A marriage of microscopy and image compression

Instead of looking at a surface pixel by pixel, a new microscope takes a more global view to achieve improved resolution. The detailed chemical structure of a surface can reveal a wealth of information about such processes as catalysis, corrosion, and wetting. One technique for getting at that surface structure is sum frequency generation (SFG), in which a pair of photons simultaneously strike a spot on the surface, interact with the material there, and convert to a single photon. Conventional SFG microscopes use a raster-scan; that is, they image the surface one pixel at a time. That approach has an inherent limitation: As the microscope samples smaller pixels to improve resolution, the intensity associated with each pixel drops, eventually becoming too small to measure. Now Steven Baldelli of the University of Houston, Kevin Kelly of Rice University, and colleagues have combined the mathematics of image compression with SFG to create a microscope that acquires less data than its conventional counterpart but generates an image with improved resolution. In the key step, the light emitted from the entire sample is focused onto a collection of tiny mirrors arranged in a random-looking pattern such as in the figure. Crucially, the light emanating from a particular pixel on the surface strikes a corresponding element of the mirror arrangement. The total intensity of the light reflected from the mirrors is then measured; it represents a type of dot product between the sample and the mirror state. It turns out that a relatively small number of such dot products suffices for faithful reproduction of the sample. Baldelli and colleagues used their proof-of-principle microscope to image 100-µm-wide gold stripes deposited onto silicon with a resolution of about 10 µm, significantly better than would be obtained in raster-scan mode. (X. Cai et al., J. Chem. Phys. 135, 194202, 2011.)