Yun Lai

Illusion Optics: The Optical Transformation of an Object into Another Object

We propose to use transformation optics to generate a general illusion such that an arbitrary object appears to be like some other object of our choice. This is achieved by using a remote device that can transform the scattered light outside a virtual boundary into that of the object chosen for the illusion, irrespective of the profile and direction of the incident light. This type of illusion device also enables people to see through walls. Our work extends the concept of cloaking as a special form of illusion to the wider realm of illusion optics.

Dirac Spectra and Edge States in Honeycomb Plasmonic Lattices

We study theoretically the dispersion of plasmonic honeycomb lattices and find Dirac spectra for both dipole and quadrupole modes. Zigzag edge states derived from Dirac points are found in ribbons of these honeycomb plasmonic lattices. The zigzag edge states for out-of-plane dipole modes are closely analogous to the electronic ones in graphene nanoribbons. The edge states for in-plane dipole modes and quadrupole modes, however, have rather unique characters due to the vector nature of the plasmonic excitations. The conditions for the existence of plasmonic edge states are derived analytically.

Manipulate the Transmissions Using Index-Near-Zero or Epsilon-Near-Zero Metamaterials with Coated Defects

We investigate the wave transmissions through an index-near-zero (INZ) or epsilon-near-zero (ENZ) metamaterial containing various kinds of coated cylindrical defects. We find that thin coatings of the defects can dramatically change the transmission behaviors. For example, perfect magnetic conductor (PMC) defects embedded in an INZ or ENZ metamaterial yield total reflections for transverse magnetic polarized waves (Hao et al., Appl Phys Lett 96:101109, 2010). However, if the PMC defects are coated with dielectric shells, total transmissions could be achieved by tuning their permittivity values or geometric sizes. The permittivity differences of dielectric shells for total reflections and transmissions in the INZ or ENZ metamaterial could be very small, implying potential applications, such as ultrasensitive sensors and switches.

Gap structures and wave functions of classical waves in large-sized two-dimensional quasiperiodic structures

By using the sparse-matrix canonical-grid method, we performed large-scale multiple-scattering calculations to study the gap structures and wave functions of classical waves in two-dimensional quasiperiodic structures. We observed many interesting phenomena arising from the quasiperiodic long-range order. In particular, a self-similar wave function with resonant structures was observed at a band edge. Our findings indicate that two-dimensional quasiperiodic systems exhibit a universal behavior that applies to both electrons or phonons in discrete lattices and classical waves in continuous media.We study thermal rectifying effect in two dimensional (2D) systems consisting of the Frenkel Kontorva (FK) lattice and the Fermi-Pasta-Ulam (FPU) lattice. It is found that the rectifying effect is related to the asymmetrical interface thermal resistance. The rectifying efficiency is typically about two orders of magnitude which is large enough to be observed in experiment. The dependence of rectifying efficiency on the temperature and temperature gradient is studied. The underlying mechanism is found to be the match and mismatch of the spectra of lattice vibration in two parts.

Nonlocality-Induced Negative Refraction and Subwavelength Imaging by Parabolic Dispersions in Metal–Dielectric Multilayered Structures with Effective Zero Permittivity

We investigate metal–dielectric multilayered structures with an effectively zero permittivity. Nonlocality induced by the surface plasmons in such structures can produce intriguing dispersions characterized by two crossing branches of parabolas. We obtain the critical conditions to set the two branches of parabolas apart, and reverse the direction of group velocity such that the system becomes capable of negative refraction as well as subwavelength imaging. Such phenomena theoretically exist in the quasistatic limit.

Oblique total transmissions through epsilon-near-zero metamaterials with hyperbolic dispersions

Zero-index metamaterials with near-zero permittivity and/or permeability usually reflect oblique incident waves due to total reflection that occurs at the air-metamaterial interface. In this work, we show that if one component of the near-zero–permittivity tensor of metamaterial turns from positive to negative due to a small disturbance, the dispersion surface changes dramatically from a tiny circle to a hyperbola, which enables oblique transmissions. A series of high-order total-transmission peaks at large incident angles is predicted and observed. These peaks are induced by Fabry-Perot effects. Our work may have potential applications for filters, sensors and switches.

Photonic crystals with broadband, wide-angle, and polarization-insensitive transparency.

Photonic crystals (PhCs) are well-known band gap materials that can block the propagation of electromagnetic waves within certain frequency regimes. Here, we demonstrate that PhCs can also exhibit the contrary property: broadband, wide-angle, and polarization-insensitive transparency beyond normal dielectric solids. Such high transparency attributes to robust impedance matching between a large group of eigen-states in PhCs and propagating waves in free space. As a demonstration, a transparent wall for broadband microwaves is designed for enhancing the transmittance of WiFi and 4G signals.

Sound-impenetrable holes in water based on acoustic complementary medium

By designing a two-dimension acoustic complementary medium of water, we demonstrate the possibility of realizing a sound-impenetrable hole that can block acoustic waves in water. The complementary medium is composed of core-shell rubber cylinders in a square lattice, and possesses the exact negative values of water in both the effective density and bulk modulus at a working frequency. The effects of negative refraction as well as the sound-impenetrable hole are verified by numerical simulations. Interestingly, by introducing a small amount of loss, we find that the functionality of such a sound-impenetrable hole becomes robust and works in a broad frequency range.

Illusion optics via one-dimensional ultratransparent photonic crystals with shifted spatial dispersions

In this work, we propose that one-dimensional ultratransparent dielectric photonic crystals with wide-angle impedance matching and shifted elliptical equal frequency contours are promising candidate materials for illusion optics. The shift of the equal frequency contour does not affect the refractive behaviors, but enables a new degree of freedom in phase modulation. With such ultratransparent photonic crystals, we demonstrate some applications in illusion optics, including creating illusions of a different-sized scatterer and a shifted source with opposite phase. Such ultratransparent dielectric photonic crystals may establish a feasible platform for illusion optics devices at optical frequencies.

Efficient way to convert propagating waves into guided waves via gradient wire structures.

We propose a method for the design of gradient wire structures that are capable of converting propagating waves into guided waves along the wire. The conversion process is achieved by imposing an additional wave vector to the scattered waves via the gradient wire structure, such that the wave vector of scattered waves is beyond the wave number in the background medium. Thus, the scattered waves turn into evanescent waves. We demonstrate that two types of gradient wire structures, with either a gradient permittivity and a fixed radius, or a gradient radius and a fixed permittivity, can both be designed to realize such a wave conversion effect. The principle demonstrated in our work has potential applications in various areas including nanophotonics, silicone photonics, and plasmonics.