Nanofocusing of mid-infrared energy with tapered transmission lines

Abstract

Mid-infrared radiation allows the analysis of a wide range of different material properties, including chemical composition and the structure of matter (Gutberlet in Science 324:1545, 2009, Li in Nature Phys. 4:532, 2008). Infrared spectroscopy is therefore an essential analytical tool in many sciences and technologies. The diffraction limit, however, challenges the study of individual molecules and nanostructures, as well as the development of highly integrated mid-infrared optical devices (Soref in Nature Photon. 4:495, 2010). Here, we experimentally demonstrate mid-infrared nanofocusing by propagating a mid-infrared surface wave along a tapered two-wire transmission line. The tapering results in a compression of the electromagnetic energy carried by the surface wave. By using infrared vector near-field microscopy (Schnell et al. in Nano Lett. 10:3524, 2010, Olmon in Phys. Rev. Lett. 105:167403, 2010), we directly visualize the evolution of the energy compression into a nanoscale confined infrared spot with a diameter of 60 nm (λ/150) at the taper apex. Our work opens the way to the development of chemical and biological sensing tools based on infrared surface waves, including miniaturized spectrometers and lab-on-a-chip integrated (bio)sensors.