Fundamental understanding and engineering of composite materials, biological structures and building blocks for electrical and optical devices of nanoscale dimensions necessitate the availability of advanced microscopy tools for mapping their local chemical, structural and free-carrier properties, as well as the near-field distribution in their vicinity. But while optical spectroscopy, particularly in the infrared (IR) and terahertz (THz) frequency range, has tremendous merit in measuring such properties optically, the diffraction-limited spatial resolution has been preventing IR and THz microscopy applications for the longest time to be used in nanoscale materials and device analysis, bio-imaging, industrial failure analysis and quality control.
During the last years, we pioneered the development of scattering-type scanning near-field optical microscopy (s-SNOM) and Fourier transform infrared nanospectroscopy (nano-FTIR), a new technique that allows for imaging and spectroscopy at visible, infrared and terahertz frequencies with a resolution that is improved by a factor 100 to 1000 compared to conventional infrared spectroscopy.
s-SNOM is typically based on atomic force microscopy (AFM). For dielectric mapping, a metallic AFM tip is illuminated with a focused laser beam and the light elastically scattered from the tip is detected simultaneously to topography. Acting as an optical antenna, the tip converts the incident light into a strongly confined near-field spot (nanofocus) at the tip apex, which locally illuminates the sample surface. Because of the strong optical near-field interaction between tip and sample, the elastically scattered light contains information about the local optical properties of the sample surface. Thus, by recording the scattered light - while scanning the sample - the local dielectric properties can be mapped. The spatial resolution is determined by size of the nanofocus, which essentially depends on radius of the tip apex, typically in the range of 10 to 30 nm. Nanoscale infrared spectroscopy can be accomplished by illuminating the tip with broadband infrared radiation and subsequent Fourier transform spectroscopy of the scattered light.