Reversed Doppler Effect Research Articles

Annular Beam Driven Metamaterial Backward Wave Oscillator

Metamaterials (MTMs) are synthetic materials designed to have characteristics that "may not be readily available in nature," such as negative permittivity, reversed Doppler Effect, reversed Cherenkov Effect, and negative Refractive Index. These characteristics have motivated researchers to analyze and investigate the use of MTMs for modelling high-power microwave (HPM) radiation sources. One of the most potential HPM sources is an annular beam-driven Backward-Wave Oscillator (BWO) based on the Cerenkov mechanism. The devices that use the Cerenkov mechanism are preferred due to their larger bandwidth. Its high output power and repetition operations make it a promising source. In this paper, an annular beam-loaded metamaterial BWO is simulated in order to investigate and comment on the possibility of metamaterial or metamaterial-inspired structures replacing the slow-wave structures (SWSs) in vacuum tube devices. Performance parameters of several BWO configurations loaded with different metamaterial-inspired conductor rings are compared to rippled SWS-based BWOs. The results suggest that fewer metamaterial rings with solid cross-section (CS) inside the BWO lead to a closer match in generated output power and frequency with the output power generated using a rippled SWS.

DESIGN AND SIMULATION ANALYSIS OF A TEXTILE BASED METAMATERIAL PERFECT ABSORBER FOR ENERGY HARVESTING APPLICATIONS AT WIFI BAND

Metamaterials, which are called as left-handed materials, exhibit electromagnetic properties that do not exist in nature, such as negative refractive index and reverse Doppler effect, at the frequency range for which they are designed. These unique properties of metamaterials have made them indispensable in a wide variety of applications such as sensor applications, signal absorption, antenna, and energy harvesting. In this study, the design and analysis of a metamaterial (MTM) perfect absorber at 5 GHz frequency band were performed to investigate the usability of the proposed structure in energy harvesting applications. The results of the study revealed that the proposed MTM structure has potentials to be used in energy harvesting applications.

Reversed Doppler effect based on hybridized acoustic Mie resonances

The realization of reversed Doppler effects in double-negative acoustic metamaterials remains challenging. This paper demonstrates the reversed Doppler effect associated with sound wave propagation in negative group velocity in hybridized metamaterial (HM) system using a simple Mie-resonator configuration. Double-negative acoustic parameters act simultaneously on the effective dynamic bulk modulus and mass density within overlapped frequency region of multiple Mie resonances. Notably, while ordinary media exhibits higher received frequency during the approach and lower during the recession, we observe that in HM the detected signals show redshift compared to the emitted frequency when approaching to the source while depict blue shift when receding from the source. On this basis, the HM exhibits negative phase velocity with reversed wavefronts and negative refraction effect for certain frequency range. Focusing of sound waves emitted from a point source is further realized with a flat lens composed by such a HM slab.

Metamaterial-Inspired Vacuum Electron Devices and Accelerators

Metamaterials (MTMs) are structured materials with subwavelength features that can be engineered to have some unique properties not found in nature, such as negative refractive index, reversed Doppler effect, and reversed Cherenkov radiation. Based on these novel MTMs, several important research groups have made great attempts to develop novel MTM-inspired vacuum electron devices (VEDs) and accelerators. Just as solid-state power devices are innovated by incessant emerging of new semiconductor materials, VEDs can also be inspired by MTMs to have very remarkable advantages, such as smaller size, higher power, higher efficiency, and/or larger gain relative to conventional VEDs, such as traveling-wave tubes, backward-wave oscillators, and klystrons. Similarly, relative to conventional accelerators, MTM-inspired accelerators have obvious advantages, such as smaller size and higher accelerating gradient. Furthermore, MTM-inspired devices have promising applications in areas such as radar, communication, electronic warfare, microwave heating, and imaging.

Optical images rotation and reflection with engineered orbital angular momentum spectrum

According to the concept of digital spiral imaging, we reconstruct an arbitrary structured light beam with information based on orbital angular momentum (OAM) eigenstates, i.e., the Laguerre-Gaussian (LG) modes. We propose a method to realize the rotation and reflection of an image by imparting an extra phase factor to each constituent OAM mode. If each OAM mode is added by a time-varying phase, the structured light beams can be manipulated to rotate with a constant speed, similar to a reverse rotational Doppler effect. While making a conjugate transformation to each OAM mode and then adding a constant phase factor to them, the structured light beams can be flipped over along any axis. In experiment, we take two typical patterns, e.g., a smiley face and a letter “e,” to demonstrate the effectiveness of our scheme. The scheme presented in this work may find potential applications in optical micromanipulation and remote sensing.

Improved RF Metamaterial Band-Pass Filter Design Using CSRR Structures on LCP Substrate

In the past decade, the emergence of man-made structures with unusual electromagnetic properties not seen in nature—commonly known as “metamaterials”—has generated much interest in designing filters, antennas, lenses, and other devices based on negative values of permittivity (ε) and permeability (μ). Manipulating negative values of these electromagnetic parameters has found applications in communication technology and cloaking research by taking advantage of interesting phenomena such as a negative index of refraction and the reverse Doppler Effect. RF and microwave filters with different frequency responses (low-pass, high-pass, band-pass, and band-stop) can be realized by varying microstrip signal line shapes at a frequency of interest due to the fact that the metamaterial frequency response is dependent on the physical dimensions of the structures. For example, the center frequency of a filter can be determined by adjusting the physical dimensions of metamaterial building blocks called split-ring resonators (SRR) or their duals, complementary split-ring resonators (CSRR). To further metamaterial applications, however, non-planar surfaces and effects of curvature on frequency response must also be considered. In this work, an RF metamaterial filter is presented to demonstrate an improvement in the band-pass frequency response from a previous design at Auburn University by enhancing the upper band behavior of the filter. This is achieved by modifying the metamaterial design on the microstrip device to incorporate new additions to the signal line to combine both high-pass and low-pass metamaterial design concepts, resulting in a band-pass response. The filter is designed using a liquid crystal polymer (LCP) slab as a substrate due in part to its dielectric properties, but also to investigate the filter's performance on a flexible structure. An exploration into the roles of different signal line and CSRR dimensions in filter design is given, and a microstrip filter designed using ANSYS HFSS is shown along with simulation results to verify band-pass filter response. LCP was selected due to its excellent RF properties, its resistance to moisture absorption, and its ability to be micromachined.

Development of Bulk Optical Negative Index Fishnet Metamaterials: Achieving a Low-Loss and Broadband Response Through Coupling

In this paper, we discuss the development of a bulk negative refractive index metamaterial made of cascaded “fishnet” structures, with a negative index existing over a broad spectral range. We describe in detail the design of bulk metamaterials, their fabrication and characterization, as well as the mechanism of how coupling of the unit cells can reduce loss in the material through an optical transmission-line approach. Due to the lowered loss, the metamaterial is able to achieve the highest figure of merit to date for an optical negative index metamaterial (NIM) in the absence of gain media. The increased thickness of the metamaterial also allows a direct observation of negative refraction by illuminating a prism made of the material. Such an observation results in an unambiguous demonstration of negative phase evolution of the wave propagating inside the metamaterial. Furthermore, the metamaterial can be readily accessed from free space, making it functional for optical devices. As such, bulk optical metamaterials should open up new prospects for studies of the unique optical effects associated with negative and zero index materials such as the superlens, reversed Doppler effect, backward Cerenkov radiation, optical tunneling devices, compact resonators, and highly directional sources.

Reversed Doppler effect in double negative metamaterials

Doppler shifts in double negative metamaterials have never been observed. This Rapid Communication presents experimental results on Doppler effect in a double negative acoustic metamaterial. We observed that frequency was downshifted when the source was approaching and upshifted when receding. Notably, while in ordinary media wavelengths corresponding to downshifted frequencies are longer, we demonstrate that in double negative metamaterials wavelengths increase as the frequencies increase. Consequently even though the frequencies were downshifted in front of the moving source, the wavelengths became shorter.

Reverse Doppler effect of magnons with negative group velocity scattered from a moving Bragg grating

We report on the observation of reverse Doppler effect in backward spin waves reflected off of surface acoustic waves. The spin waves are excited in a yttrium iron garnet (YIG) film. Simultaneously, acoustic waves are also generated. The strain induced by the acoustic waves in the magnetostrictive YIG film results in the periodic modulation of the magnetic anisotropy in the film. Thus, in effect, a travelling Bragg grating for the spin waves is produced. The backward spin waves reflecting off of this grating exhibit a reverse Doppler shift: shifting down rather than up in frequency when reflecting off of an approaching acoustic wave. Similarly, the spin waves are shifted up in frequency when reflecting from receding acoustic waves.

Doppler effect: surprises from the time domain

A full understanding of Doppler, either acoustic or electromagnetic, depends on representing the effect in the time domain as well as in the frequency domain. In the time domain, the Doppler perturbation (f - f0)/f0 for a signal becomes its time-delay time derivative. A surprising physical consequence (deduced here for an acoustic reflection problem) is that the Doppler perturbation depends on ratios of distances (source-to-echoer and echoer-to-receiver) as well as on various velocities and angles. Another surprise from the time domain concerns the reverse Doppler effect observed both for electromagnetic and acoustic signals. In certain artificial environments (metamaterials with negative refractive index), radiation can be received with a lower frequency when the source approaches a receiver and with a higher frequency when receding from it. The time-delay time derivative picture represents this effect as a paradox: The time delay (hence source-to-receiver distance) increases when its time-delay time derivative is negative. The restriction of reverse Doppler to a dissipative domain may ameliorate the paradox but does not solve it.

Three-dimensional optical metamaterial with a negative refractive index

Metamaterials are artificially engineered structures that have properties, such as a negative refractive index, not attainable with naturally occurring materials. Negative-index metamaterials (NIMs) were first demonstrated for microwave frequencies, but it has been challenging to design NIMs for optical frequencies and they have so far been limited to optically thin samples because of significant fabrication challenges and strong energy dissipation in metals. Such thin structures are analogous to a monolayer of atoms, making it difficult to assign bulk properties such as the index of refraction. Negative refraction of surface plasmons was recently demonstrated but was confined to a two-dimensional waveguide. Three-dimensional (3D) optical metamaterials have come into focus recently, including the realization of negative refraction by using layered semiconductor metamaterials and a 3D magnetic metamaterial in the infrared frequencies; however, neither of these had a negative index of refraction. Here we report a 3D optical metamaterial having negative refractive index with a very high figure of merit of 3.5 (that is, low loss). This metamaterial is made of cascaded 'fishnet' structures, with a negative index existing over a broad spectral range. Moreover, it can readily be probed from free space, making it functional for optical devices. We construct a prism made of this optical NIM to demonstrate negative refractive index at optical frequencies, resulting unambiguously from the negative phase evolution of the wave propagating inside the metamaterial. Bulk optical metamaterials open up prospects for studies of 3D optical effects and applications associated with NIMs and zero-index materials such as reversed Doppler effect, superlenses, optical tunnelling devices, compact resonators and highly directional sources.

Left-Handed Interfaces for Electromagnetic Surface Waves

We show that surface electromagnetic waves (SEMWs) propagating along two-dimensional (2D) interfaces separating different metamaterials can behave analogously to 3D electromagnetic waves in either usual or left-handed media, depending on the permeabilities and/or permittivities of the two materials forming the interface. We derive the conditions when SEMWs carry energy opposite to the phase velocity. In analogy to three-dimensional (3D) left-handed media, we derive both an anomalous Cherenkov emission and a reversed Doppler effect. We also predict a negative refraction at the boundary between two different interfaces, which can be useful for perfect 2D lensing.

Demonstration of reverse Doppler effect using a left-handed transmission line

ABSTRACT: A novel wideband crossed planar monopole antenna hav-ing the band-notched characteristic is presented. The proposed antennaconsist of a wideband crossed planar monopole and multiple U-shapedslots, producing band-notched characteristic. In order to generate theband-notched characteristic on a crossed planar monopole, we proposethat two or four U-shaped slots be required. This technique is suitablefor creating an ultra-wideband (UWB) antenna with a narrow frequencynotch or a multiband antenna. © 2006 Wiley Periodicals, Inc.Microwave Opt Technol Lett 48: 543–545, 2006; Published online inWiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.21403 crossed planar monopole antenna; band-notched antenna;wideband antenna 1. INTRODUCTION The planar monopole has recently been investigated as an antennawith wideband properties. This kind of antenna exhibits goodimpedance matching and high efficiency. However, due to thenature of planar monopole antennas, with currents confined to thelower outer edges, a variation of the radiation pattern with changeof frequency is inevitable. To solve this problem, the crossedplanar monopole has recently been studied [1–3], and improvedomnidirectional radiation patterns for frequencies across a wideoperating bandwidth have been obtained.Due to collocation of the wideband system with frequencybands reserved for narrowband wireless technologies, there is aneed for the wideband device to provide filtering in those bands inorder to avoid causing interference from or to narrowband devices.In [4–6], a planar monopole was shown to exhibit a single-frequency notch band while maintaining wideband performance.However, there is no solution for a crossed planar monopoleantenna with the frequency band-notched characteristic.In this paper, we explore the ability to achieve the band-notchedcharacteristicinawidebandcrossedplanarmonopole.Theability to provide this function in the antenna can significantlyrelax the requirements imposed upon the filtering electronicswithin the wireless device. Two types of the crossed planar mono-pole antenna with the band-notched feature are presented. Theproposed antennas are fabricated and studied.

A brief intro to metamaterials

Metamaterials are new artificial materials with unusual electromagnetic properties that are not found in naturally occurring materials. All "natural" materials such as glass, diamond and such have positive electrical permittivity, magnetic permeability and an index of refraction. In these new artificially fabricated materials - termed as negative index materials (NIM) or double negative (ONG) media or left handed (LH) materials or backward wave (BW) media - all these material parameters are negative. With these unusual material parameters, new kinds of miniaturized antennas and microwave components/devices can be created for the wireless communications and the defense industries. The electrical permittivity and the magnetic permeability are the main determinants of a material's response to electromagnetic (EM) waves. In metamaterials, both these material parameters are negative. Correspondingly, the refractive index of the metamaterials is also negative. Another strange property of metamaterials is its reverse Doppler effect.

Comment on "Observation of the Inverse Doppler Effect"

Seddon and Bearpark present a creative and exciting observation of a reversed Doppler effect when an electromagnetic shock propagates through a transmission line ([ 1 ][1]). We find that the physical origin of this anomalous effect is fundamentally different from the one suggested by Seddon and

Reversed Doppler effect in photonic crystals.

Nonrelativistic reversed Doppler shifts have never been observed in nature and have only been speculated to occur in pathological systems with simultaneously negative effective permittivity and permeability. This Letter presents a different, new physical phenomenon that leads to a nonrelativistic reversed Doppler shift in light. It arises when light is reflected from a moving shock wave propagating through a photonic crystal. In addition to reflection of a single frequency, multiple discrete reflected frequencies or a 10 GHz periodic modulation can also be observed when a single carrier frequency of wavelength 1 microm is incident.

Reversed Doppler effect under reflection from a shock electromagnetic wave

The possibility of observing the reversed Doppler effect in an electrodynamic system of coupled transmission lines with different dispersion types is demonstrated.