Gas accidents — such as toxic gas leakage in factories, carbon monoxide leakage of boilers, or toxic gas suffocation during manhole cleaning — continue to claim lives and cause injuries. Developing a sensor that can quickly detect toxic gases or biochemicals is still an important issue in public health, environmental monitoring, and military sectors. A research team has developed an inexpensive, ultra-compact, wearable hologram sensor that immediately notifies the user of volatile gas detection.
The researchers integrated a metasurface with a gas-reactive liquid crystal optical modulator to develop a sensor that provides an immediate visual holographic alarm when harmful gases are detected. Conventional gas sensing devices are not widely used due to their high manufacturing cost. 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, the team utilized the metasurface — an optical device known to have an “invisible cloak” effect by making visible objects disappear by controlling the refractive index of light. The metasurface is especially used to transmit two-way holograms or 3D video images by freely controlling light.
Using the metasurface, the team developed a gas sensor that can float a holographic image alarm in space in just a few seconds by using the polarization 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. Moreover, 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 leukemia. The sensor was confirmed to detect even the minute amount of about 200 ppm of gas. In an actual experiment using a board marker — a volatile gas source in our daily life — a visual holographic alarm popped up instantaneously the moment the marker was brought to the sensor.
A one-step nanocomposite printing method was developed to produce the flexible, 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 team is 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 is 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. It would be especially effective in more extreme work environments where acoustic and visual noise are intense.