Amphoteric Refraction Research Articles

Exploration of amphoteric and negative refraction imaging of acoustic sources via active metamaterials

The present work describes the design of three flat superlens structures for acoustic source imaging and explores an active acoustic metamaterial (AAM) to realise such a design. The first two lenses are constructed via the coordinate transform method (CTM), and their constituent materials are anisotropic. The third lens consists of a material that has both a negative density and a negative bulk modulus. In these lenses, the quality of the images is “clear” and sharp; thus, the diffraction limit of classical lenses is overcome. Finally, a multi-control strategy is developed to achieve the desired parameters and to eliminate coupling effects in the AAM. • Effective acoustic medium can produce the characteristics of negative density and bulk modulus, or anisotropic density. • Negative refraction is induced in the acoustic metamaterials, i.e., the effective medium. • The flat superlenses made of the metamaterials can overcome some shortcomings of the conventional lenses. • Realisation the metamaterials for the superlenses with a multi-control strategy.

Steering and collimating ballistic electrons with amphoteric refraction

We show that amphoteric refraction of ballistic electrons, i.e., positive or negative refraction depending on the incidence angle, occurs at an interface between an isotropic and an anisotropic medium and can be employed to steer and collimate electron beams. The steering angle is determined by the materials’ parameters, but the degree of collimation can be tuned in a significant range by changing the energy of ballistic electrons.

Tunable Amphoteric Refraction in Semiordered Nematics

This paper demonstrates amphoteric refraction in semiordered uniaxial liquid crystal (LC) and reveals the impact of the LC ordering degree and the director axis on the refraction. In particular, it elaborates more on the negative refraction and how a variation in the order parameter will affect the negatively refracted extraordinary ray, even when the director axis is undeformed. This study of the negative refraction with explicit treatment of the ordering degree is significant. It helps in tracking the LC profile deformation in LC based sensors when the molecular degree of ordering is distorted, with or without the influence of temperature. The explicit treatment of the order parameter is critical when trying to differentiate between a change in the alignment (average director deformation) and a degradation of the alignment (loss in complete uniaxial structure).

Aberration-free imaging for light and electrons

The equations for refraction of either the extraordinary wave of light or the wavefunction of an electron at a planar boundary between two misoriented uniaxially anisotropic materials are shown via raytracing to yield a transverse displacement of the object point. The displacement is independent of ray incidence angle and is thus free from spherical aberration, yielding a perfect virtual image which can have applications in birefringent optics. The general conditions for this aberration-free imaging are found to be identical to those required for amphoteric total refraction.

Construct a polarizing beam splitter by an anisotropic metamaterial slab

We present a theoretical investigation of polarizing beam splitters based on an anisotropic metamaterial (AMM) slab. At the interface associated with a certain AMM, TE- and TM-polarized waves exhibit opposite amphoteric refraction characteristics, such that one polarized wave is positively refracted whereas the other is negatively refracted. The opposite amphoteric refractions result in a large birefringence in the AMM slab. By suitably using the large birefringence, we introduce a very simple and very efficient beam splitter to route the light. We show that the splitting angle and the splitting distance between TE- and TM-polarized beams is the function of anisotropic parameters, incident angle and slab thickness.

Wave propagation in an anisotropic metamaterial with single-sheeted hyperboloid dispersion relation

We investigate the wave propagation in an anisotropic metamaterial with single-sheeted hyperboloid dispersion relation. Based on boundary conditions and dispersion relations, we find that the opposite amphoteric refraction, such that E (or H)-polarized waves are positively refracted whereas H (or E)-polarized waves are negatively refracted, can occur at the interface associated with the anisotropic metamaterial. Under a certain condition, both E- and H-polarized waves can exhibit the same single-sheeted hyperboloid or straight line dispersion relation, while the two polarized waves exhibit different propagation characteristics. We expect some potential device applications can be derived based on the unique amphoteric refraction properties.

Amphoteric refraction at the isotropic–anisotropic biaxial media interface: an alternative treatment

Amphoteric refraction of light (refraction that can be positive or negative depending on the angle of incidence) at the interface between isotropic and anisotropic biaxial media is analysed by means of an alternative method previously developed by the authors. Unlike left-handed materials (LHMs), negative (or abnormal) refraction involving anisotropic media is only due to the intrinsic properties of the media and is only exhibited by non-collinear rays (rays that do not follow an isotropic behaviour). Moreover, the axes of wavevector ellipsoids must be rotated with respect to the normal to the incidence plane. In this paper, only planes of incidence lying normal to a principal axis of the dielectric tensor are considered, and consequently comparison between traditional and alternative methods is possible because both geometrical constructions are plane. First, a very simple procedure that allows us to find the direction of wavevector k from the knowledge of non-collinear (n.c.) ray direction is outlined together with the solution of the dual problem: to find the n.c. ray direction from the k one. Then, the abnormal refraction phenomenon is studied and the equivalence between traditional and alternative methods is clearly stated. Finally, two graphical constructions to find the critical angle of incidence and the maximum angle of negative refraction are also proposed.

Amphoteric refraction exhibited by anisotropic crystals

Amphoteric refraction of light ray at the interface between isotropic materials and anisotropic materials is analyzed. Depending on the incident angle, the refractive light ray can either refract positively or negatively. This amphoteric refraction phenomenon can be quite prevalent when the difference of the two principal refractive indices is large. The reflectance under various incident angles has also been calculated, and an experimental demonstration with a calcite crystal in air is presented.

Amphoteric-like refraction in a two-dimensional photonic crystal

High-index contrast photonic crystals possess an intricate photonic band structure that is responsible for surprising phenomena as the surperprism effect, self-collimation, power splitting or negative refraction. Recently, it was reported that at the interface between an isotropic medium and a uniaxial crystal (or between two uniaxial crystals) a phenomenon known as amphoteric refraction, that is, positive as well as negative refraction, can take place. By means of a equifrequency contours analysis and finite-difference time-domain simulations, we show that a two dimensional photonic crystal can also present amphoteric refraction by properly choosing the lattice orientation and the termination. However, total transmission is difficult to achieve because a Bloch mode is excited inside the photonic crystal and the coupling efficiency from this mode to an external plane wave is lower than one.

Amphoteric refraction at the interface between isotropic and anisotropic media

It is found that the amphoteric refraction, i.e., the refraction can be either positive or negative depending on the incident angles, could occur at a planar interface associated with a uniaxially anisotropic positive index media (PIM) or an anisotropic negative index media (NIM). Particularly, the anomalous negative refraction can occur at a planar interface from an isotropic PIM to an anisotropic PIM, whereas the anomalous positive refraction occurs at the interface from an isotropic PIM to an anisotropic NIM. The optimal conditions to yield the two unusual refractions are obtained. The difference of the two types of amphoteric refraction is discussed.

Total negative refraction in real crystals for ballistic electrons and light.

It is found that there exists a category of material interfaces, readily available, that not only can provide total refraction (i.e., zero reflection) but can also give rise to amphoteric refraction (i.e., both positive and negative refraction) for electromagnetic waves in any frequency domain as well as for ballistic electron waves. These two unusual phenomena are demonstrated experimentally for the propagation of light through such an interface.