How Did Eric Betzig See Single Molecules?

Eric Betzig says one of the two experiments he did at Bell, which led to the idea for the Nobel Prize, made him the first person to see single molecules at room temperature. He localized their positions to about a 50th of the wavelength of light.

He did the work with near field microscopy. He knew the energy distribution inside the near field aperture, fit a profile to the light, and placed the molecule much better than the width of the aperture, the same way the center of a Gaussian can be found much better than the width of a Gaussian. Two facts proved a single molecule. The molecule bleaches all at once: it is there and the next instant it is gone. Fluorescent molecules are dipoles, so the dipole orientation mapped the electric fields in the subwavelength aperture to almost one nanometer precision as he drove along the molecule. The map matched a theory Hans Bethe developed in the 40 for diffraction through a subwavelength hole. Using Bethe's theory, he could determine the position of the molecules and the orientation of every dipole.