Two-beam interferometry for characterizing subwavelength layers with a negative optical path length.

In this work, we show that unequal-arm two-beam interferometers, for example, single-pass Mach–Zehnder interferometer or double-pass Michelson interferometer, illuminated with coherent point source (spherical wave front) or coherent line source (cylindrical wave front), unambiguously determine the sign of the phase shift. Theoretical consideration shows that a pattern with finite diameters of interference fringes is asymmetric with respect to the sign of the optical path difference (OPD) or phase shift (PS). In a short arm of the interferometer, the light passing through the layer of medium, characterized by positive OPD (positive PS), leads to the decrease of the diameter of interference fringe. The layer of the medium with negative OPD increases the diameter of inference fringe. When placing an object in the long arm of the interferometer, the contrary situation is observed. For variants with a coherent linear source, the above results relate to the distances between strait fringes of the same order. Experiment demonstrates that the positive optical path length of a conventional material can be compensated in air by a layer of metamaterial with a negative optical path length located at a distance significantly longer than the wavelength of light. [ABSTRACT FROM AUTHOR]