Coherent Diffraction Method for Imaging Dusty Plasma Water Ice Grains*

Water ice grains spontaneously form in the Caltech ice dusty plasma experiment [1] which involves cryogenic (~190 K) neutrals, water vapor, and weak (~10^-6) ionization. The ice grains have been imaged until now using a long-distance microscope lens—with a 3μm resolution limit—mounted on a camera. Reference [1] showed that ice grain size and ellipticity could be ascertained from diffraction patterns of a HeNe laser beam. Re-visiting diffraction methods, we aim to use the diffraction pattern to obtain a complete detailed image of the ice grains. This pattern on a screen is the absolute value of a 2D Fourier transform (FT) of the ice grain shape so phase information is lost. We will use the Fienup phase restoration method [2] where iterative numerical guesses restore the missing phase using the physical constraint that the image intensity is non-negative. By taking absolute values of FT's, we have demonstrated this method on synthetic diffraction patterns, first removing then recovering phase. Theoretical considerations indicate that this diffraction/Fienup restoration method should improve on the microscope lens resolution by an order of magnitude.

1. K. B. Chai and P. M. Bellan, Geophys. Res. Lett. 40, 6258 (2013) 

2. J. R. Fienup. Opt. Lett. 3, 27 (1978)