Detailing their work in the Journal of the Optical Society of America A, the researchers said their lens would be a hit among microelectronics and medical device manufacturers, who currently use very complex machinery to view the tiny components that they assemble.
Lead researcher Allen Yi said though they milled their prototype thermoplastic lens on a precision cutting machine, the same lens could be manufactured less expensively through traditional molding techniques.
"Ultimately, we hope to help manufacturers reduce the number and sizes of equipment they need to miniaturise products," Yi said. The prototype lens, which is about the size of a fingernail, looks at first glance like a gem cut for a ring, with a flat top surrounded by eight facets. But while gemstones are cut for symmetry, this lens is not symmetric. The sizes and angles of the facets vary in minute ways that are hard to see with the naked eye.
"No matter which direction you look at this lens, you see a different shape," Yi explained. Such a lens is called a "freeform lens," a type of freeform optics. Freeform optics have been in use for more than a decade. But the researchers were able to write a computer programme to design a freeform lens capable of imaging microscopic objects.
Then they used a commercially available milling tool with a diamond blade to cut the shape from a piece of the common thermoplastic material polymethyl methacrylate, a transparent plastic that is sometimes called acrylic glass. The machine shaved bits of plastic from the lens of 10 nanometres, or 10 billionths of a meter, which is about 5,000 times smaller than the diameter of a human hair.
The final lens resembled a rhinestone, with a faceted top and a wide, flat bottom. They installed the lens on a microscope with a camera looking down through the faceted side, and centred tiny objects beneath the flat side. Each facet captured an image of the objects from a different angle, which can be combined on a computer into a 3-D image.
The engineers successfully recorded 3-D images of the tip of a ballpoint pen and a mini drill bit with a diameter of 0.2 millimetres. "Using our lens is basically like putting several microscopes into one microscope," said Li. "For us, the most attractive part of this project is we will be able to see the real shape of micro-samples instead of just a two-dimensional projection."