Mid-infrared radiation allows the analysis of a wide range of different material properties, including chemical composition and the structure of matter1,2. 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 devices3.
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 microscopy4,5,we directly visualize the evolution of the energy compression into a nanoscale confined infrared spot with a diameter of 60 nm (l/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.