Hierarchically manufactured chiral plasmonic nanostructures with gigantic chirality for polarized emission and information encryption

Abstract

Chiral metamaterials have received significant attention due to their strong chiroptical interactions with electromagnetic waves of incident light. However, the fabrication of large-area, hierarchically manufactured chiral plasmonic structures with high dissymmetry factors (g-factors) over a wide spectral range remains the key barrier to practical applications. Here we report a facile yet efficient method to fabricate hierarchical chiral nanostructures over a large area (>11.7 × 11.7 cm²) and with high g-factors (up to 0.07 in the visible region) by imparting extrinsic chirality to nanostructured polymer substrates through the simple exertion of mechanical force. We also demonstrate the application of our approach in the polarized emission of quantum dots and information encryption, including chiral quick response codes and anti-counterfeiting. This study thus paves the way for the rational design and fabrication of large-area chiral nanostructures and for their application in quantum communications and security-enhanced optical communications.

Fig. 1. Origin of extrinsic chiral characteristic and structure characterization of the hierarchical chiral plasmonic patterns.

a, b Schematic images of the a chiral plasmonic structures under light illumination. b Enlarged image showing the incident oblique angles (θ, ϕ) for the irradiated light. c, d Schematic images of hierarchical c LH- and d RH-chiral plasmonic patterns with corresponding OM images (scale bar of OM images: 50 μm). e Enlarged OM image of the LH-chiral plasmonic patterns, showing the central (cobalt), lower left (olive), and upper right (orange) parts of the protruding area of the micropattern (scale bar: 20 μm). f SEM images of the central part (cobalt area in Fig. 1e) of the protruding area of the micropattern and an enlarged image (scale bar: 10 μm and 1 μm, respectively) g, h SEM images of the g lower left (green area in Fig. 1e), and the h upper right (amber colored area in Fig. 1e) parts of the protruding area of the micropattern (scale bar: 2 μm). i Schematic images and j enlarged images of original (blue), vertical (red), and diagonal (green) grating patterns for LH patterns. k Schematic images of original grating patterns (left), additionally formed vertical nanopatterns (middle), and diagonal nanopatterns (right) for LH-chiral plasmonic patterns with θ and ϕ for the incident light (pink arrow: incident direction of light).

Fig. 2. Schematic images and optical characteristics of plasmonic chiral nanostructures based on nanograting-patterned and flat substrates.

a, b Schematic images with corresponding OM images of a LH- and RH-chiral plasmonic patterns (based on PDMS films) and b inverted LH- and RH-chiral plasmonic patterns (based on PS films) with nanogratings (scale bars in the inset photograph images: 5 mm). c Schematic illustration of DRCD measurement system d Optical absorption spectra of chiral plasmonic patterns. e CD and f g-factor spectra for the chiral plasmonic patterns based on nanograting-patterned and flat PDMS substrates. g CD and h g-factor spectra for the inverted LH- and RH-patterned chiral plasmonic structures based on nanograting patterned and flat PS substrates.

Fig. 3. Theoretical calculation of the optical characteristics and electric field distributions of the plasmonic chiral nanostructures.

a Obliquity angle dependence of absorbance for the diagonal nanograting under (i) L-CPL and (ii) R-CPL at 550 nm wavelength. b Obliquity angle dependence of CD spectra for the diagonal nanograting at different obliquity angles: (i) θ = 20°, (ii) ϕ = 45°, and (iii) ϕ = 135°. c Macroscopic mapping of obliquity angles (i) θ(x,y) and (ii) ϕ(x,y) and (iii) their distribution histograms for the diagonal nanograting of LH- (top panels) and RH-patterned (bottom panels) plasmonic structures. d Calculated CD spectra of the chiral plasmonic structures for diagonal and vertical patterns. e Electric field intensity distributions for the diagonal nanogratings under CPL illumination at 550 nm; (i, ii) ϕ = 45° and (iii, iv) ϕ = 135°; (i, iii) L-CPL and (ii, iv) R-CPL; θ = 30°.

Fig. 4. Analyses of the PL intensity and changes in emission color of QDs according to the CPL rotation direction of the excitation light sources.

a Schematic image of QD spray coating on the chiral plasmonic patterns. b Schematic images of R-QD (left), G-QD (middle), and B-QD (right) coated LH-patterned chiral plasmonic films. c–e Polarization-sensitive PL spectra of c R-, d G-, and e B-QDs on the LH-patterned chiral plasmonic structures. f, g Polarization-sensitive PL spectra and schematic images of f GR- and g GB-QDs on the LH-patterned chiral plasmonic structures. h CIE 1931 coordinates the emitted light of the GR- and GB-QDs-based system according to CPL rotation direction.

Fig. 5. Optical properties of the large-area (11.7 × 11.7 cm²) chiral plasmonic films and small-area (1.3 × 1.3 cm²) chiral-patterned array for QR code application.

a Photograph of the large-area inverted LH-chiral plasmonic film (scale bar: 2 cm). b CD and c g-factor spectra of the 36 pieces cut from a large area of the inverted LH- and RH- chiral plasmonic film samples. Lines and the shaded areas are mean ± SD for 36 pieces. d Maximum g-factor value distribution of the 36 pieces cut from a large area inverted LH- and RH-chiral plasmonic film samples. e Comparison of the reported chiral plasmonic patterns with those in our work with respect to the g-factor and pattern area. Related references are provided in the Supplementary references 1–36. Related references are provided in the Supplementary Table 1 (NA: not available). f The chiral QR code (top) on a $1 bill: the thickness of the chiral QR code was measured using a micrometer (bottom left, scale bar: 2 cm); the chiral QR code floating in water is shown at the (bottom right, scale bar: 2 cm). g Schematic image, and h g-factor mapping results of the 54-μm-thick small-area chiral plasmonic arrays patterned in the shape of the letters “C,” “H,” “P,” and “T” for use in anti-counterfeiting. i, j Schematic diagrams illustrating the potential applications of our technique in i anti-counterfeiting and j as next-generation chiral QD codes.