Nondegenerate Modes Research Articles

An Elliptical-Core Few-Mode Fiber with Low Loss and Low Crosstalk for the MIMO-FREE Applications

In this invited paper, a novel elliptical-core few-mode fiber (EFMF) with low loss and low Crosstalk is proposed for multiple input multiple output free (MIMO-FREE) applications. The EFMF, which adopts pure silica elliptical core with a step refractive index profile and trench refractive index, can be used in MIMO-FREE operation. The result using a full-vector finite element method shows that the proposed EFMF breaks the mode degeneracy and achieves modes maintaining function. The EFMF performs five non-degenerate modes operations with the modes LP01, LP11a, LP11b, LP21b, and LP21a for the MIMO-FREE applications. The mode crosstalk of the EFMF is effectively reduced using the optimized ellipticity and the large effective refractive index difference between the modes. If the data rate or capacity is conventionally set to a multiple of 2, the transmission system can be upgraded using the EFMF with four modes operations where the effective refractive index difference between any two modes is more than 2 × 10−3. A pure silica core is employed to effectively reduce intrinsic loss. The low bending loss is realized by using the trench refractive index at the cladding. The EFMF can be applied to the good transmission of mode division multiplexing in MIMO-FREE applications while eliminating the difficulty and complexity of the MIMO digital signal processing.

Chiral valley phonons and flat phonon bands in moiré materials

We investigate the chirality of phonon modes in twisted bilayer WSe2 and demonstrate distinct chiral behavior of the $K/K^\prime$ valley phonons for twist angles close to $0^{\circ}$ and close to $60^{\circ}$. In particular, multiple chiral non-degenerate $K/K^\prime$ valley phonons are found for twist angles near $60^{\circ}$ whereas no non-degenerate chiral modes are found for twist angles close to $0^\circ$. Moreover, we discover two sets of emergent chiral valley modes that originate from an inversion symmetry breaking at the moir\'{e} scale and find similar modes in moir\'{e} patterns of strain-engineered bilayers WSe2 and MoSe2/WSe2 heterostructures. At the energy gap between acoustic and optical modes, the formation of flat phonon bands for a broad range of twist angles is observed in the twisted bilayer WSe2. Our findings are relevant for understanding electron-phonon and exciton-phonon scattering in moir\'e materials and also for the design of phononic analogues of flat band electrons.

Light propagation and magnon-photon coupling in optically dispersive magnetic media

Achieving strong coupling between light and matter excitations in hybrid systems is a benchmark for the implementation of quantum technologies. We recently proposed (Bittencourt, Liberal, and Viola-Kusminskiy, arXiv:2110.02984) that strong single-particle coupling between magnons and light can be realized in a magnetized epsilon-near-zero (ENZ) medium, in which magneto-optical effects are enhanced. Here we present a detailed derivation of the magnon-photon coupling Hamiltonian in dispersive media both for degenerate and nondegenerate optical modes, and show the enhancement of the coupling near the ENZ frequency. Moreover, we show that the coupling of magnons to plane-wave nondegenerate Voigt modes vanishes at specific frequencies due to polarization selection rules tuned by dispersion. Finally, we present specific results using a Lorentz dispersion model. Our results pave the way for the design of dispersive optomagnonic systems, providing a general theoretical framework for describing and engineering ENZ-based optomagnonic systems.

Topologically protected optomechanically induced transparency in a one-dimensional optomechanical array

We investigate the optomechanically induced transparency (OMIT) in an optomechanical lattice. By controlling the frequency of the external drivings in a periodic manner, the optomechanical lattice can be regarded as a Su-Schrieffer-Heeger model. By calculating the local photon density of states for the system, we investigate the response of the system to a weak probe field. In the nondeep topological nontrivial phase, we find that the system has two nondegenerate edge modes due to the finite size of the system. In this regime, a narrow transparency window of the probe field, which is much narrower than the scale set by photon decay, can be observed due to the destructive interference of the probe field absorption paths induced by the two nondegenerate edge modes. In the deep topological nontrivial phase, the two edge modes become degenerate and the narrow transparency window changes into a wide absorption window. The OMIT of the optomechanical array can also be observed in the presence of large disorders of the many-photon optomechanical couplings. Our work generalizes the OMIT of a single optomechanical cavity to a topological optomechanical system and might have potential applications in quantum information processing and quantum optical devices.

Design of Air-Trenches/Holes Assisted Depressed-Core 12-Mode Fiber for Less MIMO Space Division Multiplexing

An air-trenches/holes assisted depressed-core few-mode fiber (AT-AH-DC-FMF) that features some asymmetrical air trenches, air holes and a depressed step-index core is proposed in this paper. The AT and DC can produce great contribution in separating the non-degenerated LP modes and spatial modes, thus suppressing the crosstalk efficiently. Based on AT and AH, the bending loss values of AT-AH-DC-FMF can be satisfied with the ITU-T recommendations of G. 654. The simulation results indicate that our proposed fiber can support 12 spatial modes with the effective index difference between adjacent LP modes (Δneff) larger than 1.03×10-3 and the effective index difference between adjacent spatial modes (δneff) larger than 1.24×10-4. The broadband characteristics such as Δneff, δneff, chromatic dispersion and effective mode field area (Aeff) over the whole C and L band are comprehensively investigated. Moreover, fabrication methods and birefringence are detailed discussed. Our proposed AT-AH-DC-FMF possesses great potential to be applied to less multiple-input multiple-output (MIMO-less) space-division multiplexing (SDM) enlarging optical communication capacity and simplifying the system complexity.

Group theory based formation mechanism and evolution of multiple Fano resonances in dielectric nanohole arrays with lattice-perturbed

Based on the electromagnetic properties of all-dielectric optical metamaterial, an all-dielectric metasurface of lattice-perturbed nanohole array is proposed to excite a multiple Fano resonance in the near-infrared region. Combined with the group theory, the formation mechanism and evolution law of multiple Fano resonances in this structure when its unit cell is a square lattice configuration and the square lattice symmetry is broken are explored in depth. The results show that double degenerate mode directly excited by the normal incident plane wave is coupled to vertical free-space radiation continuum to form double Fano resonance when unit cell is symmetrical, while the uncoupled non-degenerate modes excited by the normal incident plane wave is coupled to vertical free-space radiation continuum to form triple Fano resonance with higher <i>Q</i> factor when the symmetry is broken. Numerical simulation is used to explore the influences of <i>x</i>-polarized and <i>y</i>-polarized plane wave on the above Fano resonances, and the results show that the Fano resonance of double degenerate resonance is polarization independent, while the non-degenerate resonance is polarization dependent. The findings in this work can provide an effective theoretical reference for designing other square lattice metasurface to realize the excitation and evolution of multiple Fano resonances.

Chirality-selective easily adjustable spin current from uniaxial antiferromagnets

Spin current generation and injection in antiferromagnetic heterostructures opens up new perspectives in the fields of nanoelectronics, spintronics, and quantum computation. Here, we theoretically investigate terahertz spin current in an antiferromagnet (AFM)/normal metal (NM) heterostructure generated by spin pumping. Under the framework of linear response theory, we compute the alternating spin current (ac spin current) inside the NM of the heterostructure arising from the spin pumping of a resonant terahertz magnetic field inside the AFM. An external direct current (dc) magnetic field is also applied to tune the resonance frequency of the AFM, allowing the resonance between terahertz field and AFM to be achieved more easily. In both uniaxial antiferromagnets MnF2 and Cr2O3, we find that once the driving THz field is resonant and circular polarized according to the resonance mode of the AFM, the amplitude of ac spin current is independent of the external magnetic field. On the other hand, in the biaxial antiferromagnet NiO under the same pumping conditions, such a phenomenon is not observed, which can be ascribed to the complicated nondegenerate elliptical chiral mode in biaxial AFMs. More interestingly, for both types of AFMs, we find that the ac component of the spin current increases linearly with the applied terahertz magnetic field distinct from the dc component that increases quadratically. The aforementioned features in uniaxial AFMs provide an easily tunable guideline to optimize the generation of high-frequency spin current by a given terahertz pumping source.

Polarization impedance measurement cavity enhanced laser absorption spectroscopy.

We present a theoretical overview and experimental demonstration of a continuous-wave, cavity-enhanced optical absorption spectrometry method to detect molecular gas. This technique utilizes the two non-degenerate polarization modes of a birefringent cavity to obtain a zero background readout of the intra-cavity absorption. We use a double-pass equilateral triangle optical cavity design with additional feed-forward frequency noise correction to measure the R14e absorption line in the 30012←00001 band of CO2 at 1572.655 nm. We demonstrate a shot noise equivalent absorption of 3 × 10-13 cm-1 Hz-1/2.

Conditions for walk-off soliton generation in a multimode fiber

It has been recently demonstrated that multimode solitons are unstable objects which evolve, in the range of hundreds of nonlinearity lengths, into stable single-mode solitons carried by the fundamental mode. We show experimentally and by numerical simulations that femtosecond multimode solitons composed by non-degenerate modes have unique properties: when propagating in graded-index fibers, their pulsewidth and energy do not depend on the input pulsewidth, but only on input coupling conditions and linear dispersive properties of the fiber, hence on their wavelength. Because of these properties, spatiotemporal solitons composed by non-degenerate modes with pulsewidths longer than a few hundreds of femtoseconds cannot be generated in graded-index fibers.

Intrinsic Vibrational Angular Momentum from Nonadiabatic Effects in Noncollinear Magnetic Molecules.

We show that in noncollinear magnetic molecules, nonadiabatic (dynamical) effects due to the electron-vibron coupling are time-reversal symmetry breaking interactions for the vibrational field. Because the electronic wave function cannot be chosen as real in these molecules, a nonzero geometric vector potential (Berry connection) arises. As a result, an intrinsic nonzero vibrational angular momentum occurs even for nondegenerate modes and in the absence of external probes. The vibronic modes can then be seen as elementary quantum particles carrying a sizeable angular momentum. As a proof of concept, we demonstrate the magnitude of this topological effect by performing nonadiabatic first principles calculations on platinum clusters and by showing that these molecules host sizeable intrinsic phonon angular momenta comparable to the orbital electronic ones in itinerant ferromagnets.

Design of steering wheel-type ring depressed-core 10-mode fiber with fully improved mode spacing.

We present a steering wheel-type ring depressed-core few-mode fiber (SWTR-DC-FMF) that features a central depressed step-index core and a novel SWTR structure consisted of two symmetrical high-index parts and low-index parts, respectively. The DC and SWTR make great contribution to separate the non-degenerated LP modes and spatial modes in the circular symmetry core, resulting in fully improved mode spacing. The designed fiber is able to support 10 spatial modes with the minimum effective index difference (Min Δneff) between adjacent spatial modes larger than 1.93 × 10-4 and the Min Δneff between adjacent LP modes above 1.51 × 10-3 at the same time, facilitating potential fiber spatial mode multiplexing transmission with less multiple-input multiple-output (MIMO-less) digital signal processing technique. The broadband performance including neff, Δneff, effective mode area (Aeff) and differential mode delay (DMD) is comprehensively investigated over the whole C and L band. Moreover, the birefringence and fabrication tolerance are discussed. The designed fiber targets emerging applications in short-reach weakly coupled space-division multiplexing (SDM) optical networking to increase transmission capacity and spectral efficiency and further reduce the system complexity effectively.

Three-Dimensional Giant Invisibility to Superscattering Enhancement Induced by Zeeman-Split Modes

We report that the fundamental three-dimensional (3-D) scattering single-channel limit can be exceeded in magneto-optical assisted systems by inducing nondegenerate magnetoplasmonic modes. In addit...

Nonlocal hydrodynamic transport and collective excitations in Dirac fluids

We study the response of a Dirac fluid to electric fields and thermal gradients at finite wave-numbers and frequencies in the hydrodynamic regime. We find that non-local transport in the hydrodynamic regime is governed by infinite set of kinetic modes that describe non-collinear scattering events in different angular harmonic channels. The scattering rates of these modes $\tau_{m}^{-1}$ increase as $\|m\|$, where $m$ labels the angular harmonics. In an earlier publication, we pointed out that this dependence leads to anomalous, L\'evy-flight-like phase space diffusion (Phys. Rev. Lett. 123, 195302 (2019)). Here, we show how this surprisingly simple, non-analytic dependence allows us to obtain exact expressions for the non-local charge and electronic thermal conductivities. The peculiar dependence of the scattering rates on $m$ also leads to a non-trivial structure of collective excitations: Besides the well known plasmon, second sound and diffusive modes, we find non-degenerate damped modes corresponding to excitations of higher angular harmonics. We use these results to investigate the transport of a Dirac fluid through Poiseuille-type geometries of different widths, and to study the response to surface acoustic waves in graphene-piezoelectric devices.

40 Gb/s High-speed mode-division multiplexing transmission employing NRZ modulation format

Abstract This report examines the performance of a high-speed MDM transmission system supporting four nondegenerate spatial modes at 10 Gb/s. The analysis adopts the NRZ modulation format to evaluate the system performance in terms of a minimum power required (PN) and the nonlinear threshold power (PTH) at a BER of 10−9. The receiver sensitivity, optical signal-to-noise ratio, and the maximum transmission distance were investigated using the direct detection by employing a multimode erbium-doped amplifier (MM-EDFA). It was found that by properly optimizing the MM-EDFA, the system performance can significantly be improved.

Main modes of the Arctic Ocean circulation and a relationship between their trends and the Atlantic water heat content

A study is presented on integral variability in Arctic Ocean characteristics, such as the volume transport and the heat content of the Atlantic water (AW) layer, obtained as results of numerical simulations using a model, SibCIOM. On the basis of an EOF decomposition three non-degenerate modes are obtained for the integral stream function and three modes for the heat content of the AW layer in the Arctic. Considering the cross-correlations of the EOF principal components a 44-yr cycle is obtained. It relates the cyclonic circulation mode in the Arctic and the second mode of AW heat content, associated with the AW warming in the area of the Beaufort Gyre. The Atlantic Multi-decadal Oscillation (AMO) statistically can act as a source of such a cycle, however, the thus obtained correlation is at the limit of the admissible significance, and the period of AMO oscillations is 1.5-2 times longer. In our opinion, a relationship with fluctuations in the Atlatic Meridional Overturning Circulation (AMOC) seems to be more plausible. However, no understanding of the role of these two mid-latitude and polar processes in relation to what extent they act as a cause or a consequence has yet been established in this study.

Raman spectroscopy and lattice dynamics calculations of tetragonally-structured single crystal zinc phosphide (Zn3P2) nanowires

Earth-abundant and low-cost semiconductors, such as zinc phosphide (Zn3P2), are promising candidates for the next generation photovoltaic applications. However, synthesis on commercially available substrates, which favors the formation of defects, and controllable doping are challenging drawbacks that restrain device performance. Better assessment of relevant properties such as structure, crystal quality and defects will allow faster advancement of Zn3P2, and in this sense, Raman spectroscopy can play an invaluable role. In order to provide a complete Raman spectrum reference of Zn3P2, this work presents a comprehensive analysis of vibrational properties of tetragonally-structured Zn3P2 (space group P42/nmc) nanowires, from both experimental and theoretical perspectives. Low-temperature, high-resolution Raman polarization measurements have been performed on single-crystalline nanowires. Different polarization configurations have allowed selective enhancement of A1g, B1g and Eg Raman modes, while B2g modes were identified from complementary unpolarized Raman measurements. Simultaneous deconvolution of all Raman spectra with Lorentzian curves has allowed identification of 33 peaks which have been assigned to 34 (8 A1g + 9 B1g + 3 B2g + 14 Eg) out of the 39 theoretically predicted eigenmodes. The experimental results are in good agreement with the vibrational frequencies that have been computed by first-principles calculations based on density functional theory. Three separate regions were observed in the phonon dispersion diagram: (i) low-frequency region (<210 cm−1) which is dominated by Zn-related vibrations, (ii) intermediate region (210–225 cm−1) which represents a true phonon gap with no observed vibrations, and (iii) high-frequency region (>225 cm−1) which is attributed to primarily P-related vibrations. The analysis of vibrational patterns has shown that non-degenerate modes involve mostly atomic motion along the long crystal axis (c-axis), while degenerate modes correspond primarily to in-plane vibrations, perpendicular to the long c-axis. These results provide a detailed reference for identification of the tetragonal Zn3P2 phase and can be used for building Raman based methodologies for effective defect screening of bulk materials and films, which might contain structural inhomogeneities.

Analytical modeling of graphene oxide based memristor

Nowadays, carbon–based materials like graphene and its derivatives consume great nano-technological applications worldwide. Graphene, due to its attractive properties such as its unusual charge transport, mechanical and thermal capabilities, is in the nano-scaling research spotlight. Additionally, to overcome the scaling limitations of large-scale charge-storage-based memories, the Memristor is focused upon as a nonvolatile memory. In the present work, the modeling of a Memristor based on graphene oxide as an active layer is considered. The drift-diffusion formalism as an electron transport mechanism, derived from ion immigration, is explored. It is concluded that by increasing the voltage between two metalelectrodes, the carrier concentration is increased, and the carrier mobility on the sandwiched layer (graphene oxide) is decreased. The conductive path by the redox of graphene oxide (GO) atoms, due to the conversion of sp3 to sp2 oxygen functionalities, is formed, which can be modeled by degenerate mode (the low resistance switching or ON state). This path is ruptured by decreasing the voltage into the reset voltage, which is modeled by the non-degenerate mode (the high resistance switching or OFF state). Finally, the model in comparison with the experimental data numerically is simulated, and acceptable results for metal and graphene–like path formation are observed.

Computing Group Velocities and Group-Velocity Dispersions of Optical Fibers Through Automatic Differentiation of Explicit Forms of Propagation Constants

We present a method to compute differential coefficients of the first and second orders, for propagation constants of nondegenerate and degenerate modes, propagating in an optical fiber to calculate dispersion characteristics of group velocities (GV) and group-velocity dispersions (GVD). This method automatically differentiates explicit forms of eigenvalues extracted from determinants of a generalized eigenvalue problem. This problem is converted from a nonlinear eigenvalue problem derived from the Trefftz method (e.g., transfer matrix and multipole methods) by the Sakurai-Sugiura method. We compute the differential coefficients of the second order of degenerate modes without computing eigenvectors and their first order differential coefficients in the generalized eigenvalue problems. Therefore, in parametric optimization, we can compute the differential coefficients of all propagating modes without remodeling the waveguide cross-section, as analyzing the whole cross-section avoids the symmetric conditions derived from the numerical model. Computed results with the proposed method for step-index and holey fibers validate the method and its effectiveness. In particular, our results show that our method computes the GV and GVD more accurately than computation using numerical differentiation.

Response of quantum Fisher information, variance entropy squeezing and entanglement to the intrinsic decoherence of two non-degenerate fields interacting with two qubits

We investigate the link between quantum Fisher information (QFI) and quantum entanglement for a scheme of two atoms that interact with two non-degenerate modes in the absence and presence of intrinsic decoherence effect. The explicit form of the density matrix approach to the dissipative systems is given when the cavity-field is in generalized coherent states. The dynamical behavior of the QFI is compared with the atom–atom concurrence entanglement. We examine the effect of the dipole–dipole interaction and intrinsic decoherence on the dynamical properties of the QFI, concurrence and variance entropy squeezing. It is found that the atom–atom concurrence and the QFI can be controlled by the intrinsic decoherence and the dipole–dipole interaction.

Spin Detection via Parametric Frequency Conversion in a Membrane Resonator

Recent demonstrations of ultracoherent nanomechanical resonators introduce the prospect of new protocols for solid state sensing applications. Here, we propose to use two coupled ultracoherent resonator modes on a Si$_3$N$_4$ membrane for the detection of small nuclear spin ensembles. To this end, we employ parametric frequency conversion between nondegenerate modes. The nondegenerate modes result from coupled degenerate resonators, and the parametric conversion is mediated by periodic inversions of the nuclear spins in the presence of a magnetic scanning tip. We analyze potential noise sources and derive the achievable signal-to-noise ratio with typical experimental parameter values. Our proposal reconciles the geometric constraints of optomechanical systems with the requirements of scanning force microscopy and brings forth a new platform for spin-phonon interaction and spin imaging.