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The strong potential of intersubband detectors in the field of mid-infrared photodetection places this technology as a relevant alternative to HgCdTe detectors in the race for ultrafast operation. While their extremely short photocarrier's lifetime opens up possibilities of detection beyond 100 GHz, it is also the main reason for their comparatively high dark current. Here, a photovoltaic quantum cascade detector at 10.3 μm, embedded in a metal–metal patch antenna is presented in both direct and heterodyne detection schemes. In DC, a responsivity of 122 mA/W at 77 K and 85 mA/W at 295 K is reached. More strikingly, the device shows temperature independent heterodyne response with a capacitance-limited cutoff frequency of 25 GHz both at 77 K and room temperature. It is modeled quantitatively by means of an equivalent RC-circuit picture stemming from microscopic transport considerations. [ABSTRACT FROM AUTHOR]

The aim of this article is to determine the best dielectric between SiO 2 , Si 3 N 4 and TiO 2 for quantum cascade laser (QCL) passivation layers depending on the operation wavelength. It relies on both Mueller ellipsometry measurement to accurately determine the optical constants (the refractive index n and the extinction coefficient k) of the three dielectrics, and optical simulations to determine the mode overlap with the dielectric and furthermore the modal losses in the passivation layer. The impact of dielectric thermal conductivities are taken into account and shown to be not critical on the laser performances.

We study the impact of interface roughness on the operation of mid-IR and THz quantum cascade lasers. Particular emphasis is given towards the differences between the Gaussian and exponential roughness distribution functions, for which we present results from simulation packages based on nonequilibrium Green's functions and density matrices. The Gaussian distribution suppresses scattering at high momentum transfer which enhances the lifetime of the upper laser level in mid-IR lasers. For THz lasers, a broader range of scattering transitions is of relevance, which is sensitive to the entire profile of the interface fluctuations. Furthermore we discuss the implementation of interface roughness within a two band model.

We use density functional theory to study the density of the 3sp semicore states in transition and noble metals. The first objective is to understand how semicore states influence cohesive properties which mainly depend on the valence density. We define a localization radius for the semicore density which is found to be a crucial parameter and to heavily influence the cohesive properties. The localization radius is found to be controlled by the occupation numbers of the 3d states. This offers the possibility of setting criteria for the construction of accurate large core pseudopotentials freezing the semicore states, and to a posteriori control the use of large core pseudopotentials in the modeling. We illustrate our findings with the examples of copper and titanium.