Invisibility or cloaking seems to only happen in fairy tales and science fiction films. In fact, a lot of creatures have versatile means to make themselves “invisible” in nature. For example, chameleons can change the color of their skin to protect themselves from their enemy or close in on their prey. Even in human society, camouflage has been used in military, including fatigues. Stealth aircrafts installed with wave-absorbing materials are blind to nearby radio stations. All these strategies are effective at making things “invisible” to brains, rather than eyes.
In the past decade, scientists have provided a new strategy to artificially create “invisibility.” This is done by using materials with precise designs to guide the propagation of light at will. Such materials are called metamaterials. The first prototype of an invisible device was fabricated by Prof. Smith of Duke University in 2006, which works in the microwave spectrum for metamaterials with split ring resonance structures. After which, a lot of effort was put into developing and pushing invisibility forward.
The requirements of a perfect invisible device are very complex because they must be inhomogeneous and anisotropic. When we consider invisibility in two-dimensional space, these materials should have an isotropic refractive index profile, just like a hot desert which can create a mirage. An isotropic refractive index profile is much easier to achieve than anisotropic metamaterials, which may be helpful to experimental invisibility.
Our research group (Prof. Chen’s group, Xiamen, China) has focused on the theory and experiments of invisibility in two-dimensional space for many years. The theory is mainly based on a complex analysis. We want to optimize the refractive index profile to bring to market an invisible device with good performance. We have already presented a proposal to design an invisible device, which is closed to experimental conditions. In a recent study, we explain why this device is invisible at both geometrical optics and wave optics with a discrete set of frequencies.
We first built a new space based on this complex analysis, a constructed environment is much easier to control. Then, we can map problems in this device to the new space. By solving problems in a new space, we can obtain the behavior of the original real device. The new space consists of a plane and a cylinder, which are sewed along a line. This structure is quite different from a single plane in topology. Light rays travel along straight lines in a plane. When the cylinder is placed adjacent to a Mikaelian lens, light rays bent to form closed curves. For waves, only eigenmodes with a discrete set of frequencies can survive in a Mikanlian lens, which determines the property of the original real device.
In summary, we demonstrated this invisible device, which arises from the excitation of eigenmodes of the cylinder with a Mikalian lens in a new space. It results in phase continuity in the device. We show a heuristic physical picture of the invisible device in geometric optics and wave optics with a discrete set of frequencies. This is helpful for optimizing the design and fabrication of invisible devices.