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Haute Ecole pédagogique Fribourg

Für eine "Kritische Pädagogik der Vielfalt" : zur Konzeption einer sozialtheoretisch gestützten Pädagogik der Vielfalt in der Lehrerinnen- und Lehrerbildung

Hofstetter, Daniel ; Duchêne, Alexandre

In: Schweizerische Zeitschrift für Heilpädagogik, 2010, p. 41-46

L’un des défis que l’école actuelle rencontre est celui de savoir comment les enseignant-e-s comprennent et gèrent la diversité de leurs élèves en salle de classe. A cet égard nous avons développé à la Haute Ecole pédagogique de Fribourg (Suisse) une approche que nous nommons « Pédagogie critique de la diversité ». Il s’agit d’une pédagogie de la diversité qui s’ancre...

Haute Ecole pédagogique Fribourg

Bildungsambitionen in Elterngesprächen der 5. Klasse, ein Jahr vor dem Übertritt in die Orientierungsschule

Hofstetter, Daniel

In: 4- bis 12-Jährige : Ihre schulischen und ausserschulischen Lern- und Lebenswelten, 2013, p. 78-86

Université de Neuchâtel

Frequency Noise and Linewidth of Mid-infrared Continuous-Wave Quantum Cascade Lasers: An Overview

Schilt, Stéphane ; Tombez, Lionel ; Di Domenico, Gianni ; Hofstetter, Daniel

In: The Wonders of Nanotechnology: Quantum and Optoelectronic Devices and Applications (chapter 12), 2013, p. 261-287

Université de Neuchâtel

Active linewidth-narrowing of a mid-infrared quantum cascade laser without optical reference

Tombez, Lionel ; Schilt, Stéphane ; Hofstetter, Daniel ; Südmeyer, Thomas

In: Optics Letters, 2013, vol. 38, no. 23, p. 5079-5082

We report on a technique for frequency noise reduction and linewidth-narrowing of a distributed-feedback mid-IR quantum cascade laser (QCL) that does not involve any optical frequency reference. The voltage fluctuations across the QCL are sensed, amplified and fed back to the temperature of the QCL at a fast rate using a near-IR laser illuminating the top of the QCL chip. A locking bandwidth of...

Université de Neuchâtel

Continuous Wave Operation of a Mid-Infrared Semiconductor Laser at Room Temperature

Beck, Mattias ; Hofstetter, Daniel ; Aellen, Thierry ; Faist, Jérôme ; Oesterle, Ursula ; Ilegems, Marc ; Gini, Emilio ; Melchior, Hans

In: Science, 2002, vol. 295, no. 5553, p. 301-305

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...

Université de Neuchâtel

Wavelength tuning and thermal dynamics of continuous-wave mid-infrared distributed feedback quantum cascade lasers

Tombez, Lionel ; Cappelli, Francesco ; Schilt, Stéphane ; Di Domenico, Gianni ; Bartalini, Saverio ; Hofstetter, Daniel

In: Applied Physics Letters, 2013, vol. 103, no. 3, p. 1-5

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...

Université de Neuchâtel

Linewidth of a quantum cascade laser assessed from its frequency noise spectrum and impact of the current driver

Tombez, Lionel ; Schilt, Stéphane ; Di Francesco, Joab F. ; Führer, Thorsten ; Rein, Benjamin ; Walther, Thomas ; Di Domenico, Gianni ; Hofstetter, Daniel ; Thomann, Pierre

In: Applied Physics B : Lasers and optics, 2012, vol. 109, no. 3, p. 407-414

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 2196.6cm−1 is used to convert the frequency fluctuations of the laser into intensity fluctuations that...

Université de Neuchâtel

Temperature dependence of the frequency noise in a mid-IR DFB quantum cascade laser from cryogenic to room temperature

Tombez, Lionel ; Schilt, Stéphane ; Di Francesco, Joab F. ; Thomann, Pierre ; Hofstetter, Daniel

In: Optics Express, 2012, vol. 20, no. 7, p. 6851-6859

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,...

Université de Neuchâtel

Si-interdiffusion in heavily doped AlN-GaN-based quantum well intersubband photodetectors

Hofstetter, Daniel ; Di Francesco, Joab F. ; Martin, Denis ; Grandjean, Nicolas ; Kotsar, Yulia ; Monroy, Eva

In: Applied Physics Letters, 2011, vol. 98, no. 241101, p. 1-3

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...

Université de Neuchâtel

Intersubband spectroscopy probing higher order interminiband transitions in AlN-GaN-based superlattices

Hofstetter, Daniel ; Di Francesco, Joab F. ; Kandaswamy, Prem K. ; Monroy, Eva

In: Applied Physics Letters, 2011, vol. 98, no. 071104, p. 1-3

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...