Electron microscopes with nanometer resolution are widely used, but they cost millions of dollars, and preparing a sample for viewing is painstaking. This state of affairs is fine for the lab but impractical for industrial applications—say, for rapidly scanning product samples to look for embedded microscopic watermarks.
A new form of holographic microscopy developed by David Grier, a physicist at New York University, and his colleagues could provide a solution. They started with an off-the-shelf Zeiss microscope and replaced its incandescent light source with a laser. The laser shines on a sample of the material under study; light scatters off the sample and creates a two-dimensional pattern of interference between the laser beam and the scattered light—a hologram—which a video camera records.
Scientists have been making holograms of microscopic objects for decades, but it has always been hard to extract useful information from them. This is where Grier's invention adds value. His team wrote software capable of quickly solving the equations that describe how light scatters off a spherical object; by finding the values of certain terms buried in those equations, the software gathers information about the object that is causing that scattering. The microscope's nanometer resolution will allow researchers to track particles floating in colloidal solutions (for example, nanoscale beads floating in a sample of paint) using equipment that is at least a tenth of the cost of an electron microscope.
Grier hopes his device will provide the first rapid, affordable way to glimpse the individual particles at the heart of modern products. Imagine a paint bucket or shampoo bottle in which every drop contains particles that have been encoded with the product's manufacturing history—how it was made, in what factory and when, “sort of like a fingerprint,” Grier says. He adds that the microscope could just as easily read a molecular message stamped into medicine, explosives or other goods.