A NANOWIRE suspended in an invisible 鈥渇orce field鈥� of laser radiation could soon be peering into biological samples, taking low-cost snapshots of viruses and proteins with unprecedented resolution.
The microscope works by scanning the superfine tip of the nanowire over an object, all the while emitting a beam of laser light from the end of the wire. Whatever light makes it through the object is captured by an optical sensor behind it to build up a picture of the object鈥檚 structure, says Peidong Yang, who built the device with his colleagues at the University of California, Berkeley.
The team鈥檚 prototype can already scan objects with a resolution of 100 nanometres, allowing it to capture the features the size of those on a silicon chip. The researchers believe they should be able to refine the device so that it will ultimately have a resolution of tens of nanometres.
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The nanowire itself is free-floating, held in place only by an 鈥渙ptical trap鈥� of infrared laser light, so it has no physical connection to the rest of the device. Building such a trap is tricky, and the team was only able to make it work by immersing the whole contraption in water. In air, intermolecular forces known as van der Waals forces pull the wires onto nearby surfaces, says Peter Pauzauskie, a chemical engineer on the team. 鈥淭he van der Waals forces are simply too strong. They overwhelm the experiment,鈥� he says.
鈥淭he team was only able to make it work by immersing the whole thing in water鈥�
To create nanowires that emit laser light, the team grew them from single crystals of potassium niobate. This is a so-called non-linear material that can combine two infrared photons to create a single, higher-frequency green photon. So in Yang鈥檚 microscope, the infrared laser both traps the nanowire and powers the generation of the visible laser photons (Nature, vol 44, p 1098).
What鈥檚 more, a single infrared laser can be used to create an array of hundreds of optical traps, with each holding its own nanowire. Such an array could quickly scan relatively large biological molecules.
Bill Brocklesby, an expert on optical microscopy at the University of Southampton, UK, cautions that the technique will need further development, but says he is excited by the work. 鈥淚t should open up a new area of imaging,鈥� he says.
