A partnership between Duke University and SRICO has created a faster, cheaper solution for gas detection using an intriguing arrangement of metamaterials
While some of the industry’s most impressive breakthroughs are the result of larger equipment, and bigger scope, it will be the smallest sensors that drive the next revolution. Having seen major advances in sensors for seismic surveys and oil analysis, gas detection is proving to be one of the most interesting fields for the technology.
Most recently, materials scientists and engineers at Duke University and optoelectronic materials company SRICO have helped develop a sensor that is fast, sensitive and efficient enough to detect specific wavelengths of electromagnetic energy while on the move – an innovation that could improve how methane or natural gas leaks are detected.
Central to the technology are metamaterials. These are carefully arranged structures of materials, usually metals and plastics, which exhibit properties not found in the base materials themselves. The exact shape, size and alignment of these patterned structures can determine these properties, and allow them to interact with electromagnetic waves in unnatural ways.
In this case, patterns of metal are interlaid with extremely thin slices of perfect crystals, allowing the tiny device to detect invisible infrared signatures emitted by various kinds of gases, plastics and other sources. “The benefit of using metamaterials is that different components required in a detector can be combined into one feature,” said Willie Padilla, professor of electrical and computer engineering at Duke. “That simplification gains you a lot of efficiency.”
That has allowed Padilla and his team to build a prototype detector that is smaller, lighter, more powerful and apparently cheaper than the current alternative. Their results were presented in the Optica journal in February 2017.
A good conductor In a typical thermal detector, infrared light waves are absorbed and converted into heat by a black substance, essentially soot. That heat is conducted to a separate component that creates an electrical signal which can be read. However, that process takes time, meaning measurements can be slow, and only by overlaying filters or a complex system of moving mirrors can the sensor be set up to identify specific wavelengths.
The metamaterial sensor avoids these issues. Each section of the detector consists of a pattern of gold sitting on top of lithium niobate crystal. This crystal is pyroelectric, generating an electrical charge when it is heated. SRICO is able to slice these lithium crystals to thicknesses of 600 nanometres using a focused ion beam (FIB), a technique common in the semiconductor and materials industry. This reduces the potential for defects in the crystalline structure – meaning the sensor is more accurate and less prone to background noise – and means the slices are thinner, allowing the crystal to heat up more quickly and producing a more sensitive detector.
The resulting crystal layer is so thin that light would normally pass through it without being absorbed. However, by adjusting the pattern in the top layer of gold, the properties of the crystal are altered, causing the pixel to absorb only a specific range of electromagnetic frequencies. This removes the need for the separate filters required by other detectors.
When the crystal heats up and generates an electric charge, the gold is also used as an electrical conductor, carrying the signal to the detector’s amplifier and eliminating the need for separate electrical leads (which add bulk and weight).
The device can be made to identify “any specific range of electromagnetic frequencies” by altering the pattern and details of these gold layers – the inference being that the system could be tuned to specific gases or other substances.
“These designs allow this technology to be 10 to 100 times faster than existing detectors because the heat is created directly by the crystal” said Jon Suen, a postdoctoral associate in Padilla’s laboratory. “This lets us create devices with fewer pixels and also presents the ability to sweep the detector across an area or capture images in motion.”
That gives the device an advantage over existing technologies. Its fast detection time would allow it to scan an area quickly while looking for methane or natural gas leaks – either in a hand-held form or potentially mounted onto an aircraft or unmanned aerial vehicle (UAV). InnovOil would also imagine there So far, SRICO has created a single-pixel prototype as a proof of concept, but it is now working to find funding from industry investors or grant to scale up and commercialise the technology.
The final form(s) of the sensor are unclear, but Padilla notes: “You could even make this into a low-cost lab instrument for spectroscopy for medical samples… I’m not sure what the eventual price point would be, but it’d be a lot less than the US$300,000 instrument we currently have in our laboratory.”