The Sight of Sound

      Although scientists often want to look at very tiny things, it is theoretically impossible to see something smaller than the wavelength of the radiation used to image it. In order to observe even smaller objects, one could either use a shorter wavelength, for instance a beam of X-rays instead of light, or play a number of tricks to get around this limit. The same principles that restrict the ability to resolve details in optical imaging also hold true for methods that use soundwaves like ultrasound. Jie Zhu, a postdoctoral fellow in Professor Xiang Zhang’s lab at UC Berkeley, and colleagues recently engineered a device capable of imaging objects smaller than the wavelength of sound used to create the acoustic image. The device has very small, square holes drilled through a block that act as perfect lenses for transmitting sound waves. When waves with the proper wavelength hit an object on one side of the block, information contained in standing waves that only exist very close to the object is faithfully transferred through the device. A microphone placed close to the output of the “holey” material can detect features up to 50 times smaller than the wavelength emitted by the source, seven times better than the best resolution previously available. This technology could spawn the next generation of probes for medical sonography and the non-destructive evaluation of materials.

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