71 research outputs found
Sensitive dependence of the linewidth enhancement factor on electronic quantum effects in quantum cascade lasers
The linewidth enhancement factor (LEF) describes the coupling between
amplitude and phase fluctuations in a semiconductor laser, and has recently
been shown to be a crucial component for frequency comb formation in addition
to linewidth broadening. It necessarily arises from causality, as famously
formulated by the Kramers-Kronig relation, in media with non-trivial dependence
of the susceptibility on intensity variations. While thermal contributions are
typically slow, and thus can often be excluded by suitably designing the
dynamics of an experiment, the many quantum contributions are harder to
separate. In order to understand and, ultimately, design the LEF to suitable
values for frequency comb formation, soliton generation, or narrow laser
linewidth, it is therefore important to systematically model all these effects.
In this comprehensive work, we introduce a general scheme for computing the
LEF, which we employ with a non-equilibrium Green's function model. This direct
method, based on simulating the system response under varying optical
intensity, and extracting the dependence of the susceptibility to intensity
fluctuations, can include all relevant electronic effects and predicts the LEF
of an operating quantum cascade laser to be in the range of 0.1 - 1, depending
on laser bias and frequency. We also confirm that many-body effects,
off-resonant transitions, dispersive (Bloch) gain, counter-rotating terms,
intensity-dependent transition energy, and precise subband distributions all
significantly contribute and are important for accurate simulations of the LEF.Comment: 8 pages, 6 figure
Highly tunable hybrid metamaterials employing split-ring resonators strongly coupled to graphene surface plasmons
Metamaterials and plasmonics are powerful tools for unconventional
manipulation and harnessing of light. Metamaterials can be engineered to
possess intriguing properties lacking in natural materials, such as negative
refractive index. Plasmonics offers capabilities to confine light in
subwavelength dimensions and to enhance light-matter interactions.
Recently,graphene-based plasmonics has revealed emerging technological
potential as it features large tunability, higher field-confinement and lower
loss compared to metal-based plasmonics. Here,we introduce hybrid structures
comprising graphene plasmonic resonators efficiently coupled to conventional
split-ring resonators, thus demonstrating a type of highly tunable
metamaterial, where the interaction between the two resonances reaches the
strong-coupling regime. Such hybrid metamaterials are employed as high-speed
THz modulators, exhibiting over 60% transmission modulation and operating speed
in excess of 40 MHz. This device concept also provides a platform for exploring
cavity-enhanced light-matter interactions and optical processes in graphene
plasmonic structures for applications including sensing, photo-detection and
nonlinear frequency generation
High-Power Directional Emission from Microlasers with Chaotic Resonators
High-power and highly directional semiconductor cylinder-lasers based on an
optical resonator with deformed cross section are reported. In the favorable
directions of the far-field, a power increase of up to three orders of
magnitude over the conventional circularly symmetric lasers was obtained. A
"bow-tie"-shaped resonance is responsible for the improved performance of the
lasers in the higher range of deformations, in contrast to
"whispering-gallery"-type modes of circular and weakly deformed lasers. This
resonator design, although demonstrated here in midinfrared quantum-cascade
lasers, should be applicable to any laser based on semiconductors or other
high-refractive index materials.Comment: Removed minor discrepancies with published version in the text and in
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