Gold-covered "Microlens" Could be Breakthrough in Infrared Imaging

Researchers at Rensselaer Polytechnic University have developed a lensless, gold-covered "microlens" that they believe will lead to breakthroughs in image-signal and infrared-imaging strength. The research uses the properties of nanoscale gold to "squeeze" light into tiny holes in the surface of the device.

The study demonstrates 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. "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 enhances the capacity of the embedded quantum dots to convert those photons into electrons," he says.

Researchers establish the detection ability of an infrared photodetector by determining how much signal it receives and dividing that signal by the noise the detector receives. Photodetectors currently employ MCT (mercury-cadmium-telluride) technology, which has a strong signal but long exposure or low-signal imaging. The study creates a plan for developing QDIPs (quantum-dot infrared photodetectors) that can outperform MCTs.

The long, flat surface plasmon QDIPs have countless holes, measuring 1.6 microns in diameter and 1 micron deep, on the surface. Approximately 50 nm of gold covers the solid surface of the structure. Quantum dots-nanoscale crystals with unique optical and semiconductor properties-fill each hole. 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 the device creates an electric field as much as 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 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 neither adds noise nor affects the device's response time. "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 says. "It's a reasonable goal and could open a new range of applications, from better night-vision goggles for soldiers to more accurate medical-imaging devices."

Rensselaer Polytechnic Institute
Reference
Chang, Chun-Chieh, Yagya D Sharma, Yong-Sung Kim, Jim A Bur, Rajeev V Shenoi, San-jay Krishna, Danhong Huang, and Shawn-Yu Lin, "A Surface Plasmon Enhanced Infrared Photodetector Based on InAs [Indium-Arsenic] Quantum Dots," Nano Letters, April 20, 2010, pg 1704.

Caption
Rensselaer Polytechnic Institute Professor Shawn-Yu Lin has developed a new nanotechnology-based “microlens”.