Optical impedance matching with scanning near-eld optical microscopy (SNOM)

We present an experimental study of the coupling of light from a waveguide into a submicron size optical transmission line. We demonstrate that the coupling e-ciency depends upon the polarization direction in the waveguide. On the condition when a quasi TEM mode is launched in the transmission line we observe a sub-stantial enhancement of the coupling eciency. We demonstrate that the coupling eciency also depends on the mismatch of the impedances of waveguide and trans-mission line, that is in turn determined by their respective geometries. We have made a prediction of the condition of the maximum coupling eciency through the approach of impedance matching. We believe these results can be of importance in the development of new concepts of probes for scanning near-eld optical microscopy. A clear distinction is made between transmission lines and waveguides in microwave theory [1]. Transmission lines consist of two or more conductors separated by a dielectric. They support TEM or quasi-TEM modes, without any cut-o frequency. Waveguides, on the other hand, are generally represented by a metal tube of circular