Electron transport in a graphene quantum well can be analogous to photon transmission in an optical fiber. In this work, we present a detailed theoretical analysis to study the transport characteristics of graphene wa...
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Electron transport in a graphene quantum well can be analogous to photon transmission in an optical fiber. In this work, we present a detailed theoretical analysis to study the transport characteristics of graphene waveguides under the influence of different edge orientations. The non-equilibrium Green's function approach in combination with the tight-binding Hamiltonian has been utilized to investigate the conductance properties of straight armchair and zigzag oriented graphene waveguides. Conductance plateaus at integer steps of 4e2/h have been observed in both orientations while the zigzag oriented waveguides present a wider first quantized plateau compared to that in the armchair oriented ones. Using various geometric and physical parameters, including side-barrier and waveguide width, and the metallic properties of terminals, we investigate the conductance profile of waveguides. In addition to the observation of valley symmetry in both edge orientations, this article explores the critical influence of drain contacts on waveguide conductance. Furthermore, we extended our transport study to three different highly bent waveguide configurations, such as U-shape, L-shape, and split-shape waveguides, in order to explore their applications in graphene-based ballistic integrated circuit devices. In the end, we also calculated the conductance of larger graphene waveguides using the scalable tight-binding model, in order to compare the results obtained from the original model.
Topological insulators are studied via tight-binding approximations of longitudinally driven photonic lattices with three lattice sites per unit cell. Two cases are considered in detail: Lieb and kagome lattices. The ...
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Topological insulators are studied via tight-binding approximations of longitudinally driven photonic lattices with three lattice sites per unit cell. Two cases are considered in detail: Lieb and kagome lattices. The lattice is decomposed into three sublattices, each of which is allowed to move independently of the others. Emphasis is placed on periodic driving induced by laser-etched helical coils along the direction of propagation. The linear Floquet bands are constructed for various intersublattice rotation patterns such as different radii, different frequency, phase offset, and quasi-one-dimensional motion. Depending on the nature of the band structure, bulk spectral bands with nonzero Chern number are found to support topologically protected edge states which can move unidirectionally. In this case, the modes move scatter-free around defects due to underlying topological protection. Intriguing mode dynamics are found including bidirectional topological modes and bulk-edge leakage, i.e., excitation of bulk modes at a defect for edge modes with dispersion frequencies near the bulk bands. Finally, certain nonlinear edge modes are also found to propagate unidirectionally and scatter-free around lattice defects.
Slow-light waveguides play an important role in optical pulse delay lines, but are practically limited by disorder-induced attenuation. Topological edge states, unidirectional and robust against disorder, have been pr...
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Slow-light waveguides play an important role in optical pulse delay lines, but are practically limited by disorder-induced attenuation. Topological edge states, unidirectional and robust against disorder, have been proposed as a way to address this issue. Here, we study the transmission phase and pulse propagation dynamics through unidirectional systems in the presence of strong discontinuities. We first investigate a magnetically biased slow-light channel, demonstrating broadband pulse delays in small footprints. We uncover a nonreciprocal etalonlike resonance, sustained by propagating forward and evanescent backward modes, affecting pulse propagation and delay. We then show the existence of these features in topologically protected edge states, highlighting their universality. These exotic resonances provide a new degree of freedom in unidirectional waveguides to engineer their response, and they may therefore prove highly useful in various nanophotonic applications.
Optical- lattice- like waveguides were fabricated in a z- cut lithium niobate crystal by an 11- MHz- repetition- rate pulsed laser. Two simple approaches based on varying the inscribing pulse energy in accordance with...
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Optical- lattice- like waveguides were fabricated in a z- cut lithium niobate crystal by an 11- MHz- repetition- rate pulsed laser. Two simple approaches based on varying the inscribing pulse energy in accordance with the position of the tracks were implemented to enhance the inscription results. Low propagation losses were observed in the visible and near- infrared parts of the spectrum. The minimum losses of less than ( 0.4 +/- 0.1) dB/ cm and ( 3.5 +/- 0.2) dB/ cm for transverse electric and transverse magnetic polarized light, respectively, in the fundamental guiding mode at 1550 nm were achieved after heat treatment at 350. C for three hours, and were preserved up to 700. degrees C.
We show the measurements of propagation loss in germanium-on-silicon (GOS) waveguides at wavelengths from 6.85 mu m to 11.25 mu m. The loss is as low as a few dB/cm at wave-lengths as high as 11 mu m for both the quas...
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We show the measurements of propagation loss in germanium-on-silicon (GOS) waveguides at wavelengths from 6.85 mu m to 11.25 mu m. The loss is as low as a few dB/cm at wave-lengths as high as 11 mu m for both the quasi-TE00 and quasi-TM00 modes. The measured waveguide transmission is consistent with our models of substrate cutoff and germanium slab cutoff. We discuss experimental and fabrication challenges unique to the long-wave infrared, as well as future prospects for GOS as a platform for long-wave infrared integrated photonics.
In anisotropic or bianisotropic waveguides, the standard coupled-mode theory fails due to the broken link between the forward- and backward-propagating modes, which together form the dual mode sets that are crucial in...
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In anisotropic or bianisotropic waveguides, the standard coupled-mode theory fails due to the broken link between the forward- and backward-propagating modes, which together form the dual mode sets that are crucial in constructing coupled mode equations. We generalize the coupled-mode theory by treating the forward- and backward-propagating modes on the same footing via a generalized eigenvalue problem that is exactly equivalent to the waveguide Hamiltonian. The generalized eigenvalue problem is fully characterized by two operators, i.e., (L¯,B¯), wherein L¯ is a self-adjoint differential operator, while B¯ is a constant antisymmetric operator. From the properties of L¯ and B¯, we establish the relation between the dual mode sets that are essential in constructing coupled-mode equations in terms of forward- and backward-propagating modes. By perturbation, the generalized coupled-mode equation can be derived in a natural way. Our generalized coupled-mode formalism (GCMF) can be used to study the mode coupling in waveguides that may contain gain, loss, anisotropy, or bianisotropy. We further illustrate how the generalized coupled theory can be used to study the modal coupling in anisotropy and bianisotropy waveguides through a few concrete examples.
We demonstrate that a photonic gauge potential, which arises from the phase degree of freedom in dynamic refractive-index modulation and hence is achormatic, can be used to achieve a broadband optical switch using the...
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We demonstrate that a photonic gauge potential, which arises from the phase degree of freedom in dynamic refractive-index modulation and hence is achormatic, can be used to achieve a broadband optical switch using the configuration of a photonic Aharonov-Bohm interferometer (ABI). The resulting ABI switch has a far larger bandwidth and lower cross talk than the conventional Mach-Zehnder interferometer. Using coupled-mode theory and full-wave numerical modeling, we compare the response of the two interferometers in the presence of nonidealities. Our results indicate the importance of the photonic gauge potential for broadband optical signal processing.
We study dc and ac transport along armchair graphene nanoribbons using the k center dot p spectrum and eigenfunctions and general linear-response expressions for the conductivities. Then, we contrast the results with ...
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We study dc and ac transport along armchair graphene nanoribbons using the k center dot p spectrum and eigenfunctions and general linear-response expressions for the conductivities. Then, we contrast the results with those for transport along ordinary waveguides. In all cases, we assess the influence of elastic scattering by impurities, describe it quantitatively with a Drude-type contribution to the current previously not reported, and evaluate the corresponding relaxation time for long- and short-range impurity potentials. We show that this contribution dominates the response at very low frequencies. In both cases, the conductivities increase with the electron density and show cusps when new subbands start being occupied. As functions of the frequency, the conductivities in armchair graphene nanoribbons exhibit a much richer peak structure than in ordinary waveguides: in the former, intraband and interband transitions are allowed, whereas in the latter, only the intraband ones occur. This difference can be traced to that between the corresponding spectra and eigenfunctions.
Production of chiral light and its manipulation down to nanolevel is a very challenging endeavor in the area of nanophotonics. In this work, the above demanding requirements are realized explicitly in two R- and S-typ...
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Production of chiral light and its manipulation down to nanolevel is a very challenging endeavor in the area of nanophotonics. In this work, the above demanding requirements are realized explicitly in two R- and S-type chiral organic optical waveguides which are self-assembled from charge-transfer type axially chiral enantiomeric molecules. These enantiomerically pure micro-optical waveguides generate one-, two-, three-photon pumped optical emissions. Remarkably, these waveguides also demonstrate pronounced circular dichroism effects in the nonlinear optical (NLO) emission which is very useful to produce molecular chirality information encoded NLO photonic structures.
The transversely confined propagating modes of an optical fiber mediate virtually infinite range energy exchanges among atoms placed within their field, which adds to the inherent free space dipole-dipole coupling. Ty...
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The transversely confined propagating modes of an optical fiber mediate virtually infinite range energy exchanges among atoms placed within their field, which adds to the inherent free space dipole-dipole coupling. Typically, the single atom free space decay rate largely surpasses the emission rate into the guided fiber modes. However, scaling up the atom number as well as the system size amounts to entering a collective emission regime, where fiber-induced superradiant spontaneous emission dominates over free space decay. We numerically study this super- and subradiant decay of highly excited atomic states for one or several transverse fiber modes as present in hollow core fibers. As particular excitation scenarios we compare the decay of a totally inverted state to the case of pi/2 pulses applied transversely or along the fiber axis as in standard Ramsey or Rabi interferometry. While a mean field approach fails to correctly describe the initiation of superradiance, a second-order approximation accounting for pairwise atom-atom quantum correlations generally proves sufficient to reliably describe superradiance of ensembles from two to a few hundred particles. In contrast, a full account of subradiance requires the inclusion of all higher order quantum correlations. Considering multiple guided modes introduces a natural effective cut-off for the interaction range emerging from the dephasing of different fiber contributions.
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