Superabsorbers and invisible sensors

Summary form only given. The optimal properties of receiving antennas and sensors have been the subject of extensive debate for many years and there has been recent renewed interest in several fundamental questions on the constraints on absorbed and scattered power of an arbitrary receiving antenna. In general, an antenna designed to receive power in an efficient way is expected to produce large scattering, significantly perturbing the signal under measurement. Bach Andersen and Frandsen [J. Bach Andersen, A. Frandsen, IEEE Trans. Antennas Propagat. 53, 2843 (2005)] theoretically showed that there is in principle no limit to the ratio between scattered and absorbed powers, yet a methodology to design a minimum-scattering receiver with specific absorption ratio does not exist. In this talk, we provide comprehensive theoretical limits on the ratio between absorbed and scattered powers in arbitrarily shaped receiving antennas and sensors of most general form, as well as a roadmap to design invisible sensors with strong absorption properties. We will discuss how strongly a sensor can interact with the electromagnetic wave without significantly perturbing it. Using the general Mie solution, we demonstrate that in principle there is no upper limit neither on the amount of absorption nor on the ratio between absorbed and scattered powers. Our studies interestingly indicate that the only parameter that may limit the efficiency of an absorber is its size, and we show that balanced non-resonant scattering modes are required to access the maximum attainable efficiency for a fixed amount of absorption and size of sensor. We show how plasmonic cloaking can be used specifically at small size limits to achieve the theoretical boundaries presented. Unlike previous studies on low-scattering absorbers, our theory also indicates a simple roadmap to design minimum-scattering sensors. To illustrate the application of our theory, we present the design of an optical sub-wavelength absorb- r with transverse cross-section smaller than 0.3λ0, an absorbed power equal to the one of a resonant dipole antenna, and a total scattering cross-section reduced by 8 folds. Our theory is applicable to broad frequency ranges from microwaves to the optical regime, of interest for many exciting applications from biomedical engineering and imaging to security and radar technology.