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Holographic metasurface gas sensors for instantaneous visual alarms

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Science Advances  07 Apr 2021:
Vol. 7, no. 15, eabe9943
DOI: 10.1126/sciadv.abe9943
  • Fig. 1 Design of gas-responsive liquid crystal (LC) cells and their optical responses.

    (A) Schematic of the proposed holographic metasurface gas sensor platform. A holographic metasurface integrated with gas-responsive LCs projects a safety signal (smiley face) in the absence of a target dangerous gas, while an alarm signal (exclamation mark) is displayed upon the detection of the gas. Right circularly polarized (RCP; yellow arrow) illumination creates a “safe signal,” and left circularly polarized (LCP; green arrow) illumination produces an “alarm signal.” (B) Schematic illustration (side view) of gas-responsive LCs that are hosted in a microwell. Initially, the LC cell has a hybrid anchoring configuration because of the vertical orientation of the LCs at the air interface and the unidirectional tangential orientation set by the rubbed polyimide coated onto a glass substrate. When volatile gases are introduced, however, the LC ordering is lowered because the isotropic gas molecules partition into the LC layer. Consequently, the nematic-to-isotropic phase transition occurs from the air interface and the isotropic layer expands as more gas molecules are diffused into the LCs. (C to E) Sequential optical micrographs (top) of the LC cell upon the exposure of IPA gas; see movie S1. Scale bar, 100 μm. The insets in (C) to (E) show the corresponding side view micrographs. The LC cell is placed in a closed chamber with a concentration of IPA gas of around 200 ppm. White arrows represent the polarization of the polarizer (input) and analyzer (output). Blue arrows represent the rubbing direction. (F) Measured retardation and calculated isotropic layer thickness over time. Data corresponding to (C) to (E) are marked by the blue, green, and red dots.

  • Fig. 2 Numerical optimization of asymmetric coupled metasurfaces.

    (A) Elements of the proposed metasurface consisting of a-Si:H nanoantennas showing the electric and magnetic field intensity distributions for the nanoantennas with their long axis parallel to the x axis (left element) and y axis (right element), under linearly polarized incidence. Height h and displacement d are fixed at 400 and 300 nm, respectively. (B) Efficiency (TLR) of the transmitted LCP component under RCP incidence as a function of length (L) and width (w) of the nanoantennas. Red dots indicate the geometries of four selected unit cells that have high diffraction efficiency while taking fabrication resolution into account. (C) Full-phase coverage and wavefront modulation using the selected set of eight unit cells. (D) Calculated holograms for safe (smiley face, left) and alarm states (exclamation mark, right) obtained from the designed asymmetric coupled metasurface.

  • Fig. 3 Demonstration of an LC-MS gas sensor.

    (A) Optical setup for an LC-MS gas sensor (HWP, half wave plate; M1, mirror 1; M2, mirror 2; P, polarizer; QWP, quarter wave plate). In the absence of IPA gas, the RCP light illuminated on the LC-MS sensor passes the LC layer without any polarization conversion and is transmitted into the metasurface. In contrast, the LC layer converts the incoming RCP into LCP light upon the exposure of IPA gas. (B) Photographs of an LC-MS gas sensor with a board marker as a source of volatile gases including IPA. Scale bar, 3 mm. Photo credit: Inki Kim, POSTECH. (C) Optical and SEM images of the integrated dielectric metasurface. Scale bar, 100 μm. (D) Resulting holographic image alarms. Upon the exposure of gases from the board marker, the LC-MS sensor rapidly exhibits the alarm sign within a few seconds and recovers the initial safety sign once the gases are removed. See movie S3.

  • Fig. 4 Demonstration of a flexible LC-MS gas sensor and an integrated safety device.

    (A) Schematic illustration of a one-step nanocasting fabrication process of a flexible metasurface. The master stamp fabricated with 1 μm-height a-Si:H metasurface is chemically treated to reduce the adhesive strength for easier demolding process. The detached polymer mold is reusable. (B) SEM image (top view) of the silicon master stamp for the nanocasting process. The inset shows a tilted view image. (C) Photograph of the resulting flexible metasurface. (D) Corresponding SEM image (top view) of the NP-resin composite (NPC) metasurface. The inset shows a tilted view image. Photo credit: Inki Kim, POSTECH. (E to G) Flexible and conformal holographic metasurface gas sensor. The complete sensor, consisting of a flexible LC cell and an NPC metasurface, is attached onto the curved surface of safety goggles. Similar to the characterization of the a-Si:H metaholograms (Fig. 3), 532 nm wavelength RCP light is illuminated onto the flexible gas sensor to display holographic images. As seen in Fig. 5 (E and F), the LC cell and NPC metasurface are combined well. Photo credit: Inki Kim, POSTECH. (H and I) Experimentally demonstrated holographic safety signal in a normal condition and alarm signal upon an exposure of IPA gas. Compared to the a-Si:H device, the NPC metasurface not only has smaller critical dimensions and a larger height, meaning a higher aspect ratio, but also has some defects during the imprinting process. Thus, the diffraction efficiency and clearness of the holographic images are degraded.

  • Fig. 5 Broadband responses of TiO2 NPC and a-Si:H metasurfaces.

    The holographic images are produced by illuminating samples with a supercontinuum laser source. As shown, the NPC metasurface exhibits a higher contrast image below 532 nm and the a-Si:H metasurface produces a higher contrast image above 600 nm. The image contrast or diffraction efficiency is related to the material refractive index and the optimized metasurface geometric parameters.

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