New ‘microlens’ could lead to ultra-powerful satellite cameras and night-vision devices
By Darren Quick
May 20, 2010
Anyone who likes to get their gear off for a spot of naked sunbathing in the backyard may have to think twice in the future. Researchers have developed a new nanotechnology-based “microlens” that could lead to a new generation of ultra-powerful satellite cameras and night-vision devices. Thankfully, the new lens is used for infrared imaging, so the technology is more likely to be used for security and monitoring climate change and deforestation than spying on naturists boosting their vitamin D levels.
Developed by researchers at Rensselaer Polytechnic Institute, the lens uses gold to boost the strength of infrared imaging. By leveraging the unique properties of nanoscale gold to “squeeze” light into tiny holes in the surface of the device, the researchers have doubled the detection cabability of a quantum dot-based infrared detector. With some refinements, the researchers expect this new technology should be able to enhance this by up to 20 times.
According to project leader Shawn-Yu Lin, professor of physics at Rensselaer, the breakthrough is the first in more than a decade to demonstrate success in enhancing the signal of an infrared detector without also increasing the noise.
“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,” said Lin.
“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. This kind of behavior has never been seen before,” he said.
The detectivity of an infrared photodetector is determined by how much signal it receives, divided by the noise it receives. 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 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.
The surface plasmon QDIPs are long, flat structures with countless 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 1 micron deep. The holes are filled with quantum dots, which are nanoscale crystals with unique optical and semiconductor properties.
The interesting properties of the QDIP’s gold surface help to focus incoming light directly into the microscale holes and effectively concentrate that light 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.
According to Lin 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’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 and use gold to boost the QDIP’s detectivity, by both widening the diameter of the surface holes and 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.”
Results of the study, titled “A Surface Plasmon Enhanced Infrared Photodetector Based on InAs Quantum Dots,” appear in the journal Nano Letters. The study was funded by the U.S. Air Force Office of Scientific Research.