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Fourier plane imaging microscopy

8 September 2014
The image generated by a microscope's objective lens can provide different information from the image that ultimately emerges from the eyepiece—with some surprising results.

These days the compound microscope is nearly as ubiquitous in physics and materials science labs as in biology and medical venues. In its simplest form, the instrument uses two-stage magnification—once with an objective lens close to the sample and once again with the eyepiece. The resulting image is formed at the real focal plane where we typically place our eye or a camera; its resolution is determined by the well-known Rayleigh diffraction limit, though various tricks can be employed to improve the resolution somewhat. The intermediate image, appearing within the microscope's barrel at the so-called rear objective focal plane or Fourier plane, is rarely considered to have its own merits. But Texas Tech graduate student Daniel Dominguez, his adviser Luis Grave de Peralta, and their colleagues decided to take a closer look by inserting a second camera to image the Fourier plane. They fabricated photonic crystals (PCs) with nicely periodic holes or pillars as test samples for their microscope setup, whose expected resolution is about 440 nm. At the instrument’s eyepiece, hole spacings of 500 nm and 450 nm in the PCs were clearly resolved, but smaller spacings were not. A very different image appeared at the Fourier plane, however, with some diffraction information clearly visible (top image, for a 250-nm PC). By carefully extrapolating the arcs into full diffraction rings, the physicists could reconstruct a real-plane image with the correct periodicity (bottom). The technique seems robust and can work for nonperiodic samples, although the origins of the Fourier-plane details are still under investigation. (D. Dominguez et al., J. Appl. Phys., in press.)

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