Nanoscale stratification of optical excitation in self-interacting one-dimensional arrays

Abstract

The major assumption of the Lorentz-Lorenz theory about uniformity of local fields and atomic polarization in dense material does not hold in finite groups of atoms, as we reported earlier [A. E. Kaplan and S. N. Volkov, Phys. Rev. Lett. 101, 133902 (2008)]. The uniformity is broken at subwavelength scale, where the system may exhibit strong stratification of local field and dipole polarization, with the strata period being much shorter than the incident wavelength. In this paper, we further develop and advance that theory for the most fundamental case of one-dimensional arrays, and study nanoscale excitation of so-called ``locsitons'' (local field excitations) and their standing waves (strata) that result in size-related resonances and related large field enhancement in finite arrays of atoms. The locsitons may have a whole spectrum of spatial frequencies, ranging from long waves, to an extent reminiscent of ferromagnetic domains, to supershort waves, with neighboring atoms alternating their polarizations, which are reminiscent of antiferromagnetic spin patterns. Of great interest is the different kind of ``hybrid'' mode of excitation, greatly departing from any magnetic analogies. We also study differences between Ising-type near-neighbor approximation and the case where each atom interacts with all other atoms in the array. We find an infinite number of ``exponential eigenmodes'' in the lossless system in the latter case. At certain ``magic'' numbers of atoms in the array, the system may exhibit self-induced (but linear in the field) cancellation of resonant local-field suppression. We also studied nonlinear modes of locsitons and found optical bistability and hysteresis in an infinite array for the simplest modes.