Coherent detection of metal-metal terahertz quantum cascade lasers with improved emission characteristics

Coherent detection of emission from quantum cascade lasers with metal-metal waveguides is demonstrated through free-space coupling of a THz pulse to the sub-wavelength waveguide. We implement a simple, monolithic planar horn antenna design on the metal-metal waveguide that reduces the impedance mis-match to the waveguide. The resulting devices show up to 10 times more directed output power than conventional metal-metal waveguides. This enhanced coupling to free-space allows a more efficient injection of broad-band THz pulses into the waveguide. Through this, we are able to seed the laser emission and coherently detect the laser emission by electro-optic sampling

Measuring the sampling coherence of a terahertz quantum cascade laser

The emission of a quantum cascade laser can be synchronized to the repetition rate of a femtosecond laser through the use of coherent injection seeding. This synchronization defines a sampling coherence between the terahertz laser emission and the femtosecond laser which enables coherent field detection. In this letter the sampling coherence is measured in the time-domain through the use of coherent and incoherent detection. For large seed amplitudes the emission is synchronized, while for small seed amplitudes the emission is non-synchronized. For intermediate seed amplitudes the emission exhibits a partial sampling coherence that is time-dependent

Optique non-linéaire résonante et contrôle de la phase d'émission des lasers à cascade quantique

Quantum cascade lasers (QCL) are semiconductor nanostructures based on inter-subband transitions between confined states in the conduction band. They filled the lack of compact and powerful sources in the mid-infrared (MIR) and in the terahertz (THz) range. This thesis presents two studies on these lasers. First part investigates resonant optical nonlinearities of QCL. We show difference frequency generation between a near-infrared (NIR) beam and the QCL THz field, i.e. at E_NIR-E_QCL. NIR excitation is resonant with interband transitions in the QCL active region, enhancing the nonlinear susceptibility of the matter. High intracavity THz field combined with the resonant NIR beam results in good efficiencies for the frequency mixing (0.13%) and in high order generation up to the third order (E_IR-3E_QCL). For the first time this nonlinear interaction is investigated within a MIR QCL. This allows demonstrating the nonlinear interaction up to 275 K, showing that the interaction is temperature independent. The second part deals with phase control of QCL emission via THz time domain spectroscopy. The novelty is the use of a Metal-Metal (MM) waveguide. These guides lead to higher temperature operations but have sub-wavelength dimensions (~10µm compared to 100µm for the emission). These dimensions make harder the coupling of a THz seed which is required to initiate the THz QCL field. "V" shape antennas are processed above QCL facets to match the impedance between free-space and guided modes. Thus, coupling efficiency and power extraction are enhanced. We demonstrate that we can phase-lock the emission of a MM QCL by THz injection seeding and resolve its amplitude and its phase.Les lasers à cascade quantique (LCQ) sont des nanostructures de semiconducteurs se basant sur des transitions intersousbandes entre états confinés de la bande de conduction. Ils ont permis de combler un manque de sources puissantes et compactes d'abord dans le moyen infrarouge (MIR) puis dans le térahertz (THz). Cette thèse présente deux études en rapport avec ces lasers. La première partie présente les propriétés d'optique non-linéaire résonantes des LCQ. Il s'agit de démontrer la génération de différence de fréquences entre un faisceau proche infrarouge (IR) et le champ THz du LCQ. L'excitation proche IR est résonante avec les transitions interbandes des puits quantiques qui composent le LCQ. Ceci exalte la susceptibilité non-linéaire du milieu. Le champ THz intense dans la cavité combiné à cette excitation résonante permet d'obtenir de bonnes efficacités (jusqu'à 0.13%) et de générer des harmoniques supérieures jusqu'à l'ordre 3. Ces interactions non-linéaires ont également été étudiées dans les LCQ MIR ce qui a permis d'augmenter la température de fonctionnement jusqu'à 275 K. Une deuxième partie traite du contrôle de la phase du champ THz d'un LCQ au moyen d'un montage de spectroscopie THz dans le domaine temporel. L'originalité résulte dans l'usage d'un LCQ ayant un guide double métal. Ces guides permettent d'avoir de meilleures performances en température mais ont des dimensions largement sous longueur d'onde. Ces dimensions compliquent le couplage d'une onde THz externe nécessaire à l'amorçage du champ THz du LCQ sur une phase fixe. Des antennes en forme de V sont déposées à la surface du LCQ pour faire une adaptation d'impédance et favoriser ainsi le couplage

Subflorescence and plaster drying dynamics

International audienceThe dynamics of plaster drying and the impact of subflorescence on the process are described through Magnetic Resonance Imaging and X-Ray Microtomography measurements. It is shown that crystals deposit around the air-liquid interface the closest to the sample free surface, which induces a recession of this interface within the sample at a rate only depending on the current saturation (water to pore volume ratio). Thus the distribution of crystals deposited during evaporation essentially depends on the history of saturation. The drying dynamics then results from vapor diffusion through the less porous layers of crystal accumulation below the sample free surface. This in particular makes it possible to predict the dramatic decrease of the drying rate after successive imbibition-drying cycles

All-optical wavelength shifting in a semiconductor laser using resonant nonlinearities

For future ultrafast all-optical networks, new optical devices are required that directly manipulate communication channels to shift their wavelength over the bandwidth of an optical fiber (50THz). 1,2 Current proposed solutions based on nonlinear processes, however, suffer from small efficiencies owing to low nonlinear susceptibilities. 3 Here, we demonstrate all-optical wavelength conversion of a near-infrared beam using a resonant non-linear process within a terahertz (THz) quantum-cascade-laser (QCL). 4 The process is based on injecting a low power CW near-infrared beam in resonance with the interband transitions of the QCL. This results in an enhanced nonlinearity allowing the efficient generation of the difference and sum frequency, shifting the frequency of the near-infrared beam by the QCL frequency. Efficiencies of 0.13 % are shown which are equivalent to those obtained using Free Electron Lasers. As well as important implications as an ultrafast wavelength shifter, this work also opens up the possibility of efficiently up-converting THz radiation to the near-infrared and the study of high THzoptical field interactions with quantum structures using QCLs. Wavelength division multiplexing (WDM) is currently used extensively in optical fibr