A crystal, i. e. an ordered lattice of atoms or molecules is normally assumed to be almost uniformly excited by an incident light on a sub-wavelength scale. An interatomic interaction produces then a uniform local field (different from that of incident laser) at each atom as well. This is a major assumption in the Lorentz-Lorenz theory of interaction of light with dense matter.

We show that at certain critical conditions on the atomic density and dipole strength, a previously unexpected phenomenon emerges: the interacting atoms break the uniformity of interaction, and in a violent switch to a strong non-uniformity, their excitation and local field form nanoscale strata with a spatial period much shorter than that of laser wavelength, thus changing the entire paradigm of light-matter interaction. The most interesting effects can be observed for relatively small 1D-arrays or 2D-lattices if the laser is almost resonant to an atomic quantum transition. The effects include huge local field enhancement at size-related resonances at the laser frequencies near the atomic line, so that the strata are readily controlled by laser tuning. A striking feature is that for the shortest strata, the nearest atomic dipoles counter-oscillate, which is reminiscent of anti-ferromagnetism of magnetic dipoles in Ising model.

Our results also show the formation of "hybrid" modes, whereby at certain atoms in the lattice, their excitation and local field get completely suppressed. Due to those modes, at certain "magic" array size or configuration, the absorption of light at the exact resonant is almost fully canceled. The simplest magic 2D-shape is a six-point star made of 13 atoms (with one atom at the center). The resonant amplitude enhancement enables optical hysteresis and bistability at low light intensities; the tiniest known optical switch can be made of just two atoms. The new phenomenon promises a potential for nanoscale non-conductive computer elements, sensors for detecting bio-molecules, etc.

Physics Bldg., room 401