Guided Mode Conversion Research Articles

Guiding acoustic waves via a gradient index meta-layer

In this work, we propose an efficient approach for controlling acoustic waves through guided mode conversion within a single meta-layer comprised of gradient index metamaterials. By harnessing the subwavelength localization characteristics of acoustic surface waves, the meta-layer can be properly shaped to accommodate arbitrary and prospective trajectories, thus enabling flexible guidance and routing of acoustic waves. Utilizing such a mode conversion mechanism, the acoustic directional transmitter and beam splitter have been devised and numerically demonstrated based on Archimedean spiral structures. These acoustic devices can exhibit a broadband response, achieving transmission efficiency exceeding 80% over a specific bandwidth that can be controlled by the geometry design. The proposed gradient index meta-layer may open new possibilities for acoustic wave manipulation, enabling some promising applications in noise control, information transmission, and energy harvesting.

Planar gradient metamaterials

Metamaterials possess exotic properties that do not exist in nature. Gradient metamaterials, which are characterized by a continuous spatial variation of their properties, provide a promising approach to the development of both bulk and planar optics. In particular, planar gradient metamaterials can be classified into three categories: gradient metasurfaces, gradient index metamaterials and gradient metallic gratings. In this Review, we summarize the progress made in the theoretical modelling of these materials, in their experimental implementation and in the design of functional devices. We discuss the use of planar gradient metamaterials for wave bending and focusing in free space, for supporting surface plasmon polaritons and for the realization of trapped rainbows. We also focus on the implementation of these materials in waveguide systems, which can enable electromagnetic cloaking, Fano resonances, asymmetric transmission and guided mode conversion. Finally, we discuss promising trends, such as the use of dielectric rather than metallic unit elements and the use of planar gradient metamaterials in 3D systems. Planar gradient metamaterials are a promising development, overcoming the limitations of both bulk and planar optics, and have been widely investigated in various domains. This Review summarizes recent progress made in the theoretical modelling, experimental implementation and design of functional devices that utilize these materials.

Quantitative evaluation of long bone fractures by ultrasonic guided waves

Using ultrasonic guided waves to assess fractured long bone has become one of the hottest topics in recent years. To quantitatively evaluate long bone fractures, this paper uses two-dimension finite-difference time-domain (FDTD) simulations to quantitatively analyze the transmission of guided wave in long bone with partially and completely diaphyseal osteotomy. It is illustrated that under the narrowband low-frequency excitation, two fundamental guided modes S0 and A0 can be excited successfully. Different from S0 mode, A0 mode is very sensitive to the fracture geometry. An amplitude ratio parameter between S0 and A0 (S0/A0) is proposed to quantify the bone fracture degree. The simulation indicates that S0/A0 significantly increases with the fracture deepening and widening. Thus, the guided mode conversion can be used to evaluate depth and width of the diaphyseal fracture, which may also be helpful to monitor the fracture healing of long cortical bone.

Axial Transmission Method for Long Bone Fracture Evaluation by Ultrasonic Guided Waves: Simulation, Phantom and in Vitro Experiments

Mode conversion occurs when the ultrasonic guided waves encounter fractures. The aim of this study was to investigate the feasibility of fracture assessment in long cortical bone using guided-mode conversion. Mode conversion behavior between the fundamental modes S0 and A0 was analyzed. The expressions proposed for modal velocity were used to identify the original and converted modes. Simulations and phantom experiments were performed using 1.0-mm-thick steel plates with a notch width of 0.5 mm and notch depths of 0.2, 0.4, 0.6 and 0.8 mm. Furthermore, in vitro experiments were carried out on nine ovine tibias with 1.0-mm-wide partial transverse gap break and cortical thickness varying from 2.10 to 3.88 mm. The study confirmed that mode conversion gradually becomes observable as fracture depth increases. Energy percentages of the converted modes correlated strongly with fracture depth, as illustrated by the frequency-sweeping experiments on steel phantoms (100–1100 kHz, r2 = 0.97, p < 0.0069) and the fixed-frequency experiments on nine ovine tibias (250 kHz, r2 = 0.97, p < 0.0056). The approaches described, including mode excitation, velocity expressions and energy percentage criteria, may also contribute to ultrasonic monitoring of long bone fracture healing.

Optically induced mode conversion in graded-index fibers using ultra-short laser pulses

We propose the use of graded-index few-mode fibers for mode conversion by long-period gratings (LPG) transiently written by ultrashort laser pulses using the optical Kerr effect. The mode interaction is studied by numerically solving the multi-mode coupled nonlinear Schrödinger equations. We present highly efficient conversion of the LP01- into the LP11-mode preserving the pulse shape in contrast to previous results in step-index fibers. Furthermore, mode conversion using different wavelengths for inducing and probing the LPG is shown. Due to the flat phase-matching curve of the examined modes in the graded-index fiber, mode conversion can be observed for probe center wavelengths of 1,100 nm up to 1,800 nm with a write beam centered around 1,030 nm. Therefore, a complete separation of the probe from the write beam should be possible as well as the application of optically induced guided-mode conversion for all-optical modulation across a broad wavelength range.

Optically induced long-period fiber gratings for guided mode conversion in few-mode fibers

We propose and demonstrate guided mode conversion by an optically induced long-period fiber grating (OLPG). The beating of two guided modes excited by a nanosecond pulsed laser beam is used to induce a refractive index grating via the Kerr effect. This grating converts a counter-propagating continuous-wave beam from the LP(01) to the LP(11) mode. Numerical simulations using a beam propagation method show that a full conversion is possible. Experimentally, a mode conversion with an efficiency of about 50% is firstly demonstrated.

Linearly and curvilinearly tapered cylindrical- dielectric-rod antennas

The body-of-revolution finite-difference time-domain (BOR-FDTD) method is applied to the analysis of a tapered cylindrical-dielectric-rod fed by a metallic waveguide with a launching horn. Before evaluating the wave propagating along the tapered rod, we design the launching horn to efficiently excite the fundamental guided-mode of a uniform rod. After confirming the effectiveness of the launching horn, guided-mode conversion properties are evaluated in linearly and curvilinearly tapered rods. As a result, the guided mode excited at the feed end is smoothly converted into that at the free end in a curvilinearly tapered rod. It is numerically revealed that the smooth guided-mode conversion leads to the expansion of the equiphase field region in a terminal aperture with a subsequent increase in the gain. The calculated radiation patterns are in good agreement with experimental data.

Hybrid modes in asymmetric periodic stratified dielectric waveguides

The conditions of occurrence of both hybrid surface modes and hybrid guided modes in a periodic stratified dielectric film in contact with a positive birefringent uniaxial material and a dielectric isotropic medium are studied by means of the 4 × 4 transfer matrix formalism. The dispersion relations of these hybrid modes are exactly calculated. The allowed values of the propagation constant of both surface modes and guided modes form a band structure. The same is true for the allowed values of the direction of propagation required for the existence of these modes. These band structures are explored with the help of numerical solutions of the eigenvalue equation. The lower cutoff thicknesses for various hybrid guided modes are obtained. The TE-dominant–TM-dominant guided-mode conversion effect is suggested.

Phase‐matched optical second‐harmonic generation in Langmuir–Blodgett film waveguides by mode conversion

Phase‐matched second‐harmonk light generation by guided mode conversion has been observed for the first time in nonlinear optically active Langmuir‐Blodgett films. The sketch of the experimental setup (Figure) shows how the infrared fundamental beam is coupled via a grating into SiO2/TiO2‐DCNAP waveguides with various thicknesses of DCNAP film. The film with the correct thickness for phase matching guides a second‐harmonic mode, which is observed as a blue beam, as illustrated in a photograph in the article. magnified image

Mode conversion and radiation loss caused by refractive-index fluctuations in an asymmetric slab waveguide

The purpose of this paper is to show how much power is converted from an incident TE 0 mode to other guided and radiation modes due to random fluctuations of the refractive index in the core region of an asymmetric slab waveguide. Power conversion is calculated by using a perturbation technique with the assumption that the perturbed field can be expanded in terms of higher guided modes together with radiation modes (air and substrate modes). Curves are presented for mode conversion and radiation loss as a function of thickness of the core region, inhomogeneity scale of the refractive index, and refractive-index difference between the core and substrate. It is shown that the radiated power depends greatly on the refractive-index inhomogeneities, and it exceeds the power of guided mode conversion at short correlation length of the medium fluctuations.