A wearable hologram sensor is able to instantly notify its user of the presence of volatile gases. The wearable device overcomes issues of high expense associated with current gas sensing technology, addressing the needs of workers in hazardous environments, such as petrochemical plants. The sensor was developed through a joint research effort at Pohang University of Science and Technology (POSTECH) led by Junsuk Rho, a professor in the departments of mechanical and chemical engineering, and Young-Ki Kim of the department of chemical engineering. It integrates a metasurface with a gas-reactive liquid crystal optical modulator to provide an immediate visual holographic alarm when harmful gases are detected. The metasurface the team used is hydrogenated amorphous silicon, known for its potential in future cloaking applications due to its control over the refractive index of light. The metasurface was chosen for its ability to transmit two-way holograms or 3D video images by freely controlling light. Left, a depiction of the sensor’s composition, layers, and readouts. Top right image shows the nanopillar structure of the metasurface. Middle right shows the device as a sticker implemented on a pair of safety glasses. Bottom right shows the sensor’s readouts: a smiley face for normal conditions and a caution symbol for elevated gas levels. Courtesy of POSTECH. When exposed to gas, molecules within the device’s liquid crystal layer change orientation, triggering the metasurface’s polarization control of light, which raises the holographic image alarm in a matter of seconds. The target gas the researchers used was isopropyl alcohol. During experimentation with a whiteboard marker, the alarm was raised the moment the marker was brought to the sensor, the researchers reported. The sensor detected gas concentrations as low as approximately 200 ppm. The device also required no support from external mechanical or electronic devices and was able to be fabricated through a one-step nanocomposite printing process. The metasurface structure, initially processed on a hard glass substrate, was designed to enable rapid production with a single-step nanocasting process on a curved or flexible structure, in this case polyethylene terephthalate (PET). In application, the sensor can be attached like a sticker onto safety glasses and display the alarm before the user’s eyes. The researchers expect that the device will be capable of integration with glasses-type AR display systems, such as those under development at Apple, Samsung, Google, and Facebook. Current efforts from the research team aim to create a high-performance environmental sensor that is able to display the type and concentration level of gases or biochemicals in the surrounding area with a holographic alarm. The team is also looking at optical design techniques that are able to encode various holographic images. “This newly developed ultracompact wearable gas sensor provides a more intuitive holographic visual alarm than the conventional auditory or simple light alarms,” Rho said. “It is anticipated to be especially effective in more extreme work environments where acoustic and visual noise are intense.” The research was published in Science Advances (www.doi.org/10.1126/sciadv.abe9943).