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$\pi$ 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

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

Bacillus pumilus severe wound infection in a healthy ten years old child: a rare case report

Bacillus pumilus is commonly isolated from a wide variety of soils, plants and environmental surfaces, but rarely from human specimens. In this report, we describe a case of infection caused by B. pumilus in a healthy 10-years-old child. The microorganism was recovered from a severe wound of the left knee after three days from trauma. Pathogen identification was carried out by mass spectrometry. The patient's outcome was positive following an ampicillin/sulbactam treatment without complications

Pelletization of composted swine manure solid fraction with different organic co-formulates: effect of pellet physical properties on rotating spreader distribution patterns

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