Novel concepts in infrared imaging at nanoscale resolution

Within the recent years, various novel optical concepts have been invented to improve the diffraction-limited resolution of optical microscopy. The first approach of scanning near-field optical microscopy (SNOM) employed a small, subwavelength-sized aperture that is scanned close to the object of interest, capable of a resolution of about 50 nm. More advanced concepts rely on the light scattering of a sharp tip probing the sample, allowing for higher resolution (10-30 nm) and the use of longer wavelengths. Another exciting new imaging device, a planar slab of a material with negative permittivity called a superlens, allows for subwavelength resolved imaging over large areas. I will focus on the latter two systems that operate with \textit{infrared light} and offer the capability of chemical sensing by directly probing molecular vibrations. Particularly, I will present the latest results on superlensing that became accessible by \textit{phase-sensitive} infrared near-field microscopy and thus provide new insight into the imaging process of a such a device [1]. I will also explain the basics of scattering-type near-field optical microscopy (s-SNOM) and present various examples of unambiguous nanoscale material characterization from various areas such as semiconductor analysis, materials science, chemistry, and biology [2-4]. In these examples, the use of infrared spectroscopy allows to sense molecular vibrations as well as collective excitation of lattice vibrations (``phonons'') in polar crystals [5]. Currently, the main limitation of this technique comprises of the low signals that demand tunable laser sources and restrict the spectral range of operation. Consequently, I will introduce new concepts for increasing the sensitivity of infrared near-field spectroscopy to ultimately allow for a broadband operation. \\[4pt] [1] T. Taubner, D. Korobkin, Y. Urzhumov, G. Shvets, R. Hillenbrand, \textit{Science} \textbf{313}, 1595 (2006). \\[0pt] [2] T. Taubner, R. Hillenbrand, F. Keilmann, \textit{Applied Physics Letters} \textbf{85}, 5064 (2004). \\[0pt] [3] A. Huber, D. Kazantsev, F. Keilmann, J. Wittborn, R. Hillenbrand, \textit{Advanced Materials} \textbf{19}, 2209 (2007). \\[0pt] [4] M. Brehm, T. Taubner, R. Hillenbrand, F. Keilmann, \textit{Nano Letters} \textbf{6}, 1307 (2006). \\[0pt] [5] R. Hillenbrand, T. Taubner, F. Keilmann, \textit{Nature} \textbf{418}, 159 (2002).