SHEGAI ET AL. VOL. 5 ’ NO. 3 ’ 2036 –2041 ’ 2011 2036 www.acsnano.org February 16, 2011 C 2011 American Chemical Society Angular Distribution of Surface- Enhanced Raman Scattering from Individual Au Nanoparticle Aggregates Timur Shegai,* Bj € orn Brian, Vladimir D. Miljkovi c, and Mikael K€ all Department of Applied Physics, Chalmers University of Technology, 412 96, G € oteborg, Sweden C ompact metal nanoantennas sup- porting plasmon resonances are able to focus light to volumes well below the diffraction limit and, through recipro- city, amplify emission from subwavelength sources. These effects are the basis for a number of surface-enhanced spectrosco- pies, including surface-enhanced Raman scattering (SERS), 1,2 fluorescence 3,4 (SEF), and infrared absorption (SEIRA). 5 Nanoplas- monic antenna effects combined with reso- nance Raman amplification even lead to single molecule sensitivity in SERS. 6-9 A number of aspects determine how suitable a particular nanoantenna is for spectrosco- py applications. First, the nanoantenna has to be a good receiver of the incident far- field radiation used for excitation. The effi- ciency of this process strongly depends on how well the excitation overlaps in energy and polarization with the local surface plas- mon resonance, which implies that the orientation of the nanoantenna in relation to the incident field is a crucial parameter. In molecular spectroscopy applications that involve an emission process, such as SERS and SEF, the nanoantenna also has to be able to efficiently convert the evanescent molecular source-field to propagating far- field radiation. This highlights the impor- tance of the position and orientation of the molecule(s) in relation to the nanoantenna. In addition, the emission frequency should match the resonance frequency of the na- noantenna. In SERS, the emission occurs at a Stokes shifted frequency, and it often turns out that the most efficient SERS systems are those for which a plasmon resonance of the antenna overlaps with both the emission and excitation frequencies of the Raman scatter- ing process. 9,10 The polarization of the SERS emission has also been extensively studied. In the case of dimer nanoantennas, for example, the light was shown to be polarized along the dimer axis, 11,12 as anti- cipated from simple field-enhancement arguments. The nanoantenna aspects of surface-en- hanced spectroscopy discussed above have been studied for the past three decades and are now comparatively well understood. Recently, however, the interest in nano- antennas has broadened to encompass the possibility of directional control over light emission, for example by means of minia- turized Yagi-Uda 13-15 and metal nanowire antennas. 16,17 Here, we study the angular distribution of light emitted by individual Raman nanoantennas located at an air-glass interface using Fourier imaging and high numerical aperture (NA) optics. We show that the SERS emission from compact clus- ters of 80 nm gold nanoparticles (NPs) peaks sharply at the air-glass critical angle, as expected for subwavelength sources lo- cated close to an interface. 18,19 This result is of practical interest because many SERS experiments are conducted on solid inter- faces, typically air-glass or water-glass, but with low NA objectives that are not capable of capturing this “forbidden” light. Further, we find that the SERS emission from dimer nanoantennas has the same directionality as the scattering from the *Address correspondence to timurs@chalmers.se. Received for review November 19, 2010 and accepted February 8, 2011. Published online 10.1021/nn1031406 ABSTRACT Nano-optical antennas based on plasmonic metal particles are well-known for their ability to dramatically concentrate electromagnetic energy. However, not much attention has been devoted to the directionality properties of nanoantennas. Here, we report on the angular distribution of surface-enhanced Raman scattering (SERS) emitted by isolated aggregates of gold nanoparticles. We find that most of the radiation appears at angles exceeding the critical angle of the air-glass interface supporting the aggregates, and we demonstrate that angle-resolved imaging can be used as a fast and facile method for determination of the three-dimensional orientation and symmetry of the nanoantenna. KEYWORDS: nanoantenna . surface-enhanced Raman scattering . Fourier imaging ARTICLE