Manipulating Nanoscale Light Fields with the Asymmetric Bowtie Nano-Colorsorter Z. Zhang, †,‡ A. Weber-Bargioni, † S. W. Wu, † S. Dhuey, † S. Cabrini, † and P. J. Schuck* ,† Molecular Foundry, Lawrence Berkeley National Lab, Berkeley, California 94720, and Department of Chemistry, UC Berkeley, Berkeley, California 94720 Received August 31, 2009; Revised Manuscript Received October 13, 2009 ABSTRACT We present a class of devices called Asymmetric Bowtie nano-Colorsorters. These devices are specifically engineered to not only capture and confine optical fields, but also to spectrally filter and steer them while maintaining nanoscale field distributions. We show that spectral properties and localized spatial mode distributions can be readily tuned by controlled asymmetry. Nano-Colorsorters can control light’s spatial and spectral distributions at the nanoscale and thus significantly impact applications ranging from broadband light harvesting to ultrafast wavelength-selective photodetection. A central goal of plasmonics is complete control over optical signals at deeply subwavelength scales. The recent invention of optical nanoantennas has led to a number of device designs that provide confinement of optical fields at nanometer length scales. 1-13 For photonic applications, however, the effective- ness of these structures would be significantly improved by the added ability to spatially sort the optical signals based on a physically accessible parameter such as energy/ color. 14-18 Here, we present our experimental and theoretical study of a class of devices, termed Asymmetric Bowtie nano- Colorsorters (ABnC), which demonstrate both the ability to efficiently capture and strongly confine broadband optical fields, as well as to spectrally filter and steer them while maintaining nanoscale field distributions. The latter property is important because it allows for manipulation while preserving the physical match, created by the optical antenna, between the localized field distribution and important physi- cal factors such as semiconductor carrier diffusion lengths and zeptoliter volumes occupied by individual nano- and quantum- objects. Because of these capabilities, ABnCs are expected to have a profound impact on a wide range of optoelectronic and plasmonic applications including ultrafast color-sensitive photodetection, solar power light harvesting, super-resolution imaging, and multiplexed chemical sensing. Our proof-of-principle ABnC devices are based on asym- metric variations of double-bowtie nanoantennas oriented in a “cross” geometry. As an essential experimental complement to our theoretical modeling, an initial step of our investigation was to demonstrate the feasibility of fabrication of these devices. The nanoantennas are fabricated using electron-beam lithography (Vistec VB300, 100 keV beam energy) and lift- off on indium-tin-oxide- (ITO) coated fused silica sub- strates (ITO thickness ) 50 nm) and consist of approximately 17 nm thick Au on top of a 3 nm Ti adhesion layer. Each constituent Au triangle in the cross nanoantenna is designed to be equilateral in shape with a perpendicular bisector length of 75 nm. Experimentally, the nanoantenna resonances are measured by collecting darkfield scattering spectra from individual structures in a transmission confocal modality: white light is focused on the back of the transparent sample with a high numerical aperture (N.A.) oil condenser (N.A. ) 1.43-1.2) and scattered light is collected with a 100×, 0.95 N.A. air objective, focused through a 150 μm diameter pinhole, then directed into 0.3 m spectrometer (PI-Acton) and dispersed onto a liquid-nitrogen-cooled charge-coupled device (CCD) camera. Calculations of the fields surrounding our devices were done using finite element method (FEM) software from COMSOL. Our analyzed volume consisted of a 500 nm (x axis) × 500 nm (y axis) × 250 nm (z axis) top layer of air (index of refraction, n, ) 1), a 500 nm × 500 nm × 50 nm middle layer of ITO (n ITO ) 1.91), and a 500 nm × 500 nm × 250 nm bottom layer of glass (n glass ) 1.5). Perfectly matched layers (PML) surrounded the volume (100 nm thick for the sides, 250 nm thick at the bottom). Twenty nanometer thick Au nanoantennas and ABnC devices were placed * To whom correspondence should be addressed. E-mail: pjschuck@lbl.gov. † Lawrence Berkeley National Lab. ‡ UC Berkeley. NANO LETTERS 2009 Vol. 9, No. 12 4505-4509 10.1021/nl902850f CCC: $40.75  2009 American Chemical Society Published on Web 11/09/2009