz (THz) radiation. The concept yields THz emitters with high efficiencies for the conversion of near-infrared into far-infrared radiation, and provides detectors that do not require tight focusing of both the THz beam and the near-infrared gating beam. GaAs substrates implanted with dual energy implants of N<sup>+</sup> and As<sup>+</sup> ions of various doses are compared with semiinsulating (SI) and low-temperature-grown GaAs. We discuss which material properties are desirable for emitters and detectors and identify which material is optimal as either emitter or detector substrate. Best results for detectors are found for implanted samples with doses in the range of 10<sup>13</sup> cm<sup>-2</sup> for GaAs:N and for LT-GaAs. Best emitters for typical excitation conditions with a Ti: sapphire oscillator system are based on SI-GaAs.
IEEE Journal of Selected Topics in Quantum Electronics 04/2008; · 3.78 Impact Factor
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ABSTRACT: Subpicosecond electron-hole recombination time and terahertz-bandwidth photoresponse in freestanding GaAs epitaxial mesoscopic structures Appl. Phys. Lett. 101, 031111 (2012) Terahertz wavefront measurement with a Hartmann sensor Appl. Phys. Lett. 101, 031103 (2012) Robust and economical multi-sample, multi-wavelength UV/vis absorption and fluorescence detector for biological and chemical contamination AIP Advances 2, 032110 (2012) Top illuminated inverted organic ultraviolet photosensors with single layer graphene electrodes Appl.
Applied Physics Letters 01/2008; 93(93):101114. · 3.84 Impact Factor
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ABSTRACT: We report on a large-area THz antenna which is based on N+ implanted GaAs as a photoconductive material. The antenna consists of an interdigitated electrode structure, where every
other electrode gap is covered by a second metallization layer. The antenna concept avoids resonances and offers a high degree
of freedom with respect to the focusing of both the THz and the gating beam.
12/2007: pages 167-169;
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ABSTRACT: We present a nonresonant photoconductive terahertz detection antenna suitable for detection of both focused and unfocused terahertz radiations. Our system consists of a scalable terahertz emitter based on an interdigitated electrode structure and a detection antenna with similar electrode geometry. While the emitter is fabricated on semi-insulating GaAs we compare different ion-implanted GaAs-based detection antennas. We studied the dependence of the measured terahertz signal on the power and spot size of the gating laser pulse. In addition we compare the performance of our antenna with that of electro-optical sampling.
Applied Physics Letters 08/2007; 91(8):081109-081109-3. · 3.84 Impact Factor
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Applied Physics Letters 01/2006; 89(89):133508. · 3.84 Impact Factor
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ABSTRACT: The third generation of infrared detection modules is characterized by advanced functionalities like more pixels (array size⩾640×512), higher frame rates (⩾800 Hz at an array size of 256×256), multicolor or multiband capability, and better thermal resolution. This paper focuses on the present status of such technologies at AIM, based on HgCdTe and quantum well focal plane array detectors.
Infrared Physics & Technology 43:257-263. · 1.30 Impact Factor
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ABSTRACT: Two-photon quantum well infrared photodetectors (QWIPs) involving three equidistant subbands take advantage of a resonantly enhanced optical nonlinearity, which is six orders of magnitude stronger than in a bulk semiconductor. This approach results in a sensitive device to measure quadratic autocorrelation of mid-infrared optical pulses from modelocked quantum cascade lasers, nonlinear optical conversion, and free-electron lasers (FEL). We report on autocorrelation measurements at wavelengths in the mid-infrared and Terahertz regimes using ps optical pulses from the FEL at the Forschungszentrum Dresden Rossendorf. In particular, quadratic detection at wavelengths around 5.5 μm is still possible at room-temperature, which is crucial for applications in practical systems. We also report on a two-photon detector which works below the Reststrahlen band at 42 μm (7.1 THz).
Infrared Physics & Technology.
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93(10):101114.
