Artificial Nonlinearity Generated from Electromagnetic Coupling Metamolecule

Abstract

A purely artificial mechanism for optical nonlinearity is proposed based on a metamaterial route. The mechanism is derived from classical electromagnetic interaction in a meta-molecule consisting of a cut-wire meta-atom nested within a splitring meta-atom. Induced by the localized magnetic field in the split-ring meta-atom, the magnetic force drives an anharmonic oscillation of free electrons in the cut-wire metaatom, generating an intrinsically nonlinear electromagnetic response. An explicit physical process of a second-order nonlinear behavior is adequately described, which is perfectly demonstrated with a series of numerical simulations. Instead of "borrowing" from natural nonlinear materials, this novel mechanism of optical nonlinearity is artificially dominated by the meta-molecule geometry and possesses unprecedented design freedom, offering fascinating possibilities to the research and application of nonlinear optics.Nonlinear optics has become thriving since the first observation of the second harmonic generation (SHG) in 1961 [1], and plays an essential role in many optical devices such as frequency up-converters and mixers, nonlinear spectrometers, and new light sources [2]. Over the past half century, with tons of efforts focused on searching new nonlinear materials [3, 4], researchers endeavor to uncover the physical mechanism behind the optical nonlinearity [5]. As a universal phenomenon, nonlinearity is exhibited by almost all materials interacting with sufficiently strong light [6], and various fundamental mechanisms were proposed, such as distortion of the electron cloud, relative motion of nuclei, reorientation of molecules, electrostriction effect, and thermal effect [7]. Despite the fact that these phenomenological theories extensively advanced the development of new nonlinear materials in the past several decades, they

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