This PDF file contains the front matter associated with SPIE Proceedings Volume 11468, including the Title Page, Copyright information, and Table of Contents.

Tip-enhanced Raman spectroscopy and microscopy are performed on a single nitrogen atom, carbon monoxide, pyridine, and benzene adsorbed on the Cu(100) surface. As the complexity of system ranging from an atomic vibrator to polyatomic molecules increases, the collective information gained from individual experiments elucidates the governing mechanism of Raman scattering in the atomistic near-field from different angles. Rich physics observed from above-mentioned systems such as dielectric screening, concomitant electromagnetic and chemical enhancement, Stark shift, dehydrogenation, and distortion and recovery of vibrational information will be discussed.

We found a gap mode plasmon induced photocatlaytic oxidation of p-alkyl thiophenol (p-AlTP) to p-mercaptobenzoic acid (p-MBA). In contrast to p-AlTP, o-methyl TP and m-methyl TP were not oxidized, indicating preferential reactivity of the para position in TP molecules. Nonetheless, the site selectivity is not always valid in this type of photocatalytic reactions, as o-, m-, and p-MBAl molecules were oxidized to corresponding MBA. With respect to the reaction mechanism, we confirmed that the oxidation of p-MeTP is not induced by thermal heating of the samples up to 373 K. Subtle temperature increase (<10 K) during the gap mode-induced oxidation was also corroborated by the observed Stokes and anti-Stokes scattering intensity of p-MeTP. Oxygen molecules accelerated the oxidation of p-MeTP at room temperature, whereas nitrogen atmosphere generated an intermediate species attributable to p-mercaptobenzyl radical. We further investigated the reaction process using density functional theory.

Spectroscopy of near-single molecular surface enhanced resonant Raman scattering (SERRS) of single plasmonic nanoparticle (NP) systems like NP dimer revealed that such systems are in strong coupling regimes between plasmon resonance and molecular exciton. We firstly show the evidence of strong coupling between plasmon and molecular exciton by calculation of classical electromagnetism. Secondly, we show reproduction of SERRS with surface enhanced fluorescence (SEF) spectra of dye molecules in the gaps of dimers under strong coupling conditions. By the way, the electronic structure of molecules in the strong coupling system is expected to be modulated by vacuum electromagnetic fields under the near single dye molecule condition. Thus, we finally show absorption spectroscopy of single strong coupling systems by combining extinction with scattering spectroscopy.

We report the first direct visualization of gap-SPPs propagating on a AgNW dimer. A self-assembled AgNW dimer loaded with a monolayer of molecules is locally excited to launch the SPPs, and the wide-field microscopy maps of surface-enhanced Raman scattering (SERS) of the molecules are acquired. The SERS images, representing the gap-field intensity distributions, reveal that the gap-plasmons of AgNW dimers with a few nm of gap can propagate up to ~8 um. The images also show oscillating components with periods of 400 ~ 800 nm, arising from the mode-beating of the two gap-SPP modes. Through a close comparison with electrodynamics simulations of NWs, we identify that the two modes are the monopole-monopole gap-modes with a propagation length of 0.5 ~ 2 um), and the dipole-dipole gap-modes with a propagation length of 5 ~ 8 um.

In monolayer transition metal dichalcogenides, quantum emitters are associated with localized strain that can be deterministically applied to create designer nano-arrays of single photon sources. Despite an overwhelming empirical correlation with local strain, the nanoscale interplay between strain, excitons, defects and local crystalline structure that gives rise to these quantum emitters is poorly understood. Here, we combine room-temperature nano-optical imaging and spectroscopy of excitons in nanobubbles of localized strain in monolayer WSe2 with atomistic structural models to elucidate how strain induces nanoscale confinement potentials that give rise to highly localized exciton states in 2D semiconductors.

A number of studies of tungsten disulfide (WS₂) have been conducted because monolayer WS₂ has a relatively high photoluminescence quantum yield. However, the defect-related Raman scattering which determines the quality of monolayer WS₂ has been rarely studied. In this study, we perform tip-enhanced Raman scattering experiments for the WS₂ monolayer to investigate the defect-induced Raman scattering properties. We demonstrate that the red-shifted A1g mode with the D and D′ modes can be attributed to the defect in monolayer WS₂. Furthermore, we also identify that the emergence of new Raman vibrational modes can be induced by sulfur vacancies through the density functional theory calculations.

In this talk we will present the use of near-field scanning probe microscopy and spectroscopy to investigate the electronic and optical quality of excitonic semiconductors. We will use two-dimensional (2D) transition metal dichalcogenides (TMDCs) of Mo and W as prototypical examples but extend our measurements to other low-dimensional excitonic systems including colloidal quantum dots, organic assemblies and layered hybrid perovskites. Via near-field photoluminescence spectroscopy we will show the nanoscale variations in quality of the contact with substrates and disorder at the interface in case of junctions or heterostructures. By placing a plasmonic metal substrate nearby and varying the distance, we will also show exciton hybridization with surface plasmons into propagating hybrid surface modes.

The control of photoluminescence processes, via the design of composite materials with engineered electromagnetic properties, is of great interest for the development of many application areas ranging from biophysics to quantum optical technologies. Approaches providing broadband enhancements of emission, not limited to resonant nanostructures, are particularly advantageous. We discuss how various photoluminescence processes, including conventional and dipolar-forbidden spontaneous emission, as well as Förster resonance energy transfer, are altered nearby and inside plasmonic hyperbolic metamaterials. They provide a flexible platform for engineering broadband Purcell enhancements due to their peculiar electromagnetic mode structure controlled by the nonlocal response of the metamaterial.

In the past two decades, various super-resolution fluorescence microscopic techniques have achieved an axial resolution on the order of tens of nanometers and been applied for a wide range of biological studies. However, these imaging techniques still face technical challenges to reach a resolution below 10 nm. Moreover, the required complex system for these techniques limits their wide applications in practice. In this talk, we present a new cellular fluorescence imaging method with a nanometer-scale axial resolution, based on a distance-dependent photobleaching suppression of fluorophores on hyperbolic metamaterial. We will show that by applying this technology to image HeLa cell membranes tagged with fluorescent proteins, an axial resolution of ~3 nm at multiple colors can be achieved, allowing for a precise determination of the architecture of cell adhesion.

The ability to monitor biochemical events in living cells is important in applications such as drug targeting and for understanding biological pathways. The enhancement of Raman signals by plasmonic nanoparticles, surface enhanced Raman scattering (SERS), or by a metalized scanning probe tip, tip enhanced Raman scattering (TERS), provide high sensitivity methods for exploring biological molecules in living cells. In this report we will examine how the Raman signals observed from functionalized nanoparticles can be used to study and differentiate protein receptor recognition based on the Raman signal observed. We will demonstrate the application of this technique in live cells. We will also examine the chemical origins of the observed signal to further elucidate the utility of these approaches.

Two-dimensional (2D) semiconducting materials such as transition metal dichalcogenides (TMDs) have been widely investigated for the potential use in optoelectronic, biosensing and photovoltaic applications. Nanoscale heterogeneous morphological, electronic and optical properties of these materials play a crucial role in the device performance. We use tip-enhanced Raman and photoluminescence near-field nanoimaging techniques to better understand the materials and hybrid biological systems such as bacterial and cancer cells on 2D TMDs and lateral heterostructures. We investigate the effects of local strain, doping, hot electron injection and transfer, as well as the electromagnetic and chemical enhancement mechanisms in these systems. The results show unusual tip-sample interaction effects that could reveal new synergistic phenomena at the nano- and picometer scales.

In SERS and TERS charge transfer can lead to the chemical signal enhancement. STM assisted TERS allows for applying DC electric fields between tip and sample comparably high to the fields experienced by molecules in regular devices. The influence of DC electric fields on tip enhanced Raman spectra has been reported by various authors who discussed different aspects such as reorientation of molecules, intensity changes and energy level shifts. Here we report on our progress towards a better understanding of a scanning DC electric field on tip enhanced Raman spectra of small molecules directly bound to the metallic substrate.

The implications of spatial coherence in tip-enhanced Raman spectroscopy of two-dimensional systems will be discussed. The prototype material is graphene. Based on massive data with improved resolution based on a plasmon-tuned tip pyramid, we show that in the tip-enhanced strong field regime, interference affects the spectral outcome utilized, for example, to quantify defects. The Raman figure of merit, i.e. the relative intensity of the defect-activated Raman band, depends on the TERS enhancement. Graphene sitting on different substrates is also analyzed. Super-resolution is shown to be due to the field configuration resulting from the coupled tip-sample-substrate system, exhibiting a non-trivial spatial surface distribution. The field distribution and the symmetry selection rules are different for non-gap versus gap mode configurations. This influences the overall enhancement which depends on the Raman mode symmetry and substrate structure.

We show how extended defects in wide bandgap semiconductors manifest in the nanoscale infrared phonon response probed by scattering-type scanning near-field optical microscopy (s-SNOM). We correlate the s-SNOM response of various defects in 4H-SiC with UV-photoluminescence, secondary electron and electron channeling contrast imaging, and transmission electron microscopy. We identify evidence of step-bunching, recombination-induced stacking faults, and threading screw dislocations, and also demonstrate the interaction of surface phonon polaritons with extended defects. Our s-SNOM results help to advance material growth efforts for electronic, photonic, phononic, and quantum optical applications.

Tip-enhanced spectroscopies have attracted considerable attention due to the capability of nanoscale optical characterization. Here, we demonstrated multimodal tip-enhanced vibrational spectroscopy for nanoscale analysis and imaging. A metallic probe tip was utilized to locally enhance not only Raman scattering but also IR absorption of sample molecules. Simultaneous detection of nano-Raman and nano-IR signal enabled us to elucidate nanoscale physicochemical properties of a variety of nanomaterials such as polymer thin films, self-assembled monolayers and biomaterials.

Tip-enhanced Raman spectroscopy (TERS) is a label-free imaging technique that combines scanning probe microscopy with Raman spectroscopy to obtain local chemical information, well below the optical diffraction limit. Discovered in the early 2000s, it has now become the tool-of-choice for the nanoscale investigation of carbon-based materials and 2D polymers. The perspective of imaging biological samples is attractive, but it is largely hampered by their low Raman cross-section and by their tendency to degrade quickly under the TERS tip. In this work, we propose to overcome these obstacles using stable tips as well as an effective sample deposition method to achieve nanoscale spatial and chemical characterization of protein/lipid membranes. As model samples, biomimetic membranes and bacteriorhodopsin (BR) membrane (from Halobacterium Salinarum) are tested.

Three-dimensional metamaterials consists of metal-insulator-metal structure were developed for a versatile platform of high-sensitive IR spectroscopy. A device with nano-fluidic channel allows the introduction and precise control of number of analyte molecules into the intense electromagnetic field of metamaterials, resulting in the improvement of sensitivity up to 2 orders compared to state-of-the-art plasmonic enhanced IR spectroscopies. High sensitive gas spectroscopy has also demonstrated by the use of vertically aligned three-dimensional MIM metamaterial device. These devices provide the capability of quantitative measurement of molecules, and open a new perspective of IR spectroscopies in bioanalysis.

Mid-infrared (MIR) spectroscopy is a powerful technique for molecular sensing through identifying the vibrational fingerprints of analyte molecules. However, the sensing efficiency drops dramatically at the nanoscale due to the poor interaction between MIR light and nanometric molecules. Here we exploit the MIR magnetic dipole resonance in a single silicon Mie antenna to demonstrate enhanced molecular sensing. We show that an ultra-sensitive measurement of a sub-10-nm PMMA layer can be achieved by positioning the antennas at the anti-node of a standing wave, which enables light-matter interactions under enhanced resonance conditions. Our results provide a new approach towards high-sensitivity MIR molecular sensing based on a miniaturized photonic platform.

A surface plasmon in a metal nanoparticle is the coherent oscillation of the conduction band electrons leading to both absorption and scattering as well as strong local electromagnetic fields. The plasmon is tunable through nanoparticel size and shape, as well as via nanoparticle interactions on different length scales that support near- and far-field coupling. Chemical synthesis and assembly of nanostructures are able to tailor plasmonic properties that are, however, typically broadened by ensemble averaging. Single particle spectroscopy together with correlated imaging is capable of removing heterogeneity in size, shape, and assembly geometry and furthermore allows one to separate absorption and scattering contributions. This talk describes how heterogeneity in crystal structure in a distribution of aluminum nanoparticles determines the damping of coherent lattice oscillations that are launched by ultrafast laser excitation.

Tremendous progress in nanotechnology has promised advances in the use of luminescent nanomaterials in imaging, sensing and photonic devices. This translational process relies on the controllable photophysical properties of the building block, that is, single luminescent nanoparticles. Among various probes, upconversion nanoparticles (UCNPs) are the unique anti-Stokes emission particles, enabling the conversion of near-infrared light to visible/UV light. In this talk, I will introduce our recent spectroscopic studies of ensemble and single UCNPs for nanothermometry and optical multiplexing.

Strong quantum-mechanical coupling between single emitters and plasmonic nanocavities has the potential to enable new classical and quantum photonic technologies at room temperature. Through a new technique of tip-enhanced strong coupling, we have demonstrated the ability to systematically study and dynamically control strong coupling between plasmons and single quantum dots at room temperature.

Polymerase chain reaction have been among the most powerful tools for many biomedical research. However, current PCR technology rely on thermocycling that uses time-consuming and expensive Peltier-block heating. Various methods including mechanical manipulation, microfluidics and nano-sized droplets have been tried and studied to improve and replace these problems. However, there are still several key limitations concerning device fabrication and complex sample preparation. Photothermal effect is a phenomenon in which energy is converted from absorbed photons to thermal energy by nanoparticles. Due to its high spatiotemporal resolution, strong and controllable optical properties, the plasmonic photothermal-based nanoparticles are expected to show great potential for the development of nucleic acid amplification-based biosensors. Here, we introduce a novel plasmonic photothermal-based PCR assay for fast, cost-effective and quantitative detection.

Fluorescent probes are widely used in biological imaging; however, their spectral properties often limit sensing and multiplexing. Instead, intracellular lasers, offer increased spectral purity, photostability and distinct spectral outputs enabling the unique tagging of multiple cells over long time periods. Here we report on the optimisation of low threshold miniature lasers, 1000-fold smaller than the eukaryotic nucleus (Vlaser<0.1μm³). The improved fabrication method has allowed us to explore more complex laser geometries such as squares and pentagons, amongst others, to optimize the optical properties to the specific sensing or tracking application. Furthermore, we demonstrate intracellular sensing, by evaluating local cellular properties.

We use a microtoroid sensing platform known as FLOWER (frequency locked optical whispering evanescent resonator) to detect small molecule drug targets at attomolar concentrations. FLOWER combines whispering gallery mode optical resonator technology with frequency locking feedback control, balanced detection, and data-processing techniques. We have previously demonstrated label-free detection down to single macromolecules. To use FLOWER as a drug screening platform, we couple the high selectivity and detection capabilities of G-protein coupled receptors (GPCRs) for small ligands with FLOWER. We demonstrate detection of the peptide dynorphin A at attomolar concentrations using a microtoroid resonator coated with a lipid bilayer containing GPCRs.

Plasmonic nanostructures can be used as nanoscale heat sources under light illumination. The heat generation leads to the increase of local temperature and thus the change of permittivities of both the plasmonic metals and surrounding dielectric media due to the thermo-optic effect. In this talk, I will present our research on using the photothermal properties of silver nanowires to modulate propagating surface plasmons and using gold nanorods with tunable surface plasmon resonances to demonstrate resonant scattering enhanced photothermal imaging.

The 2014 Nobel Prize in Chemistry is an award to praise the development of super-resolution microscopy, which has pushed the fluorescence microscopy to a new summit. However, there still exist challenges for further application of super-resolution: (1) Better spatial resolution is always preferred especially at no additional cost; (2) Deeper imaging depth inside the scattering specimen; and (3) Richer biological information.

This work reports the fabrication of large-area Au nanoantennas, tuned to 1400cm^-1 , on a Si substrate for surfaceenhanced- infrared-absorption-spectroscopy. Two different kinds of nanoantennas are fabricated, namely nano-rods and nano-slits. Fabrication is achieved by E-beam lithography (EBL). The need for an adhesion layer is eliminated using our previously reported UV-ozone pre-treatment¹. To our knowledge, this is the first time this technique is used to fabricate Au nanoantennas on Si without the need adhesion layer, while at the same time obtaining a strong adhesion. This UVozone treatment does not only speed up the fabrication process, it can potentially increase the enhancement quality due to the negative influence metallic adhesion layers can have on the plasmon resonance of Au nanoantennas^2–4. Next to using the standard positive resist for EBL lithography, we also propose a workflow using a negative photoresist to make the nano-rod antennas, potentially speeding up the process by skipping the lift off procedure. Although the negative photoresist fabrication process still requires optimization, our first fabrication attempt show promising results. In order to get the optimal enhancement for a given wavelength, we used FTDT simulations to simulate the structure length, height, width and pitch. After successful simulations, the structures were fabricated and a comparison between the simulated results and fabricated structures was made, confirming the simulation results.

Raman spectroscopy is a highly sensitive and specific technique for identifying tissue compositions. Raman-based characterization of normal and abnormal tissues is impeded due to the variability in routine tissue preparation techniques, fluorescent background, and molecular heterogeneity. Thus, sample preparation and Raman measurement conditions for tissue sections must be optimized. Here, we present an optimized Raman protocol and sample preparation method for brain tissue sections. This protocol allows the characterization of tissues and recognition of brain tumors by refining laser power, accumulation/exposure times, excitation wavelength, glass/CaF2 substrate, deparaffinization solvent, and the thickness of sections.

In this study, metasurface-based applications to enhance the resolution of the fluorescence microscopy are presented. Two different dielectric metasurfaces are designed; i) to control confinement of the excitation point spread function and ii) to provide encoded illumination patterns for patterned illumination. The results show the axial- and lateral-resolution improvement, respectively, and demonstrate applicability and practical use to conventional optical systems.

Structured illumination microscope (SIM) enables high temporal resolution wide field-of-view super-resolution imaging but typically provides only two-fold resolution improvement over the diffraction limit. We report speckle metamaterial-assisted illumination nanoscopy (Speckle-MAIN) which brings the resolution down to 40nm and beyond. A hyperbolic metamaterial structure is implemented as substrate to generate deep sub-wavelength speckle-like illumination pattern at the near field of the metamaterial. Fluorescent objects are illuminated by such high spatial-frequency near field illuminations and are reconstructed by a Blind-SIM algorithm. Speckle-MAIN provides a new route for low-cost easy-implemented super-resolution imaging with ultra-high resolution and biocompatibility.

A thin film made of organic semiconducting molecules of diphenyl derivatives dinaphthothienothiophene (DPh-DNTT) has high carrier mobility especially in the form of single layer. However, the carrier mobility can be significantly affected by molecular orientation of DPh-DNTT. Here, we present the molecular orientation analysis of single-layer DPh-DNTT by polarization Raman measurement. We analyzed the relation between the Raman intensity and polarization of incident light, and correlated them with the molecular orientation. We successfully obtained molecular orientation image of an island domain of single-layer DPh-DNTT, and revealed that they have a similar orientation within the domain.

Organoid, an in vitro model to study cell behaviours in a living organism, holds great potential for human cellular biology study, especially in disease pathology, drug delivery and drug efficacy trials. However, it remains challenging to track subcellular features inside organoid, as organoid are clusters of high-density cells that highly scatters and absorbs both excitation and emission light. Here we report a strategy on nanoscopy that applying “non-diffractive” beam and near-infrared imaging probe to minimize the light scattering and absorption inside scattering bio-tissue. Using a single Bessel-doughnut beam excitation from a 980nm diode laser and detecting at 800nm, we achieved a near-infrared, “non-diffractive” nanoscopy with high resolution under-diffractive limit in water solution. We further demonstrate that this method can image single upconversion nanoparticles inside spheroids, as deep as half-100μm, with resolution of 113nm. This method provides simple solution to inspect inter-and intra-cellular trafficking and drug release of single nanoparticles in 3D biological systems.

In this study, we will present the synthesis of self-assembled coupled Au nanorods (NRs) as substrates capable of supporting a dual modality of surface enhanced spectroscopies, SERS and SEIRAS. The AuNR arrays can be assembled either through vertical alignment or lateral alignment. We will present different assembly strategies for the Au NRs by adjusting the ionic strength of the Au NR solution. The goal is to rely on self-assembly to create organized and reproducible sensors for small molecule detection. Field enhancement criteria differs between SERS and SEIRAS. We will also present the finite-difference time-domain (FDTD) simulation of the multilayered AuNR array across visible and SWIR spectral region to explain some of the experimental observations.