Bismuth nanogratings with narrow plasmon resonances for dynamic polarized color generation and colorimetric sensing

Abstract: Bismuth nanostructures are appealing for sustainable color generation and sunlight harvesting, thanks to their non-toxicity and their tunable visible-to-near infrared interband plasmon resonances. However, owing to their broad and polarization-insensitive spectral features, the nanostructures reported so far displayed a limited color tunability and were unsuitable for other applications such as sensing. Herein, we report bismuth nanogratings with polarization-sensitive and narrow plasmon resonances (Q > 10). They were fabricated on the $cm^2$ scale following a lithography-free approach: conformal pulsed laser deposition of bismuth onto the reflective and transparent nanostructured layers of DVDs. We characterized their specular reflectance for different orientations of the plane of incidence, angles of incidence, and polarizations of light. When light is polarized in the plane perpendicular to the lines, plasmon resonances occur and shift across the visible-to-near infrared upon changing the angle of incidence. In contrast, no such resonances occur when light is polarized in the plane parallel to the lines. This results in well-contrasted, polarization-sensitive colors, which are iridescent for the former orientation of polarization, and not for the latter. Resonances strongly shift upon changing the refractive index of the surrounding medium (> 500 nm/RIU), resulting in a marked change in the plasmonic color, e.g., from green in air to red in water. This showcases the potential of these nanogratings for dynamic polarized color generation and colorimetric sensing.

• The paper presents a scalable, lithography-free method to fabricate bismuth nanogratings exhibiting narrow (FWHM ~50 nm) plasmon resonances with high polarization selectivity.
• It shows that differing substrate configurations yield mirror-responses in reflectance, enabling vivid, tunable, and iridescent color effects across visible to NIR wavelengths.
• The nanogratings demonstrate high refractive index sensitivity (exceeding 500 nm/RIU), paving the way for cost-effective, real-time colorimetric sensing applications.

Bismuth Nanogratings with Narrow Plasmon Resonances: Polarization-Sensitive Color Generation and Colorimetric Sensing

Introduction

This study demonstrates the fabrication, characterization, and application of bismuth (Bi) nanogratings exhibiting narrow, polarization-sensitive plasmonic resonances in the visible to near-infrared (NIR) spectrum. Leveraging the unique dielectric properties of Bi and a scalable, lithography-free deposition method using recycled DVD-R templates, the authors realize optically thick Bi nanogratings on cm² scales. The work provides comprehensive experimental and computational evidence for the emergence of high-quality factor ($Q > 10$) plasmon resonances, strong polarization-selective color effects, and significant sensitivity to the surrounding refractive index, positioning these nanostructures as candidates for dynamic color generation and highly sensitive colorimetric sensing.

Methodological Approach

Two distinct Bi nanogratings were fabricated:

• Bi/Ag/PC: Bi deposited on the Ag-coated polycarbonate (PC) nanograting from DVD structures
• Bi/PC: Bi deposited on the bare PC nanograting

A pulsed laser deposition approach enables conformal Bi coverage, directly replicating the underlying periodic nanostructure (period $\approx700$ nm, height $\approx70$ nm) without lithography. Optical characterization employs ellipsometry and integrating sphere spectrophotometry, with FDTD simulations implemented for theoretical support.

The polarization of incident light is systematically varied relative to the nanograting direction (E_⊥: perpendicular, E_∥: parallel), and the impact of incident angle and orientation of the incidence plane is evaluated.

Key Findings

Optical Resonance and Polarization-Selective Plasmonics

Both types of Bi nanogratings display narrow plasmonic resonances (FWHM $\approx$ 50 nm, $Q\approx15$ at 745 nm), but only under perpendicular polarization (E_⊥). No resonance occurs under parallel polarization, affirming robust polarization selectivity. The resonance wavelength strongly depends on both incident angle and plane orientation, undergoing significant blue/red shifts associated with the grating geometry.

Differential Spectral Behavior

The Bi/Ag/PC nanograting exhibits plasmon resonances as minima in specular reflectance; in contrast, Bi/PC shows maxima at corresponding wavelengths. This "mirrored" response is attributed to differences in substrate topography and Bi crystallography, as well as the relative magnitudes of specular versus diffuse scattering in each system. Notably, Bi/PC yields lower broadband reflectance and higher diffuse reflectance, leading to more saturated and vivid color responses.

Iridescence and Color Modulation

The chromatic response, quantified via CIE-1931 coordinates, reveals highly iridescent, polarization-sensitive colors in both Bi/PC and Bi/Ag/PC, most pronounced in Bi/PC due to its specular maxima over a suppressed background. The color gamut shifts from red to violet as the angle of incidence is varied (20°–70°) under perpendicular polarization, while an almost non-iridescent orange is maintained for parallel polarization.

Environmental Sensitivity and Colorimetric Sensing

Both nanogratings exhibit substantial plasmonic resonance shifts in response to changes in the surrounding refractive index, with sensitivities exceeding 500 nm/RIU ($\Delta\lambda \approx 170$ nm for transition from air to water at AOI=45°). This induces naked-eye color changes (e.g., green-to-red for Bi/PC, light pink-to-orange for Bi/Ag/PC), confirming the suitability for colorimetric sensing applications. The high sensitivity is supported by FDTD simulations demonstrating theoretical values up to 725 nm/RIU.

Theoretical and Practical Implications

The findings establish that exploiting the negative permittivity regime in Bi (comparable to Ag in the relevant spectrum), in combination with appropriate nanostructuring, achieves narrow, high-contrast, and polarization-sensitive plasmonic responses not previously accessible in Bi. This overcomes the limitations of prior Bi nanostructures, which were hindered by broad, polarization-insensitive features.

Practically, the demonstrated lithography-free, template-guided growth on commodity DVD substrates enables scalable and cost-effective manufacturing of plasmonic nanodevices for both dynamic color generation and real-time colorimetric sensing. The environmental sustainability advantages are significant: Bi is non-toxic and substantially cheaper than noble metals such as Ag or Au.

From a foundational perspective, the study elucidates mechanisms by which substrate-mediated topographical and crystallographic parameters modulate not only resonance positions but also the interplay of specular and diffuse reflectance, impacting overall chromatic and sensing behavior.

Outlook and Future Directions

These Bi-based nanogratings open avenues for:

• Integrated colorimetric sensors: for chemical, biological, or environmental monitoring, leveraging the strong and visible refractive index-induced color shifts.
• Dynamic display technologies: supporting polarization-encoded, view-angle modulated color palettes for security printing or photonic devices.
• Further material engineering: The modular approach allows exploration of other sustainable and emergent plasmonic materials (e.g., alloyed Bi, doped films).

Potential advances may include integration with microfluidics for real-time sensor arrays, exploitation in polarization-controlled optical communication systems, and tailoring of grating cross-section or multiperiodicity for engineered multi-resonant functionality.

Conclusion

This work advances the field of plasmonic nanomaterials by combining the scalable fabrication of Bi nanogratings with precise and tunable optical functionalities—narrow, polarization-selective resonances, vivid iridescent structural color, and high-efficiency colorimetric sensing. The approach leverages sustainable materials and commoditized substrates, supporting translation to large-area device technologies and offering a viable path for the development of next-generation colorimetric sensors, dynamic displays, and related photonic systems (2512.11151).