Miniature near-infrared sensor fits in a smartphone

Miniature near-infrared sensor fits in a smartphone

Technology News |
By Nick Flaherty

Researchers at the Technical University of Eindhoven have developed a near-infrared sensor that is easy to make, comparable in size to sensors in smartphones, and could be used industrial process monitoring and agriculture.

The technology is being spun out into a startup call MantiSpectra.

The sensor has 16 pixels that are sensitive in the near infra red with integrated filters and makes direct use of the signals from the pixels rather than reconstructing an image.

“Measuring the spectrum in the infrared is most interesting for applications in industry and agriculture, but there’s one major issue – current near-infrared spectrometers are just too big and expensive,” said Kaylee Hakkel, PhD researcher in the Photonics and Semiconductor Nanophysics group at the department of Applied Physics and co-first author of the study.

“Miniaturization of the sensors while keeping costs low was a major challenge. So, we designed a new wafer-scale fabrication process to achieve this. It’s low-cost because we can produce multiple sensors at the same time, and it’s ready, right now, for use in practical applications in the real world. The sensor chip is small and could even be embedded in future smartphones.”

“We’ve been investigating this technology for a number of years. And now we’ve successfully integrated the spectral sensors on a chip, while also dealing with another key issue – efficient use of the data,” said Andrea Fiore, research lead from the Department of Applied Physics and the Eindhoven Hendrik Casimir Institute.

The NIR sensor is based on an array of resonant-cavity-enhanced (RCE) photodetectors operating in the 850–1700 nm wavelength. Each pixel of the array contains a thin absorbing layer positioned inside a Fabry–Perot (FP) cavity, resulting in a strong spectral dependence of the quantum efficiency. This spectral response is controlled for the pixels individually by changing the length of each FP cavity via a tuning element inside the cavity. This provides direct integration of the filters and the detectors in a single robust device, eliminating alignment errors and the need for micromechanical tuning.

Each pixel has an active area of 150 μm × 750 μm and varying height depending n the filters. The active layers consist of a p-i-n InP photodiode with a 200 nm InGaAsabsorber layer. A SiO2 spacer layer is positioned in between the bottom Ag mirror and the active layers in order to improve the absorption in the InGaAs layer. The tuning layer is made of a dielectric layer with a 10-nm thick semi-transparent Au mirror on top. Additional metal layers employed for the n- and p-contact of the photodiode are positioned outside the cavityregion on top of two corresponding InGaAs contact layers. Two100-nm thick InP barriers separate these contact layers from theInGaAs absorber layer.

The optical response is then evaluated using finite difference time domain (FDTD) simulations.

Normally, when a sensor measures light, the generated signal is used to reconstruct the optical spectrum – or optical fingerprint – for the material. Sensing algorithms are then used to analyze the data.

In this new approach, the researchers show that the step of spectral reconstruction isn’t needed. In other words, the signals generated by the sensors can be sent straight to the analysis algorithms. “This significantly simplifies the design requirements for the device,” said Fiore.

The researchers have tested the sensor with milk and with various types of plastic. “We used the sensor to measure the nutritional properties of many materials including milk. Our sensor provided comparable accuracy in the prediction of fat content in milk as conventional spectrometers. And then we used the sensor to classify different types of plastic,” said Maurangelo Petruzzella, who is also working MantiSpectra.

“Besides these applications, we anticipate that the sensor could be used for personalized health care, precision agriculture (monitoring the ripeness of fruit and vegetable for instance), process control, and lab-on-chip testing. We now have a full development kit available based on this technology, the SpectraPod, that companies and research institutes are using to build their applications. And the great thing is that this sensor could even be commonplace in the smartphones of the future meaning that people could use it at home to check the quality of their food or check aspects of their health,” said Petruzzella.

“I’m really excited to start working on the next phase of the sensor development with MantiSpectra. This sensor could contribute to a cleaner environment and address food waste, applications that are important for everyone,” said Hakkel.

Paper: Integrated near-infrared spectral sensing;

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