Wave Collimation Research Articles

Compact Terahertz Wave Collimator Design with Photonic Crystal Slabs

A compact terahertz (THz) wave collimator is proposed, which works under the frequency from 2.4 THz to 2.7 THz with a photonic crystal (PC) slab based on the self-collimation effect. The plane wave expansion (PWE) method is used to calculate the dispersion surfaces and the equal-frequency contours (EFCs) and optimize the structure. The propagation of the THz waves in the structure is simulated and the normalized transmission is calculated by using the finite-difference time-domain (FDTD) method with perfectly matched layer (PML) absorbing boundary conditions. Numerical simulations show that the designed collimator has a good collimation property and a high transmittance.

Ponderomotive barrier for plasma particles on the boundary of astrophysical jets

AbstractWe study kinetics of plasma particles on internal inhomogeneities and on the boundary of astrophysical jets in the presence of intensive low-frequency electromagnetic (surface and bulk waves). These waves with energy density exceeding or of the same order as the particle kinetic energy density can be generated due to the non-equilibrium state of plasma and lead to the jet stratification at various scales. The main reason is a ponderomotive force which is able to change dramatically the particle behavior, the plasma density cross-section profile and the wave collimation. We present results obtained on the basis of our simple ponderomotive model of the self-consistent analysis of the electromagnetic wave propagation and the formation of the plasma density profile.

Collimation and enhancement of elastic transverse waves in two-dimensional solid phononic crystals

In this Letter, we numerically investigate the propagation characteristics of elastic transverse waves emitted by line sources embedded inside two-dimensional (2D) solid phononic crystals (PCs). The results show that collimation and enhancement of elastic transverse waves can be achieved at the band edge frequencies. We find that the collimation effect originates from the flat equifrequency contours (EFCs) at the band edge of appropriately designed 2D solid PCs. It is shown that, in addition to geometric symmetry, appropriate constituent material combination is essential to obtain flat EFCs at the band edge. A highly directional and enhanced elastic transverse wave source with a half power angular width of only 5.6° and an enhancement factor of 530 is realized simply by utilizing a finite-size 2D solid PC structure.

Bilayer Graphene Interferometry: Phase Jump and Wave Collimation

We theoretically study the phase of the reflection amplitude of an electron (massive Dirac fermion) at a lateral potential step in Bernal-stacked bilayer graphene. The phase shows an anomalous jump of pi, as the electron incidence angle (relative to the normal direction to the step) varies to pass +/-pi/4. The jump is attributed to the Berry phase associated with the pseudospin-1/2 of the electron. This Berry-phase effect is robust against the band-gap opening due to the external electric gates generating the step. We propose an interferometry setup in which collimated waves can be generated and tuned. By using the setup, one can identify both the pi jump and the collimation angle.

ANALYSIS OF A HIGH-GAIN FABRY-PÉROT CAVITY ANTENNA WITH AN FSS SUPERSTRATE: EFFECTIVE MEDIUM APPROACH

A new approach to analyze the behavior of a Fabry- Perot cavity type high-gain antenna covered with a frequency selective surface (FSS) superstrate is presented. Using an image theory and effective constitutive parameter retrieval, properties of impedance and a refractive index of the entire cavity structure are investigated. Through the analysis, we show that our antenna inherently operates in the medium whose maximum magnitude of the refractive index is lower than '0.5'. In consequence, we demonstrate that the high-gain feature of the Fabry-Perot cavity antenna is not only due to satisfy a conventional cavity resonance condition, but also for a wave collimation effect of a metamaterial with the effectively low index of refraction. To show the collimation effect of the cavity, phase properties of a radiating wave with and without the FSS superstrate are examined. Moreover, the effect of the real and imaginary parts of the refractive index on behaviors of the antenna is also investigated by analyzing the derivative of a realized gain of the antenna within an impedance matched frequency band.

Transformation optical designs for wave collimators, flat lenses and right-angle bends

The transformation optics technique is applied to design three novel devices—a wave collimator, far-zone and near-zone focusing flat optical lenses and a right-angle bend for propagating beam fields. The structures presented in this paper are all two-dimensional (2D), however, the transformation optics design methodologies can be easily extended to develop 3D versions of these optical devices. The required values of the permittivity and the permeability tensors are derived for each of the three devices considered here. Furthermore, the functional performance of each device is verified using full-wave electromagnetic simulations. A wave collimator consists of a 2D rectangular cylinder where the fields (cylindrical waves) radiated by an embedded line source emerge normal to the top and bottom planar interfaces thereby producing highly directive collimated fields. Next, a far-zone focusing lens for a 2D line source is created by transforming the equi-amplitude equi-phase contour to a planar surface. It is also demonstrated that by aligning two far-zone focusing flat lenses in a back-to-back configuration, a near-zone focusing lens is obtained. Finally, a 2D square cylindrical volume is transformed into a cylinder with a fan-shaped cross section to design a right-angle bend device for propagating beam fields.

Enhanced transmission versus localization of a light pulse by a subwavelength metal slit

The existence of resonant enhanced transmission and the collimation of light waves by subwavelength slits in metal films [for example, see Ebbesen et al., Nature (London) 391, 667 (1998) and Lezec et al., Science 297, 820 (2002)] leads to the basic question: Can a light pulse be enhanced and simultaneously localized in space and time by a subwavelength slit? To address this question, the spatial distribution of the energy flux of an ultrashort (femtosecond) wave packet diffracted by a subwavelength (nanometer-size) slit was analyzed by using the conventional approach based on the Neerhoff and Mur solution of Maxwell's equations. The results show that a light pulse can be enhanced by orders of magnitude and simultaneously localized in the near-field diffraction zone at the nm and fs scales. Possible applications in nanophotonics are discussed.

Design of interdigital transducers for crack detection in plates

Interdigital transducers (IDT) for non-destructive evaluation (NDE) of cracks in plates are designed based on an analytical model established previously. Key considerations include mode selectivity, excitation strength, collimation of wave and cost. The advantage of mode selectivity of IDT over PZT patch is presented both analytically and experimentally. Effects of parameters, namely finger spacing, width, length, number of fingers, and the size of IDT, on the excitation strength and mode selectivity are considered. This led to the design of a mobile double-sided IDT as an efficient device where excitation strength is strong and focused. The device was fabricated in-house using commercially available piezoelectric ceramics and used to develop a procedure for accurate identification of the direction and extent of cracks in plates. Three aluminum plates, one with a linear deep crack, another with a piecewise linear shallower crack and the third with a curved crack, were used to illustrate the accuracy and efficiency of both the proposed device and procedure for effective NDE.

Electron wave collimation by conical horns: computer simulation

Results are presented of extensive numerical simulations of electron wave packets transmitted by horns. A detailed quantitative analysis is given of the collimation of the electron wave by horn-like devices. It is demonstrated that the electron wave collimation effect cannot be described in terms of adiabatic wave expansion and semi-classical considerations. It is argued that, in the context of atomic-size electron beam sources, the collimation effect due to a possible horn-like structure in the metal-vacuum potential is much less than the effect stemming from the tunnel barrier itself.