Surface-enhanced Raman spectroscopy (SERS) has emerged as an ultrasensitive molecular-fingerprint-based technique for label-free biochemical analysis of biological systems. However, for conventional SERS substrates, SERS enhancement factors (EFs) strongly depend on background refractive index (RI), which prevents reliable spatiotemporal SERS analysis of living cells consisting of different extra-/intracellular organelles with a heterogeneous distribution of local RI values between 1.30 and 1.60. Here, we demonstrate that nanolaminated SERS substrates can support uniform arrays of vertically oriented nanogap hot spots with large SERS EFs (>107) insensitive to background RI variations. Experimental and numerical studies reveal that the observed RI-insensitive SERS response is due to the broadband multiresonant optical properties of nanolaminated plasmonic nanostructures. As a proof-of-concept demonstration, we use RI-insensitive nanolaminated SERS substrates to achieve label-free Raman profiling and classification of living cancer cells with a high prediction accuracy of 96%. We envision that RI-insensitive high-performance nanolaminated SERS substrates can potentially enable label-free spatiotemporal biochemical analysis of living biological systems.
Ultrasensitive surface-enhanced Raman spectroscopy (SERS) still faces difficulties in quantitative analysis because of its susceptibility to local optical field variations at plasmonic hotspots in metallo-dielectric nanostructures. Current SERS calibration approaches using Raman tags have inherent limitations due to spatial occupation competition with analyte molecules, spectral interference with analyte Raman peaks, and photodegradation. Herein, we report that plasmon-enhanced electronic Raman scattering (ERS) signals from metal can serve as an internal standard for spatial and temporal calibration of molecular Raman scattering (MRS) signals from analyte molecules at the same hotspots, enabling rigorous quantitative SERS analysis. We observe a linear dependence between ERS and MRS signal intensities upon spatial and temporal variations of excitation optical fields, manifesting the |E|4 enhancements for both ERS and MRS processes at the same hotspots in agreement with our theoretical prediction. Furthermore, we find that the ERS calibration’s performance limit can result from orientation variations of analyte molecules at hotspots.
Plasmonic nanostructures can concentrate light and enhance light-matter interactions in the subwavelength domain, which is useful for photodetection, light emission, optical biosensing, and spectroscopy. However, conventional plasmonic devices and systems are typically optimized for the operation in a single wavelength band and thus are not suitable for multiband nanophotonics applications that either prefer nanoplasmonic enhancement of multiphoton processes in a quantum system at multiple resonant wavelengths or require wavelength-multiplexed operations at nanoscale. To overcome the limitations of “single-resonant plasmonics,” we need to develop the strategies to achieve “multiresonant plasmonics” for nanoplasmonic enhancement of light-matter interactions at the same locations in multiple wavelength bands. In this review, we summarize the recent advances in the study of the multiresonant plasmonic systems with spatial mode overlap. In particular, we explain and emphasize the method of “plasmonic mode hybridization” as a general strategy to design and build multiresonant plasmonic systems with spatial mode overlap. By closely assembling multiple plasmonic building blocks into a composite plasmonic system, multiple nonorthogonal elementary plasmonic modes with spectral and spatial mode overlap can strongly couple with each other to form multiple spatially overlapping new hybridized modes at different resonant energies. Multiresonant plasmonic systems can be generally categorized into three types according to the localization characteristics of elementary modes before mode hybridization, and can be based on the optical coupling between: (1) two or more localized modes, (2) localized and delocalized modes, and (3) two or more delocalized modes. Finally, this review provides a discussion about how multiresonant plasmonics with spatial mode overlap can play a unique and significant role in some current and potential applications, such as (1) multiphoton nonlinear optical and upconversion luminescence nanodevices by enabling a simultaneous enhancement of optical excitation and radiation processes at multiple different wavelengths and (2) multiband multimodal optical nanodevices by achieving wavelength multiplexed optical multimodalities at a nanoscale footprint.
scattering, photoluminescence, and electroluminescence. [4][5][6][7][8] Nevertheless, for many nonlinear nanophotonics applications, it is highly desirable to use multiresonant plasmonic devices that can simultaneously enhance multiphoton excitation/emission processes in several different wavelength bands at the same hotspot locations. [9][10][11][12][13][14][15][16][17][18][19] For constructing multiresonant plasmonic devices, a general approach is to assemble multiple building-block plasmonic resonators within a very close distance; and the optical coupling between spectrally matched non-orthogonal elementary modes of building blocks can result in multiple hybrid plasmonic modes of different resonance wavelengths that spatially overlap. [20][21][22] Based on the geometrical configuration of building-block resonators, multiresonant plasmonic devices can be classified into three types: 1) in-plane arrangement, [9,11,12,15,18,[23][24][25][26][27] 2) core-shell arrangement, [14,[28][29][30][31][32] and 3) out-of-plane arrangement. [33][34][35][36][37] Since it is straightforward to create plasmonic systems with accurate nanoscale control of planar geometries by top-down nanolithography, most previous studies have focused on developing in-plane multiresonant plasmonic devices. [9,11,12,15,18,[23][24][25][26][27] Despite the simplicity in design and fabrication, in-plane multiresonant plasmonic devices face two severe limitations due to the planar layout of multiple building-block resonators. 1) The device footprint tends to be large, and accordingly, the surface density of multiresonant hotspots is typically low; 2) Since the nearest-neighbor coupling of elementary modes dominates between in-plane arranged building-block resonators, the planar multiresonant systems usually support a limited number (<4) of hybridized plasmonic modes with spatial overlaps. Recently, Reshef et al. demonstrated that using in-plane plasmonic metasurfaces with a finite out-of-plane dielectric cladding can increase the number of modes by creating several Fabry-Perot-like (FP-like) resonances. [38] However, such a method primarily depends on the broad electric dipolar plasmonic modes, limiting the maximum attainable absorption to 50% [39] and keeping the field enhancement factor relatively small compared to nanogap plasmonic modes. [20] As the second type of multiresonant plasmonic devices, chemically synthesized core-shell metalinsulator-metal (MIM) multilayered nanoparticles can support multiple hybrid modes by mixing the elementary modes at Effective trapping and nanolocalization of different colored photons simultaneously at the same position remain a challenge in nanophotonics research but can boost applications based on nonlinear multiphoton processes. For achieving broadband nanoscale light concentration, a promising strategy is to employ multiresonant plasmonic devices that support multiple hybridized surface plasmon modes with spatial overlap at several different resonance wavelengths. However, high-order plasmonic modes from hybri...
Surface-enhanced Raman spectroscopy (SERS) has become a powerful technique for ultrasensitive biochemical detection providing molecular fingerprint information. Due to the strong dependence of surface plasmon resonance wavelength on plasmonic nanostructures’ surrounding refractive index (RI), SERS enhancement factors (EFs) at hotspots are sensitive to changes in background RI, which is detrimental to quantitative SERS biochemical analysis in real-world applications with spatially and temporally varying RI matrices. This work reports on a tapered-shape nanolaminate plasmonic nanoantenna (TNLNA) platform that supports multiple, spatially overlapped, hybridized magnetoelectric localized surface plasmon modes revealing high SERS EFs (>107) and an insensitivity to background RI variations (between 1.30 and 1.60). Furthermore, we demonstrate that the uniform arrays of TNLNAs can be manufactured on flexible transparent polymer films to achieve backside-excitable and reversible SERS measurements for in situ label-free glucose monitoring on a skin phantom.
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