Angle-and polarization-dependent collective excitation of plasmonic nanoarrays for surface enhanced infrared spectroscopy

Abstract

Our recent work has showed that diffractively coupled nanoplasmonic arrays for Fourier transform infrared (FTIR) microspectroscopy can enhance the Amide I protein vibrational stretch by up to 10(5) times as compared to plain substrates. In this work we consider computationally the impact of a microscope objective illumination cone on array performance. We derive an approach for computing angular- and spatially-averaged reflectance for various numerical aperture (NA) objectives. We then use this approach to show that arrays that are perfectly optimized for normal incidence undergo significant response degradation even at modest NAs, whereas arrays that are slightly detuned from the perfect grating condition at normal incidence irradiation exhibit only a slight drop in performance when analyzed with a microscope objective. Our simulation results are in good agreement with microscope measurements of experimentally optimized periodic nanoplasmonic arrays.

Periodic array-based substrates for surface-enhanced infrared spectroscopy

“…Diffractively coupled nanoplasmonic arrays were also in the focus of Liberman et al [74]. Usually, the array substrates are investigated by FTIR microspectroscopy, which means that the arrays are not illuminated approximately by plane waves traveling along the normal of the substrate, but by light focused by the microscope objective onto the substrate.…”

“…In a follow-up study of [74] from Liberman et al [75], a slightly different group around the same first author investigated an approach to further optimize diffractioncoupled plasmonic arrays for SEIRS. A general problem in the search for optimized substrates is the impossibility to scan the complete parameter space by numerical methods.…”

Engineering mid-infrared nanoantennas for surface enhanced infrared absorption spectroscopy

“…While early studies have relied on metal island films, prepared either by physical vapor deposition or chemical means [15][16][17][18], recent work has shown that nanofabricated plasmonic nanoantennas are an extremely promising approach, with a number of important advantages [19][20][21][22][23][24][25][26][27][28][29][30][31][32][33][34][35][36]. These include improved repeatability [34,35,37], deterministic and well-defined sensing regions [19,20,24,32,[34][35][36] and dramatically higher absorption signal enhancement factors [19][20][21][22][23][25][26][27][32][33][34][35][36]38]. The increase in enhancement is most notable as it is a result of an important fundamental difference between the two approaches.…”

“…Specifically, due to particle sizes and geometrical fluctuations in metal island films, generally the long wavelength tail of a stochastic collection of plasmon resonances is responsible for field enhancement. In contrast, nanoantennas can be engineered to support a single, welldefined resonance directly at the frequency of interest, leading to a resonant coupling between the absorption line and the plasmonic mode [22,24,25,[27][28][29][30][31][32][33][34][35][36]39]. In this review we discuss the basic principles and key focus areas essential to moving this resonant SEIRA approach forward as a promising technique applied in various fields including biotechnology, pharmacology, biomedical research and biology to yield new insights.…”

“…In this review we discuss the basic principles and key focus areas essential to moving this resonant SEIRA approach forward as a promising technique applied in various fields including biotechnology, pharmacology, biomedical research and biology to yield new insights. While we will primarily consider our work in utilizing gold (Au) nanoantennas [24,[26][27][28][29]31,35,39,40], there are a wide range of other possibilities for controlling the interaction between mid-infrared light and a biological sample. These include other flavors of plasmonic resonances such as surface plasmon polaritons [19,20,41], alternative material systems [42][43][44] for forming nanoscale waveguides and resonators as well as integrated and fiber optics based solutions [45].…”