In the last few years Mid InfraRed (MIR) photonics has received renewed interest for a variety of commercial, scientific and military applications. This paper reports the design, the fabrication and the characterization of SiGe/Si based graded index waveguides and photonics integrated devices. The thickness and the Ge concentration of the core layer were optimized to cover the full [3 - 8 µm] band. The developed SiGe/Si stack has been used to fabricate straight waveguides and basic optical functions such as Y-junction, crossings and couplers. Straight waveguides showed losses as low as 1 dB/cm at λ = 4.5 µm and 2 dB/cm at 7.4 µm. Likewise straight waveguides, basic functions exhibit nearly theoretical behavior with losses compatible with the implementation of more complex functions in integrated photonics circuits. To the best of our knowledge, the performances of those Mid-IR waveguides significantly exceed the state of the art, confirming the feasibility of using graded SiGe/Si devices in a wide range of wavelengths. These results represent a capital breakthrough to develop a photonic platform working in the Mid-IR range.

Low loss SiGe graded index waveguides for mid-IR applications

Synchrotron radiation photoemission spectroscopy and optical transmission spectrum measurements have been performed on an HfO2 thin film grown on a Si(100) substrate to determine the band structure of the HfO2/Si stack. The result shows a valence-band offset of 2.5 eV and a conduction-band offset of 2.2 eV for the HfO2/Si interface. The Schottky barrier height between Au and HfO2 is obtained from current density–voltage measurement. The characterization reveals that the dominant conduction mechanism in the region of low field under gate injection is Schottky emission. The energy-band diagram of an Au–HfO2–Si MOS stack was obtained from these results.

Band structure and valence-band offset of HfO2 thin film on Si substrate from photoemission spectroscopy

Quantum cascade lasers are unipolar semiconductor lasers that, due to the generality of the principles they are based on and the flexibility of the underlying technology, allow to fabricate sources of coherent light from the midinfrared down to the frequencies of the terahertz. In this work we present different approaches in the domains of active region and waveguide engineering of mid-infrared quantum cascade lasers. The quantum cascade lasers with large optical cavities presented in this work allow to obtain an optical mode with a quasi-rectangular intensity profile. We show that devices based this principle reduce the effects of vertical spatial hole burning in the waveguide core and thus achieve an optimal slope efficiency. We also show that the introduction of passive layers in the waveguide core stabilizes the electric field in the latter and enhances its thermal conductivity. Moreover, due to the large spatial extension of the optical mode, the devices exhibit a much narrower farfield than devices with standard waveguides. Despite limitations encountered due to the use of non-optimized active regions and old-school device processing we could demonstrate devices that exhibit more than 14 W of peak power and 34 % slope efficiency at λ≈5μm in pulsed operation (1 % duty-cycle) at cryogenic temperatures. Devices emitting at λ ≈ 10.5μm and fabricated with more advanced techniques attained a peak power of 4.7 W and slope efficiencies up to 1.6 W/A in pulsed operation (1 % duty-cycle) at room temperature. Strain-compensated active regions developed more than 10 years ago and based on the GaInAs and AlInAs materials and grown on InP substrates have shown to be the best way to obtain high power quantum cascade lasers emitting in the first mid-infrared atmospheric window (3.5-5.0μm). After introducing the framework (including surface segregation) that allows us to quantitatively simulate the performances of our devices and to correctly predict their emission wavelength at different temperatures we present a “spiked” active region design. With this design structures are grown with a small amount of strain (±0.5 %) and AlAs and InAs spikes with sub-monolayer thicknesses are added in the barriers and wells, respectively, that are situ-

Gain and waveguide engineering in mid-infrared quantum cascade lasers

This thesis deals with linear and non-linear optical gain of quantum cascade lasers (QCLs) and the development of unique spectroscopic tools at the infrared (IR) beamline X01DC of the Swiss Light Source (SLS) synchrotron. These tools consist of (i) an ultra-broadband pumpprobe system, and (ii) a diffraction-limited micro-spectroscopy transmission setup. These tools enable the investigation of the IR response of condensed matter systems and devices under conditions of strong electrical and/or optical excitation. It is shown that infrared synchrotron sources complement conventional infrared sources by offering a high-brilliance, broadband infrared spectrum and pulsed time-structure. The first part of this thesis focusses on all-optical pump-probe experiments. A pulse of synchrotron infrared with a length of 100ps overlaps on the samples with a broadly tuneable pump pulse generated by non-linear generation from an Nd:YAG laser. This setup enables the investigation of dynamic processes down to the 100 ps limit. As a showcase, the optical gain and losses in Germanium-on-Silicon layers are investigated. A discussion is given on the possibility to realise a laser on Si using strained and doped Ge layers. Also, a brief introduction is given on the time-resolved investigation of photocatalytic processes using the here introduced pump-probe tool. The second part of the thesis deals with the characterisation of QCLs both in the linear and the non-linear regime. QCLs are based on quantum wells and emit light in the midand far-Infrared. Their structure allows to design the spectral properties of the emission with a high degree of flexibility and tuneability. Light generation results from optical intersubband transitions between electron states in the conduction band of the semiconductor heterostructure. Because such laser devices contain single-mode waveguides (typically

Linear and non-linear spectroscopy of semiconductors using synchrotron infrared

Cette these concerne l'etude theorique et experimentale des limitations de la puissance de sortie des lasers a cascade quantique (LCQ). Nous y exposons une modelisation globale de leurs proprietes electro-optiques. Le fonctionnement du laser est decrit en incluant la structure electronique, les mecanismes de diffusion responsables des transitions non radiatives des electrons et le couplage electron- photon de la cavite. Ce modele nous a permis de reproduire avec succes l'ensemble des caracteristiques (courant et puissance optique en fonction de la tension appliquee) d'un LCQ sur un large domaine de temperature de fonctionnement. Dans un premier temps, ce modele a ete utilise pour le calcul de la temperature electronique dans les LCQ. Il en ressort que la diffusion par des phonons-LO est le seul mecanisme avec lequel le gaz d'electrons peut transferer son energie vers le reseau. Les mecanismes elastiques de diffusion sont des sources d'energie pour le gaz d'electrons. Deux parametres physiques permettent de decrire completement le comportement de la temperature electronique dans le composant : la resistance thermique electronique de l'heterostructure et le coefficient de couplage courant temperature electronique. Ensuite, l'etude du couplage des electrons avec les photons de la cavite montre que ce couplage peut modifier notablement la distribution electronique sur les niveaux energetiques. Le gain de la zone active du laser diminue avec la densite de photons. Cet effet appele saturation de gain joue un role important sur les performances des LCQ. La minimisation de cet effet peut augmenter la puissance maximale de sortie du laser d'un facteur deux. Cette etude nous a permis de proposer de nouveaux dessins de zone active ayant des performances ameliorees. La fabrication de certaines de ces structures a permis de valider l'approche que nous avons suivie pour ameliorer les performances de ce type de laser.

https://pastel.archives-ouvertes.fr/pastel-00605931/document

Étude physique des limites en puissance des lasers à cascade quantique

We propose a technique based on Fabry-Perot fringes to analyze, in a broad wavelength range, the waveguide properties of semiconductor lasers in the midinfrared spectrum. This technique has been applied to strain-balanced Ga0.32In0.68As∕Al0.64In0.36As quantum cascade lasers, with a double phonon active region, in order to determine propagation losses and the effective index between 2.5 and 8μm wavelength. These results are in good agreement with the values obtained using another method.

Measurement of semiconductor waveguide optical properties in the midinfrared wavelength range

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Measurement of semiconductor waveguide optical properties in the midinfrared wavelength range