Casimir forces: Fundamental theory, computation, and nanodevice applications

Abstract

Seventy years after submission to the Physical Review of the crucial quantum electrodynamical treatment of interatomic dispersion forces by Casimir and Polder, our understanding of such interactions in both the unretarded and retarded regimes has undergone a dramatic and intricate evolution. In this contribution, we explore the ultimate physical motivations leading to this fascinating trajectory rich in momentous implications for the goal of both fabrication and operation of highly integrated micro- and nano-structures. The first and most obvious development has been the growing appreciation that, far from only representing a weak, though exotic, effect, Casimir’s “zero point pressure of electromagnetic waves” between two conducting parallel planes is actually a dominant interaction on the nanoscale. This resulted in Feynman’s unforgettable caricature – in “There’s plenty of room at the bottom” – of van der Waals forces between microparts as a “man with his hands full of molasses,” which led to such forces being understood as the leading cause of undesirable stiction for several decades. However, commencing in the 1980s, the realization that such strong dispersion interactions might offer unique technological opportunities surfaced. The second thrust was connected to the discovery that, unlike expected from London’s intermolecular force theory and the naive assumption of additivity, dispersion forces depend quite unpredictably on topology and on the interplay of dielectric properties of the interacting media. This may lead to drastic departures from results obtained through perturbative methods and indeed to the prediction, later verified both in the unretarded and retarded regimes, that dispersion forces may become repulsive. The challenge of computing Casimir forces in more general geometries different from that of two parallel planes has led to substantial progress from the numerical standpoint although open problems remain. Lastly, in one of the earliest and most significant discoveries in the history of the field, it was shown that dispersion forces can be modulated in time, for instance by illumination in semiconductors. This discovery opened the way to consideration of thermodynamical engine cycles enabled by Casimir forces and to a novel, highly effective means for energy transfer on the nanoscale.