Interferometric optical microscopy of subwavelength grooves

The amplitude and phase response of a high numerical aperture interferometric microscope to subwavelength grooves are investigated both experimentally and theoretically. It is well known that for narrow and deep structures scalar diffraction theory is no longer valid and a rigorous vector diffraction model is required. Conical diffraction results presented demonstrate significant differences in measurements taken in different polarization states. Significant light coupling occurs when the polarization state of light at the back focal plane of the microscope is aligned perpendicular to the groove (TE) whereas relatively poor coupling occurs when the polarization is aligned along the groove (TM). The stronger coupling of TE incident light in the groove means that there is much greater contrast compared to TM. Under certain circumstances an inversion of the phase occurs in TE, which is intuitively explained in terms of interference between the top and bottom of the groove. The greater coupling that occurs in TE enables the depth of narrow grooves to be measured more accurately and over a wider range of depths than in TM.