This PDF file contains the front matter associated with SPIE Proceedings Volume 12203, including the Title Page, Copyright information, Table of Contents, and Conference Committee Page.

Here we report graphene systems' nano-Raman hyperspectral imaging based on tip-enhanced Raman scattering (TERS). The vibrational and electronic structures are modulated within the graphene-related materials, leading to nano-scale changes in the behavior of electrons and phonons that can be used for spectral imaging. Furthermore, we utilize a He-focused ion beam to do nanolithography on graphene. We then show that the tiny features on graphene made by the He-focused ion beam can only be visualized under nanometer-scaled spectroscopy imaging. We have also imaged low-angle reconstructed twisted bilayer graphene, and our observations highlight the relevance of solitons and topological points for the structures' vibrational and electronic properties, relevant in the context of twistronics.

In this paper we describe our recent work in multi-excitation surface enhanced Raman spectroscopy (MX-SERS), and its application for robust strain-level bacteria identification. The development of MX-SERS follows directly from our previous work in rapid bacterial identification using multi-excitation Raman spectroscopy (MX-Raman), which enabled highly accurate (up to 99.75%) strain-level distinction of bacteria, including antibiotic resistant strains of bacteria and from within complex media. In this work we use the strong wavelength dependence of both the Raman scattering cross-section and the surface plasmon to demonstrate a novel capability in bacteria identification. Compared to MX-Raman, MX-SERS has up to 8x faster data acquisition speed as well as up to 4000x lower laser power incident on the sample. Furthermore, we fabricate SERS-active substrates with a simple and low-cost fabrication method that can be adapted to fit a chosen wavelength regime. This combination of strain-level sensitivity and high-speed detection, combined with a low-cost SERS substrate, has strong potential applications in clinical diagnostics, and could be integrated within a real-world pathogen detection workflow.

Advances in nanotechnology enable the detection of trace molecules from the enhanced Raman signal generated at the surface of plasmonic nanoparticles. We have developed technology to enable super-resolution imaging of plasmonic nanoparticles, where the fluctuations in the surface enhanced Raman scattering (SERS) signal can be analyzed with localization microscopy techniques to provide nanometer spatial resolution of the emitting molecule’s location. Additional work now enables the super-resolved SERS image and the corresponding spectrum to be acquired simultaneously. Here we will discuss how this approach can be applied to provide new insights into biological cells.

Typical surface-enhanced Raman scattering (SERS) approaches rely on localized surface plasmon resonances that provide a significant enhancement of the localized electric field. Unfortunately, this technique faces challenges in terms of repeatability, which appears due to the strong dependence of the field enhancement on the surface roughness and the presence of hot-spots in nanostructures; and adequate excitation, as the laser beam must be tuned at a very specific wavelength that corresponds to the resonant frequency of the system. Hyperbolic metamaterials (HMTMs), a type of composite materials whose effective permittivity changes as a function of the electric field polarization, can effectively address these challenges because they support bulk and surface hyperbolic modes able to drastically boost the local fields over a broadband portion of the electromagnetic spectrum. In fact, the frequency response of these artificial materials can be manipulated by adjusting the system composing materials and filling ratios.

This work aims to explore the potential of HMTMs to enhance the SERS of molecules located nearby and to address some of the challenges faced by common SERS platforms. To this purpose, we focus on Au/SiO₂ HMTMs stacks that exhibit a hyperbolic dispersion for wavelengths larger than ~580 nm. A prototype has been fabricated and characterized using TEM and ellipsometry measurements. Power-dependent SERS measurements were obtained for a monolayer of biphenyl-4,4’-dithiol (BPDT) molecules self-assembled onto the HMTM surface and a gold-based control sample. HMTMs provide repeatable SERS detection with low laser powers <900µW and integration times <97ms (~30X and ~100X lower than control, respectively) over a large surface area, exhibiting a performance like complex TERS systems.

The lateral and vertical resolution in conventional optical microscopy is restricted by fundamental diffraction limits. One direction towards super-resolution optical microscopy is the use of photonic nanojets (PNJs) for sample illumination. Here, the aim is to exploit the high spatial localization of PNJs to allow measurements of sub-classical particles and features in spite of their small size compared to the operating wavelength. The applications of super-resolution methods include fluorescence and Raman microscopy, scatterometric measurements, and optical imaging. As a step towards PNJ scanning microscopy, we here apply our recently proposed method for fast and precise steering of PNJs over a large dynamical range in the near field. In a proof-of-concept computation, we use the steerable optical probe to extract information on structures beyond the classical lateral and vertical resolution limits.

This study describes a sensitivity analysis of absorption spectra for NIR-SWIR absorbing dyes relative to the inverse analysis of measured spectra. Absorption spectra of NIR/SWIR-absorbing dyes obtained by inverse spectral analysis provide information for estimating the dielectric response functions. Sufficient sensitivity of absorption spectra relative to inverse spectral analysis implies that estimated dielectric response functions can be used for the construction of approximate effective medium models capable of estimating reflectance from dye formulations on substrates, e.g., fabrics. The results of this study provide the foundational concepts for ensembles of absorption functions, which support the modeling and prediction of dielectric responses.