49 research outputs found
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
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
Optical frequency combs in SiN hybrid lasers
We model the dynamical behavior of a III-V SiN hybrid laser accounting for the narrowband mirror effective reflectivity. We characterize the laser as a function of bias current and detuning of the laser emission frequency with respect to the effective reflectivity peak. Numerical simulations allow to address the potential physical triggers for longitudinal multimode dynamics. Among these, we assess the mechanism of generation of frequency combs and we preliminarily characterize this regime in terms of bandwidth and line separation
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
Self-pulsing in single section ring lasers based on Quantum Dot materials: theory and simulations
We studied theoretically coherent phenomena in the multimode dynamics of
single section semiconductor ring lasers with Quantum Dots (QDs) active region.
In the unidirectional ring configuration our simulations show the occurrence of
self-mode-locking in the system leading to ultra-short pulses (sub-picoseconds)
with a THz repetition rate. As confirmed by the Linear Stability Analysis (LSA)
of the Traveling Wave (TW) Solutions this phenomenon is triggered by the
analogous of the Risken-Nummedal-Graham-Haken (RNGH) instability affecting the
multimode dynamics of two-level lasers
Reconfigurable photoinduced metamaterials in the microwave regime
We investigate optically reconfigurable dielectric metamaterials at gigahertz
frequencies. More precisely, we study the microwave response of a subwavelength
grating optically imprinted into a semiconductor slab. In the homogenized
regime, we analytically evaluate the ordinary and extraordinary component of
the effective permittivity tensor by taking into account the photo-carrier
dynamics described by the ambipolar diffusion equation. We analyze the impact
of semiconductor parameters on the gigahertz metamaterial response which turns
out to be highly reconfigurable by varying the photogenerated grating and which
can show a marked anisotropic behavior.Comment: 6 figures, 7 page
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
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