"Artist’s conception of the metamaterial array – the power absorption in the lithium niobate pyroelectric element is shown" (credit: Kebin Fan)Engineers from Duke University in the US in collaboration with SRICO have developed a fast and sensitive pyroelectric mobile sensor that can detect specific wavelengths of electromagnetic energy. Using gold-plated crystals, the technology could provide a low-cost alternative to current infrared sensors that scan for methane or natural gas leaks, as well as monitoring the health of crops or even sorting through plastics for recycling.
The multi-functional prototype detector, which was reported in Optica [Suen et al. Optica (2017) DOI: 10.1364/OPTICA.4.000276], depends on metamaterials, engineered structures that comprise designed repeating cells that interact with electromagnetic waves in various ways. With metamaterials, the components required for a detector can be combined into one feature, providing much-needed efficiency; here, in combining patterns of metal with very thin slices of perfect crystals, the team was able to create a device that detects invisible infrared signatures emitted by a range of gasses, plastics and other sources.
In most thermal detectors, infrared light waves are absorbed and converted into heat, which is conducted to a separate component, creating an electrical signal that can then be read out. Such a process brings speed limitations, so it is only by overlaying filters or using a complicated system of moving mirrors that enable specific wavelengths to be singled out. However, every part of this new detector consists of a pattern of gold located on top of lithium niobate crystal – as the crystal is pyroelectric, when it gets hot it creates an electrical charge. The device was designed to detect any particular range of electromagnetic frequencies just by redesigning the details of the gold pattern.
“We found that pyroelectric detectors are ideal since their optimization requires the coordination of a number of factors including cost, optical absorption, heat generation and transport, and electrical readout.”Jonathan Suen
This crystal would usually be so thin that light would travel through without being absorbed. Here, they designed the top layer of gold into a pattern that works along with the crystal’s properties to result in the pixel absorbing only a specific range of electromagnetic frequencies, which eliminates the need for separate filters. When the crystal heats up and generates an electric charge, the gold transports the signal to the detector's amplifier; as the heat is created directly by the crystal, fewer pixels are necessary.
As researcher Jonathan Suen said “We found that pyroelectric detectors are ideal since their optimization requires the coordination of a number of factors including cost, optical absorption, heat generation and transport, and electrical readout.” The team has already developed a single-pixel prototype as proof of concept, research that could lead to new multi-functional metamaterial designs that combine optical properties with other physical phenomena, and also plan to improve further the sensitivity and speed of the detector.