NEAR-FIELD OPTICAL-DETECTION OF ASPERITIES IN DIELECTRIC SURFACES

A finite-difference time-domain code is used to model. near-zone electromagnetic probe fields of subwavelength dimensions and the interactions of these fields with a dielectric sample. The magnitude and the phase of the electric and the magnetic fields are determined in the region in which the energy leaving the probe interacts with the sample. An angular-spectrum code is then used to propagate the electric field into the far zone, in which signal detection takes place. TE and TM polarizations in a two-dimensional waveguide are modeled. We examine the effects of scanning the probe over a surface asperity in a dielectric sample. Two different far-zone detection schemes (total-energy detection and differential detection with a split-cell detector) are studied. When the probe scans a well, total-energy detection by TE polarization yields the closest estimate of the well's actual width, whereas differential detection by TM polarization yields the sharpest profile of the well's edges. Differential detection is shown to be less sensitive to variations in the probe-to-sample separations during a scan and has minimal distortions with both TE and TM polarizations.