Quasi-transverse Modes Research Articles

High-Efficiency Coupling of Free Electrons to Sub-λ3 Modal Volume, High-Q Photonic Cavities.

We report on the design, realization, and experimental investigation by spatially resolved monochromated electron energy loss spectroscopy (EELS) of high-quality-factor cavities with modal volumes smaller than λ3, with λ being the free-space wavelength of light. The cavities are based on a slot defect in a 2D photonic crystal slab made up of silicon. They are optimized for high coupling of electrons accelerated to 100 kV to quasi-transverse electrical modes polarized along the slot direction. We studied the cavities in two geometries and took advantage of the deep sub-optical wavelength spatial resolution of the electron microscope and high spectral resolution of the monochromator to comprehensively describe the optical excitations of the slab. The first geometry, for which the cavities have been designed, corresponds to an electron beam traveling along the slot direction. The second consists of the electron beam traveling perpendicular to the slab. In both cases, a large series of modes is identified. The dielectric slot mode energies are measured to be in the 0.8-0.85 eV range, as per design, and surrounded by two bands of dielectric and air modes of the photonic structure. The dielectric even slot modes, to which the cavity mode belongs, are highly coupled to the electrons with up to 3.2% probability of creating a slot photon per incident electron. Although the experimental spectral resolution (around 30 meV) alone does not allow to disentangle cavity photons from other slot photons, the excellent agreement between the experiments and finite-difference time-domain simulations allows us to deduce that among the photons created in the slot, around 30% are stored in the cavity mode. A systematic study of the energy and coupling strength as a function of the photonic band gap parameters permits us to foresee an increase of coupling strength by fine-tuning phase-matching. Our work demonstrates free electron coupling to high-quality-factor cavities with low mode densities and sub-λ3 modal volumes, making it an excellent candidate for applications such as quantum nano-optics with free electrons.

The role of shear waves in electron-phonon relaxation and electrical resistivity of noble metals

• Shear waves make a main contribution to the resistivity of noble metals • At low temperatures, it is 95, 91, and 95% for crystals Au, Ag, and Cu, respectively • Results is aligns well with experiment in the temperature range from 10 to 1000 K The influence of elastic energy anisotropy on electron-phonon relaxation and electrical resistivity of noble metals has been studied within the Bloch-Grüneisen theory. Accounting for the influence of elastic anisotropy on spectrum, phonon polarization vectors, and electron relaxation by quasi-transverse modes makes it possible to quantitatively put in agreement the outcomes of calculating the temperature dependence of the electrical resistivity of noble metals with experimental data in the temperature range from 10 to 1000 K. A constant characterizing the interaction of electrons with shear waves in noble metals has been determined. The role of shear waves in electrical resistivity has been analyzed. It has been shown that the relaxation of electrons by shear waves, which was not previously taken into account, makes a dominant contribution to the electrical resistivity of noble metals in the entire temperature range from 10 to 1000 K. At temperatures well below the Debye temperature, this contribution is 95%, 91%, and 95% of the electrical resistivity for crystals Au, Ag, and Cu, respectively, and at T = 1000 K it decreases to 73%, 44%, and 68%.

Compact TE-pass polarizer based on silicon-on-insulator platform with bimetallic rhodium–silver grating

An ultra-compact hybrid plasmonic grating based transverse electric (TE) pass polarizer using SOI platform is proposed and analyzed. The reported structure utilizes bi-metallic grating of noble metals; rhodium (Rh) and silver (Ag) as plasmonic materials to get benefit from their attractive properties. The rhodium is a corrosion-free, nonoxidizing, and chemically inert against aggressive chemicals noble metal. In conjunction with the use of Ag, Rh ensures better plasmonic response. The bi-metallic combination Ag/Rh exhibits large coupling between the quasi transverse magnetic mode confined in the silicon core and the surface plasmon mode constructed at the dielectric/metal interface, while the TE-mode can propagate with minimal losses. The modal characteristics of the proposed device are numerically investigated using full vectorial finite difference method, while the propagation analysis is performed using finite difference time domain method via Lumerical software package. The numerical results show that the TE-pass polarizer of a compact device length (LD) = 3.5 μm offers an extinction ratio (ER) of 36.6 dB and an insertion loss (IL) of about 0.8 dB at λ = 1.55 μm. Additionally, the reported polarizer is robust to fabrication errors keeping the ER larger than 20 dB and the IL less than 1 dB despite changing the geometrical parameters by pm hspace{0.17em}10% in the wavelength range of 1.4 to 1.7 μm.

Magnetohydrodynamic Kelvin–Helmholtz instabilities of supersonic shear layers with finite interface thickness and heat flux in anisotropic space plasmas

ABSTRACT The linear magnetohydrodynamic Kelvin–Helmholtz instability (KHI) in an anisotropic plasma is studied. The governing equations obtained as the 16 moments of Boltzmann–Vlasov kinetic equations, including the heat flow, are applied. In the case of tangential discontinuity between the supersonic flows along the magnetic field, the calculated growth rates as functions of the anisotropic plasma properties allow us to conclude that quasi-transverse modes grow faster. Then, dispersion equations for the KHI of quasi-transverse modes are derived, considering the finite width of the transition zone with different velocity profiles. For these modes, when the role of heat flow is not important, the plasma parameters are controlled so that the fundamental plasma instabilities (firehose and mirror) do not affect the KHI. The problem is solved analytically, which will be helpful in verifying numerical simulations. In contrast to the tangential discontinuity, the finite width of the transition layer confines KHI excitation as the wavenumber increases. In the general case of oblique propagation (when heat flux complicates the problem), the boundary value problem is solved to determine the spectral eigenvalues. In particular, it is observed that the fundamental plasma instabilities that arise in the transition zone between flows with a finite width can modify and considerably enhance the KHI.

Long-wavelength phonon dynamics in incommensurate Bi2Sr2CaCu2O8+δ crystals by Brillouin light scattering spectroscopy

Room temperature phonon dynamics in crystals of the high-Tc superconductor Bi2Sr2CaCu2O(8+delta) were probed using Brillouin light scattering spectroscopy. Eight distinct acoustic modes were observed and identified, including two quasi-longitudinal bulk modes and four quasi-transverse bulk modes. A peak at a frequency shift of ~95 GHz with behaviour reminiscent of an optic phonon was also observed in the spectra. The existence and nature of these modes is a manifestation of the incommensurate structure of Bi2Sr2CaCu2O(8+delta) and suggests that it may be categorized as a so-called composite incommensurate crystal comprised of two weakly interacting sublattices. A mass ratio of m1/m2=0.36 obtained from the two measured quasi-longitudinal acoustic velocities led to sublattice assignments of Bi2Sr2O4 and CaCu2O4. The Brillouin data also places an upper limit of ~10 GHz on the crossover frequency between commensurate and incommensurate phonon dynamics.

Phonon focusing and anisotropy of drag thermopower in potassium thin films at low temperatures

We have investigated the influence of elastic energy anisotropy on the dependences of drag thermopower in potassium thin films on the orientation of the film planes, directions of heat flux, and their geometric parameters. It is shown that the drag thermopower in square-shaped potassium films with orientations {100} and {111} is isotropic in the film planes. Their anisotropy is determined by the orientation of the film plane. As for square-shaped films with {110} orientation, the drag thermopower dependences appear ellipsoidal, with a long axis that points towards the [100] direction. As the transverse parameters of potassium films with {100} and {111} orientations are increased, the drag thermopower anisotropy of the films in the [110] direction varies in a non-monotonic manner. This effect is due to the competition between the contributions of slow quasi-transverse modes and longitudinal phonons. The relaxation rate of the latter by electrons has proved to be 30 times higher than that for the slow mode. The drag thermopower and phonon mean free paths in potassium films with lengths much larger than their widths in the boundary scattering regime are found to become anisotropic. However, accounting for the electron–phonon relaxation leads to isotropization of these quantities in thin films with {100} and {111} film plane orientations, with the orientation anisotropy being retained. For long films with {110} orientation, the shape of the drag thermopower dependences remains ellipsoidal with a long axis along the [100] direction.

On-chip Optical Phase Monitoring in Multi-Transverse-Mode Integrated Silicon-based Optical Processors

We design a Multi-Transverse-Mode Optical Processor (MTMOP) on 220 nm thick Silicon Photonics exploiting the first two quasi-transverse electric modes (TE0 and TE1). The objective is to measure the optical phase, required for programming the optical processor, without use of conventional optical phase detection techniques (e.g., coherent detection). In the proposed design, we use a novel but simple building block that converts the optical phase to optical power. Mode TE0 carries the main optical signal while mode TE1 is for programming purposes. The MTMOP operation relies on the fact that the group velocity of TE0 and TE1 propagating through a mode-sensitive phase shifter are different. An unbalanced Mach-Zehnder interferometer (MZI) consists of a mode-sensitive and mode-insensitive phase shifters in the two arms. We set the bias of the phase shifters so that TE0 propagating in the two arms constructively interfere while this will not be the case for TE1. Hence, we detect the phase shift applied to TE0 by measuring the variation in the optical power of TE1. To the best of our knowledge, this design is the first attempt towards realizing a programmable optical processor with fully integrated programming unit exploiting multimode silicon photonics.

On-Chip Non-Blocking Optical Mode Exchanger for Mode-Division Multiplexing Interconnection Networks

Optical mode exchange is an important technology for advanced mode-division multiplexing (MDM) interconnection networks, which provides enhanced flexibility by swapping signals between different mode channels. However, the available dual-mode exchange devices cannot fully meet the demands of MDM systems applying multiple orthogonal guided modes for parallel transmission. In this paper, an on-chip non-blocking mode exchanger is proposed based on a mesh network of interconnected Mach-Zehnder interferometers with demonstrated reconfigurability and scalability, and thus, the device is capable of swapping signals among arbitrary multiple optical modes at will. As a proof of concept, the non-blocking mode exchange among the first three order quasi-transverse electric modes is successfully demonstrated on a silicon-on-insulator platform. The device shows a large bandwidth covering the whole C band (1525–1565 nm), low insertion loss (less than 7.6 dB including the auxiliary mode coupling loss) and low inter-modal crosstalk (less than −14.3 dB) for all mode conversions. In order to demonstrate the data processing capabilities of this device, 40 Gbps data transmission experiment at the wavelength of 1550 nm is successfully carried out, and the obtained clear and wide-open eye diagrams indicate its good performance.

Dipole-Fed Rectangular Dielectric Resonator Antennas for Magnetic Resonance Imaging at 7 T: The Impact of Quasi-Transverse Electric Modes on Transmit Field Distribution

Shortened dipole antennas based on rectangular dielectric blocks play an important role in ultrahigh field magnetic resonance imaging (UHF-MRI) radio frequency (RF) coil design. However, the generally assumed direct contact with the subject is difficult to maintain in typical in vivo settings. We have previously observed that certain dielectrically shortened dipole antennas can produce a substantially altered transmit field distribution with a very low transmit efficiency when the block and the sample are physically separated. Therefore, the aim of this study was to determine a) why certain designs of dielectrically shortened dipole antennas can produce an inefficient transmit field when the block and the sample are physically separated and b) how this depends on key parameters such as rectangular block geometry, dielectric constant, loading geometry, and RF feeding. In this work, two main types of quasi-transverse dielectric modes were found in different rectangular block geometries and interpreted as TE11δz (MR efficient) and TE1δδy (MR inefficient), and their impact on in vivo MRI experiments involving the human head, calf, and wrist was explored. This study shows, for the first time, why certain antennas preserve their transmit field efficiency despite physical separation from the sample. We conclude that the proposed approach has the potential to provide new insights into dipole antenna design for UHF-MRI.

On-chip silicon photonic controllable 2\u2009\xd7\u20092 four-mode waveguide switch

Multimode optical switch is a key component of mode division multiplexing in modern high-speed optical signal processing. In this paper, we introduce for the first time a novel 2 × 2 multimode switch design and demonstrate in the proof-of-concept. The device composes of four Y-multijunctions and 2 × 2 multimode interference coupler using silicon-on-insulator material with four controllable phase shifters. The shifters operate using thermo-optic effects utilizing Ti heaters enabling simultaneous switching of the optical signal between the output ports on four quasi-transverse electric modes with the electric power consumption is in order of 22.5 mW and the switching time is 5.4 µs. The multimode switch exhibits a low insertion loss and a low crosstalk below − 3 dB and − 19 dB, respectively, in 50 nm bandwidth in the third telecom window from 1525 to 1575 nm. With a compact footprint of 10 µm × 960 µm, this device exhibits a relatively large width tolerance of ± 20 nm and a height tolerance of ± 10 nm. Furthermore, the conceptual principle of the proposed multimode switch can be reconfigurable and scalable in multifunctional on-chip mode-division multiplexing optical interconnects.

Conditions for photonic bandgaps in two-dimensional materials

Conditions that define the spectral location of bandgaps in the quasi-transverse magnetic surface plasmon polariton modal dispersion for 2D/quasi-2D materials with a tensor response function, embedded in a simple isotropic medium, are obtained. In the isotropic case, transverse magnetic surface plasmon polariton modes propagate if the surface conductivity is inductive. However, in the anisotropic case considered here, we find that quasi-transverse magnetic modes are supported by surfaces with an inductive effective conductivity seen by the wave along the direction of propagation (written as a weighted sum of the diagonal elements). Examples of natural anisotropic 2D/quasi-2D materials are presented to demonstrate the effectiveness of the method.

Compact, Highly Efficient, and Controllable Simultaneous 2 × 2 Three-Mode Silicon Photonic Switch in the Continuum Band

Multimode switch (MMS) allows realizing multimode optical communication enabling high-speed communication. However, to develop such an ultrafast switch for simultaneous multimode with a compact size is very challenging. In this paper, we design and demonstrate a compact multimode 2×2 MMS based on numerical simulation methods using silicon Ψ-junctions and multimode interference (MMI) couplers. The switch is controlled by thermal-optic phase shifters which is able to switch simultaneously states of the optical signal between three quasi-transverse electric modes. The MMS exhibits a low insertion loss from -1.5 dB to -3 dB, low crosstalk below -22 dB, and high extinction ratio larger than 22 dB in 40-nm bandwidth in the third telecom window from 1520 to 1560 nm, respectively. With a compact footprint of 12 μm ×1300 μm, the MMS exhibits relatively large dimensional tolerances. Besides, the MMS provides total electric power consumption levels smaller than 103 mW at an ultrafast switching time of 4.4 μs without the impact of the plasmonic effect. Furthermore, the conceptual principle of the proposed MMS can be reconfigurable and scalable in dimensional multifunctional on-chip mode-division multiplexing optical interconnects and promising potential for photonic large scale integration circuits in the continuum band.

Ultrafast light-induced shear strain probed by time-resolved x-ray diffraction: Multiferroic BiFeO3 as a case study

Enabling the light-control of complex systems on ultra-short timescales gives rise to rich physics with promising applications. While crucial, the quantitative determination of both the longitudinal and shear photo-induced strains still remains challenging. Here, by scrutinizing asymmetric Bragg peaks pairs $(\pm h01)$ using picosecond time-resolved X-ray diffraction experiments in BiFeO$_3$, we simultaneously determine the longitudinal and shear strains. The relative amplitude of those strains can be explained only if both thermal and non-thermal processes contribute to the acoustic phonon photogeneration process. Importantly, we also reveal a difference of the dynamical response of the longitudinal strain with respect to the shear one due to an interplay of quasi-longitudinal and quasi-transverse acoustic modes, well reproduced by our model.

A Flexible and Reconfigurable Optical Add-Drop Multiplexer for Mode Division Multiplexing Systems

Reconfigurable optical add-drop multiplexer (ROADM) is one of the key building blocks for on-chip optical networks, which can download the desired signals from the bus waveguide to the corresponding drop port and upload the local signals from the add port to the bus waveguide to achieve the signals dropping/adding between the local and the bus waveguide. In this letter, we propose a ROADM based on a Benes network for mode-division multiplexing systems. As a proof of the concept, a ROADM with three quasi-transverse electric modes (TE0-TE2) is fabricated and experimentally demonstrated, which can download signals from TEi (i = 0, 1, 2) mode channel in the bus waveguide to an arbitrary local drop port or upload local signals to TEi mode channel via an arbitrary add port. The excess loss of the fabricated device is less than 10.7 dB, and the crosstalk among different space or mode channels is lower than -13.7 dB. Finally, a 12 Gbps data transmission experiment is carried out to verify the performance of the device. The proposed device is reconfigurable and scalable, and thus, it is expected to be used for optical data processing in the mode-division multiplexing systems.

On-chip switchable and reconfigurable optical mode exchange device using cascaded three-waveguide-coupling switches.

Data exchange between different data channels can offer more flexible and advanced functions for many optical networks. In this paper, we propose a switchable and reconfigurable data exchange device for arbitrary two optical mode channels based on three-waveguide-coupling (TWC) switches in mode-division multiplexing (MDM) networks. The working principle of the TWC switches is numerically analyzed using the coupled supermode theory. As a proof of concept, switchable data exchange between arbitrary two mode channels among the first three-order quasi-transverse electric modes is experimentally demonstrated successfully. The insertion losses of the device are less than 5.6 dB, including the coupling loss of the multiplexer and demultiplexer, while the mode crosstalk is less than -13.0 dB for all functions. The proposed device is expected to offer more flexibility to on-chip MDM networks due to its low loss, low crosstalk and good scalability.

Attenuation and Phase Velocity of Elastic Wave in Textured Polycrystals with Ellipsoidal Grains of Arbitrary Crystal Symmetry

This study extends the second-order attenuation (SOA) model for elastic waves in texture-free inhomogeneous cubic polycrystalline materials with equiaxed grains to textured polycrystals with ellipsoidal grains of arbitrary crystal symmetry. In term of this work, one can predict both the scattering-induced attenuation and phase velocity from Rayleigh region (wavelength >> scatter size) to geometric region (wavelength << scatter size) for an arbitrary incident wave mode (quasi-longitudinal, quasi-transverse fast or quasi-transverse slow mode) in a textured polycrystal and examine the impact of crystallographic texture on attenuation and phase velocity dispersion in the whole frequency range. The predicted attenuation results of this work also agree well with the literature on a textured stainless steel polycrystal. Furthermore, an analytical expression for quasi-static phase velocity at an arbitrary wave propagation direction in a textured polycrystal is derived from the SOA model, which can provide an alternative homogenization method for textured polycrystals based on scattering theory. Computational results using triclinic titanium polycrystals with Gaussian orientation distribution function (ODF) are also presented to demonstrate the texture effect on attenuation and phase velocity behaviors and evaluate the applicability and limitation of an existing analytical model based on the Born approximation for textured polycrystals. Finally, quasi-static phase velocities predicted by this work for a textured polycrystalline copper with generalized spherical harmonics form ODF are compared to available velocity bounds in the literature including Hashin–Shtrikman bounds, and a reasonable agreement is found between this work and the literature.

Design and analysis of a grating assisted hybrid mode and polarization division (de)multiplexer.

In this paper, a hybrid mode and polarization division (MDM-PDM) (de)multiplexer based on a buried strip waveguide, operating at 1550 nm, is proposed on the silicon-on-insulator platform. The proposed device can (de)multiplex three quasi-transverse electric and two quasi-transverse magnetic modes of a multimode waveguide concurrently. The multiplexing and demultiplexing operations are achieved by coupling the multimode waveguide modes with the fundamental modes of the single-mode waveguides. The couplings between the modes are realized by employing grating structures in between the multimode and single-mode waveguides. A 2.5D finite-difference time-domain simulation technique is used to examine the performance of the device. An insertion loss of >-0.76dB, return loss of <-11.23dB, and crosstalk of <-12.42dB are obtained for the proposed hybrid MDM-PDM (de)multiplexer. Moreover, the fabrication tolerance of the proposed device structure is studied by keeping the center-to-center distance between the waveguides constant and by varying the waveguide width.

Parametric analysis and comparative study of multimode waveguides on lithium niobate-on-insulator and silicon-on-insulator platforms

Multimode waveguides on lithium niobate-on-insulator (LNOI) and silicon-on-insulator (SOI) platforms are numerically investigated in buried, rib, and strip configurations. Performance of waveguides is compared in terms of waveguide cross-sectional area, dispersion, mode hybridization, and power confinement for both quasi-transverse electric and quasi-transverse magnetic modes. Tall waveguides with single mode in the horizontal direction, supporting higher order modes in the vertical direction, are analyzed. Also, wide waveguides with single mode in the vertical direction, supporting higher order modes in the horizontal direction, are studied. Designs that overcome mode hybridization are proposed, which are well suited for applications such as optical interconnects. LNOI waveguides were found to exhibit lower dispersion in all the configurations, with power confinement and physical dimensions comparable to those of SOI waveguides. The results are instrumental in design optimization of multimode components for on-chip mode-division multiplexing schemes and multiparameter sensing applications. Electro-optic effect is also illustrated in a buried multimode LNOI waveguide that is largely useful in modulation and switching applications.

Quintuple-mode wideband bandpass filters with improved out-of-band rejection

AbstractThis paper presents design of quintuple-mode wideband bandpass filters, implemented with off-centered perturbed metallic cylinders in a rectangular waveguide cavity. Three perturbation cylinders are placed at the bottom of the rectangular waveguide cavity, along with a pair of perpendicularly fed coaxial lines; excite five quasi-transverse magnetic modes to realize the desired passband. The height of the waveguide cavity and the shape of the perturbation cylinders are exploited to shift the resonant modes far away from the passband and achieve a good out-of-band rejection and sharp skirt selectivity. The filter operates at the center frequency of 2.68 GHz with a wide fractional bandwidth of 43%. The proposed filter is fabricated with aluminum. The measured and simulated results are in good agreement with each other.

On-chip reconfigurable and scalable optical mode multiplexer/demultiplexer based on three-waveguide-coupling structure.

Reconfigurable optical mode multiplexers/demultiplexers have attracted increasing attention in the academic community, because they enable convenient construction of flexible and complex on-chip optical networks. Here, we propose and demonstrate a scheme of reconfigurable and scalable optical mode multiplexer/demultiplexer with large operation bandwidth, based on three-waveguide-coupling structures. As proof of concept, a reconfigurable device that can multiplex input signals to the fundamental and first-order quasi-transverse electric mode is fully fabricated and demonstrated successfully. Static response spectra show that the optical crosstalk at the output ports of the device are less than -14.3 dB and -13.7 dB over the entire C band (> 40 nm), respectively. The dynamic performance with data transmission speeds of 40 Gbps for each multiplexing channel are also demonstrated successfully. The presented device is believed to be a potential candidate for future on-chip optical network with large-scale integration, flexible functionality, and low cost.