Hydrogen: a non-toxic gas that stores three times the energy of oil, yet releases only water when burnt, making it a promising option for greener future vehicles. However, hydrogen’s highly flammable nature can pose more safety challenges than fossil fuels, and its lack of colour or smell can make gas leaks difficult to detect.
A trick of the light stands to change that. In a recent joint work, researchers from the A*STAR Institute of Materials Research and Engineering (A*STAR IMRE) and Nanyang Technological University, Singapore, demonstrated a rare optical phenomenon known as a dual-phase singularity using a scalable nanophotonic cavity. Based on this concept, they proposed developing a compact, low-cost and highly sensitive hydrogen sensor.
“In optics, a phase singularity is a point on a wave of light where its phase becomes undefined, typically when the light’s intensity is at its minimum,” explained KV Sreekanth and Jinghua Teng, A*STAR IMRE Senior Scientists. “This point is exceptionally sensitive to external disturbances, making it a powerful tool for precise sensing.”
As a phase singularity reacts to even slight changes in the light-bending ability (refractive index) of its surrounding medium, it can potentially detect even minute traces of hydrogen gas. However, researchers have found it challenging to devise a practical sensor based on this effect.
“Most optical structures are sensitive to the polarisation of light, so it’s difficult to achieve phase singularity in both forms of polarised light (s- and p-) with one light frequency and incident angle,” said Sreekanth. “As such, only single-phase singularities have been achieved so far with light sources from high and impractical incident angles.”
To solve the issue, Sreekanth, Teng and colleagues looked to Tamm cavities: special nanostructures that resonate with s- and p-polarised light across a wide range of incident angles. They proposed that Tamm resonance could achieve a novel dual-phase singularity using just one light frequency at a lower incident angle than other methods.
Using thin-film deposition techniques, the team fabricated tuneable Tamm cavities from alternating layers of silicon dioxide and antimony trisulfide. These cavities successfully induced dual-phase singularities at much lower incident angles than single-phase singularity devices.
Going further, the team simulated their Tamm cavity design in a prototype hydrogen sensor. They found that it was able to detect extremely small refractive index changes from hydrogen exposure at five times the sensitivity of single-phase singularity devices. The dual-phase singularities essentially acted as alarms tuned to two different signal wavelengths, covering a wider bandwidth than a single alarm would.
“The dual-phase singularity concept can enable low-cost, ultrasensitive sensors,” Teng said. “Our metal-free tuneable Tamm cavities have significant potential for thin film and flat optics development, especially for multispectral light modulators and filters.”
With the help of machine learning, the team plans to explore alternative Tamm cavity designs that produce multiple-phase singularities across different bands of the light spectrum, including ultraviolet, visible and mid-infrared light.
The A*STAR-affiliated researchers contributing to this study are from the A*STAR Institute of Materials Research and Engineering (A*STAR IMRE).
