Angle-multiplexed all-dielectric metasurfaces for broadband molecular fingerprint retrieval

Abstract

Infrared spectroscopy resolves the structure of molecules by detecting their characteristic vibrational fingerprints. Subwavelength light confinement and nanophotonic enhancement have extended the scope of this technique for monolayer studies. However, current approaches still require complex spectroscopic equipment or tunable light sources. Here, we introduce a novel metasurface-based method for detecting molecular absorption fingerprints over a broad spectrum, which combines the device-level simplicity of state-of-the-art angle-scanning refractometric sensors with the chemical specificity of infrared spectroscopy. Specifically, we develop germanium-based high-Q metasurfaces capable of delivering a multitude of spectrally selective and surface-sensitive resonances between 1100 and 1800 cm^-1. We use this approach to detect distinct absorption signatures of different interacting analytes including proteins, aptamers, and polylysine. In combination with broadband incoherent illumination and detection, our method correlates the total reflectance signal at each incidence angle with the strength of the molecular absorption, enabling spectrometer-less operation in a compact angle-scanning configuration ideally suited for field-deployable applications.

Fig. 1. Angle-multiplexed broadband fingerprint retrieval.

(A) A germanium-based high-Q all-dielectric metasurface delivers ultrasharp on-demand resonances with specific resonance frequency ν for every incidence angle θ with broad spectral coverage. Continuous scanning of the incidence angle produces a multitude of resonances over a target fingerprint range, realizing an angle-multiplexed configuration ideally suited for surface-enhanced mid-IR molecular absorption spectroscopy. (B) Strong near-field coupling between the dielectric resonators and the molecular vibrations of the analyte induces a pronounced attenuation of the resonance line shape correlated with the vibrational absorption bands. (C) Angle multiplexing combined with the spectral selectivity of high-Q resonances allows for broadband operation and straightforward device implementation. (D) Chemically specific output signal of the device scheme from (C), which is determined by the imaginary part k of the analyte’s complex refractive index $\tilde{n}$ (a.u., arbitrary units).

Fig. 2. Working principle of the angle-multiplexed metasurface.

(A) The dielectric metasurface design consists of a zigzag array of elliptical germanium resonators on a calcium fluoride substrate. When varying the incidence angle, we consider both the TM_x (red plane) and the TE_y (blue plane) modes. (B) Simulated reflectance spectra for normal incidence and for an incidence angle of θ = 55° in both TM_x and TE_y modes. The two modes show similar spectral shifts of around 300 cm⁻¹, but in opposite shift directions, enabling wide spectral coverage with a single metasurface design. (C) Full resonance dispersion curves (color-coded reflectance) versus incidence angle illustrate the continuous tunability of ultrasharp resonances over the target wave number range together with the spectrally opposite behavior of the TM_x and TE_y modes. (D) Because of the highly accessible and strongly enhanced electromagnetic near fields around the resonators, our design is ideally suited for amplifying and detecting the molecular vibrations of adsorbed analytes. (E) Zigzag array of line dipoles embedded in a homogeneous environment with a unity refractive index. The length and the orientation angle of the dipoles are 2.7 μm and α = ±10°, respectively. (F) Effective reactance Im(Z_eff) of the quasi-BIC as a function of the incident angle under different excitation modes. The circles mark the positions where the reactance is equal to zero, which determine the angle-dependent resonance frequencies of the quasi-BIC.

Fig. 3. Angle-multiplexed molecular fingerprint detection.

(A) Photograph of a fabricated large-area all-dielectric metasurface used for reflection experiments. (B and C) Scanning electron microscopy micrographs of the metasurface confirm the homogeneity of the nanofabrication. (D) Normalized reflectance spectra of the metasurface before analyte coating. The angular range is from θ = 13° to θ = 60°, which corresponds to a wide spectral tuning range of 1120 to 1800 cm⁻¹. (E) Normalized reflectance spectra after deposition of a spin-coated PMMA thin film. Multiple molecular absorption bands of the PMMA are clearly visible as a distinct modulation of the reflectance spectra. (F) Absorbance spectrum in optical density (OD) units calculated from the reflectance envelopes before and after analyte coating. Agreement with independent IRRAS measurements is excellent, and a signal enhancement factor of around 50 is observed.

Fig. 4. Spectrometer-less angle-scanning molecular fingerprint detection and application to a multistep bioassay.

(A and B) The high resonance sharpness and low reflectance background of our metasurface design over a broad tuning range enable the retrieval of vibrational fingerprint information from the total reflectance signals I₀ and I_A. (C) The AFR signal clearly reproduces the PMMA absorption fingerprint over 600 cm⁻¹, confirming the spectrometer-less detection capability of our approach. (D) The broad spectral coverage of the angle-multiplexed method enables chemically specific fingerprint detection of a wide range of analytes in a bioassay involving interactions of polylysine, DNA, and ODAM protein molecules. Multiple distinct absorption bands of these biomolecules are well resolved. ssDNA, single-stranded DNA.