Thermal radiation scanning tunnelling microscopy

In standard near-field scanning optical microscopy (NSOM), a subwavelength probe acts as an optical 'stethoscope' to map the near field produced at the sample surface by external illumination(1). This technique has been applied using visible(1,2), infrared(3), terahertz(4) and gigahertz(5,6) radiation to illuminate the sample, providing a resolution well beyond the diffraction limit. NSOM is well suited to study surface waves such as surface plasmons(7) or surface-phonon polaritons(8). Using an aperture NSOM with visible laser illumination, a near-field interference pattern around a corral structure has been observed(9), whose features were similar to the scanning tunnelling microscope image of the electronic waves in a quantum corral(10). Here we describe an infrared NSOM that operates without any external illumination: it is a near-field analogue of a night-vision camera, making use of the thermal infrared evanescent fields emitted by the surface, and behaves as an optical scanning tunnelling microscope(11,12). We therefore term this instrument a 'thermal radiation scanning tunnelling microscope' (TRSTM). We show the first TRSTM images of thermally excited surface plasmons, and demonstrate spatial coherence effects in near-field thermal emission.