Short-sighted microscopes to get superlens specs

 作者:习瘩阀     |      日期:2019-03-02 09:13:00
By Colin Barras (Image: Stefan Mendach) Rolls of a new material could be used to upgrade conventional light microscopes, allowing them to image objects usually only visible to electron microscopes. If it can be made to perform as predicted, the device has much to offer biology. It could for the first time make it possible to view viruses and small objects inside cells, without having to kill the organisms and set them solid to survive the vacuum of an electron microscope. The very best light microscopes have a magnification limit at around half the wavelength of light. Finer detail is lost as light bounces, interferes and diffracts off the object, limiting resolution to around 0.2 micrometres – enough to view some of the smallest bacteria, but not to see the internal components of cells. In fact, the finer detail is recorded. But only in so-called “evanescent” waves that decay rapidly as they move away from an object. Effectively, they only exist within a few nanometres of a surface, a zone dubbed the “near field”. In 2007 two US research teams made superlenses that can collect these evanescent waves, by directly touching the object being imaged. The superlenses convert them into normal light waves that can propagate over large distances and be collected by a light microscope’s lens. But one of those devices worked only with ultraviolet light, and the other in only one dimension over a small region of the visible spectrum. Now Stefan Mendach‘s team at the University of Hamburg in Germany has designed a superlens that could produce 2D images of 3D nanoscale objects across the whole of the visible, and some of the infrared, spectrum. Their superlens consists of thin strips of silver interleaved with a semiconductor made from silicon, gallium, indium and arsenic. A mismatch in the atomic structure of the two layer materials makes the structure roll itself up like a carpet, leaving a hollow tube some 2 micrometres wide at its centre. If an object is placed inside that tube, the evanescent waves near its surface are collected by the lens’ inner surface and travel through the layers. Because light can only travel perpendicularly through the rolled lens, when evanescent waves arrive at the outer surface they appear bigger, in much the same way that a pattern printed on a balloon appears larger as the balloon is inflated. That process magnifies nanoscale features, which are too small to be detected using a standard lens, to a point where they can be seen by a light microscope positioned above the superlens (see image, right). “This is a nice paper which demonstrates another way of making a [superlens],” says Igor Smolyaninov, who designed a superlens in 2007 that works using infrared light, and also made the world’s first true invisibility cloak. Making a superlens that can operate over a wide range of the visible spectrum is important, Smolyaninov says. But he adds that Mendach’s team need to make the walls of the rolled tube thicker if they are to achieve significant magnification. At the moment, their prototype works but does not function as a superlens, because its walls are not thick enough to magnify an image enough. Mendach says the team is now trying to thicken up the walls – paradoxically, by thinning down the silver and semiconductor layers in the device. That will help the device to roll up more tightly, creating a tube with thicker walls, he says. Journal reference: