A team at the University of Twente in Enschede, the Netherlands, has developed a technique that may aid the design and analysis of photonic structures. Reporting in the July 10 issue of Physical Review Letters, the researchers describe how they used a heterodyne interferometric photon scanning tunneling microscope to observe the phase singularities in an optical waveguide predicted by theory. The researchers split a laser beam into two components: a reference and a beam that they injected into the 4-nm-high, 3-µm-wide Si3N4 channel. They scanned a fiber in a raster pattern 20 nm over the structure to probe the evanescent field at the waveguide-air interface by photon tunneling. By combining the reference and the tunneled signals in a 50/50 fiber coupler, they determined the amplitude and phase of the local optical field, and identified the phase singularities caused by the interference of transverse electric and magnetic modes.