Ultimate performance of Quantum Well Infrared Photodetectors in the tunneling regime

Thanks to their wavelength diversity and to their excellent uniformity, Quantum Well Infrared Photodetectors (QWIP) emerge as potential candidates for astronomical or defense applications in the very long wavelength infrared (VLWIR) spectral domain. However, these applications deal with very low backgrounds and are very stringent on dark current requirements. In this paper, we present the full electro-optical characterization of a 15 micrometer QWIP, with emphasis on the dark current measurements. Data exhibit striking features, such as a plateau regime in the IV curves at low temperature (4 to 25 K). We show that present theories fail to describe this phenomenon and establish the need for a fully microscopic approach

Integral representation of one dimensional three particle scattering for delta function interactions

The Schr\"{o}dinger equation, in hyperspherical coordinates, is solved in closed form for a system of three particles on a line, interacting via pair delta functions. This is for the case of equal masses and potential strengths. The interactions are replaced by appropriate boundary conditions. This leads then to requiring the solution of a free-particle Schr\"{o}dinger equation subject to these boundary conditions. A generalized Kontorovich - Lebedev transformation is used to write this solution as an integral involving a product of Bessel functions and pseudo-Sturmian functions. The coefficient of the product is obtained from a three-term recurrence relation, derived from the boundary condition. The contours of the Kontorovich-Lebedev representation are fixed by the asymptotic conditions. The scattering matrix is then derived from the exact solution of the recurrence relation. The wavefunctions that are obtained are shown to be equivalent to those derived by McGuire. The method can clearly be applied to a larger number of particles and hopefully might be useful for unequal masses and potentials.Comment: 18 pages, 2 figures, to be published in J. Math. Phy

Intermanifold similarities in partial photoionization cross sections of helium

Using the eigenchannel R-matrix method we calculate partial photoionization cross sections from the ground state of the helium atom for incident photon energies up to the N=9 manifold. The wide energy range covered by our calculations permits a thorough investigation of general patterns in the cross sections which were first discussed by Menzel and co-workers [Phys. Rev. A {\bf 54}, 2080 (1996)]. The existence of these patterns can easily be understood in terms of propensity rules for autoionization. As the photon energy is increased the regular patterns are locally interrupted by perturber states until they fade out indicating the progressive break-down of the propensity rules and the underlying approximate quantum numbers. We demonstrate that the destructive influence of isolated perturbers can be compensated with an energy-dependent quantum defect.Comment: 10 pages, 10 figures, replacement with some typos correcte

Space Charge at Nanoscale: Probing Injection and Dynamic Phenomena Under Dark/Light Configurations by Using KPFM and C-AFM

International audienc

Re-thinking green roof design : The prospect of a carbon sink structure investigated through a life cycle cost-benefit analysis

Green Roofs (GRs) have been used worldwide as a solution for greening cities, while providing useful services to society such as storm water management, improved air quality, energy savings, and increased biodiversity, to name a few. However, the environmental impact of their structure has been understudied, and more importantly, the carbon cost related to the use of materials that have a high carbon footprint has received little to no attention. In the context of an aspiring carbon neutral Sweden, public and private decision makers need better guidance when considering the implementation of GR systems. While cost-benefit analysis (CBA) is a decision support tool that has been commonly used to assess GR projects from an environmental, social, and economic point of view, the carbon impact of the structure layers has been left unaccounted for. This thesis aims to propose an analytical framework which incorporates the carbon footprint of each component used in the GR structure into CBA, to support more informed decision making. This analytical framework is tested on two extensive GRs, one made with commonly used materials and one experimental prototype which is almost entirely made with carbon sink materials. This case study is designed to reveal the socio-economic trade-offs resulting from the use of conventional materials versus materials that have a lower carbon footprint. This comparison is also made to support future innovations in GR design and to bring forward the most cost-effective materials from a socio-economic perspective. The results of this thesis show that the two GR alternatives have a positive net present value (NPV) indicating that both projects bring more socio-economic benefits than they incur costs in their lifetime. However, the key findings reveal that while the structure of the conventional GR is a source of carbon which incurs socio-economic costs of 42 SEK/m2, the structure of the prototype is a carbon sink which brings socio-economic benefits of 63 SEK/m2. A comparison of the two extensive GRs shows that the avoided emissions from using the prototype instead of a traditional GR has a value of 105 SEK/m2. Moreover, when considering each component of the GR structure it becomes evident that while it is possible to replace traditional materials with carbon sink materials this generally comes with higher material costs. However, once the avoided CO2 emissions are accounted for, this choice seems of potential interest from a socio-economic perspective

Etude expérimentale et théorique de la réponse angulaire et de la réponse spectrale hors bande de détecteurs infrarouges hautes performances.

Infrared detectors have proved to be of particular interest in very various application fields (military, medical, astronomy, environment). Each of them exhibits specific requirements, such that one unique type of infrared detectors can n! ot be suited for all applications. Nowadays, many different detectors coexist. Yet the traditional figures of merit don't allow one to compare the performance of different technologies for a specific application. Within the framework of this study, carried out on infrared quantum detectors of high performances such as the quantum well infrared photodetectors (QWIP) and the MCT detectors, we focused on two new figures of merit : the off-band spectral response and the angular response. The first one consists in studying the spectral response of a detector far from the wavelength the detector has been optimised for, thanks to a great dynamics of measurement. The second figure of merit represents the variation of the response of the detector according to the angle of incidence of the incoming light. The set up of these electro-optical characterizations required the development of innovating test benches. Among the results obtained during our work on the angular response, let us underline that the shape of a QWIP angular response doesn't depend on the etch depth of grating or that the angular response of a MCT detector is flat on a wide angular range. The study of the off-band spectral response, as for it, made it possible to highlight physical phenomena never observed until then on a QWIP photocurrent spectrum. This work shows how important the comprehension of the physical phenomena inside the pixels is. Measurements associated with an accurate model of physical phenomena lead thus to a complete study of the infrared quantum detectors ultimate performances.Les domaines d'application de la détection infrarouge sont tellement nombreux (militaire, médical, astronomie, environnement) et spécifiques qu'un seul type de détecteur infrarouge ne peut pas répondre à l'ensemble des besoins. De nombreuses filières coexistent à ce jour. Or, les fonctions de mérite classiques ne sont pas suffisantes pour comparer les performances de détecteurs de filières différentes pour une application spécifique. Dans le cadre de cette étude, réalisée sur des détecteurs quantiques infrarouges de hautes performances tels que les détecteurs à multipuits quantiques (MPQ) et les détecteurs quantiques HgCdTe, nous nous sommes intéressés à deux nouvelles fonctions de mérite : la réponse spectrale dite « hors bande » et la réponse angulaire. La premièr! e consiste à étudier la réponse spectrale d'un détecteur loin de la gamme de longueur d'onde pour laquelle il a été optimisé, grâce à une très grande dynamique de mesure. La deuxième fonction de mérite traduit la variation de la réponse du détecteur en fonction de l'angle d'incidence du flux lumineux. La mise en place de ces caractérisations électro-optiques a nécessité le développement de bancs de test innovants. Parmi les résultats obtenus lors du travail sur la réponse angulaire, citons le fait que la forme de la réponse angulaire est indépendante de la profondeur de gravure du réseau d'un détecteur MPQ ou encore que la réponse d'un détecteur HgCdTe est quasi-plate sur une large plage angulaire. L'étude de la réponse spectrale a, quant à elle, permis de mettre en évidence des phénomènes physiques jamais observés jusque là sur un spectre de photocourant d'un détecteur MPQ. Ce travail montre l'importance de la compréhension des phénomènes physiques au sein des pixels. Les mesures associées à une modélisation fine des phénomènes physiques aboutissent ainsi à une étude complète des performances ultimes des détecteurs quantiques infrarouges