37 - Scanning optical microscopy

Summary

The diffraction-limited focusing of a laser beam to either explore or modify a surface is the basis of several important technologies. Examples include scanning optical microscopy, optical disk data storage, and laser printing. The size of the focused spot and the corresponding depth of focus are important factors in determining the performance characteristics of these systems. In this chapter we examine methods of forming the focused spot, and clarify the relation between spot size and depth of focus.

Principle of operation

The essential features of a scanning optical microscope are shown in Figure 37.1. A laser beam is sent through an objective lens to form a focused spot on the sample. Ideally, the objective is corrected for all aberrations, yielding a diffraction-limited focused spot. The light reflected from the sample returns through the objective and is redirected by the beam-splitter to a detection module. The detection module may be designed to monitor the power, the phase, or the polarization state of the returning beam. The electrical signal S(x, y) produced by the detector is thus representative of the small area of the sample illuminated by the focused spot at and around the point (x, y). The sample is moved to different locations by the XY stage on which it is mounted; the signal S(x, y), plotted against the sample's position, yields an image of the sample's surface over the desired area.