SNOM measures spatial phase distribution

Researchers at the University of Bath (Bath, England) have built a scanning near-field optical microscope (SNOM) that measures not only the intensity distribution of an optical field, but its phase distribution, as well. Such information is needed for a complete characterization of an optical field. The microscope contains a Mach-Zehnder interferometer in which the measurement signal is combined with a reference signal via a fiberoptic 50:50 coupler. The resulting signals from the two output ports of the coupler are sent to two detectors. During scanning, the sum of the intensities measured by the two detectors gives the near-field intensity plus a constant, while the difference in intensities gives phase information.

The SNOM probe is a chemically etched single-mode fiberoptic tip with an apex diameter of 50 nm, providing a resolution of 110 nm in the evanescent regime. Near-field phase was accurately measured near a prism surface for a wave traveling parallel to the prism face. In addition, phase and intensity profiles were taken across the face of an optical fiber supporting the LP₁₁ mode, which has two lobes of opposite phase. Measurements of this type will aid in the development of waveguides, photonic crystals, and lasers. Contact Pepe Phillips at [email protected].