1,721,076 research outputs found
A Mortar Element Method for the Analysis of Electromagnetic Passive Devices
Full text linkThe 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, Netherland
A Mortar-Element Method for the Analysis of Cylindrical Guiding Structures
No full textIn this work a novel formulation for the analysis of structures with axial symmetry is presented. A multi-domain spectral method based on mortar-matching is developed. The technique is applied to the analysis and design of smooth-wall horn antennas. A comparison of the simulation results obtained with this numerical scheme and with a mode-matching code is presented
Bimodal Resonance Phenomena—Part I: Generalized Fabry-Pérot Interferometers
Full text linkThe operation of several optical components, such as high-contrast gratings, is based on the interference between two oscillation modes. Therefore, this paper is devoted to the complete characterization of bimodal Fabry-Pérot interferometers, which can effectively model such two-mode interactions. Thanks to a novel parameterization of the mirror scattering matrices, this paper presents for the first time explicit expressions of the bimodal interferometer response, proving phenomena such as 100% reflection peaks, and predicting their positions. For this reason, this paper, which complements—rather than replaces—the existing numerical techniques, provides a completely new perspective on high-contrast gratings
High-Contrast Gratings Performance Issues in Tunable VCSELs
Full text linkTunable vertical-cavity surface-emitting lasers (VCSELs) are becoming key components in several sensing applications, where single-mode, single polarization light is required. In such devices, high-contrast gratings (HCGs) are very promising mirrors, because of their high reflectivity, strong polarization discrimination, and low-volume occupation. However, their introduction might lead to unexpected increases in the threshold gain, destroying the tuning performance. In this paper, these phenomena are investigated and explained in detail. Although VCSEL modes are computed by means of our full 3-D vectorial code, a Gaussian beam model is used to obtain deeper insight into the HCG reflectivity mechanism in the presence of a finite beam
Bimodal Resonance Phenomena—Part II: High/Low-Contrast Grating Resonators
Full text linkSeveral groups are currently working on integrated optical resonators. In this context, one of the most interesting implementations is based on high-contrast gratings, featuring high-Q Fano resonances. Mastering these phenomena can potentially open up the possibility to exploit such devices for the implementation of vertical-coupled filters or compact monolithic lasers. To this aim, in this paper, the novel analytic framework introduced in Part I is applied to complement the design guidelines present in the literature. Through explicit expressions of the loop gain eigenvalues, it is possible to predict the positions and quality factors of resonances in crossing and anticrossing points. Moreover, a quantitative connection between these points and Fano resonances is established. The framework presented in this paper has been useful for identifying, in a low-contrast grating, resonance features similar to those well known in high-contrast structures
New features of HCG reflectivity under finite excitation
No full textThe use of vertical-cavity surface-emitting lasers based on high-contrast gratings (HCG-VCSELs) as light sources is getting diffused over a very broad range of applications. For this reason, nowadays, their design-oriented modeling is a very hot research topic. Usually HCGs are designed on the basis of their plane-wave response. On the other hand, their reflection performance has been recently characterized for an incident Gaussian beam, to account for the source finiteness. This leads to an excitation similar to the VCSEL mode, without using the entire, complicated 3-D model. It has been observed that reflectivity is much lower compared to the normal-incident plane-wave case. Moreover, as reported in Fig. 1, it has been shown that the plane-wave reflectivity exhibits a very strong angular dependence, causing a degeneration of the well-known reflectivity peak to a trench. This mechanism impacts strongly on the reflection performance for a finite beam, as shown in Fig. 2, where reflectivity is reported as a function of the Gaussian beam waist, for two operating wavelengths. In this work the reflectivity degradation is investigated through theoretical analyses that will be discussed during the presentation. These results allow to obtain additional design guideline
Multiscale and Multiphysics Solvers for AlGaAs TJ-VCSEL
Full text linkAmong possible strategies to improve the performance of near infrared AlGaAs/GaAs vertical-cavity surface-emitting lasers (VCSELs) for short-reach interconnects, current injection schemes based on tunnel junctions (TJs) may be an enabling technology to meet the high temperature requirements of data-center applications. To assess the merits of TJs in this context, we perform a comparative simulation-based study of a commercial pin VCSEL and a modified structure where holes are injected into the active region through a TJ. Band-to-band tunneling probabilities are computed within a multiband nonequilibrium Green’s function (NEGF) approach. The resulting generation rates are included in a quantum-corrected drift-diffusion model for carrier transport. The optical modes of the cavity are found with an electromagnetic solver, and self-heating effects are studied with a thermal model. The comparative multiphysical 1D and 3D simulations of pin and TJ-VCSELs predict that the voltage penalty introduced by the reverse-biased TJ is compensated by the higher output optical power
Extraction of Mobility from Quantum Transport Calculations of Type-II Superlattices
Full text linkType-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
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