Holographic sensor designed to detect harmful gases

Researchers at Pohang University of Science and Technology (POSTECH) have developed a wearable hologram sensor by integrating a metasurface with a gas-reactive liquid crystal optical modulator. Described in the journal Science Advances, the sensor provides an immediate visual holographic alarm when harmful gases are detected.

For those working in hazardous environments such as petrochemical plants, gas sensors are essential. However, conventional gas sensing devices are not widely used due to their high cost of being made with complex machines and electronic devices. In addition, commercial gas sensors have limitations in that they are difficult to use and have poor portability and reaction speed.

To solve these issues, POSTECH researchers utilised an optical device known as a metasurface, which has an ‘invisibility cloak’ effect in that it makes visible objects disappear by controlling the refractive index of light. Metasurfaces can be used to transmit two-way holograms or 3D video images by freely controlling light.

Using the metasurface, the research team developed a gas sensor that can float a holographic image alarm in space in just a few seconds by using the polarisation control of transmitted light that transforms due to the change in orientation of liquid crystal molecules in the liquid crystal layer inside the sensor device when exposed to gas. The gas sensor requires no support from external mechanical or electronic devices, unlike other conventional commercial gas sensors. The researchers used isopropyl alcohol as the target hazardous gas, known as a toxic substance that can cause stomach pain, headache, dizziness and even leukaemia.

The sensor was confirmed to detect even a minute amount of gas at about 200 ppm. In an actual experiment using a board marker, a volatile gas source in everyday life, a visual holographic alarm popped up instantaneously the moment the marker was brought to the sensor.

The research team also developed a one-step nanocomposite printing method to produce the flexible and wearable gas sensor. The metasurface structure, which was previously processed on a hard substrate, was designed to enable rapid production with a single-step nanocasting process on a curved or flexible substrate. When the flexible sensor fabricated using this method attaches like a sticker on safety glasses, it can detect gas and display a hologram alarm. It is anticipated to be integrable with glass-type AR display systems under development at Apple, Samsung, Google and Facebook.

The researchers are now developing a high-performance environmental sensor that can display the type and concentration level of gases or biochemicals in the surroundings with a holographic alarm, and are studying optical design techniques that can encode various holographic images. If these studies are successful, they can be used to reduce accidents caused by biochemical or gas leaks.

“This newly developed, ultracompact wearable gas sensor provides a more intuitive holographic visual alarm than the conventional auditory or simple light alarms,” said research co-leader Professor Junsuk Rho. “It is anticipated to be especially effective in more extreme work environments where acoustic and visual noise are intense.”

Image credit: ©stock.adobe.com/au/Vectorman2017

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