277 research outputs found
Universal lineshapes at the crossover between weak and strong critical coupling in Fano-resonant coupled oscillators
In this article we discuss a model describing key features concerning the
lineshapes and the coherent absorption conditions in Fano-resonant dissipative
coupled oscillators. The model treats on the same footing the weak and strong
coupling regimes, and includes the critical coupling concept, which is of great
relevance in numerous applications; in addition, the role of asymmetry is
thoroughly analyzed. Due to the wide generality of the model, which can be
adapted to various frameworks like nanophotonics, plasmonics, and
optomechanics, we envisage that the analytical formulas presented here will be
crucial to effectively design devices and to interpret experimental results
Coherent perfect absorption in photonic structures
The ability to drive a system with an external input is a fundamental aspect
of light-matter interaction. The coherent perfect absorption (CPA) phenomenon
extends to the general multibeam interference phenomenology the well known
critical coupling concepts. This interferometric control of absorption can be
employed to reach full delivery of optical energy to nanoscale systems such as
plasmonic nanoparticles, and multi-port interference can be used to enhance the
absorption of a nanoscale device when it is embedded in a strongly scattering
system, with potential applications to nanoscale sensing. Here we review the
two-port CPA in reference to photonic structures which can resonantly couple to
the external fields. A revised two-port theory of CPA is illustrated, which
relies on the Scattering Matrix formalism and is valid for all linear two-port
systems with reciprocity. Through a semiclassical approach, treating two-port
critical coupling conditions in a non-perturbative regime, it is demonstrated
that the strong coupling regime and the critical coupling condition can indeed
coexist; in this situation, termed strong critical coupling, all the incoming
energy is converted into polaritons. Experimental results are presented, which
clearly display the elliptical trace of absorption as function of input
unbalance in a thin metallo-dielectric metamaterial, and verify polaritonic CPA
in an intersubband-polariton photonic-crystal membrane resonator. Concluding
remarks discuss the future perspectives of CPA with photonic structures.Comment: arXiv admin note: substantial text overlap with arXiv:1605.0890
Design and Simulation of THz Quantum Cascade Lasers
Strategies and concepts for the design of THz emitters based on the quantum
cascade scheme are analyzed and modeled in terms of a fully three-dimensional
Monte Carlo approach; this allows for a proper inclusion of both
carrier-carrier and carrier-phonon scattering mechanisms. Starting from the
simulation of previously published far-infrared emitters, where no population
inversion is achieved, two innovative designs are proposed. The first one
follows the well-established chirped-superlattice scheme whereas the second one
employs a double-quantum well superlattice to allow energy relaxation through
optical phonon emission. For both cases a significant population inversion is
predicted at temperatures up to 80 K.Comment: 4 pages, 2 figures, 2 table
Thermal noise and optomechanical features in the emission of a membrane-coupled compound cavity laser diode
We demonstrate the use of a compound optical cavity as linear displacement
detector, by measuring the thermal motion of a silicon nitride suspended
membrane acting as the external mirror of a near-infrared Littrow laser diode.
Fluctuations in the laser optical power induced by the membrane vibrations are
collected by a photodiode integrated within the laser, and then measured with a
spectrum analyzer. The dynamics of the membrane driven by a piezoelectric
actuator is investigated as a function of air pressure and actuator
displacement in a homodyne configuration. The high Q-factor ( at mbar) of the fundamental mechanical mode at kHz guarantees a detection sensitivity high enough for direct measurement
of thermal motion at room temperature ( pm RMS). The compound cavity
system here introduced can be employed as a table-top, cost-effective linear
displacement detector for cavity optomechanics. Furthermore, thanks to the
strong optical nonlinearities of the laser compound cavity, these systems open
new perspectives in the study of non-Markovian quantum properties at the
mesoscale
Photocurrent-based detection of Terahertz radiation in graphene
Graphene is a promising candidate for the development of detectors of
Terahertz (THz) radiation. A well-known detection scheme due to Dyakonov and
Shur exploits the confinement of plasma waves in a field-effect transistor
(FET), whereby a dc photovoltage is generated in response to a THz field. This
scheme has already been experimentally studied in a graphene FET [L. Vicarelli
et al., Nature Mat. 11, 865 (2012)]. In the quest for devices with a better
signal-to-noise ratio, we theoretically investigate a plasma-wave photodetector
in which a dc photocurrent is generated in a graphene FET. The rectified
current features a peculiar change of sign when the frequency of the incoming
radiation matches an even multiple of the fundamental frequency of plasma waves
in the FET channel. The noise equivalent power per unit bandwidth of our device
is shown to be much smaller than that of a Dyakonov-Shur detector in a wide
spectral range.Comment: 5 pages, 4 figure
Stretching graphene using polymeric micro-muscles
The control of strain in two-dimensional materials opens exciting
perspectives for the engineering of their electronic properties. While this
expectation has been validated by artificial-lattice studies, it remains
elusive in the case of atomic lattices. Remarkable results were obtained on
nanobubbles and nano-wrinkles, or using scanning probes; microscale strain
devices were implemented exploiting deformable substrates or external loads.
These devices lack, however, the flexibility required to fully control and
investigate arbitrary strain profiles. Here, we demonstrate a novel approach
making it possible to induce strain in graphene using polymeric micrometric
artificial muscles (MAMs) that contract in a controllable and reversible way
under an electronic stimulus. Our method exploits the mechanical response of
poly-methyl-methacrylate (PMMA) to electron-beam irradiation. Inhomogeneous
anisotropic strain and out-of-plane deformation are demonstrated and studied by
Raman, scanning-electron and atomic-force microscopy. These can all be easily
combined with the present device architecture. The flexibility of the present
method opens new opportunities for the investigation of strain and
nanomechanics in two-dimensional materials
Linear and nonlinear capacitive coupling of electro-opto-mechanical photonic crystal cavities
We fabricate and characterize a microscale silicon electro-opto-mechanical
system whose mechanical motion is coupled capacitively to an electrical circuit
and optically via radiation pressure to a photonic crystal cavity. To achieve
large electromechanical interaction strength, we implement an inverse shadow
mask fabrication scheme which obtains capacitor gaps as small as 30 nm while
maintaining a silicon surface quality necessary for minimizing optical loss.
Using the sensitive optical read-out of the photonic crystal cavity, we
characterize the linear and nonlinear capacitive coupling to the fundamental 63
MHz in-plane flexural motion of the structure, showing that the large
electromechanical coupling in such devices may be suitable for realizing
efficient microwave-to-optical signal conversion.Comment: 8 papers, 4 figure
Periodic Structural Defects in Graphene Sheets Engineered via Electron Irradiation
Artificially-induced defects in the lattice of graphene are a powerful tool for engineering the properties of the crystal, especially if organized in highly-ordered structures such as periodic arrays. A method to deterministically induce defects in graphene is to irradiate the crystal with low-energy (<20 keV) electrons delivered by a scanning electron microscope. However, the nanometric precision granted by the focused beam can be hindered by the pattern irradiation itself due to the small lateral separation among the elements, which can prevent the generation of sharp features. An accurate analysis of the achievable resolution is thus essential for practical applications. To this end, we investigated patterns generated by low-energy electron irradiation combining atomic force microscopy and micro-Raman spectroscopy measurements. We proved that it is possible to create well-defined periodic patterns with precision of a few tens of nanometers. We found that the defected lines are influenced by electrons back-scattered by the substrate, which limit the achievable resolution. We provided a model that takes into account such substrate effects. The findings of our study allow the design and easily accessible fabrication of graphene devices featuring complex defect engineering, with a remarkable impact on technologies exploiting the increased surface reactivity
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