128 research outputs found
Energy of eigen-modes in magnetohydrodynamic flows of ideal fluids
Analytical expression for energy of eigen-modes in magnetohydrodynamic flows
of ideal fluids is obtained. It is shown that the energy of unstable modes is
zero, while the energy of stable oscillatory modes (waves) can assume both
positive and negative values. Negative energy waves always correspond to
non-symmetric eigen-modes -- modes that have a component of wave-vector along
the equilibrium velocity. These results suggest that all non-symmetric
instabilities in ideal MHD systems with flows are associated with coupling of
positive and negative energy waves. As an example the energy of eigen-modes is
calculated for incompressible conducting fluid rotating in axial magnetic
field.Comment: 10 pages, 3 figure
Peculiarities and evolution of Raman spectra of multilayer Ge/Si(001) heterostructures containing arrays of low-temperature MBE-grown Ge quantum dots of different size and number density: Experimental studies and numerical simulations
Ge/Si(001) multilayer heterostructures containing arrays of low-temperature
self-assembled Ge quantum dots and very thin SiGe layers of varying
composition and complex geometry have been studied using Raman spectroscopy and
scanning tunneling microscopy. The dependence of Raman spectra on the effective
thickness of deposited Ge layers has been investigated in detail in the range
from 4 to 18 \r{A}. The position and shape of both Ge and SiGe vibrational
modes are of great interest since they are closely related to the strain and
composition of the material that plays a role of active component in
perspective optoelectronic devices based on such structures. In this work, we
present an explanation for some peculiar features of Raman spectra, which makes
it possible to control the quality of the grown heterostructures more
effectively. A dramatic increase of intensity of both the GeGe and SiGe
bands for the structure containing Ge layers of 10 \r{A} and anomalous shift
and broadening of the SiGe band for structures comprising Ge layers of 8 and
9 \r{A} thick were observed. In our model, the anomalous behavior of the Raman
spectra with the change of thickness of deposited Ge is connected with the
flatness of Ge layers as well as transitional SiGe domains formed via the
stress-induced diffusion from {105} facets of quantum dots. The conclusions are
supported by the STM studies and the numerical calculations.Comment: 17 pages, 11 figure
Effect of self-consistent electric field on characteristics of graphene p-i-n tunneling transit-time diodes
We develop a device model for p-i-n tunneling transit-time diodes based on
single- and multiple graphene layer structures operating at the reverse bias
voltages. The model of the graphene tunneling transit-time diode (GTUNNETT)
accounts for the features of the interband tunneling generation of electrons
and holes and their ballistic transport in the device i-section, as well as the
effect of the self-consistent electric field associated with the charges of
propagating electrons and holes. Using the developed model, we calculate the dc
current-voltage characteristics and the small-signal ac frequency-dependent
admittance as functions of the GTUNNETT structural parameters, in particular,
the number of graphene layers and the dielectric constant of the surrounding
media. It is shown that the admittance real part can be negative in a certain
frequency range. As revealed, if the i-section somewhat shorter than one
micrometer, this range corresponds to the terahertz frequencies. Due to the
effect of the self-consistent electric field, the behavior of the GTUNNETT
admittance in the range of its negativity of its real part is rather sensitive
to the relation between the number of graphene layers and dielectric constant.
The obtained results demonstrate that GTUNNETTs with optimized structure can be
used in efficient terahertz oscillators.Comment: 8 pages, 9 figure
Chiral photonic super-crystals based on helical van der Waals homostructures
Chirality is probably the most mysterious among all symmetry transformations.
Very readily broken in biological systems, it is practically absent in
naturally occurring inorganic materials and is very challenging to create
artificially. Chiral optical wavefronts are often used for the identification,
control and discrimination of left- and right-handed biological and other
molecules. Thus, it is crucially important to create materials capable of
chiral interaction with light, which would allow one to assign arbitrary chiral
properties to a light field. In this paper, we utilized van der Waals
technology to assemble helical homostructures with chiral properties (e. g.
circular dichroism). Because of the large range of van der Waals materials
available such helical homostructures can be assigned with very flexible
optical properties. We demonstrate our approach by creating helical
homostructures based on multilayer AsS, which offers the most
pronounced chiral properties even in thin structures due to its strong biaxial
optically anisotropy. Our work showcases that the chirality of an
electromagnetic system may emerge at an intermediate level between the
molecular and the mesoscopic one due to the tailored arrangement of non-chiral
layers of van der Waals crystals and without additional patterning
Broadband optical properties of monolayer and bulk MoS2
Layered semiconductors such as transition metal dichalcogenides (TMDs) offer endless possibilities for designing modern photonic and optoelectronic components. However, their optical engineering is still a challenging task owing to multiple obstacles, including the absence of a rapid, contactless, and the reliable method to obtain their dielectric function as well as to evaluate in situ the changes in optical constants and exciton binding energies. Here, we present an advanced approach based on ellipsometry measurements for retrieval of dielectric functions and the excitonic properties of both monolayer and bulk TMDs. Using this method, we conduct a detailed study of monolayer MoS2 and its bulk crystal in the broad spectral range (290–3300 nm). In the near- and mid-infrared ranges, both configurations appear to have no optical absorption and possess an extremely high dielectric permittivity making them favorable for lossless subwavelength photonics. In addition, the proposed approach opens a possibility to observe a previously unreported peak in the dielectric function of monolayer MoS2 induced by the use of perylene-3,4,9,10-tetracarboxylic acid tetrapotassium salt (PTAS) seeding promoters for MoS2 synthesis and thus enables its applications in chemical and biological sensing. Therefore, this technique as a whole offers a state-of-the-art metrological tool for next-generation TMD-based devices
Giant and tunable excitonic optical anisotropy in single-crystal CsPbX halide perovskites
During the last years, giant optical anisotropy demonstrated its paramount
importance for light manipulation which resulted in numerous applications
ranging from subdiffraction light guiding to switchable nanolasers. In spite of
recent advances in the field, achieving continuous tunability of optical
anisotropy remains an outstanding challenge. Here, we present a solution to the
problem through chemical alteration of the ratio of halogen atoms (X = Br or
Cl) in single-crystal CsPbX halide perovskites. It turns out that the
anisotropy originates from an excitonic resonance in the perovskite, which
spectral position and strength are determined by the halogens composition. As a
result, we manage to continually modify the optical anisotropy by 0.14. We also
discover that the halide perovskite can demonstrate optical anisotropy up to
0.6 in the visible range -- the largest value among non-van der Waals
materials. Moreover, our results reveal that this anisotropy could be in-plane
and out-of-plane, depending on perovskite shape -- rectangular and square.
Hence, it can serve as an additional degree of freedom for anisotropy
manipulation. As a practical demonstration, we created perovskite anisotropic
nanowaveguides and show a significant impact of anisotropy on high-order
guiding modes. These findings pave the way for halide perovskites as a
next-generation platform for tunable anisotropic photonics.Comment: 18 pages, 3 figure
Transition metal dichalcogenide nanospheres for high-refractive-index nanophotonics and biomedical theranostics
Recent developments in the area of resonant dielectric nanostructures have created attractive opportunities for concentrating and manipulating light at the nanoscale and the establishment of the new exciting field of all-dielectric nanophotonics. Transition metal dichalcogenides (TMDCs) with nanopatterned surfaces are especially promising for these tasks. Still, the fabrication of these structures requires sophisticated lithographic processes, drastically complicating application prospects. To bridge this gap and broaden the application scope of TMDC nanomaterials, we report here femtosecond laser-ablative fabrication of water-dispersed spherical TMDC (MoS2 and WS2) nanoparticles (NPs) of variable size (5 to 250 nm). Such NPs demonstrate exciting optical and electronic properties inherited from TMDC crystals, due to preserved crystalline structure, which offers a unique combination of pronounced excitonic response and high refractive index value, making possible a strong concentration of electromagnetic field in the NPs. Furthermore, such NPs offer additional tunability due to hybridization between the Mie and excitonic resonances. Such properties bring to life a number of nontrivial effects, including enhanced photoabsorption and photothermal conversion. As an illustration, we demonstrate that the NPs exhibit a very strong photothermal response, much exceeding that of conventional dielectric nanoresonators based on Si. Being in a mobile colloidal state and exhibiting superior optical properties compared to other dielectric resonant structures, the synthesized TMDC NPs offer opportunities for the development of next-generation nanophotonic and nanotheranostic platforms, including photothermal therapy and multimodal bioimaging
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