Vignette d'image

Hofstetter, Daniel

Nom
Hofstetter, Daniel
Affiliation principale
Fonction
Ancien.ne collaborateur.trice
Identifiants
https://libra.unine.ch/handle/123456789/472

Résultat de la recherche

Filtres

31
118
11341
Réinitialiser les filtres

Paramètres

Voici les éléments 1 - 10 sur 118
  • Publication
    Accès libre
    Wavelength tuning and thermal dynamics of continuous-wave mid-IR distributed feedback quantum cascade laser
    (2013-7-17)
    Tombez, Lionel 
    ;
    Cappelli, Francesco
    ;
    Schilt, Stephane 
    ;
    Di Domenico, Gianni 
    ;
    Bartalini, Saverio
    ;
    Hofstetter, Daniel 
    We report on the wavelength tuning dynamics in continuous-wave distributed feedback quantum cascade lasers (QCLs). The wavelength tuning response for direct current modulation of two mid-IR QCLs from different suppliers was measured from 10 Hz up to several MHz using ro-vibrational molecular resonances as frequency-to-intensity converters. Unlike the output intensity, which can be modulated up to several gigahertz, the frequency-modulation bandwidth was found to be on the order of 200 kHz, limited by the laser thermal dynamics. A non-negligible roll-off and a significant phase shift are observed above a few hundred hertz already and explained by a thermal model.
  • Publication
    Accès libre
    Linewidth of a quantum cascade laser assessed from its frequency noise spectrum and impact of the current driver
    (2012-4-21)
    Tombez, Lionel 
    ;
    Schilt, Stephane 
    ;
    Di Francesco, Joab 
    ;
    Di Domenico, Gianni 
    ;
    Hofstetter, Daniel 
    ;
    Thomann, Pierre 
    We report on the measurement of the frequency noise properties of a 4.6-μm distributed-feedback quantum-cascade laser (QCL) operating in continuous wave near room temperature using a spectroscopic set-up. The flank of the R(14) ro-vibrational absorption line of carbon monoxide at 2196.6 cm^−1 is used to convert the frequency fluctuations of the laser into intensity fluctuations that are spectrally analyzed. We evaluate the influence of the laser driver on the observed QCL frequency noise and show how only a low-noise driver with a current noise density below ≈1 nA/√Hz allows observing the frequency noise of the laser itself, without any degradation induced by the current source. We also show how the laser FWHM linewidth, extracted from the frequency noise spectrum using a simple formula, can be drastically broadened at a rate of ≈1.6 MHz/(nA/√Hz) for higher current noise densities of the driver. The current noise of commercial QCL drivers can reach several nA/√Hz , leading to a broadening of the linewidth of our QCL of up to several megahertz. To remedy this limitation, we present a low-noise QCL driver with only 350 pA/√Hz current noise, which is suitable to observe the ≈550 kHz linewidth of our QCL.
  • Publication
    Accès libre
    Temperature dependence of the frequency noise in a mid-IR DFB quantum cascade laser from cryogenic to room temperature
    (2012)
    Tombez, Lionel 
    ;
    Schilt, Stephane 
    ;
    Di Francesco, Joab F.
    ;
    Thomann, Pierre 
    ;
    Hofstetter, Daniel 
    We report on the measurement of the frequency noise power spectral density in a distributed feedback quantum cascade laser over a wide temperature range, from 128 K to 303 K. As a function of the device temperature, we show that the frequency noise behavior is characterized by two different regimes separated by a steep transition at ≈200 K. While the frequency noise is nearly unchanged ~200 K, it drastically increases at lower temperature with an exponential dependence. We also show that this increase is entirely induced by current noise intrinsic to the device. In contrast to earlier publications, a single laser is used here in a wide temperature range allowing the direct assessment of the temperature dependence of the frequency noise.
  • Publication
    Accès libre
    Frequency noise of free-running 4.6 um distributed feedback quantum cascade lasers near room temperature
    (2011-8-10)
    Tombez, Lionel 
    ;
    Di Francesco, Joab 
    ;
    Schilt, Stephane 
    ;
    Di Domenico, Gianni 
    ;
    Faist, J.
    ;
    Thomann, Pierre 
    ;
    Hofstetter, Daniel 
    The frequency noise properties of commercial distributed feedback quantum cascade lasers emitting in the 4.6 um range and operated in cw mode near room temperature (277K) are presented. The measured frequency noise power spectral density reveals a flicker noise dropping down to the very low level of <100 Hz2/Hz at 10 MHz Fourier frequency and is globally a factor of 100 lower than data recently reported for a similar laser operated at cryogenic temperature. This makes our laser a good candidate for the realization of a mid-IR ultranarrow linewidth reference.
  • Publication
    Accès libre
    Frequency noise of free-running 4.6 μm distributed feedback quantum cascade lasers near room temperature
    (2011)
    Tombez, Lionel 
    ;
    Di Francesco, Joab F.
    ;
    Schilt, Stephane 
    ;
    Di Domenico, Gianni 
    ;
    Thomann, Pierre 
    ;
    Hofstetter, Daniel 
    ;
    Faist, Jérôme
    The frequency noise properties of commercial distributed feedback quantum cascade lasers emitting in the 4.6 μm range and operated in cw mode near room temperature (277 K) are presented. The measured frequency noise power spectral density reveals a flicker noise dropping down to the very low level of <100 Hz2/Hz at 10 MHz Fourier frequency and is globally a factor of 100 lower than data recently reported for a similar laser operated at cryogenic temperature. This makes our laser a good candidate for the realization of a mid-IR ultranarrow linewidth reference.
  • Publication
    Accès libre
    Intersubband spectroscopy probing higher order interminiband transitions in AlN-GaN-based superlattices
    (2011)
    Hofstetter, Daniel 
    ;
    Di Francesco, Joab 
    ;
    Kandaswamy, Prem K.
    ;
    Monroy, Eva
    We investigate midinfrared intersubband photodetectors based on short-period AlN/GaN superlattices with different quantum well thicknesses. Band structure calculations, as well as optical transmission and photovoltage measurements, underline the importance of higher order interminiband transitions. In particular, it was found that optical transitions between the second and third minibands benefit from much larger electron displacements and oscillator strengths than those between the first and second minibands. Our results suggest that optical rectification is therefore much more efficient for devices based on a higher order interminiband transition.
  • Publication
    Accès libre
    Si-interdiffusion in heavily doped AlN-GaN-based quantum well intersubband photodetectors
    (2011)
    Hofstetter, Daniel 
    ;
    Di Francesco, Joab F.
    ;
    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.
  • Publication
    Accès libre
    Mid-infrared quantum cascade detectors for applications in spectroscopy and pyrometry
    (2010)
    Hofstetter, Daniel 
    ;
    Giorgetta, Fabrizio R.
    ;
    Baumann, Esther
    ;
    Yang, Quankui
    ;
    Manz, Christian
    ;
    Köhler, Klaus
    An overview of quantum cascade detector technology for the near- and mid-infrared wavelength range will be given. Thanks to photovoltaic instead of photoconductive operation, quantum cascade detectors offer great opportunities in terms of detection speed, room temperature operation, and detectivity. Besides some crucial issues dealing with fabrication and general characteristics, some possibilities for performance improvement will also be briefly presented. In a theory section, some basic considerations adopted from photoconductive detectors confirm the necessity of various trade-offs for the optimization of such devices. Nevertheless, we will show several possible measures to push the key performance figures of these detectors closer to their physical and technological limits.
  • Publication
    Accès libre
    Intersubband Transition-Based Processes and Devices in AlN/GaN-Based Heterostructures
    (2010)
    Hofstetter, Daniel 
    ;
    Baumann, Esther
    ;
    Giorgetta, Fabrizio R.
    ;
    Théron, Ricardo
    ;
    Wu, Hong
    ;
    Schaff, William J.
    ;
    Dawlaty, Jahan
    ;
    George, Paul A.
    ;
    Eastman, Lester F.
    ;
    Rana, Farhan
    ;
    Kandaswamy, Prem K.
    ;
    Guillot, Fabien
    ;
    Monroy, Eva
    We report on the physics, epitaxial growth, fabrication, and characterization of optoelectronic devices based on intersubband transitions in the AlN/GaN material system. While in 1999, only results of optical absorption experiments could be shown, photodetectors and modulators with operation frequencies beyond 10 GHz as well as optically pumped light emitters have been demonstrated recently. This is the reason for a comprehensive report on the most important properties of such devices. Beside some basic theoretical considerations, we will concentrate on the fabrication and characterization of modulators, switches, photodetectors, and light emitters. At the end of this paper, an outlook to future trends and developments in this emerging field will be given.