NEAR-FIELD OPTICAL-DETECTION OF ASPERITIES IN DIELECTRIC SURFACES

A finite-difference time-domain code is used to model. near-zone elect romagnetic probe fields of subwavelength dimensions and the interactio ns of these fields with a dielectric sample. The magnitude and the pha se of the electric and the magnetic fields are determined in the regio n in which the energy leaving the probe interacts with the sample. An angular-spectrum code is then used to propagate the electric field int o the far zone, in which signal detection takes place. TE and TM polar izations in a two-dimensional waveguide are modeled. We examine the ef fects of scanning the probe over a surface asperity in a dielectric sa mple. Two different far-zone detection schemes (total-energy detection and differential detection with a split-cell detector) are studied. W hen the probe scans a well, total-energy detection by TE polarization yields the closest estimate of the well's actual width, whereas differ ential 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 m inimal distortions with both TE and TM polarizations.