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Optimized sub-wavelength grating mirror design for mid-infrared wavelength range

We present the design of a sub-wavelength grating mirror optimized for integration in mid-IR VCSEL. The definition of a quality factor adapted to VCSEL requirements and maximized by an optimization algorithm allowed us to obtain a polarization selective and high reflectivity structure showing several percent of tolerance. The design of the SGM mirror can easily be adapted to the technological constraints of the manufacturers by adjusting the limitations defined for the automated optimization process. Indeed, the comparison between different technological constraints shows that a better accuracy of the etching process results in an increase of the quality of the mirror.

Nicolas Fressengeas

@fresseng 11 yr. ago

Optimized sub-wavelength grating mirror design for mid-infrared wavelength range

Several designs of sub-wavelength grating mirrors adapted to mid-infrared operation are reported with several percents of tolerance for the grating fabrication. These designs have been automatically optimized by the use of a genetic-based algorithm to maximize a quality factor defined to meet the requirements of a VCSEL cavity mirror. These mirrors are devoted to an integration in VCSEL operating near λ=2.3 µm, with a large bandwidth, very high reflectivity coefficient for transverse magnetic mode only, polarization selectivity and a thickness as low as 2 µm.

Nicolas Fressengeas

@fresseng 11 yr. ago

Optimized sub-wavelength grating mirror design for mid-infrared wavelength range

Nicolas Fressengeas

@fresseng 11 yr. ago

Optimized subwavelength grating mirror design for mid-infrared wavelength range

Nicolas Fressengeas

@fresseng 11 yr. ago

Optimized GaAs High Contrast Grating Design and Fabrication for Mid-infrared Application at 2.3 μm

A GaAs high contrast grating design has been optimized for a mid-infrared VCSEL application through the use of an optimization algorithm. Reflection spectra of an experimental grating with dimensions within the computed tolerances are in a correct agreement with the simulations.

Nicolas Fressengeas

@fresseng 11 yr. ago

Experimental control of steady state photorefractive self-focusing in InP:Fe at infrared wavelengths

This paper reports an experimental study of the self-focusing process in iron doped indium phosphide at an 1.06 micron wavelength, identifying the influence of temperature, beam intensity and background illumination \RefereeOne{for two different iron dopings}. We point out that the iron ionization ratio is at the origin of different qualitative behavior previously reported and we show that it is possible to reproduce the said behaviors in the same crystal by applying a uniform illumination, allowing their eventual control for dynamic wave-guiding.

Nicolas Fressengeas

@fresseng 11 yr. ago

Robust design of subwavelength grating mirror for mid-infrared VCSEL application

We present the design of a Si/Si3N4 subwavelength grating mirror optimized for an integration in mid-infrared VCSEL. The definition of a quality factor adapted to VCSEL requirements and maximized by an optimization algorithm allowed us to obtain a polarization selective and high reflectivity structure. The robustness with respect to fabrication error has been enhanced thanks to a precise study of the grating dimension tolerances.

Nicolas Fressengeas

@fresseng 11 yr. ago

Mid-infrared sub-wavelength grating mirror design: tolerance and influence of technological constraints

High polarization selective Si/SiO2 mid-infrared sub-wavelength grating mirrors with large bandwidth adapted to VCSEL integration are compared. These mirrors have been automatically designed for operation at λ = 2.3 µm by an optimization algorithm which maximizes a specially defined quality factor. Several technological constraints in relation with the grating manufacturing process have been imposed within the optimization algorithm and their impact on the optical properties of the mirror have been evaluated. Furthermore, through the tolerance computation of the different dimensions of the structure, the robustness with respect to fabrication errors has been tested. Finally, it appears that the increase of the optical performances of the mirror imposes a less tolerant design with severer technological constraints resulting in a more stringent control of the manufacturing process.

Nicolas Fressengeas

@fresseng 11 yr. ago

Interface states effects in GaN Schottky diodes

Current voltage (I-V) and capacitance voltage (C-V) measurements have been performed versus temperature on GaN Schottky diodes. The results show an increase of the Schottky barrier height ϕb and a decrease of the ideality factor n both with the increase of the temperature. We show that this behavior originates in the existence of an interface state density distribution, which is determined via the analysis of the temperature dependence of the I-V measurements, and allows the tunneling of the carriers from the semiconductor to the metal. Those interface states are shown to be responsible for interface inhomogeneities which result in two Gaussian voltage dependent Schottky barrier distributions. We show also that, in the presence of this interface state distribution, C-V measurements, without the correction of the built in voltage by taking into account the effect of both the high values of the ideality factor and series resistance, lead to erroneous values of the Schottky barrier height ϕb.

Nicolas Fressengeas

@fresseng 11 yr. ago

Robust design of Si/Si3N4 high contrast grating mirror for mid-infrared VCSEL application

A Si/Si3N4 high contrast grating mirror has been designed for a VCSEL integration in mid-infrared (λ = 2.65 µm). The use of an optimization algorithm which maximizes a VCSEL mirror quality factor allowed the adjustment of the grating parameters while keeping large and shallow grating pattern. The robustness with respect to fabrication error has been enhanced thanks to a precise study of the grating dimension tolerances. The final mirror exhibits large high reflectivity bandwidth with a polarization selectivity and several percent of tolerance on the grating dimensions.

Nicolas Fressengeas

@fresseng 11 yr. ago

Gaussian Beams

This is a set of two documents useful for a lecture on Gaussian Optics : - one is a set of slides that can be projected for a lecture - the other one is a handout useful for students following the lecture Both are complementary, not identical.

Nicolas Fressengeas

@fresseng 11 yr. ago

Optimized Si/SiO2 high contrast grating mirror design for mid-infrared wavelength range: robustness enhancement

A high reflectivity and polarization selective high contrast grating mirror has been designed with the use of an automated optimization algorithm. Through a precise study of the tolerance of the different lengths of the structure, the robustness with respect to the fabrication errors has been enhanced to high tolerance values between 5 % and 210 %. This adjustment of the dimensions of the structure leads to a 250 nm large bandwidth mirror well adapted for a VCSEL application at λ = 2.65 µm and can easily be scaled for other wavelengths.

Nicolas Fressengeas

@fresseng 11 yr. ago

Robust design for parameter tolerant GaAs High Contrast Grating mirror for mid infrared VCSEL application

Grating structures made of materials which provide a high optical index contrast between the grating slabs and the surrounding medium have shown many extraordinary properties. The resonance of Bloch modes between the two grating interfaces and their coupling can be adjusted to obtain different and very promising properties in a large range of applications such as broadband mirrors, high-Q filters, planar lenses, circular polarizer... The development of VCSEL devices based on GaSb for a mid-infrared emission is still limited today at 2.6 µm. The high performances of high contrast grating (HCG) mirrors combined with a good polarization selectivity and a thickness lower than 2 µm should be very promising characteristics to enhance electrical, thermal and optical properties of VCSEL devices emitting above 2.3 µm and would allow spectroscopic measurements of polluting gas in the 2-3 µm wavelength range. In this paper, we present the design of a GaAs HCG to replace the top Bragg mirror of mid infrared based VCSEL. A robust optimization algorithm is used to find the best dimensions of the HCG structure for a VCSEL application. Thus, the homemade algorithm searches not only for the most efficient mirror but also for a robust design structure with large tolerances with respect to the errors of fabrication on the grating dimensions.

Nicolas Fressengeas

@fresseng 11 yr. ago

AlOx/GaAs high contrast grating mirrors for mid infrared VCSELs

High contrast gratings (HCG) have drained recently highly attractive perspectives for VCSEL devices through the wide potentialities for obtaining unique optical properties with these integrable optical elements, in contrast to conventional Bragg reflectors. Since most of the HCG approaches on III-V based photonic devices are based on semiconductor/air-gap structures, strong limitations are foreseen for the integration in practical lasers. We proposed to exploit the high refractive index contrast available in the AlOx/GaAs system for which the oxide is formed with the mature thermal wet oxidation technique. High reflective and polarization selective mirrors can thus been achieved monolithically by means of a VCSEL compatible process technology, in a wide spectral range from near to mid infrared. Jointly, the AlOx lateral confinement scheme is proposed for implementation in mid infrared emitters. We present in this paper the fabrication and performances of AlOx/GaAs HCG operating between 2.3 and 2.6 μm and their integration within oxide-confined mid-infrared electrically pumped VCSELs.

Nicolas Fressengeas

@fresseng 11 yr. ago

Oxide confinement and high contrast grating mirrors for Mid-infrared VCSELs

A new mid-infrared (MIR) Vertical Cavity Surface Emitting Laser (VCSEL) structure is proposed. We have integrated to the VCSEL structure both an oxide aperture for lateral confinement, and a sub-wavelength high-contrast-grating top mirror. Upon the GaSb-based half-VCSEL, we have grown a metamorphic AlGaAs heterostructure to enable thermal oxidation and grating mirror fabrication steps. A methodology based on optimization and anti-optimization methods has been used to design the optical grating, with improved parameter tolerances regarding processing errors. Finally, we show the complete fabrication of an electrically-pumped MIR monolithic VCSEL structure implementing both oxide confinement and a subwavelength grating top mirror.

Nicolas Fressengeas

@fresseng 11 yr. ago

Rigorous optimization of InGaN multijunction solar cells efficiency

The InGaN ternary alloy has the potentiality to achieve high efficiency solar cells. Indeed the bandgap of the InGaN alloy covers the whole solar spectrum including the visible region by changing the Indium composition. The InGaN alloy also counts among its advantages a relatively low effective mass of electrons and holes, their high mobilities, high absorption coefficients as well as radiation tolerance. These very promising characteristics give the possibility to design and develop next-generation high-efficiency thin films solar cells, based on multijunction technology. However, the main drawback is the difficulty of p-doping and the high defects density in these III-Nitride alloys. In this report, we develop a novel and rigorous optimization approach based on state-of-the-art optimization algorithms, to design an optimal multijunction solar cell based on InGaN alloys. This approach allows to automatically determine the values of the cell parameters, among which are the geometries of the active layers, their doping and defect levels, to maximize its efficiency within the possibilities of actual device fabrication. We have found that the use of this rigorous optimization approach is necessary because of the high dependence of the cell efficiency on the device geometry, doping and defects, particularly concerning the top and bottom P layers properties

Nicolas Fressengeas

@fresseng 11 yr. ago

Microsecond infrared beam bending in photorefractive iron doped indium phosphide

A time resolved study of the behavior of a single beam in photorefractive iron doped indium phosphide is provided down to the microsecond range, showing that infrared beam bending does occur on the microsecond time scale for moderate beam intensities. Two distinct time scales are evidenced, the behavior of which are the sign of two different photorefractive mechanisms.

Nicolas Fressengeas

@fresseng 11 yr. ago

Cellular Computing and Least Squares for partial differential problems parallel solving

This paper shows how partial differential problems can be solved thanks to cellular computing and an adaptation of the Least Squares Finite Elements Method. As cellular computing can be implemented on distributed parallel architectures, this method allows the distribution of a resource demanding differential problem over a computer network.

Nicolas Fressengeas

@fresseng 11 yr. ago

Robust Design by Antioptimization for Parameter Tolerant GaAs/AlOx High Contrast Grating Mirror for VCSEL Application

A GaAs/AlOx high contrast grating structure design which exhibits a 99.5% high reflectivity for a 425nm large bandwidth is reported. The high contrast grating (HCG) structure has been designed in order to enhance the properties of mid-infrared VCSEL devices by replacing the top Bragg mirror of the cavity. A robust optimization algorithm has been implemented to design the HCG structure not only as an efficient mirror but also as a robust structure against the imperfections of fabrication. The design method presented here can be easily adapted for other HCG applications at different wavelengths.

Nicolas Fressengeas

@fresseng 11 yr. ago

Bandgap energy bowing parameter of strained and relaxed InGaN layers,

This paper focuses on the determination of the bandgap energy bowing parameter of strained and relaxed InxGa1−xN layers. Samples are grown by metal organic vapor phase epitaxy on GaN template substrate for indium compositions in the range of 0