Researchers claim gold-covered 'microlens' could be breakthrough for infrared imaging

Suzanne Deffree -- EDN, May 19, 2010

Researchers from Rensselaer Polytechnic Institute have developed lens-less, gold-covered "microlens" that they believe will lead to breakthroughs in image signal and infrared imaging strengths, without increasing noise.

The nanotechnology-based microlens use gold to boost the strength of infrared imaging. Leveraging the properties of nanoscale gold to "squeeze" light into tiny holes in the surface of the device, the researchers claim to have doubled the detectivity of a quantum dot-based infrared detector. Within years, the researchers expect this new technology will be able to enhance detectivity by up to 20 times, allowing for a new generation of ultra-powerful satellite cameras and night-vision devices.

The study is the first in more than a decade to demonstrate success in enhancing the signal of an infrared detector without also increasing the noise, according to project leader Shawn-Yu Lin, professor of physics at Rensselaer and a member of the university's Future Chips Constellation and Smart Lighting Engineering Research Center. Lin in 2008 was credited with creating the world's darkest material, as well as a coating for solar panels that absorbs 99.9% of light from nearly all angles.

"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.

As the research describes, 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 challenges including long exposure times for low-signal imaging. Lin claimed the study creates a roadmap for developing quantum dot infrared photodetectors (QDIP) that can outperform MCTs.

The surface plasmon QDIPs are long, flat structures with countless tiny holes on the surface. According to Rensselaer, the solid surface of the structure that Lin built is covered with about 50 nanometers of gold. Each hole is about 1.6 microns in diameter and 1-micron deep, and is filled with quantum dots (nanoscale crystals with unique optical and semiconductor properties).

Rensselaer said properties of the QDIP's gold surface help to focus incoming light directly into the microscale holes and concentrate that light in the pool of quantum dots. That 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% stronger than the raw energy that enters the QDIP, according to Rensselaer.

The research uses the term "microlens," as 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 the hassle and cost of installing and calibrating millions of microscopic lenses.

Rensselaer said that 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.

"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," were published online by the journal Nano Letters. The United States Air Force Office of Scientific Research funded this study, available here.

Co-authors of the paper are Rensselaer Senior Research Scientist James Bur, graduate student Chun-Chieh Chang, and Research Associate Yong-Sung Kim; Yagya D. Sharma, Rajeev V Shenoi, and Sanjay Krishna of the Center for High Technology Materials at the University of New Mexico, Albuquerque; and Danhong Huang of the Space Vehicles Directorate at the Air Force Research Laboratory, Kirtland Air Force Base.