Broadband polygonal invisibility cloak for visible light

Shifty invisibility cloaks

We recently presented what we believe are new cloaking strategies [Bělín et al., Opt. Express27, 37327 (2019)10.1364/OE.27.037327], abstracted from the properties of an ideal-lens cloak that exists in theory only. Key to the cloaking strategies is that objects on the cloak's inside are imaged to its outside. In the simplest case, interior objects appear simply shifted, forming a "shifty cloak". Here we connect our work to several previous investigations of shifty cloaks and other shifty devices, designed using standard transformation optics, thereby bringing our cloaking strategies closer to experimental realization. We investigate to the best of our knowledge novel combinations of shifty cloaks, specifically Janus devices and optical wormholes. Finally, we demonstrate an experimental realization of a paraxial shifty cloak.

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.

Transformation Metamaterials

Based on the form-invariance of Maxwell's equations under coordinate transformations, mathematically smooth deformation of space can be physically equivalent to inhomogeneous and anisotropic electromagnetic (EM) medium (called a transformation medium). It provides a geometric recipe to control EM waves at will. A series of examples of achieving transformation media by artificially structured units from conventional materials is summarized here. Such concepts are firstly implemented for EM waves, and then extended to other wave dynamics, such as elastic waves, acoustic waves, surface water waves, and even stationary fields. These shall be cataloged as transformation metamaterials. In addition, it might be conceptually attractive and practically useful to control diverse waves for multi-physics designs.

Cloaking object on an optofluidic chip: its theory and demonstration

Recently, the design of metamaterial guided by transformation optics (TO) has emerged as an effective method to hide objects from optical detection, based on arranging a bended light beam to detour. However, this TO-based solution involves fabrication of material with complicated distribution of permittivity and permeability, and the device falls short of tunability after fabrication. In this work, we propose an optofluidic model employing the method of streamline tracing-based transformation optofluidics (STTOF) to hydrodynamically reconfigure light propagation in a given flow field for object-cloaking purposes. The proof-of-concept is demonstrated and tested on an optofluidic chip to validate our proposed theory. Experimental results show that our proposed STTOF method can be used to successfully detour the light path from the object under cloaking in a mathematically pre-defined manner.

Active Cloaking of a Non-Uniform Scatterer

An object illuminated by an electromagnetic wave can be actively cloaked using a surface conformal array of radiating sources to cancel out scattering. This method is promising as elementary antennas can be used as sources while its active nature can surpass passivity-based performance limitations. While this technique has been conceptually extended to accommodate complex geometries, experimental validation past simple uniform scatterers is lacking. To address this scarcity, the design and experimental demonstration of a low-profile, active cloak capable of concealing a complex, metallic, polygonal target is presented. This cloak is constructed with commercially available monopoles and enclosed within a parallel-plate waveguide-based apparatus to approximate a quasi-2D environment. Performance is then assessed when the target is illuminated at either frontal or oblique incidence by a 1.2 GHz cylindrical wave. Overall, the cloak reduces the target’s scattering cross-section by an average of 7.2 dB at frontal incidence and 8.6 dB at oblique incidence. These results demonstrate the feasibility of this kind of active cloaking for more complex scatterers containing flat surfaces and edges. Further analysis shows that the cloak possesses a functional bandwidth of 14% and can be reconfigured for single frequency operation over 0.8–1.8 GHz.

Ideal-lens cloaks and new cloaking strategies

Previously [Courtial et al., Opt. Express 26, 17872 (2018)] we presented the theory of transformation optics (TO) with ideal lenses and demonstrated an example, an omnidirectional lens. Here we interpret this omnidirectional lens in two different parameter regimes as ideal-lens cloaks that employ different cloaking strategies: a standard "shrink cloak" in which objects appear smaller (ideally zero) and a novel "abyss cloak" in which interior physical-space positions are mapped to the exterior and thus are visible only from certain directions. We proceed to combine two nested abyss cloaks into another novel, omnidirectional, "bi-abyss cloak." Our work significantly extends the arsenal of cloaking strategies.

Electromagnetic time-harmonic and static field polygonal rotator with homogeneous materials

We propose a scheme of designing polygonal rotator with homogenous materials by using linear coordinate transformation. Our strategy is available for both time-harmonic electromagnetic field case and static field case. In particular, we found that only one anisotropic material is needed in static field case, and the density of field in the central region can be altered to be denser or sparser, or stay the same. The magnetostatic field rotator can be realized by multilayered structure composed of ferromagnetic materials and superconductor, and the direct current rotator can be realized by metals with different conductivity. Numerical results verify the effectiveness of our strategy in both time-harmonic field case and static field case.

Experimental Realization of an Extreme-Parameter Omnidirectional Cloak

An ideal transformation-based omnidirectional cloak always relies on metamaterials with extreme parameters, which were previously thought to be too difficult to realize. For such a reason, in previous experimental proposals of invisibility cloaks, the extreme parameters requirements are usually abandoned, leading to inherent scattering. Here, we report on the first experimental demonstration of an omnidirectional cloak that satisfies the extreme parameters requirement, which can hide objects in a homogenous background. Instead of using resonant metamaterials that usually involve unavoidable absorptive loss, the extreme parameters are achieved using a nonresonant metamaterial comprising arrays of subwavelength metallic channels manufactured with 3D metal printing technology. A high level transmission of electromagnetic wave propagating through the present omnidirectional cloak, as well as significant reduction of scattering field, is demonstrated both numerically and experimentally. Our work may also inspire experimental realizations of the other full-parameter omnidirectional optical devices such as concentrator, rotators, and optical illusion apparatuses.

Long distance invisibility system to hide dynamic objects with high selectivity

With the development of invisibility technology, invisibility devices have now become more practical, with properties such as working at visible wavelengths, using natural materials, and hiding macroscopic objects. Recently, the cloaking of dynamic objects was experimentally realized using ray-optics. Here, based on a phase retrieval algorithm and phase conjugation technology, we design and fabricate a system to hide dynamic objects that changes at speeds faster than 8 seconds per frame. Different from shell cloaks and carpet-like cloaks, which conceal the entire region covered by the cloak, our system works when the object is at a distance and hides only the selected part of an object when the entire object is within the working area of the system. We experimentally demonstrate the concealment of a millimeter-scale object at different wavelengths. We believe that our work may provide a new approach to hiding objects in real life and may also be applicable in biological imaging and atmospheric imaging.

Invisibility cloak with image projection capability

Investigations of invisibility cloaks have been led by rigorous theories and such cloak structures, in general, require extreme material parameters. Consequently, it is challenging to realize them, particularly in the full visible region. Due to the insensitivity of human eyes to the polarization and phase of light, cloaking a large object in the full visible region has been recently realized by a simplified theory. Here, we experimentally demonstrate a device concept where a large object can be concealed in a cloak structure and at the same time any images can be projected through it by utilizing a distinctively different approach; the cloaking via one polarization and the image projection via the other orthogonal polarization. Our device structure consists of commercially available optical components such as polarizers and mirrors, and therefore, provides a significant further step towards practical application scenarios such as transparent devices and see-through displays.

Concealing arbitrary objects remotely with multi-folded transformation optics

An invisibility cloak that can hide an arbitrary object external to the cloak itself has not been devised before. In this Letter, we introduce a novel way to design a remote cloaking device that makes any object located at a certain distance invisible. This is accomplished using multi-folded transformation optics to remotely generate a hidden region around the object that no field can penetrate and that does not disturb the far-field scattering electromagnetic field. As a result, any object in the hidden region can stay in position or move freely within that region and remain invisible. Our idea is further extended in order to design a remote illusion optics that can transform any arbitrary object into another one. Unlike other cloaking methods, this method would require no knowledge of the details of the object itself. The proposed multi-folded transformation optics will be crucial in the design of remote devices in a variety of contexts.

Creation of an anti-imaging system using binary optics

We present a concealing method in which an anti-point spread function (APSF) is generated using binary optics, which produces a large-scale dark area in the focal region that can hide any object located within it. This result is achieved by generating two identical PSFs of opposite signs, one consisting of positive electromagnetic waves from the zero-phase region of the binary optical element and the other consisting of negative electromagnetic waves from the pi-phase region of the binary optical element.

Detecting Thermal Cloaks via Transient Effects

Recent research on the development of a thermal cloak has concentrated on engineering an inhomogeneous thermal conductivity and an approximate, homogeneous volumetric heat capacity. While the perfect cloak of inhomogeneous κ and inhomogeneous ρc_p is known to be exact (no signals scattering and only mean values penetrating to the cloak’s interior), the sensitivity of diffusive cloaks to defects and approximations has not been analyzed. We analytically demonstrate that these approximate cloaks are detectable. Although they work as perfect cloaks in the steady-state, their transient (time-dependent) response is imperfect and a small amount of heat is scattered. This is sufficient to determine the presence of a cloak and any heat source it contains, but the material composition hidden within the cloak is not detectable in practice. To demonstrate the feasibility of this technique, we constructed a cloak with similar approximation and directly detected its presence using these transient temperature deviations outside the cloak. Due to limitations in the range of experimentally accessible volumetric specific heats, our detection scheme should allow us to find any realizable cloak, assuming a sufficiently large temperature difference.

Full-Polarization 3D Metasurface Cloak with Preserved Amplitude and Phase

A full-polarization arbitrary-shaped 3D metasurface cloak with preserved amplitude and phase in microwave frequencies is experimentally demonstrated. By taking the unique feature of metasurfaces, it is shown that the cloak can completely restore the polarization, amplitude, and phase of light for full polarization as if light was incident on a flat mirror.

Linear transformation method to control flexural waves in thin plates

In this paper, the linear transformation method (LTM) to control flexural waves propagating in thin plates is presented. Unlike earlier studies, only a small number of homogeneous materials with no requirement of in-plane forces or pre-stress are needed, which tremendously simplifies the implementation of devices for flexural waves. An invisibility cloak with homogeneous materials is studied to confirm the validity of the present approach, and to show its imperfection due to impedance mismatch at interfaces. Required materials can be further simplified as layered isotropic materials using the effective medium theory. Finally, the LTM can be extended to the case of flexural waves propagating in anisotropic thin plates. The present method opens a promising avenue toward the realization of advanced structured shields and other devices.

A broadband polygonal cloak for acoustic wave designed with linear coordinate transformation

Previous acoustic cloaks designed with transformation acoustics always involve inhomogeneous material. In this paper, a design of acoustic polygonal cloak is proposed using linear polygonal transformation method. The designed acoustic polygonal cloak has homogeneous and anisotropic parameters, which is much easier to realize in practice. Furthermore, a possible acoustic metamaterial structure to realize the cloak is proposed. Simulation results on the real structure show that the metamaterial acoustic cloak is effective to reduce the scattering of the object.

Omnidirectional optical attractor in structured gap-surface plasmon waveguide

An optical attractor based on a simple and easy to fabricate structured metal-dielectric-metal (SMDM) waveguide is proposed. The structured waveguide has a variable thickness in the vicinity of an embedded microsphere and allow for adiabatic nano-focusing of gap-surface plasmon polaritons (GSPPs). We show that the proposed system acts as an omnidirectional absorber across a broad spectral range. The geometrical optics approximation is used to provide a description of the ray trajectories in the system and identify the singularity of the deflection angle at the photon sphere. The analytical theory is validated by full-wave numerical simulations demonstrating adiabatic, deep sub-wavelength focusing of GSPPs and high local field enhancement. The proposed structured waveguide is an ideal candidate for the demonstration of reflection free omnidirectional absorption of GSPP in the optical and infrared frequency ranges.

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.

Broadband unidirectional cloak designed by eikonal theory

A method for designing optical device is derived based on the eikonal theory, which could obtain the eikonal distribution on a curved surface according to the propagation characteristics of the subsequent light wave. Then combining with the phase matching condition, we designed a broadband unidirectional cloak. Different from the reported unidirectional cloaks, the proposed one could be used for coherent wave and has continuous broadband performance. Moreover, it has three cloaked regions. Full-wave simulation results verify the properties of the cloak.

Topology optimized all-dielectric cloak: design, performances and modal picture of the invisibility effect

We present the design of an all-dielectric cloaking device at microwave frequencies. A gradient based topology optimization is employed to find a dielectric permittivity distribution that minimizes the diffracted field in free space. The layout is binary, i.e. made either of standard ABS plastic or air and is designed to reduce the scattering from an ABS cylinder excited by a line source for TE polarization. We study the performances of cloaks optimized for one, two and three frequencies in terms of scattering reduction and correlations with respect to the free space propagation case. Finally, a modal analysis is carried out providing physical insights on the resonant cloaking mechanism at stake.

A broadband polarization-insensitive cloak based on mode conversion

In this work, we demonstrate an one-dimensional cloak consisting of parallel-plated waveguide with two slabs of gradient index metamaterials attached to its metallic walls. In it objects are hidden without limitation of polarizations, and good performance is observed for a broadband of frequencies. The experiments at microwave frequencies are carried out, supporting the theoretical results very well. The essential principle behind the proposed cloaking device is based on mode conversion, which provides a new strategy to manipulate wave propagation.

Single frequency microwave cloaking and subwavelength imaging with curved wired media

We consider the cloaking properties of electromagnetic wired media deduced from arbitrary coordinate transformations. We propose an interpretation of invisibility via sub-wavelength imaging features. The quality of cloaking is assessed by the level of deformation of the image of a P-shaped source through the stretched wired media: the lesser the image deformation, the more effective the cloaking. We numerically and experimentally demonstrate a tetrahedral wired cloak with longer edge length about 7cm at a frequency of 1GHz (the cloak is thus subwavelength). The wired cloak has two functionalities: it can serve as a high-resolution imaging system over long distances, and it can also perform space transformations such as, but not limited to, cloaking at a single operation frequency.

Active control of electromagnetic radiation through an enhanced thermo-optic effect

The control of electromagnetic radiation in transformation optical metamaterials brings the development of vast variety of optical devices. Of a particular importance is the possibility to control the propagation of light with light. In this work, we use a structured planar cavity to enhance the thermo-optic effect in a transformation optical waveguide. In the process, a control laser produces apparent inhomogeneous refractive index change inside the waveguides. The trajectory of a second probe laser beam is then continuously tuned in the experiment. The experimental results agree well with the developed theory. The reported method can provide a new approach toward development of transformation optical devices where active all-optical control of the impinging light can be achieved.

Direct stigmatic imaging with curved surfaces

We study the possibilities of direct (using one intersection with each light ray) stigmatic imaging with a curved surface that can change ray directions in an arbitrary way. By purely geometric arguments we show that the only possible case of such imaging is the trivial one where the image of any point is identical to the point itself and the surface does not perform any change of the ray direction at all. We also discuss an example of a curved surface which performs indirect stigmatic imaging after twice intersecting each light ray.

Geometric optics-based multiband cloaking of large objects with the wave phase and amplitude preservation

The geometric optics principles are used to develop a unidirectional transmission cloak for hiding objects with dimensions substantially exceeding the incident radiation wavelengths. Invisibility of both the object and the cloak is achieved without metamaterials, so that significant widths of the cloaking bands are provided. For the preservation of wave phases, the λ-multiple delays of waves passing through the cloak are realized. Suppression of reflection losses is achieved by using half-λ multiple thicknesses of optical elements. Due to periodicity of phase delay and reflection suppression conditions, the cloak demonstrates efficient multiband performance confirmed by full-wave simulations.

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.

Although many electromagnetic cloaking schemes exist at different wavelengths, realizing a broadband visible wavelength device is hard. By relaxing the need for phase preservation inherent to most methods, Chen et al. present a ray-optics scheme for cloaking large-scale objects from the human eye.

Concealing with structured light

While making objects less visible (or invisible) to a human eye or a radar has captured people's imagination for centuries, current attempts towards realization of this long-awaited functionality range from various stealth technologies to recently proposed cloaking devices. A majority of proposed approaches share a number of common deficiencies such as design complexity, polarization effects, bandwidth, losses and the physical size or shape requirement complicating their implementation especially at optical frequencies. Here we demonstrate an alternative way to conceal macroscopic objects by structuring light itself. In our approach, the incident light is transformed into an optical vortex with a dark core that can be used to conceal macroscopic objects. Once such a beam passed around the object it is transformed back into its initial Gaussian shape with minimum amplitude and phase distortions. Therefore, we propose to use that dark core of the vortex beam to conceal an object that is macroscopic yet small enough to fit the dark (negligibly low intensity) region of the beam. The proposed concealing approach is polarization independent, easy to fabricate, lossless, operates at wavelengths ranging from 560 to 700 nm, and can be used to hide macroscopic objects providing they are smaller than vortex core.

Experimental demonstration of a bilayer thermal cloak

Invisibility has attracted intensive research in various communities, e.g., optics, electromagnetics, acoustics, thermodynamics, dc, etc. However, many experimental demonstrations have only been achieved by virtue of simplified approaches due to the inhomogeneous and extreme parameters imposed by the transformation-optic method, and usually require a challenging realization with metamaterials. In this Letter, we demonstrate a bilayer thermal cloak made of bulk isotropic materials, and it has been validated as an exact cloak. We experimentally verified its ability to maintain the heat front and its heat protection capabilities in a 2D proof-of-concept experiment. The robustness of this scheme is validated in both 2D (including oblique heat front incidence) and 3D configurations. The proposed scheme may open a new avenue to control the diffusive heat flow in ways inconceivable with phonons, and also inspire new alternatives to the functionalities promised by transformation optics.

Homogeneous thermal cloak with constant conductivity and tunable heat localization

Invisible cloak has long captivated the popular conjecture and attracted intensive research in various communities of wave dynamics, e.g., optics, electromagnetics, acoustics, etc. However, their inhomogeneous and extreme parameters imposed by transformation-optic method will usually require challenging realization with metamaterials, resulting in narrow bandwidth, loss, polarization-dependence, etc. In this paper, we demonstrate that thermodynamic cloak can be achieved with homogeneous and finite conductivity only employing naturally available materials. It is demonstrated that the thermal localization inside the coating layer can be tuned and controlled robustly by anisotropy, which enables an incomplete cloak to function perfectly. Practical realization of such homogeneous thermal cloak has been suggested by using two naturally occurring conductive materials, which provides an unprecedentedly plausible way to flexibly realize thermal cloak and manipulate heat flow with phonons.

Making waves round a structured cloak: lattices, negative refraction and fringes

Using the framework of transformation optics, this paper presents a detailed analysis of a non-singular square cloak for acoustic, out-of-plane shear elastic and electromagnetic waves. Analysis of wave propagation through the cloak is presented and accompanied by numerical illustrations. The efficacy of the regularized cloak is demonstrated and an objective numerical measure of the quality of the cloaking effect is provided. It is demonstrated that the cloaking effect persists over a wide range of frequencies. As a demonstration of the effectiveness of the regularized cloak, a Young's double slit experiment is presented. The stability of the interference pattern is examined when a cloaked and uncloaked obstacle are successively placed in front of one of the apertures. This novel link with a well-known quantum mechanical experiment provides an additional method through which the quality of cloaks may be examined. In the second half of the paper, it is shown that an approximate cloak may be constructed using a discrete lattice structure. The efficiency of the approximate lattice cloak is analysed and a series of illustrative simulations presented. It is demonstrated that effective cloaking may be obtained by using a relatively simple lattice structure, particularly, in the low-frequency regime.

First experimental demonstration of an isotropic electromagnetic cloak with strict conformal mapping

In the past years quasi-conformal mapping has been generally used to design broadband electromagnetic cloaks. However, this technique has some inherit practical limitations such as the lateral beam shift, rendering the device visible or difficult to hide a large object. In this work we circumvent these issues by using strict conformal mapping to build the first isotropic cloak. Microwave near-field measurement shows that our device (with dielectric constant larger than unity everywhere) has a very good cloaking performance and a broad frequency response. The present dielectric approach could be technically extended to the fabrication of other conformal devices at higher frequencies.

A self-assembled three-dimensional cloak in the visible

An invisibility cloak has been designed, realized and characterized. The cloak hides free-standing sub-wavelength three-dimensional objects at the short wavelength edge of the visible spectrum. By a bottom-up approach the cloak was self-assembled around the object. Such fabrication approach constitutes a further important step towards real world applications of cloaking; leaving the realm of curiosity. The cloak and the way it was fabricated opens an avenue for many spectacular nanooptical applications such as non-disturbing sensors and photo-detectors, highly efficient solar cells, or optical nanoantenna arrays with strongly suppressed cross-talk to mention only a few. Our results rely on the successful combination of concepts from various disciplines, i.e. chemistry, material science, and plasmonics. Consequently, this work will stimulate these fields by unraveling new paths for future research.

Photorealistic ray tracing of free-space invisibility cloaks made of uniaxial dielectrics

The design rules of transformation optics generally lead to spatially inhomogeneous and anisotropic impedance-matched magneto-dielectric material distributions for, e.g., free-space invisibility cloaks. Recently, simplified anisotropic non-magnetic free-space cloaks made of a locally uniaxial dielectric material (calcite) have been realized experimentally. In a two-dimensional setting and for in-plane polarized light propagating in this plane, the cloaking performance can still be perfect for light rays. However, for general views in three dimensions, various imperfections are expected. In this paper, we study two different purely dielectric uniaxial cylindrical free-space cloaks. For one, the optic axis is along the radial direction, for the other one it is along the azimuthal direction. The azimuthal uniaxial cloak has not been suggested previously to the best of our knowledge. We visualize the cloaking performance of both by calculating photorealistic images rendered by ray tracing. Following and complementing our previous ray-tracing work, we use an equation of motion directly derived from Fermat's principle. The rendered images generally exhibit significant imperfections. This includes the obvious fact that cloaking does not work at all for horizontal or for ordinary linear polarization of light. Moreover, more subtle effects occur such as viewing-angle-dependent aberrations. However, we still find amazingly good cloaking performance for the purely dielectric azimuthal uniaxial cloak.

Radiation-suppressed plasmonic open resonators designed by nonmagnetic transformation optics

How to confine light energy associated with surface plasmon polaritons (SPPs) in a physical space with minimal radiation loss whereas creating maximum interacting section with surrounding environment is of particular interest in plasmonic optics. By virtue of transformation optics, we propose a design method of forming a polygonal surface-plasmonic resonator in fully open structures by applying the nonmagnetic affine transformation optics strategy. The radiation loss can be suppressed because SPPs that propagate in the designed open structures will be deceived as if they were propagating on a flat metal/dielectric interface without radiation. Because of the nonmagnetic nature of the transformation strategy, this design can be implemented with dielectric materials available in nature. An experimentally verifiable model is subsequently proposed for future experimental demonstration. Our design may find potential applications in omnidirectional sensing, light harvesting, energy storage and plasmonic lasing.

Recent advances in transformation optics

Within the past a few years, transformation optics has emerged as a new research area, since it provides a general methodology and design tool for manipulating electromagnetic waves in a prescribed manner. Using transformation optics, researchers have demonstrated a host of striking phenomena and devices; many of which were only thought possible in science fiction. In this paper, we review the most recent advances in transformation optics. We focus on the theory, design, fabrication and characterization of transformation devices such as the carpet cloak, "Janus" lens and plasmonic cloak at optical frequencies, which allow routing light at the nanoscale. We also provide an outlook of the challenges and future directions in this fascinating area of transformation optics.