February 2022 Issue
Research Highlights
Gas sensing
Spectroscopic gas detection
Small gas sensors are used in many settings for detecting specific gases — for example, recognizing explosive or smelly gases — or for analysing the composition of atmospheric gas. Most gas-sensing methods require a separate sensor for every type of gas. Yet, in light of on-going efforts to miniaturize such devices, a single detector that can sense different gases is highly desirable. One approach towards ‘unified’ sensing relies on spectroscopy: by analysing the absorption frequency spectrum of light passing through a gas mixture, the components of the mixture can in principle be identified. Now, Tetsuo Kan from the University of Electro-Communications and colleagues report the successful application of spectroscopic sensing in a miniaturized setup. The principle behind the sensor is so-called reconstructive spectroscopy, in which a spectrum is not directly measured, but reconstructed by converting other measured quantities. The scientists’ spectrometer reaches a wavelength resolution of 20 nm, and, by way of test, was able to detect ethanol gas.
The difficulty with miniaturizing a normal spectroscopic device lies in the need for a long optical path length in order to disperse light into its different wavelengths. Reconstructive spectrometers overcome this issue by letting light interact with a photonic material — a structure with dispersive properties. The photonic material is located close to the optical detector, enabling compact construction. Kan and colleagues developed a spectrometer with a gold grating as the photonic material. Upon irradiation with light (having passed through the gas mixture being examined), electrons in the grating start to oscillate, a process called surface plasmon resonance. The grating is made to rotate, and because of the irradiation-induced surface plasmon resonance, an electrical current develops, which can be measured and converted into a frequency spectrum. (The current depends on both the angle of incidence and the wavelength.)
For the light source, the researchers used near-infrared light — electromagnetic radiation with wavelengths in the range 800 nm – 2500 nm. First, they tested how their device performed spectroscopically; that is, how well it could reconstruct spectra, and with what resolution. The measurements showed that wavelength peaks spaced 20 nm apart could still be clearly distinguished.
Then, the scientists examined how well their reconstructive spectrometer can detect a gas. They choose to perform a test with ethanol gas, which is known to absorb near-infrared light with a wavelength of 1392 nm. The drop in transmittance at that wavelength was clearly seen in the reconstructed spectrum.
Kan and colleagues conclude that their reconstructive spectrometer has the capacity for gas measurement. Although there is room for further improvement — for example, in the current experiments, a combination of single-wavelength sources was used, whereas a continuous-wavelength source is preferable — the researchers are confident that the “advancement of the proposed method will result in the development of a new micro-sized integrated spectrometer that will provide rich information on our environment”.
Schematic of the working principle of a reconstructive spectrometer for gas sensing.
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