Blooming of algae and more generally phytoplankton in water ponds or marine environments can lead to hyper eutrophication and lethal consequences on other organisms. The selective recognition of invading species is investigated by automatic recognition algorithms of optical and fluorescence imaging. On one hand, morphological characteristics of algae of microscopic imaging are treated. The image processing lead to the identification the genus of aquatic organisms and compared to a morphologic data base. On the other hand, fluorescence images allow an automatic recognition based on multispectral data that identify locally the ratio of different photosynthetic pigments and gives a unique finger print of algae. It is shown that the combination of both methods are useful in the recognition of aquatic organisms.
Mid-infrared Vertical cavity surface emitting lasers (MIR-VCSEL) are very attractive compact sources for spectroscopic measurements above 2μm, relevant for molecules sensing in various application domains. A long-standing issue for long wavelength VCSEL is the large structure thickness affecting the laser properties, added for the MIR to the tricky technological implementation of the antimonide alloys system. In this paper, we propose a new geometry for MIR-VCSEL including both a lateral confinement by an oxide aperture, and a high-contrast sub-wavelength grating mirror (HCG mirror) formed by the high contrast combination AIOx/GaAs in place of GaSb/A│AsSb top Bragg reflector. In addition to drastically simplifying the vertical stack, HCG mirror allows to control through its design the beam properties. The robust design of the HCG has been ensured by an original method of optimization based on particle swarm optimization algorithm combined with an anti-optimization one, thus allowing large error tolerance for the nano-fabrication. Oxide-based electro-optical confinement has been adapted to mid-infrared lasers, byusing a metamorphic approach with (Al) GaAs layer directly epitaxially grown on the GaSb-based VCSEL bottom structure. This approach combines the advantages of the will-controlled oxidation of AlAs layer and the efficient gain media of Sb-based for mid-infrared emission. We finally present the results obtained on electrically pumped mid-IR-VCSELs structures, for which we included oxide aperturing for lateral confinement and HCG as high reflectivity output mirrors, both based on AlxOy/GaAs heterostructures.
This work is devoted to the design of high contrast grating mirrors taking into account the technological constraints and tolerance of fabrication. First, a global optimization algorithm has been combined to a numerical analysis of grating structures (RCWA) to automatically design HCG mirrors. Then, the tolerances of the grating dimensions have been precisely studied to develop a robust optimization algorithm with which high contrast gratings, exhibiting not only a high efficiency but also large tolerance values, could be designed. Finally, several structures integrating previously designed HCGs has been simulated to validate and illustrate the interest of such gratings.
Towards the development of high efficient GaN-based Vertical Cavity devices, the fabrication of cracks-free
high reflective semiconductor mirrors is still an issue. For near-UV operating devices, one of the best solution is
the use of AlGaN/GaN materials family. With a relatively high Al molar fraction in AlGaN, a large enough
index contrast can be achieved to fabricate high reflectivity mirrors. However, the lattice mismatch between
AlGaN and GaN increases with the Al molar fraction and induces a lot of cracks in the structure which affect its
optical and electrical properties. Moreover, for a regrowth of an active layer on the top of the mirror, it is
necessary to suppress crack generations to achieve a smooth surface. In this work, asymmetrical designs were
investigated for the modeling of fully-strained AlGaN/GaN distributed Bragg Reflectors with crack-free
surfaces. First, the critical thickness of MOVPE-grown AlGaN on GaN-on-sapphire templates was
experimentally determined and modeled. Then, several AlGaN/GaN mirrors with various Al molar fractions and
asymmetry factors were simulated demonstrating that non relaxed DBRs could be obtained with adequate
parameters. Finally, it has also been shown that there is a best suited Al molar fraction in AlGaN for each DBR
centering wavelength.
One of the main problem for the realization of high reflectivity GaN-based Bragg mirrors operating in the near-UV
wavelength range is to limit the crack formation due to the lattice mismatch between the different nitride compounds
while keeping a large refractive index contrast. Recent works have demonstrated that the introduction of several
AlN/GaN superlattices (SLs) in a classical AlN/GaN quarter wavelength layers mirror structure strongly improved the
crystalline quality and therefore the optical properties of such a mirror. In this work, several AlN/GaN SLs were studied
for their direct use as pseudo-alloy layers pair material in a Bragg mirror. Such a configuration should allow combining
the limitation of cracks by SLs with the improvement of the index contrast. First, the band structure of different
AlN/GaN SLs was simulated using a self-consistent 8-band-k.p Schrödinger-Poisson solver. Then, the influence of
surrounding layers such as AlN bulk ones on the band structure were considered. Using miniband-to-miniband
transitions deduced from these calculations, refractive indices of these SLs were finally estimated for the design of an
optimized high reflective Bragg mirror at 450 nm.
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