Object characterization with a differential heterodyne microscope

One-dimensional image formation theory is presented for a scanning differential heterodyne microscope based on a balanced scheme for the Mach-Zehnder interferometer with a Bragg cell as the beam splitter. The interferometer allows coherent detection of a complex signal in the Fourier plane. The scattering problem of illuminating beams focused on the object was solved with rigorous diffraction theory for a perfectly conducting groove-corrugated grating using the modal method. The responses from step-like micro-objects have been calculated for both TE and TM (transverse electric and magnetic) polarizations for a range of heights. The experimental amplitude and phase responses from thick dielectric steps on silicon substrates (with and without evaporation of a thin gold film) are presented for various intervals between illuminating spots.