NAVARRO-CIA AND MAIER VOL. 6 ’ NO. 4 ’ 3537–3544 ’ 2012 www.acsnano.org 3537 March 19, 2012 C 2012 American Chemical Society Broad-Band Near-Infrared Plasmonic Nanoantennas for Higher Harmonic Generation Miguel Navarro-Cia * and Stefan A. Maier Experimental Solid State Group, Department of Physics, Imperial College London, London SW7 2AZ, United Kingdom A ntennas 1 are widely used at radio and microwave frequencies to cou- ple/radiate energy from a source to free space with a far-field emission of re- duced angular divergence. Due to recipro- city, they also collect radiation efficiently from defined directions. These functional- ities are performed via the manipulation of enhanced fields in subdiffraction volumes. With the advent of nanotechnology, anten- nas operating at optical/near-infrared fre- quencies have become accessible, 2 exceeding the energy localization capabilities of tradi- tional optical elements such as mirrors and lenses. 3 Optical antennas 2 can be advanta- geously used to either harvest or focus light in nanoscale volumes not limited by diffrac- tion. This results in a large electromagnetic field enhancement in the proximity of the antenna. Likewise, they tailor the excitation and emission processes of nearby fluores- cent molecules or quantum dots. 4À6 These abilities hold significant promise for optical emitters, photovoltaics, spectroscopy, and nonlinearities. 2,7À12 Most nanoantennas reported in the lit- erature so far (nanorods and nanodipoles) show a narrow-band response because of their dipolar nature. 1,7 However, it would be highly desirable to have a nanoantenna with significant bandwidth of operation, for example, more than an octave. Nanoanten- nas with such properties are expected to make significant contributions, 2 for in- stance, in the burgeoning areas of surface- enhanced linear/nonlinear vibrational spec- troscopy 13,14 and spontaneous two-photon emission, 15,16 which are intrinsically multi- wavelength and broad-band in nature, and in higher harmonic generation. We will focus on the latter in this article. Indeed, a broad-band plasmonic nanoantenna could be used for more specific applications linked to harmonic generation, such as perfect lensing via phase conjugation and time reversal by enhancing the efficiency of non- linear processes not only at the funda- mental 17 but also at the harmonic frequency. In an effort to surpass the inherently limited narrow-band operation of infrared frequencies, nanorods and the bow-tie to- pology have been extensively explored. Nevertheless, their bandwidths of operation are significantly below an octave. 11,15,16,18,19 Other attempts less explored involve the use of fractal topologies, 20 multielement arrangements, 21À23 or the interaction with grating modes. 24 However, they also experi- ence limitations. For instance, the fractal Sierpinski nanocarpet 20 has strong field en- hancement at several wavelengths, but the spatial position of the hot spot is wavelength- dependent; multi-nanodipoles of different * Address correspondence to m.navarro@imperial.ac.uk. Received for review February 7, 2012 and accepted March 19, 2012. Published online 10.1021/nn300565x ABSTRACT We propose a broad-band near-infrared trapezoidal plasmonic nanoantenna, analyze it numerically using finite integration and difference time domain techniques, and explain qualitatively its performance via a multidipolar scenario as well as a conformal transforma- tion. The plasmonic nanoantenna reported here intercepts the incoming light as if it were of cross-sectional area larger than double its actual physical size for a 1500 nm bandwidth expanding from the near-infrared to the visible spectrum. Within this bandwidth, it also confines the incoming light to its center with more than 1 order of magnitude field enhancement. This wide-band operation is achieved due to the overlapping of the different dipole resonances excited across the nanoantenna. We further demonstrate that the broad- band field enhancement leads to efficient third harmonic generation in a simplified wire trapezoidal geometry when a Kerr medium is introduced, due to the lightning rod effect at the fundamental and the Purcell effect at the induced third harmonic. KEYWORDS: broad-band . conformal transformation . nanoantenna . plasmonic . third harmonic generation ARTICLE