Field enhancement in subnanometer metallic gaps

Motivated by recent experiments, we present here a theoretical analysis of the optical response of sharp gold electrodes separated by a subnanometer gap. In particular, we have used classical finite difference time domain simulations to investigate the electric-field distribution in these nanojunctions upon illumination. Our results show a strong confinement of the field within the gap region, resulting in a large enhancement compared to the incident field. Enhancement factors exceeding 103 are found for interelectrode distances on the order of a few angstrom, which are fully compatible with the experimental findings. Such huge enhancements originate from the coupling of the incident light to the evanescent field of hybrid plasmons involving charge density oscillations in both electrodesA. G. M. acknowledges financial support from the EU (Grant No. NMP3- SL-2008- 214107-Nanomagma) and the Spanish MICINN (“MAGPLAS” Grant No.MAT2008-06765-C02-01/NAN and “FUNCOAT” CONSOLIDER INGENIO Grant No. 2010 CSD2008-00023). J. C. C. acknowledges support from the EU through the BIMORE network (Grant No. MRTN-CT- 2006-035859) and the Spanish MICINN (Grant No. FIS2008- 04209). D. N. and D. R. W. acknowledge support from the Robert A. Welch Foundation (Grant No. C-1636) and the Lockheed Martin Advanced Nanotechnology Center of Excellence at Rice University (LANCER