377 research outputs found
Coherent multi-mode dynamics in a Quantum Cascade Laser: Amplitude and Frequency-modulated Optical Frequency Combs
We cast a theoretical model based on Effective Semiconductor Maxwell-Bloch
Equations and study the dynamics of a multi-mode mid-Infrared Quantum Cascade
Laser in Fabry Perot with the aim to investigate the spontaneous generation of
optical frequency combs. This model encompasses the key features of a
semiconductor active medium such as asymmetric,frequency-dependent gain and
refractive index as well as the phase-amplitude coupling of the field dynamics
provided by the linewidth enhancement factor. Our numerical simulations are in
excellent agreement with recent experimental results, showing broad ranges of
comb formationin locked regimes, separated by chaotic dynamics when the field
modes unlock. In the former case, we identify self-confined structures
travelling along the cavity, while the instantaneous frequency is characterized
by a linear chirp behaviour. In such regimes we show that OFC are characterized
by concomitant and relevant amplitude and frequency modulation
Terahertz optically tunable dielectric metamaterials without microfabrication
We theoretically investigate the terahertz dielectric response of a
semiconductor slab hosting an infrared photoinduced grating. The periodic
structure is due to the charge carries photo-excited by the interference of two
tilted infrared plane waves so that the grating depth and period can be tuned
by modifying the beam intensities and incidence angles, respectively. In the
case where the grating period is much smaller than the terahertz wavelength, we
numerically evaluate the ordinary and extraordinary component of the effective
permittivity tensor by resorting to electromagnetic full-wave simulation
coupled to the dynamics of charge carries excited by infrared radiation. We
show that the photoinduced metamaterial optical response can be tailored by
varying the grating and it ranges from birefringent to hyperbolic to
anisotropic negative dielectric without resorting to microfabrication.Comment: 3 pages, 2 figure
Extrinsic electromagnetic chirality in all-photodesigned one-dimensional THz metamaterials
We suggest that all-photodesigned metamaterials, sub-wavelength custom
patterns of photo-excited carriers on a semiconductor, can display an exotic
extrinsic electromagnetic chirality in terahertz (THz) frequency range. We
consider a photo-induced pattern exhibiting 1D geometrical chirality, i.e. its
mirror image can not be superposed onto itself by translations without
rotations and, in the long wavelength limit, we evaluate its bianisotropic
response. The photo-induced extrinsic chirality turns out to be fully
reconfigurable by recasting the optical illumination which supports the
photo-excited carriers. The all-photodesigning technique represents a feasible,
easy and powerful method for achieving effective matter functionalization and,
combined with the chiral asymmetry, it could be the platform for a new
generation of reconfigurable devices for THz wave polarization manipulation.Comment: 11 page
Retrieval of the Dielectric Properties of a Resonant Material in the Terahertz Region via Self-Detection Near Field Optical Microscopy
We present a numerical and analytical study of the self-detection scattering type near field optical microscopy (SD s-SNOM), a recently demonstrated technique based on a combination of self-mixing interferometry and scattering near-field microscopy. This scheme, which exploits a terahertz (THz) quantum cascade laser as both a laser source and detector, allows to investigate the optical properties of resonant materials in the THz range with resolution far beyond the diffraction limit. Our study, developed by using a modified version of the Lang-Kobayashi model, is focused on the weak feedback regime (Acket parameter C approximate to 10(-1)), where we derive an approximated method for the retrieval of the scattering coefficient of the SD s-SNOM configuration applied to a sample of Cesium Bromide (CsBr). These results were used in turn to derive the dielectric permittivity of the sample, reporting a good accuracy in the estimation of its phonon resonances
Dissipative phase solitons in semiconductor lasers
We experimentally demonstrate the existence of non dispersive solitary waves
associated with a 2 phase rotation in a strongly multimode ring
semiconductor laser with coherent forcing. Similarly to Bloch domain walls,
such structures host a chiral charge. The numerical simulations based on a set
of effective Maxwell-Bloch equations support the experimental evidence that
only one sign of chiral charge is stable, which strongly affects the motion of
the phase solitons. Furthermore, the reduction of the model to a modified
Ginzburg Landau equation with forcing demonstrates the generality of these
phenomena and exposes the impact of the lack of parity symmetry in propagative
optical systems.Comment: 5 pages, 5 figure
Bioengineering the Pancreas: Cell-on-Scaffold Technology
Nowadays, type I diabetes mellitus is a pathology afflicting millions of people globally with a dramatic assessment in the next future. Current treatments including exogenous insulin, pancreas transplantation and islets transplantation, are not free from important lifelong side effects. In the last decade, tissue engineering and regenerative medicine have shown encouraging results about the possibility to produce a functional bioengineered pancreas. Among many technologies, decellularization offers the opportunity to produce an organ-specific acellular matrix that could subsequently repopulate with endocrine cellular population. Herein, we aim to review the state-of-art and this technology highlighting the diabetes burden for the healthcare system and the major achievements toward the manufacturing of a bioengineered pancreas obtained by cell-on-scaffold technology
Dynamics of Optical Frequency Combs in Ring and Fabry-Perot Quantum Cascade Lasers
Since the demonstration that multimode Quantum Cascade Lasers (QCLs) can operate as sources of Optical Frequency Combs (OFC) [1] , an extended class of theoretical models, based on standard two or three level Maxwell-Bloch equations, has been proposed to interpret such phenomenology
Scaling collaborative policymaking: how to leverage on digital co-creation to engage citizens
n recent years, new methods to engage citizens in deliberative processes of governments and institutions have been studied. Such methodologies have become a necessity to assure the efficacy and sustainability of policies. Several tools and solutions have been proposed while trying to achieve such a goal. The dual problem to citizen engagement is how to provide policymakers with useful and actionable insights and data stemming from those processes. The following paper has the aim to share with the audience of the Data for Policy Conference 2021 an innovative tool based on the concept of participatory policymaking with the scope of collecting feedback and comments to enhance the consistency and the usefulness of the tool.
We propose research featuring a method and implementation of a crowdsourcing and co-creation technique that can provide value to both citizens and policymakers engaged in the policy-making process.
Thanks to our methodology, policymakers can design challenges for citizens to take part, cooperate and provide their input to policymakers. We also propose a web-based tool that allows citizens to participate and produce content to support the policymaking processes through a gamified interface that focuses on emotional and vision-oriented content
Machine learning methods to predict outcomes of pharmacological treatment in psychosis
In recent years, machine learning (ML) has been a promising approach in the research of treatment outcome prediction in psychosis. In this study, we reviewed ML studies using different neuroimaging, neurophysiological, genetic, and clinical features to predict antipsychotic treatment outcomes in patients at different stages of schizophrenia. Literature available on PubMed until March 2022 was reviewed. Overall, 28 studies were included, among them 23 using a single-modality approach and 5 combining data from multiple modalities. The majority of included studies considered structural and functional neuroimaging biomarkers as predictive features used in ML models. Specifically, functional magnetic resonance imaging (fMRI) features contributed to antipsychotic treatment response prediction of psychosis with good accuracies. Additionally, several studies found that ML models based on clinical features might present adequate predictive ability. Importantly, by examining the additive effects of combining features, the predictive value might be improved by applying multimodal ML approaches. However, most of the included studies presented several limitations, such as small sample sizes and a lack of replication tests. Moreover, considerable clinical and analytical heterogeneity among included studies posed a challenge in synthesizing findings and generating robust overall conclusions. Despite the complexity and heterogeneity of methodology, prognostic features, clinical presentation, and treatment approaches, studies included in this review suggest that ML tools may have the potential to predict treatment outcomes of psychosis accurately. Future studies need to focus on refining feature characterization, validating prediction models, and evaluate their translation in real-world clinical practice
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