Localized Surface Plasmon Coupling between Mid-IR-Resonant ITO Nanocrystals

Sn-doped indium oxide (ITO) nanocrystals (NC) provide tunable localized surface plasmon resonance in the mid-infrared. To evaluate the applicability of these n-doped plasmonic semiconductors in field-enhanced spectroscopies, it is necessary to assess how the low, free-electron density affects the E-field localization and plasmon coupling in NC films when compared to metal nanoparticles (NP). In this article, we investigate plasmon coupling between approximate 6 nm diameter ITO NC on the collective resonance and quantify the effect of the electromagnetic field enhancement on the absorbance signal of surface-attached ligands in NC films and monolayers with different ratios of doped and undoped indium oxide NC

Identification of Tumor Cells through Spectroscopic Profiling of the Cellular Surface Chemistry

A key challenge for molecular cancer diagnostics is the identification of appropriate biomarkers and detection modalities. One target of interest is the cell surface, which is involved in important steps of cancerogenesis. Here, we explore the feasibility of a label-free spectroscopic profiling of the cell surface chemistry for the detection of tumor cells. Vibrational spectra of the cellular surfaces of tumor and nontumor breast and prostate cell lines were recorded on silver nanoparticle substrates using surface-enhanced Raman spectroscopy (SERS). The quantitative analysis of the spectra revealed characteristic differences, especially in the spectral range between 600 and 900 cm⁻¹. The detection of tumor-cell-specific differences in the recorded SERS spectra indicates the possibility of improving the precision of current cancer detection and staging approaches through a spectroscopic profiling of cell surface cancer biomarkers

Enhanced Optical Chirality through Locally Excited Surface Plasmon Polaritons

Plasmonic nanostructures provide unique opportunities to improve the detection limits of chiroptical spectroscopies by enhancing chiral light–matter interactions. The most significant of such interaction occur in ultraviolet (UV) range of the electromagnetic spectrum that remains challenging to access by conventional localized plasmon resonance based sensors. Although surface plasmon polaritons (SPPs) on noble metal films can sustain resonances in the desired spectral range, their transverse magnetic nature has been an obstacle for enhancing chiroptical effects. Here we demonstrate, both analytically and numerically, that SPPs excited by near-field sources can exhibit rich and nontrivial chiral characteristics. In particular, we show that the excitation of SPPs by a chiral source not only results in a locally enhanced optical chirality but also achieves manifold enhancement of net optical chirality. Our finding that SPPs facilitate a plasmonic enhancement of optical chirality in the UV part of the spectrum is of great interest in chiral biosensing

Harnessing Leaky Modes for Fluorescence Enhancement in Gold-Tipped Silicon Nanowires

Hybrid structures containing plasmonic and photonic components can enhance light–matter interaction through both photonic and plasmonic modes and potentially their interactions. In this article, we systematically investigate the optical properties of nanoparticle-tipped silicon nanowires (NPTWs) and characterize their ability to enhance the fluorescence signal of two cyanine dyes (Cy3 and Cy5) selectively bound to the gold tip. Although both gold and silicon components contribute to the fluorescence signal enhancement, the experimentally observed strong morphology dependence of the fluorescence emission enhancement is demonstrated to depend on tunable leaky modes of the silicon nanowires. The ability to encode emission fluorescence enhancement in the structure of NPTWs is of high relevance for multiparametric biosensing and imaging

Generating Optical Birefringence and Chirality in Silicon Nanowire Dimers

Narrow high refractive index nanowires sustain weakly guided modes with significant mode volume outside of the nanowire. This modal spillover makes them interesting photonic materials for a multitude of applications. In this article we fabricate dimers of nanowires with lengths up to 1.4 μm, radii down to 55 nm, and edge-to-edge separation down to 60 nm through anisotropic etching from crystalline silicon (Si). We investigate how the properties of the weakly confined fundamental HE_1,1 mode in Si nanowires are modified by their integration into dimers. In particular, we characterize through a combination of experimental spectroscopy and numerical electromagnetic simulations how the lifting of the degeneracy of HE_1,1<i>x</i> and HE_1,1<i>y</i> modes in dimers of Si nanowires generates linear birefringence, spin angular momentum, and superchirality. Achiral Si nanowire dimers are found to create locations of strongly enhanced near-field chirality in the gap between the nanowires, where the field can interact with the ambient medium

Probing DNA Stiffness through Optical Fluctuation Analysis of Plasmon Rulers

The distance-dependent plasmon coupling between biopolymer tethered gold or silver nanoparticles forms the foundation for the so-called plasmon rulers. While conventional plasmon ruler applications focus on the detection of singular events in the far-field spectrum, we perform in this Letter a ratiometric analysis of the continuous spectral fluctuations arising from thermal interparticle separation variations in plasmon rulers confined to fluid lipid membranes. We characterized plasmon rulers with different DNA tethers and demonstrate the ability to detect and quantify differences in the plasmon ruler potential and tether stiffness. The influence of the nature of the tether (single-stranded versus double-stranded DNA) and the length of the tether is analyzed. The characterization of the continuous variation of the interparticle separation in individual plasmon rulers through optical fluctuation analysis provides additional information about the conformational flexibility of the tether molecule(s) located in the confinement of the deeply subdiffraction limit interparticle gap and enhances the versatility of plasmon rulers as a tool in Biophysics and Nanotechnology

Quantification of Differential ErbB1 and ErbB2 Cell Surface Expression and Spatial Nanoclustering through Plasmon Coupling

Cell surface receptors play ubiquitous roles in cell signaling and communication and their expression levels are important biomarkers for many diseases. Expression levels are, however, only one factor that determines the physiological activity of a receptor. For some surface receptors, their distribution on the cell surface, especially their clustering, provides additional mechanisms for regulation. To access this spatial information robust assays are required that provide detailed insight into the organization of cell surface receptors on nanometer length scales. In this manuscript, we demonstrate through combination of scattering spectroscopy, electron microscopy, and generalized multiple particle Mie theory (GMT) simulations that the density- and morphology-dependent spectral response of Au nanoparticle (NP) immunolabels bound to the epidermal growth factor receptors ErbB1 and ErbB2 encodes quantitative information of both the cell surface expression and spatial clustering of the two receptors in different unliganded in vitro cancer cell lines (SKBR3, MCF7, A431). A systematic characterization of the collective spectral responses of NPs targeted at ErbB1 and ErbB2 at various NP concentrations indicates differences in the large-scale organization of ErbB1 and ErbB2 in cell lines that overexpress these receptors. Validation experiments in the scanning electron microscope (SEM) confirm that NPs targeted at ErbB1 on A431 are more strongly clustered than NPs bound to ErbB2 on SKBR3 or MCF7 at overall comparable NP surface densities. This finding is consistent with the existence of larger receptor clusters for ErbB1 than for ErbB2 in the plasma membranes of the respective cells

Photonic–Plasmonic Mode Coupling in On-Chip Integrated Optoplasmonic Molecules

We investigate photonic–plasmonic mode coupling in a new class of optoplasmonic materials that comprise dielectric microspheres and noble metal nanostructures in a morphologically well-defined on-chip platform. Discrete networks of optoplasmonic elements, referred to as optoplasmonic molecules, were generated through a combination of top-down fabrication and template-guided self-assembly. This approach facilitated a precise and controllable vertical and horizontal positioning of the plasmonic elements relative to the whispering gallery mode (WGM) microspheres. The plasmonic nanostructures were positioned in or close to the equatorial plane of the dielectric microspheres where the fields associated with the plasmonic modes can synergistically interact with the evanescent fields of the WGMs. We characterized the far-field scattering spectra of discrete optoplasmonic molecules that comprised two coupled 2.048 μm diameter polystyrene microspheres each encircled by four 148 nm diameter Au nanoparticles (NPs), through far-field scattering spectroscopy. We observed a broadening of the TE and TM modes in the scattering spectra of the optoplasmonic dimers indicative of an efficient photonic–plasmonic mode coupling between the coupled photonic modes of the WGM resonators and the localized surface plasmon modes of the NPs. Our experimental findings are supported by generalized multiple particle Mie theory simulations, which provide additional information about the spatial distributions of the near fields associated with the photonic–plasmonic hybrid modes in the investigated optoplasmonic molecules. The simulations reveal partial localization of the spectrally sharp hybrid modes outside of the WGM microspheres on the Au NPs where the local E-field intensity is enhanced by approximately 2 orders of magnitude over that of an individual Au NP

Electromagnetic Field Enhancement and Spectrum Shaping through Plasmonically Integrated Optical Vortices

We introduce a new design approach for surface-enhanced Raman spectroscopy (SERS) substrates that is based on molding the optical powerflow through a sequence of coupled nanoscale optical vortices “pinned” to rationally designed plasmonic nanostructures, referred to as Vortex Nanogear Transmissions (VNTs). We fabricated VNTs composed of Au nanodiscs by electron beam lithography on quartz substrates and characterized their near- and far-field responses through combination of computational electromagnetism, and elastic and inelastic scattering spectroscopy. Pronounced dips in the far-field scattering spectra of VNTs provide experimental evidence for an efficient light trapping and circulation within the nanostructures. Furthermore, we demonstrate that VNT integration into periodic arrays of Au nanoparticles facilitates the generation of high E-field enhancements in the VNTs at multiple defined wavelengths. We show that spectrum shaping in nested VNT structures is achieved through an electromagnetic feed-mechanism driven by the coherent multiple scattering in the plasmonic arrays and that this process can be rationally controlled by tuning the array period. The ability to generate high E-field enhancements at predefined locations and frequencies makes nested VNTs interesting substrates for challenging SERS applications