Efficient Waveguide Mode Conversions Based on Phase-Gradient Metasurfaces

In this paper, waveguide mode conversions in a parallel plate waveguide, rectangular waveguide, and circular waveguide are all demonstrated at terahertz frequencies. Different from the traditional ways to realize mode conversions, an ultracompact design method based on the inner-embedded subwavelength metallic gratings is provided. The principle of the method relies on modulating the phase velocities of spoof surface plasmon polaritons (SSPPs) supported on the gratings to generate π phase difference. Owing to the gradient matching the phase velocities of the waveguide modes and SSPPs, the efficiencies of the mode conversions are considerable. The output mode purities of the both demonstrations are very good. Besides, waveguide bend is also achieved by this method. The work in this paper hopes to establish a universal technique for compact waveguide mode control, especially in the terahertz frequency range.

Phased antenna arrays patterned on optical waveguides can strongly affect mode coupling and propagation in the waveguides. The unidirectional effective wavevector provided by the phased antenna array can facilitate phase matching between different waveguide modes; the effective wavevector can also breaks the symmetry of optical power flow in waveguides. These effects can be used to realize novel integrated photonic circuits. We used full-wave simulations to demonstrate broadband, small-footprint optical waveguide mode converters, polarization converters, and optical power diodes in the telecom and mid-infrared spectral ranges. The designed optical devices have small footprints (i.e., length of antenna arrays ranges from a few to ten times of the free-space wavelength). The efficiency of mode conversion (defined as the percentage of optical power coupled from one waveguide mode to the other) ranges from 10% to 60%, and the purity of converted modes approaches unity. The on-off ratio of optical power diodes (defined as the ratio between optical power transmission in two opposite directions) can be larger than 20dB over a broad wavelength range. We developed a coupled mode theory to model the asymmetric coupling between different waveguide modes and between waveguide modes and surface waves propagating along antenna arrays. The results calculated using the coupled mode theory can successfully explain mode evolution in our waveguide structures observed in simulations. We believe the platform of optical waveguides integrated with phased antenna arrays offers great opportunity to study novel physics originating from interaction between guided waves and designer structures. The platform also has great implication for integrated photonics device applications.

Plasmonic waveguides of tapered structures such as metallic V-groove and cone have the capability to confine terahertz (THz) electromagnetic waves into an area smaller than the diffraction limit. We experimentally demonstrated that parallel-plate waveguides (PPWGs) coupled to V-groove tapered waveguides are very simple and useful structures to increase the sensitivity of electrooptic sampling technique due to the enhancement of electric field of linearly polarized THz waves1). However, a mechanism of the enhancement is not enough to provide theoretically because plasmonic effects are frequently ignored under the condition of perfect conductor approximation for metallic materials in the THz region. This paper provides a theoretical method to calculate enhancement factors of electric field for THz waves in PPWGs coupled to V-groove waveguides as shown in Fig.1 by considering the voltage reflection coefficient of two plasmonic modes in parallel-plate and V-groove waveguides.

The symmetrically truncated right angle E-plane corner is investigated as a function of the position of the truncation plane. The problem is treated by placing electric and magnetic walls in the plane of symmetry of the bend and solving the resulting boundary value problems assuming an incident TEM mode in parallel-plate waveguide. The results are directly applicable to the same type of corner in rectangular waveguide. In analyzing the problem an infinite set of first order differential equations arise which are appropriately truncated and solved on an electronic computer by means of a fourth order Runge-Kutta scheme.

The characteristics of guided modes in chiral negative refraction grounded slab, slab and parallel-plate waveguides are studied theoretically. The dispersion curves, electromagnetic fields distribution, power flow distribution and the power of several low-order guided modes in these chiral negative refraction waveguides are calculated. Some novel features such as the existence of surface wave mode, the power flows opposite to the wave propagation direction in the chiral negative refraction waveguides are found.

A detailed investigation of the spectrum of the waves propagating in the parallel-plate waveguide is carried out. It is shown that four families of waves can propagate in the planar gyrotropic plasma waveguide: plasma, cyclotron, anisotropic and waveguide modes. It is proved that the singular waves propagate in the crossing points of the waves with the complex zone boundary. The influence of the width of the waveguide and the magnetization of the plasma on the spectrum of the waves is analysed. As a rule the, cut-off frequencies of the propagating modes decrease with an increase in the width, and increase with the increase of magnetization. The fields of different kinds of the waves are plotted and a comparison is made between them. The evolution of the fields along the dispersion curves is shown.

Studied in this paper are two-dimensional guided wave reflections from normal boundaries in an isotropic elastic media. By making use of the transverse resonance concept, the reflections of the waveguide modes from normal interfaces are interrogated. A general condition is obtained under which the guided waves in an isotropic medium will undergo no mode conversion when interaction occurs with a normal traction free or fixed end. Under some circumstances, similarities are obtained between waveguide modes and bulk-wave modes, for example, doubling of the displacement field at a free end and doubling of the stress field at a fixed end. The results obtained are applicable to all two-dimensional, guided-wave modes, along one waveguide direction with lossless boundaries on the surface(s) parallel to the waveguide direction, including all possible guided-wave modes, propagating and nonpropagating, in plates, one half space, interface of two different half spaces, layers on a half space, multilayer structures, and all axisymmetric modes in cylindrical structures. In addition, the function of displacement potentials is analyzed in the course of guided-wave mode conversion at a normal end.

In this paper, an numerical model for studying the wave propagations in the parallel plate waveguides excited by the slant incident waves with arbitrary modes is developed. In this model, first, the slant incident wave is decomposed into Ez and Hz components and both the reflected and transmitted fields are expressed as the combination of the Ez and Hz modes, where the z direction is normal to the aperture on the junction plane. Second, the system matrix equation is obtained and solved by a hybrid matching method in which the field continuity at the discontinuity aperture is tested with hybridization of the point matching and modal matching. This hybrid method is complete for it can be used for solving the wave propagations in the symmetric stepped parallel waveguide excited by arbitrary modes. This work is useful in modeling the wave propagations in the electric-large dielectric cylinder antenna arrays which have a stepped parallel plate waveguide structure.

In this contribution, the orthogonality relations between complex modes are investigated to demonstrate the coupling effect which exists between TE/sup z/ and TM/sup z/ hybrid modes when power storage, loss or leakage is presented in the longitudinal direction of a transmission line. As an example, the Parallel Plate Waveguide (PPW) modes are used to study a laterally-shielded top-open microstrip line (LShMSW). The study of leaky-waves is a very interesting application of the novel orthogonality relations derived in this paper. This study is carried out using the correct and incorrect orthogonality formulations, and a comparison of results is made to check the validity of the proposed technique.

This letter presents the first implementation of the concurrent complementary operators method (C-COM) for mesh truncation in finite-element simulations, in general, and to waveguide structures in particular. In addition to presenting the procedure of the implementation, the strength of the C-COM is demonstrated by focusing on the absorption of spurious evanescent waves that are typically the most challenging to eliminate when truncating computational boundaries. Numerical results for both evanescent TE and TM modes in parallel-plate waveguides are provided.

Reconfigurable mode converters are essential components in efficient higher-order mode sources for on-chip multimode applications. We propose an on-chip reconfigurable silicon waveguide mode conversion scheme based on the nonvolatile and low-loss optical phase change material antimony triselenide (Sb2Se3). The key mode conversion region is formed by embedding a tapered Sb2Se3 layer into the silicon waveguide along the propagation direction and further cladding with graphene and aluminum oxide layers as the microheater. The proposed device can achieve the TE0-to-TE1 mode conversion and reconfigurable conversion (no mode conversion) depending on the phase state of embedded Sb2Se3 layer, whereas such function could not be realized according to previous reports. The proposed device length is only 2.3 μm with conversion efficiency (CE) = 97.5%, insertion loss (IL) = 0.2 dB, and mode crosstalk (CT) = -20.5 dB. Furthermore, the proposed device scheme can be extended to achieve other reconfigurable higher-order mode conversions. We believe the proposed reconfigurable mode conversion scheme and related devices could serve as the fundamental building blocks to provide higher-order mode sources for on-chip multimode photonics.

A solution for coupling through a double slit between two parallel-plate waveguides is obtained in analytic series form. The Fourier transform is used to express the scattered field in the spectral domain in terms of waveguide modes. The simultaneous equations for the modal coefficients are solved to obtain the transmission and reflection coefficients in simple, rigorous, and numerically efficient series. Numerical computations are performed to illustrate the coupling and directivity characteristics of a double-split coupler. © 2000 John Wiley & Sons, Inc. Microwave Opt Technol Lett 24: 182–185, 2000.

Simple normalized dispersion relations for transverse magnetic (TM) and transverse electric (TE) propagating modes in parallel-plate waveguides fllled with DPS/DPS or DNG/DNG, and DNG/DPS bilayers are presented. The evanescent TE0 mode of the waveguide fllled with a DNG/DPS bilayer is characterized also by a simple normalized dispersion relation. Since an important behavior of the modes in the waveguide fllled with a DNG/DPS bilayer is the existence of a turning point (TP) at which the power carried by the respective mode on the propagation direction equals zero and changes the sign, we present also implicit relations for determining the normalized parameters of the TM and TE modes at that TP. We show that the TP begins to exist at certain values of the normalized parameter v2 characterizing the DPS layer. For both the TM and TE modes, the higher is the mode order, the greater is the v2 parameter at which the TP begins to exist, but the behavior of the TP is difierent for the TM and TE modes.

An S‐band sectoral waveguide (SWG) mode converter was presented for simulation and electromagnetic analysis. A mode‐matching technique (MMT) for the electromagnetic analysis of an SWG mode converter has been proposed. The problem of trifurcation of the coaxial waveguide to shape SWGs and bifurcation of SWGs is discussed. In this framework, the selections of appropriate testing modes at each junction are justified. The simulation time has improved and the complexity of the proposed problem has reduced by the proposed approach. The numerical results of the MMT are compared against simulation results and are found to be in good agreement. The efficiency of both the simulated and analyzed results shows a mode conversion efficiency higher than 98.0%. An experimental evaluation shows the mode conversion of TM01 to TE11 mode using far‐field radiation patterns and has been compared with simulation results. Also, the MMT and simulation validate the mode conversion of TM01 to TE11 mode.

In this work, a numerical simulation and experimental results using a modified prism technique are presented. It is based on pulse-echo technique and involves the measurement of both longitudinal and transversal waves velocities in trapezoidal-prism shaped specimens with only one transducer. Both longitudinal and transversal waves are generated through mode conversion at the interface between water and the specimen under test. Based on numerical simulation with the 2-D Elastodynamic Finite Integration Technique (EFIT), several snapshots are generated to illustrate the functionality of this new technique, and the experiment was performed on cement paste with various water/cement ratios.