Commentary: Controlling electromagnetic metamaterials

Abstract

The last two decades have been witness to two exciting and independent developments that have forever changed our conventional view of how light interacts with matter. One relates to coherent control via quantum interference, wherein the possibility of making an otherwise opaque medium transparent [1], now known as electromagnetically induced transparency (EIT), set off intense research activity. EIT essentially requires careful creation of atomic coherence, that results in diverse effects varying from almost freezing light in its tracks (slow light) to freezing atoms to nanoKelvin temperatures via velocity-selective coherent population trapping. The second development relates to metamaterials whose origins are very classical in nature. In electromagnetics, these designer materials were originally proposed for realizing a super-lens wherein the evanescent field becomes the work horse that accords sub-wavelength resolution in imaging [2]. Since then a variety of metamaterials have been proposed, where even the propagating fields can be dramatically controlled, as in electromagnetic cloaks wherein the fields are maneuvered around an obstacle so as to make it invisible. The biggest technological contraints in realizing large-scale device applications of metamaterials have been two. The first is the large dissipation associated with an inherently resonant phenomenon. The second arises due to the very design of metamaterial; once the metamaterial structures (inclusions) are fabricated, they offer little maneuverability in terms of the operating frequency. However, both EIT and metamaterials have truly lifted the tedium associated with the usual classical linear phenomena involving light. It is commonly believed this is just the beginning of a long journey, where our inherent drive to control gainfully these and many other wondrous effects will be the prime mover of future developments. The marriage of these two diverse developments is highlighted here, with a word of caution: one can only naively guess the surprises this relationship will bring forth. In order to achieve a composite material combining the attributes of EIT and metamaterials, one simple design involves immersing the metamaterial in a dilute atomic gas whose frequency-selective absorption can be exploited to manipulate the metamaterial response [3]. Furthermore, a combination of light fields accords extra control over the metamaterial via the absorption and dispersion of the atomic gas through the atomic coherence (quantum) route, based on effects like EIT. The price of working at near-resonant conditions is the large dispersion with frequency accompanied by large loss. EIT-based control exploits the large frequency-dispersion and yet provides substantially decreased loss which is even lower than the metallic losses in a narrow-bandwidth regime. The large variation of the refractive index of the EIT medium results in the freezing of currents in the metallic inclusions of the metamaterial, thereby lowering loss. The most critical issues that govern this alliance arise from the very nature of the two partners. EIT is a quantum phenomenon, whereas metamaterials are described classically. Quantum phenomena are extremely susceptible to the surroundings, and using a solid or liquid medium severely restricts the performance of the quantum partner. For example, rareearth ions implanted in crystals have been shown to exhibit EIT only at extremely low temperatures where the phonon noise is sufficiently suppressed such that the ground-state