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Hofstetter, Daniel

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Hofstetter, Daniel
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Ancien.ne collaborateur.trice
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https://libra.unine.ch/handle/123456789/472

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  • Publication
    Métadonnées seulement
    Si-Interdiffusion in heavily doped AIN-GaN-based quantum well intersubband photodetectors
    (2011)
    Hofstetter, Daniel 
    ;
    Di Francesco, Joab 
    ;
    Martin, Denis
    ;
    Grandjean, Nicolas
    ;
    Kotsar, Yulia
    ;
    Monroy, Eva
    We demonstrate the effect of rapid thermal annealing on heavily Si-doped AlN/GaN quantum wells. After 1000 °C annealing during 5, 10, and 20 min, the dominant effect was interdiffusion of Si rather than intermixing between the Al and Ga atoms. Both their original value and the magnitude of the changes after annealing reveal that intersubband absorption and photovoltage are related to two different optical transitions as follows: absorption occurs in the 1 to 2 intersubband transition, whereas photovoltage is due to a subsequent process from the 1 to 2 and the manifold of 2 to higher order transitions. © 2011 American Institute of Physics.
  • Publication
    Métadonnées seulement
    CO2 isotope sensor using a broadband infrared source, a spectrally narrow 4.4 μm quantum cascade detector and a Fourier spectrometer
    (2011)
    Hofstetter, Daniel 
    ;
    Di Francesco, Joab 
    ;
    Hvozdara, L.
    ;
    Herzig, H.-P.
    ;
    Beck, M.
    Abstract: We report a prototype CO2 gas sensor based on a simple blackbody infrared source and a spectrally narrow quantum cascade detector (QCD). The detector absorption spectrum is centered at 2260 cm(-1) (4.4 um) and has a full width at half maximum of 200 cm(-1)(25 meV). It covers strong absorption bands of two spectrally overlapping CO2 isotopomers, namely the P-branch of 12CO2 and the R-branch of 13CO2. Acquisition of the spectral information and data treatment were performed in a Fourier transform infrared (FTIR) spectrometer. By flushing its sample compartment either with nitrogen, dry fresh air, ambient air, or human breath, we were able to determine CO2 concentrations corresponding to the different gas mixtures. A detection limit of 500 ppb was obtained in these experiments.
  • Publication
    Métadonnées seulement
    Performance Improvement of AIN-GaN-Based Intersubband Detectors by Using Qantum Dots
    (2010)
    Hofstetter, Daniel 
    ;
    Di Francesco, Joab 
    ;
    Kandaswamy, Prem K.
    ;
    Das, Aparna
    ;
    Valdueza-Felip, Sirona
    ;
    Monroy, Eva
  • Publication
    Métadonnées seulement
    Resonant tunnelling and intersubband absorption in AlN - GaN superlattices
    (2005)
    Baumann, Esther
    ;
    Giorgetta, Fabrizio R.
    ;
    Hofstetter, Daniel 
    ;
    Wu, Hong
    ;
    Schaff, William J.
    ;
    Eastman, Lester F.
    ;
    Kirste, Lutz
    We report on intersubband absorption and photovoltage measurements on regular GaN/AlN-based superlattice structures at 1.55 µm. For high barriers, the photovoltage peaks at a higher energy than the absorbance spectrum due to the decrease of the tunnelling probability. The observed photovoltage is thus the macroscopic manifestation that the 2-dimensional electron gas at the top of the superlattice gets depleted by a vertical transport of electrons.
  • Publication
    Métadonnées seulement
    Continuous-wave operation of quantum cascade laser emitting near 5.6
    (2003)
    Yarekha, Dmitri A
    ;
    Beck, Mattias
    ;
    Blaser, Stéphane
    ;
    Aellen, Thierry
    ;
    Gini, Emilio
    ;
    Hofstetter, Daniel 
    ;
    Faist, Jérôme
    Buried heterostructure quantum cascade lasers emitting at 5.64 mum are presented. Continuous-wave (CW) operation has been achieved at -30degreesC for junction down mounted devices with both facets coated. A 750 mum-long laser exhibited 3 mW of CW power with a threshold current density of 5.4 kA/cm(2).
  • Publication
    Métadonnées seulement
    Bound-to-continuum and two-phonon resonance quantum-cascade lasers for high duty cycle, high-temperature operation
    (2002)
    Faist, Jérôme
    ;
    Hofstetter, Daniel 
    ;
    Beck, Mattias
    ;
    Aellen, Thierry
    ;
    Rochat, Michel
    ;
    Blaser, Stéphane
    Recent advances in quantum-cascade (QC) laser active-region design are reviewed. Based on a rate equation model of the active region, we show why new gain regions based on a two-phonon resonance or a bound-to-continuum transition exhibit significantly better performance than the traditional design based on a three-quantum-well active region. Threshold current densities as low as 3 kA/cm(2) at T = 300 K, operation with a peak power of 90 mW at 425 K, single-mode high-power operation up to temperatures above 330 K at lambda approximate to 16 mum and continuous wave operation up to T = 311 K are demonstrated. QC lasers able to operate at high duty cycles (50%) on a Peltier cooler were used in a demonstration of a 300-MHz free-space optical link between two buildings separated by 350 m.
  • Publication
    Métadonnées seulement
    Continuous wave operation of a mid-infrared semiconductor laser at room temperature
    (2002)
    Beck, Mattias
    ;
    Hofstetter, Daniel 
    ;
    Aellen, Thierry
    ;
    Faist, Jérôme
    ;
    Oesterle, Ursula
    ;
    Ilegems, Marc
    ;
    Gini, Emilio
    ;
    Melchior, Hans
    Continuous wave operation of quantum cascade lasers is reported up to a temperature of 312 kelvin. The devices were fabricated as buried heterostructure lasers with high-reflection coatings on both laser facets, resulting in continuous wave operation with optical output power ranging from 17 milliwatts at 292 kelvin to 3 milliwatts at 312 kelvin, at an emission wavelength of 9.1 micrometers. The results demonstrate the potential of quantum cascade lasers as continuous wave mid-infrared light sources for high-resolution spectroscopy, chemical sensing applications, and free-space optical communication systems.
  • Publication
    Métadonnées seulement
    Continuous wave operation of a 9.3 mu m quantum cascade laser on a Peltier cooler
    (2001)
    Hofstetter, Daniel 
    ;
    Beck, Mattias
    ;
    Aellen, Thierry
    ;
    Faist, Jérôme
    ;
    Oesterle, Ursula
    ;
    Ilegems, Marc
    ;
    Gini, Emilio
    ;
    Melchior, Hans
    High average power quantum cascade lasers at 9.3 mum using InP top cladding layers and both junction up and junction down mounting are presented. A 3 mm long, junction up mounted device emitted 54 mW average power at 30 degreesC and 11.5% duty cycle with a threshold current density of 3.72 kA/cm(2). A similar, but only 1.5 mm long device with high reflection coating on both facets was mounted junction down and tested at even higher duty cycles. At -27 degreesC, we achieved continuous wave operation with a threshold current density of 3.3 kA/cm(2). (C) 2001 American Institute of Physics.
  • Publication
    Métadonnées seulement
    Long-wavelength (
    (2001)
    Rochat, Michel
    ;
    Hofstetter, Daniel 
    ;
    Beck, Mattias
    ;
    Faist, Jérôme
    Room-temperature operation of long-wavelength, Fabry-Perot and single-mode quantum-cascade lasers at lambda approximate to 16 mum is reported. Multimode emission with pulsed peak power up to 400 mW at -40 degreesC and 220 mW at 30 degreesC is demonstrated. Single-mode emission up to 60 mW peak power has been achieved at 30 degreesC. (C) 2001 American Institute of Physics.
  • Publication
    Métadonnées seulement
    Edge- and surface-emitting 10.1
    (2000)
    Hofstetter, Daniel 
    ;
    Faist, Jérôme
    ;
    Beck, Mattias
    ;
    Muller, Antoine
    ;
    Oesterle, Ursula
    We present measurement results on high-power low-threshold quantum cascade-distributed feedback lasers emitting infrared radiation at 10.16 mu m. A lateral current injection scheme allowed the use of a strongly coupled surface grating without metal coverage and epitaxial re-growth. Although this design resulted in a simplified processing, the fabrication of high-performance edge- and surface-emitting devices was demonstrated. For the edge-emitting laser, we used a standard first-order grating with a period of 1.57 mu m, and for the surface emitter, a second-order grating with a period of 3.15 mu m was used. Maximal output powers in excess of 200 mW at 85 K and 70 mW at 300 K were achieved for both configurations. The threshold current densities at 85 K (300 K) were 1.85 kA/cm(2) (5.4 kA/cm(2)) and 2.1 kA/cm(2) (5.6 kA/cm(2)) for edge and surface emitters, respectively. (C) 2000 Elsevier Science B.V. All rights reserved.