Large Enhancement of Nonlinear Optical Phenomena by Plasmonic Nanocavity Gratings Patrice Genevet, †,§ Jean-Philippe Tetienne, † Evangelos Gatzogiannis, ‡ Romain Blanchard, † Mikhail A. Kats, † Marlan O. Scully, §,| and Federico Capasso* ,†,§ † School of Engineering and Applied Sciences and ‡ Center for Nanoscale Systems, Harvard University, Cambridge, Massachusetts 02138, United States, § Institute for Quantum Studies and Department of Physics, Texas T&M University, College Station, Texas 77843, United States, and | College of Engineering and Applied Science, Princeton University, Princeton, New Jersey 08544, United States ABSTRACT Enhancing nonlinear processes at the nanoscale is a crucial step toward the development of nanophotonics and new spectroscopy techniques. Here we demonstrate a novel plasmonic structure, called plasmonic nanocavity grating, which is shown to dramatically enhance surface nonlinear optical processes. It consists of resonant cavities that are periodically arranged to combine local and grating resonances. The four-wave mixing signal generated in our gold nanocavity grating is enhanced by a factor up to ≈2000, 2 orders of magnitude higher than that previously reported. KEYWORDS Surface plasmons, localized plasmons, plasmonic nanocavity, grating, nonlinear optics P lasmonics is concerned with the manipulation of light at the nanoscale. It involves the study of the coupling between electromagnetic radiation and collective electronic oscillations in metals, known as surface plas- mons. 1,2 The resulting field enhancement makes them particularly attractive for surface nonlinear optics, since the efficiency of most nonlinear phenomena can be greatly enhanced. 3-10 Potential applications include nanophotonics and spectroscopy where nonlinear optical processes play an important role, either for frequency conversion or to detect chemical fingerprints. While isolated metal particles have been extensively studied, 5 patterned substrates have received much less attention. Recently, Renger et al. showed that four-wave mixing (4WM) in a gold substrate can be enhanced by patterning the surface with a grating and exploiting evanes- cent waves. 10 Gratings have also been used to enhance the 4WM process by phase matching the generated signal with the pump beam. 11 Here we report on a novel approach based on plasmonic nanocavity gratings to enhance surface nonlinear processes. We have designed plasmonic nano- cavities, made of nanogrooves in a gold film, 12,13 and arranged them periodically to combine local resonances of the nanocavities with grating resonances that involve surface plasmon polaritons (SPPs) propagating on the corrugated surface. Since the corrugation is composed of resonant structures, the properties of SPPs along such an interface are considerably affected. To avoid any confusion with SPPs propagating at a flat metal-dielectric interface, we generally refer to these modes as surface waves. Several works already proposed to interpret such an interaction between localized and delocalized plasmons in the context of Fano resonances. 14-16 We apply this concept of coupled resonances to 4WM in gold and demonstrate an enhancement of the generated signal of up to ≈2000 compared with an unpatterned surface, 2 orders of magni- tude higher than that reported in ref 10. This result shows that plasmonic nanocavity gratings are a promising route to enhancing optical nonlinearities in the metal and also in any material filling the cavities, with potential applications to nanoscale frequency conversion and highly sensitive vibra- tional spectroscopy and microscopy. A single narrow groove (tens of nanometers wide) defined on a metal surface can be viewed as a portion of a metal- insulator-metal (MIM) waveguide terminated by a metallic mirror on one side and a dielectric mirror (air) on the other. It therefore forms a cavity that sustains Fabry-Pe ´rot modes. 13 Because the cavity mode is confined in a subwave- length volume, large fields are established both in the metal and in the dielectric under resonant excitation. 13 In addition to these localized surface plasmons (LSPs), a grating of nanogrooves enables the coupling of free space light to surface waves (SWs) propagating on the corrugated surface. Figure 1 illustrates schemes to couple light into (a) or out of (b) a nanocavity via excitation of SWs. Because they com- bine both LSPs and SWs and can exhibit a very high density of electromagnetic energy, nanogroove gratings are an ideal tool for enhancing various nonlinear optical phenomena. In our experiment, two laser beams, E 1 at frequency ω 1 and * To whom correspondence should be addressed. Received for review: 08/4/2010 Published on Web: 11/03/2010 pubs.acs.org/NanoLett © 2010 American Chemical Society 4880 DOI: 10.1021/nl102747v | Nano Lett. 2010, 10, 4880–4883