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John O | April 2017

Researchers use new metamaterial to create improved infrared sensors


engineers at duke university have used metamaterials composed of gold sitting atop a layer of lithium niobate crystal to create a thermal sensor that is fast, sensitive, and efficient enough to detect specific wavelengths of electromagnetic energy while on the move, which opens it up for many possible applications including scanning for gas leaks, monitoring fields of crops or sorting plastics for recycling.

 



a layer-by-layer look at the new metamaterial infrared detector, with simulated views
as to its temperature distribution (top right), electric field (bottom left), and
how it absorbs power (bottom right). (duke university)

 

according to a report on the duke website, the engineers built a prototype sensor with optoelectronic materials company srico that is better than current competitors in weight, size, power, speed, and cost.

 

typical thermal detectors absorb infrared light waves and convert them into heat by a black substance. the heat moves into a separate component that creates the electric signal for a read out of the heat signature. obviously, this process can lag and requires a complex system of moving mirrors to single out the required wavelength.

 

the article explained, “each tiny section of the detector consists of a pattern of gold sitting on top of lithium niobate crystal. this crystal is pyroelectric, meaning that when it gets hot, it creates an electrical charge. like shaving a piece of cheese off a block, engineers at srico use an ion beam to peel a slice of crystal just 600 nanometers thick.

 

“this technique eliminates potential defects in the crystalline structure, which reduces background noise. it also creates a thinner slice than other approaches, allowing the crystal to heat up more quickly.”

 

the gold is engineered into a pattern that combines with the crystal to absorb only a specific range of electromagnetic frequencies and the gold conducts the electric signal to the amplifier. this removes the need for separate filters or electrical leads.

 

as the article noted, “the researchers can fabricate the device to detect any specific range of electromagnetic frequencies simply by redesigning the details of the gold pattern.”

 

the work was published in optica. the abstract read:

 

“pyroelectric materials enable the construction of high-performance yet low-cost and uncooled detectors throughout the infrared spectrum. these devices have been used as broadband sensors and, when combined with an interferometric element or filter, can provide spectral selectivity.

 

“here we propose the concept of and demonstrate a new architecture that uses a multifunctional metamaterial absorber to directly absorb the incident longwave ir (8–12 μm) energy in a thin-film lithium niobate layer and also to function as the contacts for the two-terminal detector.

 

“our device achieves a narrowband (560 nm fwhm at 10.73 μm), yet highly efficient (86%) absorption. the metamaterial creates high field concentration, reducing temperature fluctuation noise, and lowering device capacitance and loss tangent noise.

 

“the metamaterial design paradigm applied to detectors thus results in a very fast planar device with a thermal time constant of 28.9 ms with a room temperature detectivity, ?∗d*, of 107  cm w/hz−−−√107  cm w/hz.”

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