Active Metamaterials with Negative Static Electric Susceptibility.

Although well-established textbook arguments suggest that static electric susceptibility χ must be positive in "all bodies," it has been pointed out that materials that are not in thermodynamic equilibrium are not necessarily subject to this restriction. Media with inverted populations of atomic and molecular energy levels have been predicted theoretically to exhibit a χ < 0 state, however the systems envisioned require reduced temperature, reduced pressure, and an external pump laser to maintain the population inversion. Further, the existence of χ < 0 has never been confirmed experimentally. Here, a completely different approach is taken to the question of χ < 0 and a design concept to achieve "true" χ < 0 is proposed based on active metamaterials with internal power sources. Two active metamaterial structures are fabricated that, despite still having their power sources implemented externally for reasons of practical convenience, provide evidence in support of the general concept. Effective values are readily achieved at room temperature and pressure and are tunable throughout the range of stability -1 < χ < 0, resulting in experimentally-determined magnitudes that are over one thousand times greater than those predicted previously. Since χ < 0 is the missing electric analog of diamagnetism, this work opens the door to new technological capabilities such as stable electrostatic levitation.