Investigations of plasmonic devices and thermal emission at sub-λ scale using NSOM microscopy

Abstract

Infrared near-field scanning optical microscopy (NSOM) has been used to investigate surface plasmon polaritons (SPPs) which are generated on an integrated device by means of a semi-conductor laser. Devices which realize a slit-doublet experiment with SPPs have been investigated. Using NSOM imaging, we demonstrate in situ generation of SPPs with quantum cascade lasers at λ=7.5 µm and with laser diodes at λ=1.3 µm. Using thermal radiation scanning optical microscopy (TRSTM), we measure the infrared thermal emission of a sample, which is scattered in the near-field by means of a tip. We have recorded images and Fourier transform infrared (FTIR) spectra of the near-field thermal emission of materials which support surface polaritons. We show that the TRSTM probes both the spatial and frequency dependence of the electromagnetic local density of states (EM-LDOS). The modes of SPPs confined on metallic patterns are observed with TRSTM imaging. At the surface of materials supporting surface phonon polaritons, the measured near-field thermal emission spectra are quasi-monochromatic, in striking contrast with blackbody-like far-field thermal emission. TRSTM combined with FTIR spectroscopy is a promising method for infrared spectroscopy with nanoscale resolution, i.e. well beyond the diffraction limit.