Tuesday, 28 December 2010

Antennas or imaging array?

Via last week's New Scientist magazine I came across an interesting announcement from Idaho National Laboratory, which is part of the US Department of Energy. Researchers there, at Microcontinuum in Cambridge (Mass) and Patrick Pinhero of the University of Missouri have been working on an alternative way of gathering solar energy using minute antennas rather than solar cells.

They have produced an array of what they call nanoantennas which are tuned to infrared radiation and, as antennas do, resonate and pass the energy into electrical circuits. So far they have a six-inch circular 'stamp' that can emboss a plastic substrate which can then be coated with metal. The six-inch stamp can produce an array of ten million antennas. So far they are only able to produce arrays small enough to match infrared wavelengths but they plan to go finer and so reach visible wavelengths. One problem is how to handle the energy from the resonating antennas, since this is at light-like frequencies (a few hundred teraHerz) and rather beyond your average radio set!

Why is this of interest to us? Let's follow this through logically and refer to that six-inch stamp as a ten megapixel array and you see what I'm thinking. Could a tuned array of nanoantennas be an alternative way of making an image sensor? Ironically it is easier to make such an array for far infrared wavelengths than for visible light; so this might be a way of producing an inexpensive thermal imaging sensor. The down side is that (I assume) the energy arrays collect energy in serial or parallel and don't give access to individual 'pixel' antennas. For thermal wavelengths there is also the problem of ambient radiation: this is something they wish to capture for a solar array but it would contaminate an image. However, hopefully the former is a question of nano-engineering and the latter is already a problem with some existing kinds of thermal camera so we know how to address it (by cooling the sensor).

Using antennas for imaging has some advantages. To filter wavelengths you simply adjust the size of the antenna to match whichever wavelength you wish to receive. It might even be possible to do this dynamically. The antennas may be inherently polarised as well, which would have interesting applications. They might even be more efficient in harvesting the light energy since the researchers estimate the efficiency as being close to 80% and perhaps you could even make miniature Yagis for more gain. And the cost? "As cheap as inexpensive carpet" says the report. The INL report is actually from 2008 although New Scientist reported some more recent developments.

For over 150 years we have relied on energetic photons changing chemical or electronic structures for our imaging: the idea of tuning in to the energy directly is an interesting alternative. Perhaps the biggest barrier to use as an imaging sensor is scale. Don't forget that ten megapixel array was six inches across. It'd be ironic if a large format view camera was needed to use it.

[Later: It was pointed out to me that the plural of antenna is antennas if it's radio and antennae if it's insects. YLSED!]

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