AN OPTICAL memory chip that senses, stores and then displays the light it has received has sent a buzz of excitement round the imaging world. 鈥淲e鈥檝e never seen anything like this before,鈥� says Trevor Whittley, an expert in optical devices at the University of Edinburgh in the UK.
Such a device could delay light signals in optical-fibre networks, or allow the police to compare mug shots 1000 times faster than with today鈥檚 computers.
The chip works in a similar way to the charge-coupled device (CCD) image sensors in digital cameras. A CCD turns light falling on its surface into an array of electrical charges, which in a camera are shunted off into a memory card for storage. The difference with the new photon storage device (PSD) is that it holds onto the charges, and then converts them back into light.
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The idea for a chip that senses, stores and displays an image came from J枚rg Kotthaus of the Centre for Nanoscience at the Ludwig Maximilian University in Munich, Germany. Now he has made it a reality alongside colleagues from the University of California at Santa Barbara, the Technical University in Munich and the University of Augsburg.
Like a CCD, the PSD is made from semiconducting gallium arsenide with an array of electrodes on the surface that define the pixels. When a photon crashes into a pixel it creates a positively charged hole and a negatively charged electron, which are dragged apart into separate storage areas by an electric field (see Graphic). The amount of charge stored for each pixel depends on the number of photons hitting it, which in turn is proportional to the brightness of the image at that point.
All that is needed to retrieve the picture from the PSD is to switch off the electric field. This allows the hole and electron to reunite, shooting out excess energy in the form of photons.
The team has demonstrated the device by focusing a picture of Kotthaus鈥檚 face onto the chip for a few microseconds, then releasing it a few microseconds later and photographing the image that flashed out (Applied Physics Letters, vol 85, p 5830). The image was preserved perfectly, Kotthaus told 快猫短视频.
Kotthaus has spent five years getting the idea to work but he is not planning to patent it. This is because there is still a major hurdle standing in the way of turning the PSD into a practical device. In its current form, it only works if it is kept very cold. Above -2oo 掳C, the temperature of liquid nitrogen, the electrons and holes have so much energy the electric field can鈥檛 contain them. So they leak out through the electrodes and the image can鈥檛 be recovered.
New semiconductor materials might retain the image at higher temperatures, but finding them is likely to take time, Kotthaus warns. 鈥淚t鈥檚 not a device that is going to make me a lot of money in five years, and I鈥檓 too old to wait for 20 years.鈥�
