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Our theoretical examination of second and third harmonic generation frommetal-based nanostructures predicts that nonlocal and quantum tunnelingphenomena can significantly exceed expectations based solely on local,classical electromagnetism. Mindful that the diameter of typical transitionmetal atoms is approximately 3{\AA}, we adopt a theoretical model that treatsnanometer-size features and/or sub-nanometer size gaps or spacers by takinginto account: (i) the limits imposed by atomic size to fulfill the requirementsof continuum electrodynamics; (ii) spillage of the nearly-free electron cloudinto the surrounding vacuum; and (iii) the increased probability of quantumtunneling as objects are placed in close proximity. Our approach also includesthe treatment of bound charges, which add crucial, dynamical components to thedielectric constant that are neglected in the conventional hydrodynamic model,especially in the visible and UV ranges, where interband transitions areimportant. The model attempts to inject into the classical electrodynamicpicture a simple, perhaps more realistic description of the metal surface byincorporating a thin patina of free-electrons that screens an internal,polarizable medium.

Nonlocal and Quantum Tunneling Contributions to Harmonic Generation in Nanostructures: Electron Cloud Screening Effects