Near field imaging with magnetic metamaterials: Theory and experiment

Abstract

Ever since Pendry's paper on perfect lens [1] the idea of near field subwavelength imaging has been a driving force in the rapid development of the field of metamaterials as the search for suitable structures capable of near field imaging continues. A large variety of physical mechanisms were suggested to be responsible for near field imaging, ranging from excitation of surface plasmon-polariton resonances [1] and effective medium theory [1,2] to imaging based on the curvature of the dispersion characteristics [3], phase conjugation [4] or propagation of magnetoinductive (MI) waves [5]. Experimentally, the so-called swiss roll lens (a single layer of resonant swiss rolls) was the first example of a magnetic metamaterial shown to be able to transfer subwavelength information from the object to the image plane [2]. First explanations of imaging with a single layer of swiss rolls were based on the existence of a negative permeability region [2]. A competing explanation is based on the microscopic description of the subwavelength physics of the structure taking into account strongly anisotropic magnetic coupling between individual elements. This coupling leads to propagation of slow MI waves in the vicinity of the resonant frequency thus preventing good imaging [6]. Freire and Marques [5] proposed to use a structure made of two parallel layers of magnetic metamaterial elements and suggested that in this case MI waves may play a positive role resulting in evanescent amplification of the fields in the inter-layer spacing. In this work, we review our recent theoretical and experimental results on magnetoinductive near field lenses. Our theoretical model is based on a coupled-mode approach for magnetoinductive waves [8] developed to describe "bi-atomic" metamaterials with two magnetically coupled elements per unit cell [7]. The model explains deficiencies of the swiss roll subwavelength lens of Wiltshire [2] and clarifies the physical mechanism of the magnetoinductive lens of Freire and Marques [5]. In order to achieve subwavelength imaging magnetic field spreading in the plane perpendicular to the axis connecting object and image must be prevented so that the field distribution of a subwavelength object can be transmitted through the device to the image plane with little distortion. We identify regimes of "pseudo" and "true" subwavelength imaging. The "pseudo" subwavelength imaging is possible outside the pass bands of magnetoinductive waves i.e. far away from the resonance frequency of individual resonators which results in a weak signal accompanied by a poor signal-to-noise ratio. The "true" subwavelength imaging is shown to be a highly resonant phenomenon manifested in a narrow frequency range close to the resonance frequency. Experimental results confirm theoretical predictions on a variety of structures designed to operate at frequencies around 46 MHz. Our approach can be employed for the design of three-dimensional subwavelength near field lenses made of periodically arranged loop-shaped resonators suitable for applications in Magnetic Resonance Imaging systems.