Engineering ghost images

When someone claims to have seen a ghost, the phenomenon may be caused by an optical illusion happening through a wild stroke of nature. Engineering such a phenomenon is the holy grail of researchers in the field of optical illusions, and electromagnetic and radar detection, not only because of the thrill and excitement of being able to create a ‘ghost’ but because of the implications it will have for science and applications. Transformation optics, a novel design tool based on space transformation, has enabled a number of unconventional optical devices, including perfect invisibility cloaks,^1–6 carpet cloaks,^7–13illusion-optics devices,^14–18 Eaton lenses,¹⁹and flattened Luneburg lenses.^20,21

Figure 1. Schematic of the ghosting phenomenon. (a) The original ‘metallic’ (i.e., conducting) object with radius a. (b) The metallic object covered by the designed ghost device of internal radius aand external radius c, showing where the ghosts will appear, with two ghosts at the wings separated from a central ghost by a distance b. (c) The apparent objects as deduced from the scattering pattern of the original metal object and the ghost device: a shrunk metallic object at the original center with two dielectric objects at the wings.

Creating wave-dynamic illusions is of great interest to various scientific communities. Controlling perception could enable unprecedented applications in fields such as advanced materials science, camouflage, cloaking, optical and microwave cognition, and defense and security.^11–15So far, however, scientists experimenting with metamaterials (artificial materials with properties not found in nature) in ‘ghosting’ have not had much success in changing the perception of a real object or defining where a ghost should appear. They can only create one ghost, in the same location (as the real object).

We have developed and demonstrated an electromagnetic scheme to ‘engineer’ ghosts in the microwave (gigahertz) frequency range.²²For the first time, we have created ghosts using electromagnetic scattering and metamaterials. Our device is capable of creating more than one ghost image from an actual object. The geometric shape, position, and equivalent material properties of these ghost images can be pre-designed and controlled. They are also able to appear in distributed places away from the location of the real object.

Our functional illusion device manipulates the wave dynamics of electromagnetic waves. The material used is inhomogeneous, anisotropic, and made up of thousands of varying unit cells working at non-resonance. It virtually transforms an arbitrary object's electric field scattering signature to that of two or more (multiple) isolated ghost objects (i.e., not existing physically). In other words, the ghost illusion device can make the scattering signature of one object equivalent to that of two or more other objects, which are arbitrarily pre-designed, not only in the near-field scattering but also in the far-field scattering cross section.

The device transforms the perception of a perfectly conducting (or metallic) object into three apparent objects, including a shrunken metallic object in the original position and two ‘virtual’ dielectric objects (which can also be made to appear metallic) on each side: see Figure 1. The functionality can be directly derived using transformation optics theory.²³

Figure 2. The experimental prototype for the ghosting coating.

Our prototype ghost device comprises eight concentric layers of low-loss printed circuit boards (PCBs): see Figure 2. Each contains split-ring resonators (SRRs) of 35μm-thick copper coated on one side of the 0.25mm-thick substrate (F4B). Although we apply different structures in regions I and II, the height of each layer and the distance between adjacent layers remain the same. Each layer carries three rows of SRRs, whose length and height are 3.14 and 3.6mm. The designed SRRs create the required radial permeability and z-direction permittivity at 10GHz. The PCB rings are adhered to a 2mm-thick hard-foam board cut with concentric circular grooves to fit the concentric layers exactly.

In region I, the permittivity is 2.82 and the permeability ranges from 0.065 to 0.226, controlled by conventional SRRs etched on a dielectric substrate. In region II, the permittivity is 6.29, and the permeability ranges from 0.226 to 0.355, controlled by a class of modified SRRs, which can raise the permittivity remarkably. We use two methods to generate the required high electric permittivity without using any auxiliary high-permittivity materials. One is to add some curved structures to the SRRs, and the other is to reduce the wire width of the SRRs.

Figure 3. (a) Photograph of the parallel-plate waveguide mapping system. (b) Photograph of the fabricated ghost-illusion device. ϕ₀: Angle of the arc area marked as region II.

We illuminated our device (surrounding a disk-shaped conducting object) and measured the resulting electric field around the ghost device. The experimental setup is shown in Figure 3. We used a vector network analyzer to excite and receive the microwave signals. We used a parallel-plate mapping system to scan the electric field distributions. The electric fields were polarized in the vertical direction confined by two large aluminum plates, whose distance is set as 12mm. The electric field was detected and scanned by a monopole probe embedded in the top aluminum plate. We connected the feeding probe (i.e., the microwave source) and the detection probe to the network analyzer by thin coaxial cables. The ghost-device sample was fixed on the center of the bottom plate, which was mounted on a computer-controlled stage that can move in two dimensions. We scanned a region of 128×140mm with a step resolution of 1mm.

Once we had measured the electric field, we were able to compare the experimental results with calculations and to interpret what an ‘observer’ looking in the plane of the device toward it would see. Our experiments agreed with our calculations that such an observer would see not just the image of the real disk-shaped conducting object but also two ‘ghost’ images on either side of it as in Figure 1.

This electromagnetic proof-of-concept experiment was chosen to demonstrate the phenomena in a more accessible way. In fact, the mechanism can be adapted to operate at higher frequencies (terahertz, near-IR, or visible) as long as fabrication can be supported. Our work solves several major issues associated with ghost illusions, and will pave the way for future applications of advanced optical illusions, camouflage, and cloaking in an interestingly new sense. Our work has enormous potential to enhance our ability to mold, harness, and perceive waves at will. In future work we will explore the use of optical frequencies, which can also make the real object or person ‘disappear.’