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We investigate the intensity and phase noise properties of GaAs-based 1060 nm oxide-confined single-mode vertical-cavity surface-emitting lasers (VCSELs) and their dependence on slope efficiency and current spreading, parameters that control the achievable output power. We find strong dependence of the linewidth on slope efficiency because it affects the optical resonator loss and therefore the spontaneous emission rate and the photon density. Likewise, we find strong dependence of the relative intensity noise on the slope efficiency since the optical resonator loss controls the photon lifetime, and therefore the damping of the relaxation oscillations. There is no noticeable dependence on transverse current confinement and current spreading. We measure linewidths as small as 6 MHz which we attribute to a small linewidth enhancement factor. This assumption is supported by calculations of the linewidth enhancement factor from optical resonator and optical gain simulations. The dependencies of noise on design parameters are general and therefore valid for single-mode VCSELs at other wavelengths and in other material systems.

International audience; In this work, the thermo-optical properties of a nematic liquid crystal are determined through reflectance measurements performed on a high finesse tunable filter fabricated using a polymer-based microcell technology. The final aim is to insert such material in the optical cavity of a 850 nm tunable VCSEL device, in which local self-heating due to CW pumping must be taken into account. These localized interferometric experiments are performed in the near-infra-red range and at temperatures up to 115 °C. A thermal model is derived from the acquired data. Finally, we demonstrate that the birefringence of QYPDLC-36 liquid crystal remains higher than 0.18 at 60 °C, feature well suited to real device operation.

One of the more obscure areas of Roman Republican early coinage are debased quadrigati, which traditionally are deemed to represent the last stage of this coin type. Starting from a pre-screening, based on low specific gravity samples, we first performed neutron diffraction analyses on a larger sample, which allowed us to single out the more debased series. Subsequently, we focused on those series and applied various additional physical investigations on 18 specimens. The focus of this paper is 17 samples displaying very low silver content. They all belong to a very particular group, refereed as “Apulian” quadrigati in previous literature. They are selected for their numismatic differences and to put in evidence the relevant details of their silver quality, which ranges from “apparently good” to “plain bronze.” In this work, we combine rigorous analytical investigations like X-ray fluorescence (both ordinary and micro-), scanning electron microscope, and neutron diffraction to our accurate numismatic classification of the specimens, leading to a clear correlation between series and debasement. This work aims in particular to gain better insights into these mysterious “silver” emissions, bringing new results that can disclose unknown financial and political facts pertaining to the Second Punic War.

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.

Probing the thermal effects that limit the performance of vertical-cavity surface-emitting lasers (VCSELs) reveals an intricate interplay between carrier transport, recombination mechanisms, thermal conduction, and optical features. An understanding of this interplay requires an accurate yet computationally-efficient multiphysical approach at the microscopic level. We address this task by characterizing oxide-confined multimode 850 nm VCSELs over a wide temperature range, and by simulating them with VENUS, our in-house multiphysics simulator. The agreement with the experimental results in the whole explored temperature range provides physical insight into the mechanisms that determine VCSEL rollover and turn off, and demonstrates the predictive capabilities of the model. A complete set of model parameters determined through this combined experiment-simulation approach is presented.

The properties of vertical-cavity surface-emitting lasers (VCSELs) are investigated by means of a multiphysical Vcsel Electro-opto-thermal NUmerical Simulator (VENUS). VENUS includes a three-dimensional vectorial electromagnetic code, a description of the quantum well optical response, a heat equation solver, and a quantum-corrected drift-diffusion simulator. The proposed suite includes coupling mechanisms often overlooked in VCSEL simulation and allows to reassess the impact of parameters, which can be critically dependent on implementation details inaccessible in commercial codes. The agreement with experimental results holds the promise of the application of this framework to the computer-aided design of innovative VCSEL concepts.

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. less

The aim of this work is to investigate the scattering losses induced by the buried tunnel junction (BTJ) in a VCSEL operating at 4 µm for gas spectroscopy [1] . In this device, losses are even more important than in a standard VCSEL, due to the presence of an active region (AR) of type II which displays lower gain compared to type I. This is due to a reduced carrier wavefunction overlapped integral. Therefore a robust optical design should guarantee minimal losses. In fact, the presence of the BTJ inside the cavity enables scattering losses, which is a very well known problem in long wavelength VCSEL; in [2] the effect of the BTJ size in a VCSEL emitting at 2.3 µm was investigated by keeping the etching height constant and varying the step aperture and smoothness.

International audience; Relying on preliminary experiments, in this work we design a tunable 850 nm laser based on a hybrid combination of a liquid crystal micro-cell with nanoimprinted grating and an electrically-pumped GaAs half-VCSEL. The optical design is challenging due to the inherent tolerances of this hybrid technology, the presence of metals in the cavity and the need for single fundamental extraordinary mode emission over the whole tuning range. To ensure proper operation, we introduce for the first time the new concept of anisotropic transverse confinement design. The overall performance is verified by our multiphysics VCSEL suite. Beyond tuning features, we predict side mode suppression ratio and optical power under all working conditions, comparing the performance of liquid crystals with different clearing temperatures. The results qualifies these lasers as viable tunable sources with interesting performance and complementary features compared to other technologies.

Vertical-cavity surface-emitting lasers (VCSELs) are widely used in optical data communication mainly in data centers for short-haul transmissions. However, their intensity modulation resonance frequency does not exceed 40 GHz which also limits the achievable modulation bandwidth and data rate. In contrast, spin-VCSELs can overcome these bandwidth limitations by modulating spin and polarization instead of current and intensity. In spin-VCSELs, the birefringence determines the resonance frequency of the polarization dynamics as well as the modulation bandwidth. We control the birefringence and thus the polarization dynamics via the elasto-optic effect by mechanically or thermally induced strain providing polarization oscillation frequencies up to more than 200 GHz. Detailed analysis shows that spin-VCSELs offer polarization dynamics with good signal strength even when operating close to threshold and at high temperatures. Here, we analyze devices with integrated surface gratings. VCSELs with different grating periods as well as mesa diameters and resulting different oxide apertures were investigated. © 2021 SPIE.

A preliminary study of the dynamic behaviour of a GaAs/AlGaAs 850 nm VCSEL is presented, with the focus on the small-signal analysis and in particular on the optical amplitude modulation response. Simulations are performed with our in-house quantum-corrected one-dimensional drift-diffusion code D1ANA, updated to perform the AC analysis. The-3 dB cutoff frequency of the device is extracted as a function of VCSEL bias current and compared with experimental results.

In this work we investigate carrier transport in tunnel junctions for vertical-cavity surface-emitting lasers by a novel self-consistent simulation framework for semiconductor quantum devices. Based on a Poisson-drift-diffusion foundation, in this approach quantum features are described through a nonequilibrium Green's function formalism. The simulator is validated through a comparison with experimental results.

International audience; An InP-based Vertical-Cavity Surface-Emitting 1 Laser (VCSEL) with a liquid crystal (LC) microcell monolithically integrated on its surface for spectral tuning is investigated. Unlike tunable VCSELs integrating a movable membrane, here the physical length of the cavity remains unchanged and only the voltage applied on the LC ensures a refractive index modification for a particular polarization emitted by the VCSEL. This tunable VCSEL operates in CW at room temperature and exhibits more than 23 nm wavelength tuning around 1.55 µm at a maximum applied voltage of 20 V. The measured laser threshold around 6.5 mW is still comparable to VCSEL without LC microcell, a clear indication that the optical losses related to the LC are very low. On the other hand, for this first optically pumped device, the lasing characteristics suggest that the LC birefringence is lower than expected. To assess this hypothesis, thermo-optical simulations have been conducted.

In single-mode vertical-cavity surface-emitting lasers (VCSELs) the frequency difference between the two orthogonal modes, which is defined by the birefringence present in the cavity, is the key factor to enable ultrafast polarization dynamics in spin-lasers. This could be a promising alternative to overcome the bandwidth limitations in short-haul data transmission. Therefore, controlling the birefringence is indispensable to utilize the full potential of the polarization dynamics. Splittings of around 100GHz were realized with an on-chip approach by integration of a surface grating in an oxide-confined AlGaAs-based VCSEL. In this paper we present further details of the parameter search process using a three-dimensional vectorial optical VCSEL electro-magnetics (VELM) model. We also show the geometrical properties of the processed grating structure.

International audience; In this work, the thermo-optical properties of nematic liquid crystals are investigated through localized reflectance spectra measurements performed on a tunable LC-filter. The final aim is to insert such LC in the cavity of a tunable VCSEL device, in which local self-heating due to optical or electrical pumping must be taken into account. We demonstrate that, whereas standard LC E7 is not suited for real device operation, the maximal operating temperature of QYDPLC-36 LC is 95°C, with a birefringence at 1.55 μm higher than 0.18 at 60°C.

This work investigates carrier transport in tunnel junctions for vertical-cavity surface-emitting lasers (VCSELs). The study is performed with a quantum-corrected semiclassical approach, where tunneling is described rigorously with a nonequilibrium Green's function formalism based on a multiband description of the electronic structure. Validated with experimental results, the proposed approach provides a quantum -kinetic perspective of the tunneling process and paves the way toward a comprehensive theory of VCSELs, bridging the gap between semiclassical and quantum simulations.

International audience; In this work we investigate a tunable 850nm laser based on a hybrid combination of a liquid crystal micro-cell and a half GaAs VCSEL. The target application is optical coherence tomography. The inherent tolerances of the hybrid technology, the presence of metals in the cavity and the need for a pure extraordinary mode lasing make challenging the optical design of this laser. Hence, VELM, a full 3D and vectorial VCSEL optical solver, is utilized to understand loss mechanisms and provide an optimized design.

On our way to develop a comprehensive simulator of vertical-cavity surface-emitting lasers including carrier transport, optical modes, light-matter interaction and heat conduction, we address some critical multiscale aspects of the adopted coupling strategy. The quantum corrections to the semiclassical carrier transport framework are the bridge from nanometer to micrometer scales. In this paper, such corrections are shown to be fundamental in view of predicting the mode competition ruling VCSEL operation. Nevertheless, they can generate unforeseen features such as swirling electron flows in the active region, which are discussed here in detail. The simulation approach has been finally tested through a successful comparison with a large set of experimental results.

Several 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.

The lasing operation of vertical-cavity surface-emitting lasers (VCSELs) results from the interaction of diverse physical mechanisms. For this reason, a reliable model must solve the coupled carrier transport, thermal and optical problems. With the aim of removing the present lack of established simulation tools, in this work we present our in-house electro-opto-thermal VCSEL numerical simulator VENUS. After a description of its four constituents, a validation with experimental results obtained at different temperatures is presented, demonstrating the predictive capabilities of VENUS.

Increasing the birefringence splitting in single-mode vertical-cavity surface-emitting lasers (VCSELs) enables high-speed polarisation dynamics which can be the basis to overcome the current bandwidth limitations in short-haul data transmission. The authors observe large birefringence splittings of up to 98 GHz in an oxide-confined AlGaAs-based VCSEL with a tailored integrated surface grating. Since surface gratings are routinely used in VCSEL production, there is a great potential of this technique to realise spin-VCSELs for ultrafast optical communication.

On our way to develop a multiphysical model of VCSELs including carrier transport, electromagnetic propagation, and heat conduction, we discuss some critical aspects of the adopted multiscale strategy. In particular, we address the inclusion of non-classical corrections within a classical carrier-transport framework, and the possibilities offered by rigorous approaches such as the nonequilibrium Green’s function (NEGF) towards the determination of some critical parameters, bridging from the nanometer to the macroscopic scale. This seems a promising line of action towards the simulation and optmization of 3D highly nanostructured devices such as VCSELs.

The measurement of the fineness of debased ancient silver coins has proven to be a very difficult issue, which has been studied for a long time. In this paper, this subject is analysed, and the various consequences of the silver surface enrichment (SSE) are discussed exploiting the most recent investigations. A new model is proposed for the complex ob- ject that is an ancient-debased silver coin, based on the silver profiles measured on some sectioned specimens. The model is applied to a sample of 43 coins, mainly Roman victoriati, Cisalpine and Illyrian drachms (from late III to I century B.C.). The coins are investigated in two different ways: neutron diffraction (ND) and specific gravity (SG). The results of the two measurements are combined via the proposed model to provide a more complete numismatic information of the original fineness of the monetary alloy. As a result, a relation between SSE thickness and SG is derived, which, for these coinages, allows to estimate the original alloy silver content from a simple SG measurement; the same method can be used to study other debased coinages, provided that all the procedure (ND and SG) is applied

The extraordinary reflectivity and resonance features of high-contrast gratings are consequences of the interference of two grating modes. From this observation, an extension of the Fabry-P´erot interferometer to the bimodal case is proposed, in view of describing rigorously the physical mechanisms occurring in the grating. The closedform expressions of the interferometer reflection and transmission, obtained starting from a novel parametrization of the unitary symmetric 3 × 3 scattering matrix characterizing the bar-air interface, allow a complete exploration of the device parameter space, explaining and predicting phenomena such as ultra-broadband quasi-100% reflectivity.

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.

Based on the study of a large sample of 1,150 coins, this paper proposes a new classification, in several distinct groups, of the anonymous denarii classed by Crawford as RRC 53/ 2. For each of these groups, a place, date and/ or their circumstances of emission will be proposed according to their stylistic differences, die links and metrology, as well as on the basis of archaeological and hoard evidence. Special attention will be drawn to the links between some of these anonymous coins and issues with symbols. Within the overall framework of the Second Punic War it is suggested that these large issues of denarii, of which some important new varieties are identified, were produced in Apulia, perhaps between 209 and 205 BC, rather than in the mint of Rome at an earlier starting date proposed by Crawford., Fondé sur l’étude d’un large échantillon de 1 150 monnaies, cet article propose un nouveau classement, selon plusieurs groupes distincts, des deniers anonymes regroupés par Crawford sous la référence 53/ 2. Pour chacun de ces groupes, un lieu, une date et/ ou des circonstances d’émission seront proposés en fonction de leurs différences stylistiques, des liaisons de coins, de leur métrologie ainsi que de la documentation archéologique et des trouvailles monétaires. Une attention particulière sera portée aux relations entre certaines de ces monnaies anonymes et plusieurs émissions avec symboles. Replacées dans le contexte des événements et relations numismatiques intervenus au cours de la deuxième guerre punique, ces larges émissions de deniers, dont plusieurs importantes nouvelles variétés ont pu être identifiées, ont été produites en Apulie, peut-être entre 209 et 205 av. J.-C. pour la plupart d’entre elles, une date plus tardive que celle proposée par Crawford qui les pense émises par l’atelier de Rome., Debernardi Pierluigi, Brinkman Steve. A New Arrangement for RRC 53/2. In: Revue numismatique, 6e série - Tome 175, année 2018 pp. 193-239.

Recently, we reported the first successful fabrication of a marker-free microparticle detector with a unique working principle: when a particle is introduced into the cavity of an electrically pumped vertical-external-cavity surface-emitting laser, both the change of electrical operation as well as optical output parameters can be directly monitored, which enables phenotyping of properties, such as particle volume and refractive index. In the present contribution, we investigate this mechanism by numerical modeling of the wave-optical characteristics of the sensor.

The book chapter reviews some of the main models aimed at describing electron transport in optoelectronic semiconductor devices. In the first section the hydrodynamic, energy-balance and drift-diffusion models are derived from the Boltzmann transport equation. Then, the second section is focused on quantum transport, and, after reviewing the semiconductor Bloch equations, introduces the nonequilibrium Green's function formalism.

Carrier transport in GaAs-based vertical-cavity surface-emitting lasers (VCSELs) is investigated by means of an in-house multiphysics code, with particular emphasis on the description of many-valley effects in the conduction band of AlGaAs barriers. These effects, which are revealed to have a significant impact on the overall VCSEL performance, are accounted for by an effective density of states obtained with a closed-form model. This description has been included in a simplified simulation framework, where most of the distributed Bragg reflector pairs are replaced by an equivalent homogeneous layer. This leads to a major reduction of the computational cost, especially important in view of the computer-aided design of 3D devices.

A modal analysis of both molecular beam epitaxy (MBE)- and metal-organic vapor phase epitaxy (MOVPE)-planarized short-cavity (SC) vertical-cavity surface-emitting lasers (VCSELs) emitting at 1.3 μm is presented. The comparison of simulated threshold gains with experimental threshold current densities, as well as modal gain difference with side-mode suppression ratios, allows the clear identification of design-related limitations with respect to single-mode emission for different active diameters. In particular, the influence of the radial profile of the effective refractive index on the strength of index-guiding is found to depend on the regrowth-type (MBE or MOVPE). Moreover, the impact of strongly absorbing contact layers and surface relief structures on the modal properties of the fundamental mode is investigated. A design proposal for a MOVPE-regrown SC-VCSEL with optimized surface relief structure is given, predicting reduced threshold current densities and increased single-mode optical output powers.

A computationally efficient theoretical approach is used to assess the performance of novel devices with directly emitting optical vortex modes.

We present polarization-resolved intensity noise investigations of oxide-confined vertical-cavity surface-emitting lasers (VCSELs) with an integrated small-area surface grating. We demonstrate that the reduction of the complex polarization and transverse mode behavior via the introduced high dichroism of the grating relief lead to an improved, quite simple noise behavior. The observed noise behavior is in good qualitative agreement with a two-mode noise model.

The problem of the origin of silver plated coins during Roman Republican times is analyzed. All the information available to settle the long-lasting discussion about the subject will be addressed, by giving particular relevance especially to what the coins can directly tell us. First of all, to put the matter in a clear framework, a review of the past opinions by scholars is presented. The central part of the paper is devoted to an analysis of a previously published extended catalogue of the plated coins of the Roman Republic. Here it will be investigated under a different perspective and, moreover, its temporally resolved information is compared with the silver content policy in the Republican times, recently studied in detail by the author. The overall analysis leads to the conclusion that the main stream of plated coins, during Republican times, was produced under the control of the Roman mint. In such a perspective the many oddities which arise in the option of a private origin of plated coins and which are thoroughly discussed in the paper, can be easily solved., Le problème de l’origine des monnaies fourrées pendant la République romaine est analysé. Toutes les informations disponibles pour régler les longues débats sur le sujet seront utilisés, en accordant une importance particulière à ce que les pièces peuvent directement nous le dire. Tout d’abord, un examen des différentes opinions exprimées dans le passé par divers étudiants est présenté. La partie centrale de l’article analyse un catalogue déjà publié des deniers fourrés de la République romaine. Ici, il sera étudié sous un point de vue différent et, par ailleurs, ses informations temporellement résolues sont comparées avec la politique d’avilissement monétaire de l’époque républicaine, récemment étudié en détail par l’auteur. L’analyse globale conduit à la conclusion que la majorité des pièces fourrées, à l’époque républicaine, sont monnaies officielles. Dans une telle perspective les bizarreries qui dérivent de l’hypothèse d’une origine privée des monnaies fourrées, qui sont discutés en profondeur dans le document, peuvent être facilement résolues., Debernardi Pierluigi. Plated coins, false coins?. In: Revue numismatique, 6e série - Tome 166, année 2010 pp. 337-381.

We discuss some of the key issues to be addressed along the way to complement, and possibly to replace, the standard semiclassical Boltzmann picture with genuine quantum approaches for the simulation of carrier transport and recombination in GaN-based LEDs, with the goal of gradually removing the fitting parameters presently required by semiempirical "quantum corrections" and to better understand the processes responsible for the efficiency droop. As examples of augmented semiclassical models, we present a three-step description of trap-assisted tunneling, especially relevant below the optical turn-on, and a carrier-density-dependent estimate of the phonon-assisted capture rate from bulk states to quantum wells (QWs). Moving to genuine quantum models, we solve the semiconductor Bloch equations to calculate the gain/absorption spectra of AlGaN/GaN QWs, and we discuss our first simulations of spatially and energetically resolved currents across the active region of a single-QW LED based on the nonequilibrium Green's function approach.

The 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.

A detailed parametric investigation of the modal performance of single-mode, single-polarization vertical-cavity surface-emitting lasers (VCSELs) loaded with a microspiral phase plate is reported. This paper stems from a recent publication, where the structure is technologically implemented and tested. Starting from the actual structure parameters, we investigate by VCSEL ELectroMagnetic (VELM), our in-house fully 3-D and vectorial solver, the features of such a novel approach to produce orbital angular momentum modes, in view of next-generation high-capacity optical communications.