Gold 'Microlens' Could Boost Satellite Vision

Researchers have developed a new nanotechnology-based "microlens" that uses gold to boost the strength of infrared imaging and could lead to a new generation of ultra-powerful satellite cameras and night-vision devices.

The power of an infrared detector was doubled by using nanoscale gold to funnel light into tiny holes in the surface of the device. With some refinements, the researchers expect this technology to enhance infrared detection by up to 20 times.

Project leader Shawn-Yu Lin, a professor of physics at Rensselaer Polytechnic Institute, said his team enhanced the signal of an infrared detector without also increasing the noise, or the unwanted signals from other light sources.

"Infrared detection is a big priority right now as more effective infrared satellite imaging technology holds the potential to benefit everything from homeland security to monitoring climate change and deforestation," Lin said.

Gold and quantum dots

At the heart of the device are so-called quantum dots, which are tiny crystals with unique optical and semiconductor properties. When struck by incoming infrared light, quantum dots produce an electrical signal.

"We have shown that you can use nanoscopic gold to focus the light entering an infrared detector, which in turn enhances the absorption of photons and also enhances the capacity of the embedded quantum dots to convert those photons into electrons," Lin said. "This kind of behavior has never been seen before."

For an infrared detector to be effective, it must receive more signal than noise.

The current state-of-the art in photodetectors is based on mercury-cadmium-telluride (MCT) technology, which has a strong signal but faces several challenges including the need for long exposure times for low-signal imaging.

Lin said his new study creates a roadmap for developing quantum dot infrared photodetectors (QDIP) that can outperform MCTs, and bridge the innovation gap that has stunted the progress of infrared technology over the past decade.

Hole-y light

The QDIPs are long, flat structures with millions of tiny holes on the surface. The solid surface of the structure that Lin built is covered with about 50 nanometers – or 50 billionths of a meter – of gold. Each hole is about 1.6 microns – or 1.6 millionths of a meter – in diameter, and one micron deep.

The QDIP’s gold surface focuses incoming light directly into the tiny holes and effectively concentrates it in the pool of quantum dots. This concentration strengthens the interaction between the trapped light and the quantum dots, and in turn strengthens the dots' ability to convert those photons into electrons.

The end result is that Lin's device creates an electric field up to 400 percent stronger than the raw energy that enters the QDIP.

The effect is similar to what would result from covering each tiny hole on the QDIP with a lens, but without the extra weight, and minus the hassle and cost of installing and calibrating millions of microscopic lenses, Lin said.

Lin's team also demonstrated that the nanoscale layer of gold on the QDIP does not add any noise or negatively impact the device’s response time.

Lin plans to continue honing this new technology by both widening the diameter of the surface holes and using more effective placement of the quantum dots.

"I think that, within a few years, we will be able to create a gold-based QDIP device with a 20-fold enhancement in signal from what we have today," Lin said. "It's a very reasonable goal, and could open up a whole new range of applications from better night-vision goggles for soldiers to more accurate medical imaging devices."

The research appeared recently in the journal Nano Letters.

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