A Mortar Element Method for the Analysis of Electromagnetic Passive Devices

The thesis consists of two blocks. The first and main block concerns the application of multi-domain spectral methods to the analysis of electromagnetic guiding structures. A general scattering formulation for vector differential problems is developed. The boundary-value problems are discretized using basis functions synthesized according to the mortar-element method. An analysis technique of the scattering generated by skew-incident plane waves on 2-D dielectric periodic structures based on this idea is proposed; the boundary-value problem describing these devices is given by the system of two coupled Helmholtz equations, therefore it exhibits a vector nature. Then, a technique aimed at analyzing axisymmetric structures using the same concept has been developed; in this case, the boundary-value problem arises from the transversalization of Maxwell’s equations written in cylindrical coordinates with respect to the angular coordinate. Half of the second block concerns the design of a low-frequency Vivaldi antenna in the framework of the Sardinia Array Demonstrator project. This antenna has been realized and preliminarily characterized with a prototypical measurement system developed by CNR-IEIIT. The second half of this block is focused on the development of a boundary-integral equation method aimed at analyzing dielectric lens antennas. A preliminary version of this code has been implemented and compared with commercial simulators. This activity has been performed in the THz Sensing Group of TU-Delft, Delft, Netherlands

Bimodal Resonance Phenomena. Part III: High-Contrast Grating Reflectors

The extraordinary broadband high-reflectivity features of high-contrast gratings are stimulating great interest in many opto-electronic applications. In view of obtaining a simple simulation framework, the analogy of high-contrast grating reflectors with bimodal Fabry-Pérot interferometers is proposed. The closed-form expressions of the interferometer reflectivity, obtained starting from a novel parametrization of the scattering matrices characterizing the bar-air interface, allow a complete exploration of the device parameter space, explaining and predicting the phenomenon of ultra-broadband quasi-100% reflectivity. In this paper an optimized and numerically efficient design procedure is described and compared with the standard rigorous coupled wave analysis, both for the classical "bar-in-air" configuration and for a more robust and practical one, with bars lying on a dielectric support. It is shown that the model can be applied also in the more realistic case of lossy gratings

Nonequilibrium Green’s function study of phonon-dressed Auger transitions in superlattice infrared photodetectors

Antimonide-based type-II superlattices represent a promising alternative to mercury cadmium telluride in infrared imaging applications. Among the original motivations for the development of type-II superlattices as a material system for infrared photodetectors is Auger engineering, the possibility of tailoring Auger processes to increase the "intrinsic"minority carrier lifetime at long wavelengths. We investigate Auger transitions in antimonide-based superlattices by means of a nonequilibrium Green's function model based on a multiband description of the electronic structure and a GW treatment of carrier-carrier interactions. As carrier-phonon interactions mediated by acoustic deformation potential and polar optical scattering are included by means of appropriate self-energies, the resulting GW Auger rates may be considered as "phonon-dressed."The spatial and spectral resolution afforded by the model provides a clear, graphical representation of the transitions contributing to the recombination process. A detailed study of Auger transitions in conventional and polytype superlattice absorbers demonstrates the effectiveness of the proposed approach for the design of Auger-suppressed infrared photodetectors. The calculated Auger coefficients are in good agreement with measurements extracted from pulsed threshold current densities of midwave infrared lasers with active regions realized with type-II quantum wells

Mastering Lateral Radiation Losses in Tunable VCSELs

This paper deals with the loss mechanisms in tunable vertical-cavity surface-emitting lasers (VCSELs). The strong increase of the threshold gain at the tuning range edges motivates the investigation of the degradation mechanisms presented in this paper. A campaign of simulations performed with our in-house VCSEL ELectroMagnetic (VELM) simulation code, combined with a novel approach based on the study of the Poynting vector, allowed to identify significant lateral radiation losses. The proposal of technology-affordable countermeasures rests on the proper understanding of these phenomena

Extraction of Mobility from Quantum Transport Calculations of Type-II Superlattices

Type-II superlattices (T2SLs) are being investigated as an alternative to traditional bulk materials in infrared photodetectors due to predicted fundamental advantages. Subject to significant quantum effects, these materials require the use of quantum transport methodologies, such as the nonequilibrium Green’s function (NEGF) formalism to fully capture the relevant physics without uncontrolled approximations. Carrier mobility is a useful parameter that affects carrier collection in photodetectors. This work investigates the application of mobility extraction methodologies from quantum transport simulations in the case of T2SLs exemplified using an InAs/GaSb midwave structure. In a resistive region, the average velocity can be used to calculate an apparent mobility that incorporates both diffusive and ballistic effects. However, the validity of this mobility for predicting device properties is limited to cases of diffusive limited transport or when the entire device can be included in the simulation domain. Two methods that have been proposed to extract diffusive limited mobility, one based on approximating the ballistic component of transport and the other which considers the scaling of resistance with simulation size, were also studied. In particular, the resistance scaling approach is demonstrated to be the method most physically relevant to predicting macroscopic transport. We present a method for calculating the mobility from resistance scaling considerations that accounts for carrier density variation between calculations, which is particularly relevant in the case of electrons. Finally, we comment on the implications of applying the different mobility extraction methodologies to device property predictions. The conclusions of this study are not limited to T2SLs, and may be generally relevant to quantum transport mobility studies

Simulation of Spiral Phase Plate VCSELs

Several groups belonging to different scientific communities are focusing their efforts on the theoretical and applied research on the orbital angular momentum of light (OAM). Among the most significant examples in the field of ICT, the OAM-division multiplexing recently emerged as a viable approach for increasing the capacity of the next generation optical networks. Optical beams featuring an OAM are usually generated through the conversion of a standard laser mode; such techniques usually rely on bulk optical components such as computer-generated holograms, spatial light modulators or spiral phase plates (SPPs). In a recent paper Li et al. proposed the possibility to load a commercial GaAs VCSEL with a micro-SPP [1]. The resulting device, which can emit directly OAM modes, is extremely appealing in view of reducing manufacturing costs, even opening up the possibility to introduce it in mass market applications, with an eye to the “green technology” paradigm in which VCSELs perfectly fit, due to their well-known low consumption features. The experimental work described in [1] has been complemented by a theoretical study performed by means of our in-house Vcsel ELectroMagnetic simulator (VELM) [2]. The possibility to estimate the OAM mode efficiency with full-wave simulations allowed to perform a thorough analysis of the sensitivity of the device to the possible manufacturing issues. As an example of the several numerical results that will be presented during the conference, Fig. 1 shows the simulated field profiles for the unprocessed VCSEL and for three micro-SPP geometries producing various OAM orders. Furthermore, the flexibility of VELM opens up the possibility to explore alternative implementations of SPP-VCSELs. Such solutions, supported by a technological affordability study, may allow to further develop these devices, in view of improving the emitted OAM mode purity while lowering the manufacturing costs

Nonequilibrium Green’s Function Modeling of type-II Superlattice Detectors and its Connection to Semiclassical Approaches

Theoretical investigations of carrier transport in type-II superlattice detectors have been mostly limited to simplified semiclassical treatments, due to the computational challenges posed by quantum kinetic approaches. For example, interband tunneling in broken-gap configurations calls for a multiband description of the electronic structure, and spatially indirect optical transitions in superlattice absorbers require fully nonlocal carrier-photon self-energies. Moreover, a large number of iterations is needed to achieve self-consistency between Green’s functions and self-energies in the presence of strongly localized states not directly accessible from the contacts. We demonstrate an accurate, yet computationally feasible nonequilibrium Green’s function model of superlattice detectors by formulating the kinetic equations in terms of problem-matched maximally localized basis functions, numerically generated from few modes representing the main conductive channels of the nanostructure. The contribution of all the remaining modes is folded in an additional self-energy to ensure current conservation. Inspection of spatially and energetically resolved single particle properties offers insight into the complex nature of carrier transport in type-II superlattice detectors, and the connection to semiclassical approaches enables the interpretation of mobility experiments

A multiscale approach for BTJ-VCSEL electro-optical analysis

This paper presents a theoretical comparison of the electro-optical characteristics of 850nm GaAs/AlGaAs pin-and BTJ-based VCSELs. The calculations are based on a drift-diffusion model coupled with a NEGF formalism, able to model accurately the tunneling across the TJ. The resulting LIV characteristics demonstrate promising improvements, at both 25 and 80°C, enabled by TJ confinement scheme