Optic-null Medium Research Articles

A metamaterial-free omnidirectional invisibility cloak based on thrice transformations inside optic-null medium

A full space omnidirectional cloak with an optic-null medium (ONM) shell and an isotropic homogeneous restoring core is designed by thrice coordinate transformations inside ONM. The designed cloak can be realized by subwavelength dielectric channels enclosed with metallic plates and a uniform dielectric restoring core in air, i.e., no metamaterials or waveguide structures are required. Different from previous studies, the electromagnetic parameters of the designed cloak in this study are strict in accordance with transformation optics (no simplifications), which can achieve a nearly perfect omnidirectional cloaking effect at the design frequency by natural materials. Due to the transformation-invariant feature of the ONM, the shape, size and position of the concealed region can be adjusted freely, while the material parameters of the corresponding cloak remain unchanged.

Full-space omnidirectional cloak by subwavelength metal channels filled with homogeneous dielectrics.

Cloaks can greatly reduce the scattering cross-section of hidden objects through various mechanisms, thereby making them invisible to outside observers. Among them, the full-space omnidirectional cloak based on transformation optic with full parameters are difficult to realize without metamaterials and often needs to be simplified before realization, while most cloaks with simplified parameters have limited working direction and cannot achieve omnidirectional cloaking effect. In this study, a full-space omnidirectional cloak is designed based on transformation optics and optic-null medium, which only needed natural materials without metamaterials. The designed omnidirectional cloak is realized by subwavelength metal channels filled with isotropic dielectrics whose refractive indices range from 1 to 2, which is homogeneous in each channel. The numerical simulation results verify good scattering suppression effect of the designed cloak for various detecting waves.

Full-view imaging on dynamic closed surface by curved-to-flat conversion lens.

Conventional full-view imaging systems, which often need complicated image processing algorithms to reconstruct full-view images captured by motional/multiple cameras from different views, cannot have good real-time imaging capability. We design curved-to-flat conversion lens (CFCL) based on optic-null medium, which can directly project/image optical patterns from closed object surface onto image plane (e.g., the focal plane of microscopy), and shows good real-time full-view imaging performance. To realize the CFCL, the reduced optic-null medium is designed by subwavelength metal channels filled with homogeneous isotropic dielectrics. Numerical simulation results verify the function of the designed CFCL, which can image various dynamic optical patterns from the closed object surface to the finite-view image plane. The designed CFCL may have many applications in real-timely observing dynamic closed surfaces in full view, e.g., living tissue/cell and soft material's surface.

Experimental Verification of Shape‐Independent Surface Cloak Enabled by Nihility Transformation Optics

AbstractThe manipulation of electromagnetic surface waves on interfaces that have arbitrary shape geometrical disorders has gained a lot of interest owing to its wealth of advantages in modern photonics and plasmonics devices. In this paper, based on the well‐established method of transformation optics, a reconfigurable cloak that is competent in guiding the propagation of surface plasmon polaritons (SPPs) at a distorted metal‐dielectric interface is introduced. The presented cloak consists of a homogeneous anisotropic material, dubbed as optic null medium (ONM) that is independent of the cloak geometrical shape. Whilst the homogeneity property of ONMs leads to the feasibility in fabrication, the shape independency feature yields an on‐demand control over the flow of SPPs, simply by changing the shape of the cloak while leaving its constituent material unchanged. In order to authenticate the idea, a triangular‐shape SPP cloak is designed and fabricated experimentally at microwave frequency. It is observed that by utilizing the designed cloak almost total transmission can be obtained from the distorted interface. The unique designing approach introduced here may open a new horizon to nano‐optics and downscaling of photonic circuits.

Planar hyper-lens with uniform pre-designed magnification factor by homogeneous medium

A hyper-lens with planar object/image surfaces and uniform magnification factor is designed based on directional projecting property of optic-null medium (ONM). The proposed planar hyper-lens only requires one homogeneous anisotropic medium, i.e. ONM. We use metallic plates inside ϵ-near-zero medium to realize the proposed hyper-lens. 0.191λ 0 resolution is observed (0.238λ 0 can be clearly resolved) in experimental measurement, which consists very well with 3D numerical simulations and verifies the performance of the proposed planar hyper-lens. The planar hyper-lens with uniform pre-designed magnification factor may have some applications in super-resolution imaging technology.

Reflectionless spatial beam benders with arbitrary bending angle by introducing optic-null medium into transformation optics* *Project supported by the National Natural Science Foundation of China (Grant Nos. 61971300, 11604292, 61905208, 11674239, and 11621101), the Scientific and Technological Innovation Programs (STIP) of Higher Education Institutions in Shanxi Province, China (Grant Nos. 2019L0159 and 2019L0146), and the Postdoctoral Science Foundation of China

By introducing an optic-null medium into the finite embedded transformation, a reflectionless spatial beam bender is designed, which can steer the output beam by a fixed pre-designed angle β for an arbitrary incident angle. The bending angle β of the beam bender is determined by the geometrical angle of the device, which can be changed by simply choosing different geometrical angles. For various bending angles, the designed spatial beam bender can be realized by the same materials (i.e., an optic-null medium), which is a homogenous anisotropic material. Numerical simulations verify the reflectionless bending effect and rotated imaging ability of the proposed beam bender. A reduction model of the optic-null medium is studied, which can also be used for a reflectionless spatial beam bender with a pre-designed bending angle.

Geometry free materials enabled by transformation optics for enhancing the intensity of electromagnetic waves in an arbitrary domain

The concentration of electromagnetic waves is of utmost importance in many engineering applications such as energy harvesting devices. According to the transformation optics methodology, a feasible approach for obtaining arbitrary shape concentrators is proposed. Contrary to the previous works, the obtained materials are homogeneous and independent of the concentrator shape. That is, regardless of the input geometry, always one constant material, which is optic-null medium (ONM), is used, and the performance of the device will not alter. This competency will ease the design process and circumvent the sophisticated calculations of the necessitating materials. To demonstrate such a capability, several numerical full-wave simulations were performed for different shapes of concentrators. Furthermore, with the aid of effective medium theory (EMT), the ONM was realized using split-ring resonator-meander line metamaterial blocks. Then, as a proof of concept, an arbitrary shape concentrator was designed according to EMT and by exploiting the retrieved parameters of the realized metamaterials. It was observed that the realization results exhibit strong agreement with the theoretical investigations and corroborate the generality and effectiveness of the proposed designing method.

Arbitrarily shaped retro-reflector by optics surface transformation

A novel way to design arbitrarily shaped retro-reflectors by optics surface transformation is proposed. The entire design process consists of filling an optic-null medium between the input and output surfaces of the retro-reflector, on which the points have 180 deg reverse corresponding relations. The retro-reflector can be designed to be very thin (a planar structure) with high efficiency. The effective working angles of our retro-reflector are very large (from −80 deg to +80 deg), which can, in principle, be further extended. Layered metal plates and zero refractive index materials are designed to realize the proposed retro-reflector for a TM polarized beam.

Subwavelength focusing by optical surface transformation

We propose a novel beam-compression device to achieve sub-wavelength focusing (SF) by optical surface transformation (a new theoretical branch derived from transformation optics). When a Gaussian beam is incident on the input surface of our device, we can obtain a sub-wavelength focused spot at the output surface of the compression device. The focused mode can still propagate to the far-field and keep very good directivity as incident Gaussian beam. The size of the sub-wavelength beam can be tuned by changing the size of the output surface. We only need one kind of homogeneous anisotropic medium, the optic-null medium (ONM), to realize the proposed beam-compression device. After some simplifications, we design a layered structure of two isotropic homogeneous media to realize the device. Numerical simulations verify the performance of our beam-compression device.

Waveguide bends by optical surface transformations and optic-null media

We propose what we believe is a novel method to design waveguide bends for metallic waveguides with arbitrary bending angles. The proposed method is based on a new theoretical branch from transformation optics that is referred as to optic surface transformation. Compared with waveguide bends designed by traditional transformation optics, the design process of our method can be made in a graphical way that is very simple and convenient. To realize any waveguide bend designed by the method proposed in this study, one needs only one homogeneous material, i.e., an optic-null medium (even if the bending angles are different for various cases). After some reductions, we find that the optic-null media here can be approximately realized by some anisotropic zero refractive index materials. 2D numerical simulations verify the performance of the designed waveguide bends. The design principle can be extended to the 3D case.

Anti-optic-null medium: Achieving the optic-null medium effect by enclosing an air region with relatively low-anisotropy media

A so-called anti-optic-null medium (anti-ONM), which can be utilized to cancel the optic-null medium (ONM) and create many novel optical illusions, is introduced and designed by transformation optics (TO). Optical separation illusions can be achieved with an anti-ONM. With the help of the anti-ONM, we can achieve the same optical illusions where ONM is required via a shelled structure filled with low anisotropic medium, which is easier to realize for some novel optical devices designed by TO and optical surface transformation. The special function of the anti-ONM will lead to a new way to design optical devices or simplify the material requirements. Overlapping illusions, and wave-front reshapers are designed to demonstrate the function of the proposed method.

Optic-null space medium for cover-up cloaking without any negative refraction index materials.

With the help of optic-null medium, we propose a new way to achieve invisibility by covering up the scattering without using any negative refraction index materials. Compared with previous methods to achieve invisibility, the function of our cloak is to cover up the scattering of the objects to be concealed by a background object of strong scattering. The concealed object can receive information from the outside world without being detected. Numerical simulations verify the performance of our cloak. The proposed method will be a great addition to existing invisibility technology.

Overlapping illusions by transformation optics without any negative refraction material.

A novel method to achieve an overlapping illusion without any negative refraction index material is introduced with the help of the optic-null medium (ONM) designed by an extremely stretching spatial transformation. Unlike the previous methods to achieve such an optical illusion by transformation optics (TO), our method can achieve a power combination and reshape the radiation pattern at the same time. Unlike the overlapping illusion with some negative refraction index material, our method is not sensitive to the loss of the materials. Other advantages over existing methods are discussed. Numerical simulations are given to verify the performance of the proposed devices.

Optical Surface Transformation: Changing the optical surface by homogeneous optic-null medium at will.

A new theory on designing electromagnetic/optical devices is proposed, namely, an optical surface transformation (OST). One arbitrary surface can establish the corresponding relationship with another surface entirely optically with an optic-null medium (ONM), (i.e. the electromagnetic wave propagates from one surface to its equivalent surface without any phase delay). Many novel optical devices can be designed by an OST with the help of an ONM. Compared with traditional devices designed by Transformation Optics, our optical surface-reshaping devices have two main advantages. Firstly, the design process is very simple (i.e. we do not need to consider any mathematics on how to make a coordinate transformation, and what we need to do is simply to design the shapes of the input and the output surfaces of the devices). Secondly, we only need one homogeneous anisotropic medium to realize all devices designed by this method. Our method will explore a new way to design novel optical devices without considering any coordinate transformations.

SURFACE TRANSFORMATION WITH HOMOGENOUS OPTIC-NULL MEDIUM

An optical surface transformation (OST) method is proposed briefly in this short paper. Compared with Transformation Optics (TO), we do not need to consider what kinds of coordinate transformation should be used when designing a novel device, but only need to choose the shapes of two end surfaces (namely, the input and output surfaces of the device), which are linked by an optic-null medium (ONM) that is a highly anisotropic homogeneous medium. All devices designed by OST can be realized by some ONM. The design process of an optical device with some pre-designed function can be converted to the simple choice of the shape and size of the optical surfaces with the OST, which will become a simple and yet innovative way to design electromagnetic/optical devices.

Optic-null medium: realization and applications

Optic-null medium (ONM), an electromagnetic (EM) space representing optically nothing, has many interesting applications but is difficult to realize practically due to its extreme EM parameters. Here we demonstrate that a holey metallic plate with periodic array of subwavelength apertures can well mimic an ONM. We develop an effective-medium theory to extract the EM parameters of the designed ONM, and employ full-wave simulations to demonstrate its optical functionalities. Microwave experiments, in excellent agreement with full-wave simulations, are performed to illustrate several applications of the ONM, including the radiation cancellation effect and the hyperlensing effect.