Scientists gain better understanding of physics behind invisibility cloaks
experimental trial, the cloak hid the concealed object from the electromagnetic microwaves in two dimensions. Chen and his colleagues wanted to know whether perfect invisibility could be achieved under any wavelength. The scientists explained that perfect invisibility is achieved when the scattering cross section is zero, which indicates that the cloak exhibits zero scattering. The group found that the parameters for a perfect cloak are very difficult to realize, and that when some specific type of loss is included, the three dimensional spherical cloak wrapped around a hidden object exhibits zero backscattering while a two dimensional cylindrical cloak does not. “The cloak is both anisotropic and inhomogeneous: all of the components in the permittivity and permeability tensor are functions of the radius, which implies that the perfect invisibility cloak is very difficult to design,” Chen explained. “If we introduce a specific type of loss both in a spherical cloak and a cylindrical cloak, only the spherical cloak exhibits a zero backscattering, which indicates only the spherical cloak can still be rendered invisible with a monostatic (transmitter and receiver in the same location) detection. This is because the impedance of the spherical cloak is still matched to the free space in this particular loss case.”
Because imperfect cloaks have less stringent requirements than perfect cloaks, they may offer a more realistic alternative for engineers. Yes, imperfect cloaks have non-zero scattering, but the objects they cloak can still appear isolated from the outside field under certain specific conditions and the incident fields cannot penetrate into the hidden object. What is more, for an imperfect cloak with matched impedance, it can still be rendered invisible with monostatic detection, which is most widely used in current radar. “For a monostatic detection, no reflection wave will be received by the detector if the imperfect cloak has a matched impedance with the free space,” Chen explained. “Therefore, the imperfect cloak, even with its parameters deviated far from the ideal parameters, still can be made completely invisible with the monostatic detection as long as it satisfied the impedance requirement.”
Almost all radars currently in use belong to the monostatic class, so Chen explained that his group’s research can offer a more realistic alternative for engineers. In the future, applications of invisible cloaks could include military uses such as making planes and weapons invisible to radar, enabling the possibility of looking out through walls as if they were windows, and hiding ugly factories for aesthetic reasons. “The effectiveness of the cloak based on the analytical solutions of the electromagnetic wave interactions with metamaterial cloaks (ideal or non-ideal) can be quantitatively provided,” Chen said. “Our research work therefore provides a new way for the cloak design and to qualify the effectiveness or performance of a non-ideal cloak.”
-read more in Hongsheng Chen et al., “Electromagnetic Wave Interactions with a Metamaterial Cloak,” Physical Review Letters 99, 063903 (6 August 2007)
