Lian Shen

Ray-optics cloaking devices for large objects in incoherent natural light

A cloak that can hide living creatures from sight is a common feature of mythology but still remains unrealized as a practical device. To preserve the wave phase, the previous cloaking solution proposed by Pendry and colleagues required transformation of the electromagnetic space around the hidden object in such a way that the rays bending around the object inside the cloak region have to travel faster than those passing it by. This difficult phase preservation requirement is the main obstacle for building a broadband polarization-insensitive cloak for large objects. Here we propose a simplified version of Pendry’s cloak by abolishing the requirement for phase preservation, as it is irrelevant for observation using incoherent natural light with human eyes, which are phase and polarization insensitive. This allows for a cloak design on large scales using commonly available materials. We successfully demonstrate the cloaking of living creatures, a cat and a fish, from the eye.

Hyperbolic spoof plasmonic metasurfaces

Hyperbolic metasurfaces have recently emerged as a new research frontier because of the unprecedented capabilities to manipulate surface plasmon polaritons (SPPs) and many potential applications. However, thus far, the existence of hyperbolic metasurfaces has neither been observed nor predicted at low frequencies because noble metals cannot support SPPs at longer wavelengths. Here, we propose and experimentally demonstrate spoof plasmonic metasurfaces with a hyperbolic dispersion, where the spoof SPPs propagate on complementary H-shaped, perfectly conducting surfaces at low frequencies. Thus, non-divergent diffractions, negative refraction and dispersion-dependent spin-momentum locking are observed as the spoof SPPs travel over the hyperbolic spoof plasmonic metasurfaces (HSPMs). The HSPMs provide fundamental new platforms to explore the propagation and spin of spoof SPPs. They show great capabilities for designing advanced surface wave devices such as spatial multiplexers, focusing and imaging devices, planar hyperlenses, and dispersion-dependent directional couplers, at both microwave and terahertz frequencies. An artificial optical material known as a hyperbolic metasurface that operates at low frequencies has been made. Metamaterials can be designed to have optical properties not found in nature. One example is the hyperbolic metasurface, so called because the strongly anisotropic electric or magnetic response of the material creates a hyperbolic dispersion in the photons momemtum space. So far, only hyperbolic metasurfaces that operate at relatively high frequencies have been created. Now, Hongsheng Chen from Zhejiang University in China and co-workers has created a spoof plasmonic metasurface with exotic optical properties and that have low-frequency operation. They achieved this by using so-called spoof surface-plasmon polaritons that arise as light interacts with capacitances and inductances created by an array of H-shaped perfectly conducting surfaces. We propose and experimentally demonstrate spoof plasmonic metasurfaces with a hyperbolic dispersion, where the spoof SPPs propagate on complementary H-shaped perfectly conducting surfaces at low frequencies. In this way, non-divergent diffractions, negative refraction, and dispersion-dependent spin-momentum locking are observed as the spoof SPPs travel over the hyperbolic spoof plasmonic metasurfaces. They show great capabilities to design advanced surface wave devices such as spatial multiplexers, focusing and imaging devices, planar hyperlenses, and dispersion-dependent directional couplers, at both microwave and terahertz frequencies.Metasurfaces, with intrinsically planar nature and subwavelength thickness, provide us unconventional methodologies to not only mold the flow of propagating waves but also manipulate near-field waves. Plasmonic metasurfaces with topological transition for controlling surface plasmon polaritons (SPPs) recently have been experimentally demonstrated, which, however, are limited to optical frequencies. In this work, we proposed and experimentally characterized an ultrathin metasurface with the topological transition for manipulating spoof SPPs at low frequency. We demonstrated rich interesting phenomena based on this metasurface, including frequency-dependent spatial localization, non-diffraction propagation, negative refraction, and dispersion-dependent spin-momentum locking of spoof SPPs. Comparing with traditional three-dimensional metamaterials, our metasurface exhibits low propagation loss and compatibility with the photonic integrated circuit, which may find plenty of applications in spatial multiplexers, focusing and imaging devices, planar hyperlens, and dispersion-dependent directional couplers, in microwave and terahertz frequencies.

Free-space carpet cloak using transformation optics and graphene.

Free-space carpet cloak designed with transformation optics requires materials exhibiting simultaneously anisotropic properties and plasma-like behaviors, but materials that simultaneously meet these requirements are rarely found in nature. The recently discovered graphene has shown unique anisotropic plasma-like behavior benefitting from its natural two-dimensional structure and in-plane ultrahigh electron mobility, and therefore, can be a good candidate for the free-space carpet cloak design. In this Letter, we theoretically propose a novel free-space carpet cloak by using periodically stacking layered graphene for the first time. Simulation results show an object under the graphene-based carpet cloak becomes invisible in the THz frequencies. By exploiting the large tunability of graphenes conductivity, we also demonstrate the working frequency of the designed cloak is continuously tunable in a wide spectrum.

Hyperbolic-polaritons-enabled dark-field lens for sensitive detection

Sensitive detection of features in a nanostructure may sometimes be puzzled in the presence of significant background noise. In this regard, background suppression and super-resolution are substantively important for detecting weakly scattering nanoscale features. Here, we present a lens design, termed hyperbolic-polaritons-enabled dark-field lens (HPEDL), which has the ability to accomplish straightforward sensitive detection. This HPEDL structure consists of type I and type II hyperbolic media that support high-k field waves via hyperbolic polaritons (HPs). We show that the cone-like characteristics of the HPs could be manipulated while the influence of the low-k field waves would be removed. Numerical simulations demonstrate that this proposed structure can successfully realize straightforward sensitive detection by modifying its thickness under the phase compensation condition. Besides, the minimum resolvable length and angular-dependent performance for sensitive detection are also demonstrated by simulations. Remarkably, these findings are very promising for propelling nanophotonics technologies and constitute a further important step towards practical applications of optical microscopy.

A Simple Unidirectional Optical Invisibility Cloak Made of Water

Previous invisibility cloaks were based on metamaterials, which are di-cult for practical realization in visible light spectrum. Here we demonstrate a unidirectional invisibility cloak in visible light spectrum. By using water as the efiective material and separated into several regions by glass sheets, a simplest and cheapest invisible device is realized. This device can hide macroscopic objects with large scale and is polarization insensitive. Owing to simple fabrication and easily acquisitive materials, our work can be widely applied in our daily life.

Non-contact method to freely control the radiation patterns of antenna with multi-folded transformation optics

In this paper, we propose to use multi-folded transformation optics method to design a non-contact illusion device that can distantly and freely manipulate the radiation behavior of antenna located at a certain distance and such manipulation is enabled by the use of mapped electromagnetic medium coated with the transformed medium. The proposed design aims to achieve the radiation pattern of our choice from the antenna that does not possess any electromagnetic medium. Based on this, the functionality of parabolic antenna is distantly achieved from the point source. We further extended our idea to array of antennas in which the proposed device distantly makes the linear array of antennas behave like a geometrically different array of antennas. Our work extends the concept of illusion optics for active scatterer that will be very helpful for future antenna design.

Magnetized Plasma as a Versatile Platform for Switching

We study the magneto-permittivity effect in a magnetized plasma with appropriately designed parameters. We show that at frequencies near the plasma frequency, magneto-optical activity plays an important role to manipulate and control the wave propagations in the magnetized plasma. Such a unique feature can be utilized to establish sensitive magnetic field switching mechanism, which is confirmed by detailed numerical investigations. Switching by magnetic field based on magnetized plasma is flexible and compatible with other optical system; moreover it is applicable to any frequency by tuning the plasma density. For these reasons, our work shows the possibility for developing a new family of high frequency and ultrasensitive switching applications.

Photonic transport in a graphene van der Waals homojunction

Layered structures of two-dimensional (2D) materials stacked by van der Waals interactions such as homojunctions and heterojunctions have shown a number of microelectronic applications in the realm of ultrathin and highly flexible devices. Since van der Waals interaction is the dominant strength among these layered structures and can induce the anisotropic behavior of incident electromagnetic waves or photons, a novel concept of photonic transport can be proposed based on the physical picture of transferring both far-field and near-field components of an object to a certain distance and rebuilding a subwavelength image. Herein, through delicately designing and engineering the layered structures based on graphene, we have achieved a flexible graphene van der Waals homojunction for subwavelength imaging. Such an approach is the first time an optical lens from the atomic level has been designed and constitutes a further important step towards practical applications of subwavelength imaging.

Highly Directional Small-Size Antenna Designed with Homogeneous Transformation Optics

Achieving high directivity antenna usually requires a large size antenna aperture in traditional antenna design. Previous work shows that, with the help of metamaterials and transformation optics, a small size antenna can perform as high directivity as a large size antenna, but the material parameters are inhomogeneous and difficult to realize. In this paper, we propose a linear homogeneous coordinate transformation to design the small size antenna. Distinguishing from inhomogeneous transformation, we construct a regular polygon in virtual space and then divide it into several triangle segments. By applying linear homogeneous coordinate transformation, the antenna devices can be greatly compressed without disturbing the radiation patterns by using homogeneous metamaterial substrates. The material parameters of the antenna designed from this method are homogeneous and easy to fabricate. Square and hexagonal antenna structures are numerically demonstrated to illustrate the validity of our methodology.

Interferenceless Polarization Splitting Through Nanoscale van der Waals Heterostructures

The ability to control the polarization of light at the extreme nanoscale has long been a major scientific and technological goal for photonics. Here we predict the phenomenon of polarization splitting through van der Waals heterostructures of nanoscale thickness, such as graphene-hexagonal boron nitride (hBN) heterostructures, at infrared frequencies. The underlying mechanism is that the designed heterostructures possess an effective relative permittivity with its in-plane (out-of-plane) component being unity (zero); such heterostructures are transparent to the transverse-electric (TE) waves while opaque to the transverse-magnetic (TM) waves, without resorting to the interference effect. Moreover, the predicted phenomenon is insensitive to incident angles. Our work thus indicates that van der Waals heterostructures are a promising nanoscale platform for the manipulation of light, such as the design of polarization beam nano-splitters and epsilon-near-zero materials, and the exploration of superscattering for TM waves while zero scattering for TE waves from deep-subwavelength nanostructures.