This PDF file contains the front matter associated with SPIE Proceedings Volume 9925, including the Title Page, Copyright information, Table of Contents, and Conference Committee listing.

Raman microscopy is a powerful tool for analytical imaging. The wavelength shift of Raman scattering corresponds to molecular vibrational energy. Therefore, we can access rich chemical information, such as distribution, concentration, and chemical environment of sample molecules. Despite these strengths of Raman microscopy, the spatial resolution has been a limiting factor for many practical applications. In this study, we developed a large-area, high-resolution Raman microscope by utilizing structured illumination microscopy (SIM) to overcome the spatial resolution limit. A structured line-illumination (SLI) Raman microscope was constructed. The structured illumination is introduced along the line direction by the interference of two line-shaped beams. In SIM, the spatial frequency mixing between structured illumination and Raman scattering from the sample allows access to the high spatial frequency information beyond the conventional cut-off. As a result, the FWHM of 40-nm fluorescence particle images showed a clear resolution enhancement in the line direction: 366 nm in LI and 199 nm in SLI microscope. Using the developed microscope, we successfully demonstrated high-resolution Raman imaging of various kinds of specimens, such as few-layer graphene, graphite, mouse brain tissue, and polymer nanoparticles. The high resolution Raman images showed the capability to extract original spectral features from the mixed Raman spectra of a multi-component sample because of the enhanced spatial resolution, which is advantageous in observing complex spectral features. The Raman microscopy technique reported here enables us to see the detailed chemical structures of chemical, biological, and medical samples with a spatial resolution smaller than 200 nm.

We present experimental studies of live cancer cells via microscopic imaging ellipsometry (MIE). The Rotating Compensator Ellipsometry (RCE) is used for our measurements. Ellipsometry spectra with signals integrated over a 20μm×20μm area in visible range (450nm to 750nm) are obtained with the Optrel MULTISKOP system for both specular reflection and off-specular scattering. The microscopic ellipsometry (ME) images at a few fixed wavelengths within the same range were also analyzed. Dielectric constants for glass slide, culture fluid, and glass cover plate were firstly characterized by spectroscopic ellipsometry measurements, which can be used in the simulation for MIE measurements of cancer cells embedded in the culture fluid and sandwiched between a glass slide and cover plate. The measured ME spectra and images of cancer cells before and after medicine injection are measured and analyzed.

In this letter, we present the principle behind nanoscale photoacoustic tomography (nPAT), in addition to simulation results demonstrating the thermal safety and the diagnostic potential of such a modality. Nanoscale photoacoustic tomography is a novel biomedical imaging modality that can allow for the 3D imaging of cells at nanometer resolutions. This modality also allows for the imaging of single red blood cells (RBCs) such that the hemoglobin concentration quantities can be visualized within the cell. As a result, we believe that nPAT can allow for diagnostic information at unprecedented resolutions and enable the visualization of previously unseen phenomenon in RBCs.

Tip-enhanced Raman spectroscopy (TERS) in air and ultra-high vacuum (UHV) has been refined over the years through the study of various adsorbates in different experimental configurations. Developing the technique toward more realistic working conditions would render possible the investigation of more complex solid/liquid systems like bio-membranes or energy conversion and storage devices, providing a powerful tool to characterize nanoscale electrochemical processes occurring at the interface with high sensitivity and resolution. However, the extension to solid/liquid interfaces and electrochemical conditions still remains a challenge and few reports have been published. We have built an electrochemical TERS setup with side-illumination geometry that adapts easily to different experimental conditions such as opacity, shape and dimensions of the sample. The instrument features a specially designed solid/liquid sample holder that is implemented in a standard commercial STM. The experimental scheme can, in principle, be adapted to upgrade classic air TERS setups for work in liquids. Here, we show potential-dependent EC-TER spectra of a monolayer of adenine adsorbed on Au(111). The intensity of the ring-breathing mode at 735 cm-1 decreases with increasing sample potential and is recovered again upon potential reversal. The intensity variation is attributed to orientational changes of adenine upon (dis)charging of the Au substrate.

Near-field scanning optical microscopy (NSOM) offers subwavelength optical resolution beyond the diffraction limit, enabling practical applications in optical imaging, sensing and nanolithography. However, due to the sub-100 nm size of apertures, conventional NSOM aperture probes suffer from the constrains of the strong attenuation of the throughput and limited the spatial resolution. To solve the problem, we designed a novel scheme for apertureless plasmonic probes with radial internal illumination. Employing non-periodic multi-rings geometry for plasmonic excitations, surface plasmons adiabatically nanofocuse energy at tip and the full width at half maximum of the optimal design is ∼18 nm. The proposed probe was optimized with 2D finite-difference time-domain (FDTD) analysis and realistic parabolic probe geometries. Comprehensive electromagnetic simulation shows that the optimal probe feature obeys Fabry-Pérot condition on the plasmonic metallic wall, giving rise to substantial field enhancement up to 6 orders of magnitude greater than conventional aperture probes without degrading its spatial resolution. We fabricated the proposed probe which possesses apex angle (∼ 22 degree) and tip radius (∼ 30 nm). Finally, the proposed near field plasmonic probe effectively combining the high resolution of apertureless probes with high throughput can enable the proposed plasmonic NSOM probe as a practical tool for applications in near field optical microscopy.

We report results of TERS characterization of graphene oxide and the 2D semiconductors, MoS2 and WS2. The gap mode TERS signal of these 2D materials becomes dramatically enhanced over wrinkles and creases, as well as over nanopatterns imprinted into flakes using a sharp diamond probe. The resonant Raman signal of MoS2 contains additional peaks normally forbidden by selection rules. TERS maps of few-layer-flakes of this 2D semiconductor show that the spatial distribution of Raman intensity across the flake varies for different peaks, providing interesting insights into the structure of such 2D semiconductors with 10-20 nm spatial resolution.

Strong coupling regime is reached when SPPs and matter exchange energy coherently and reversibly before losses take place, resulting in the formation of new hybrid exciton-plasmon states formed by lower and upper energy bands [1,2]. In this field most of the works focus on steady-state observations, indeed experiments on the dynamics of such hybrid systems are relatively scarce and their intrinsic photophysics is still far from being understood. Here, in order to improve our understanding of the dynamics of hybrid exciton-plasmon states, we have studied through ultrafast pump-probe approach, a hybrid system composed by gold hole arrays and J-aggregate molecules while modifying the lattice constant of the metallic array. Under upper hybrid band resonant excitation, transient absorption spectra provide the evidence that exciton-plasmon hybrid states are formed. Meanwhile, kinetics analysis led to the discovery of a remarkably long-lived upper band, at least one order of magnitude at 1/e than bare J-aggregate molecules. This result was explained with the identification, in the transient absorption spectra, of a trap state combined with the negligible relaxation effect from vibrational modes. The intrinsic long lifetime of hybrid states is of crucial importance both from a fundamental and applicative point of view, having implications in the use of exciton-plasmon states for technological purposes. The understanding of the dynamics on strong coupling systems can provide indeed a promising route towards novel ultrafast plasmonic devices with coherent functionalities.

Spintronics and spin-based technology rely on the ultra-fast unbalance of the electronic spin population in quite localized spatial regions. However, as a matter of fact, the low susceptibility of conventional materials at high frequencies strongly limits these phenomena, rendering the efficiency of magnetically active devices insufficient for application purposes. Among the possible strategies which can be envisaged, plasmonics offers a direct approach to increase the effect of local electronic unbalancing processes. By confining and enhancing free radiation in nm-size spatial regions, plasmonic nano-assemblies have demonstrated to support very intense electric and magnetic hot-spots. In particular, very recent studies have proven the fine control of magnetic fields in Fano resonance condition. The near-field-induced out-of-phase oscillation of localized surface plasmons has manifested itself with the arising of magnetic sub-diffractive hot-spots. Here, we show how this effect can be further boosted in the mid-infrared regime via the introduction of higher order plasmonic modes. The investigated system, namely Moon Trimer Resonator, combines the high efficiency of a strongly coupled nano-assembly in Fano interferential condition with the elevated tunability of the quadrupolar resonance supported by a moon-like geometry. The fine control of the apical gap in this unique nanostructure, characterizes a plasmonic device able to tune its resonance without any consequence on the magnetic hot-spot size, thus enabling an efficient squeezing in the infrared.

Localized surface plasmon resonance (LSPR) on metallic nanostructures is able to enhance photoluminescence (PL) emission significantly. However, the mechanism for anomalous blue-shifted peak of PL emission from metallic nanostructures, relative to the corresponding scattering spectra, is still unclear so far. In this paper, we presented the detailed investigations on both the Lorentz-like PL profile with blueshifted peak and Fano-like one with almost unshifted dip, as observed on dolmen-like metallic nanostructures. Such anomalous PL emission profile is the product of the density of plasmon states (DoPS) with Lorentz-/Fano-like profile and the population distribution of the relaxed collective free electrons during relaxation. To be more specific, the fast relaxation process of these collective free electrons contributes to the PL shifting characteristics of both Lorentz-like and Fano-like emission profiles. We believed that our results provide a general solid foundation and guidance for analyzing and manipulating the physical processes of the PL emission from various plasmonic nanostructures.

Micro/nanofluidics have attracted much attention as ideal platforms for bioanalysis. The interest in micro/nanofluidics has resulted in a high demand of non-label detections in ultra-small volume(aL-fL). Among existing optical detections in micro/nanofluidics, photothermal spectroscopy (PTS) is an important approach, which allows detection in non-label fashion. Its principle is based on the detection of refractive index change following the thermal relaxation when molecules absorb light. Many types of PTS have been developed for microfluidics, yet the sensitivity of PTS becomes an issue in sub-micrometer spaces where thermal diffusion is dominant. On the other hand, plasmonic metamaterials, which offers unique surface condition with tailored absorption properties and strong plasmonic enhancement is widely utilized to improve the sensitivity of absorption, fluorescence, Raman spectroscopies, etc. Recently, we proposed the integration of metamaterials to improve the sensitivity of PTS. However, the strong absorption of metamaterials itself is an obstacle for detection. In this study, we propose an idea of exploiting the electromagnetically induced transparence (EIT) phenomena to suppress the absorption in metamaterials. As the EIT peak is tailored to the absorption peak of detecting molecules or the excitation light, the absorption from metamaterials is negligibly small while the strong field enhancement can be achieved. The numerical calculation of absorption and PTS signals in case of metamaterials only, and under the existence of molecules were carried out by COMSOL. The results showed the improvement of signal-to-background ratio to 3-4 orders, while the sensitivity was improved to 2 orders. The experiments are ongoing to verify the calculation.

We have investigated the tip-enhanced Raman spectra of carbon nanotubes on the metal substrate. The relationship between the 2D and G band intensities and the number of walls of a multiwalled carbon nanotube was elucidated, which enables spatial resolution of nanometer order. The 2D band to G band integrated intensity ratio was found to be sensitive to the number of carbon nanotube walls. Furthermore, the 2D band intensity was enhanced by the intertube interaction between carbon nanotubes. This phenomenon can be explained by quantum interference between Raman scattering paths resulting from the strong interaction between carbon nanotube walls.

We investigate the optical response of individual porphyrin (TPPS₃) nanoaggregates anchored onto a glass substrate by using a specific configuration of Polarization-Modulation Scanning Near-Field Optical Microscopy (PM-SNOM). Subdiffraction spatial resolution and sensitivity to the chiroptical properties of the material is reported. By demodulating the transmitted signal at the first and the second harmonics, the response of the nanoggregates to circular and linear polarization is simultaneously acquired in the same scan. In order to evaluate the relevant dichroic coefficients, we analyze a sample comprising several tens of individual nanoaggregates by using a model based on the Mueller matrix formalism. Circular dichroism in the nanoaggregates is demonstrated to stem from their molecular structure, whereas linear dichroism occurs as a consequence of the strongly anisotropic shape of the deposited nanoaggregates.

We present a dynamic approach to scanning near field optical microscopy that extends the measurement technique to the third dimension, by strobing the illumination in sync with the cantilever oscillation. Nitrogen vacancy (NV) centers in nanodiamonds placed on cantilever tips are used as stable emitters for emission enhancement. Local field enhancement and modulation of optical density states are mapped in three dimensions based on fluorescence intensity and spectrum changes as the tip is scanned over plasmonic nanostructures. The excitation of NV centers is done using a total internal reflection setup. Using a digital phase locked loop to pulse the excitation in various tip sample separations, 2D slices of fluorescence enhancement can be recorded. Alternatively, a conventional SNOM tip can be used to selectively couple wideband excitation to the collection path, with subdiffraction resolution of 60 nm in x and y and 10 nm in z directions. The approach solves the problem of tip-sample separation stabilization over extended periods of measurement time, required to collect data resolved in emission wavelength and three spatial dimensions. The method can provide a unique way of accessing the three dimensional field and mode profiles of nanophotonics structures.

Near field scanning optical microscopy (NSOM) has been used in many fields to see the optical properties of various materials with highly improved resolution over the diffraction limit. On the other hand, an alternative way to measure the optical properties of nanostructures were proposed using photosensitive polymer. By irradiation irradiating light on certain nanostructures on photosensitive polymer, electric field can be recorded depending on the irradiation time, intensity or nanostructure. [1] Though this method has some limits compared to the conventional NSOM, it gives high spatial resolution with relatively simple setup. Until now, most of the researches were done by simply coating photosensitive polymer on nanostructure or nanoparticles. Here, we measured near field intensity of superspherical gold nanoparticles (AuNPs) by varying the embedding depth of AuNPs in photosensitive polymer. Changed electric field from as transferred one (non-embedded) to fully embedded one was measured by Atomic Force Microscopy (AFM). By simply changing the irradiation angle on AuNPs, we mapped angle-dependent localized near field. Also, we placed thin gold film underneath the photosensitive polymer to measure image dipole of AuNPs. Furthermore, sub-nanometer gap from monolayer graphene can be differentiated by placing graphene sheet between gold film and polymer. Along with single AuNP, near field mapping of AuNP dimers were also measured by varying the interparticle distance. Finally, near field mapping of simple artificial structures manipulated with AuNPs for possible plasmonic applications were measured.

Accurate measurement of Rayleigh scattering is crucially important for fundamental understanding of the plasmonic properties of meltimeric (≥ 3) nanoparticles that can be served as efficient SERS sensing platforms and nanophotonic materials. Thus, using the laser-scanning assisted dark-field microscopy that enabled to precisely collect far-field (Rayleigh) scattering from the centers of individual trimeric nanoparticles, we monitored spectral redistributions of oscillating coupled plasmonic modes as a function of trimer symmetry. As a consequence of the precise measurement of the polarization-resolved Rayleigh scattering spectra obtained from triangular trimers to linear trimers via elongated triangular trimers, the in-phase horizontally oscillating plasmonic mode with the largest dipole moment is found to be greatly increased by 20-folds, whereas the axially oscillating plasmonic mode with the second-largest dipole moment is dramatically decreased by 70-folds. Consequently, the overall quantity of the far-field scattering, the total sum of the individual coupled plasmonic modes, was gradually increased by 2-folds. The precise polarization-resolved Rayleigh scattering measurement also visualizes directly the directions of the radiation fields of individual oscillating coupled plasmonic modes, which would be valuable information in systematic controlling the polarization direction of the scattered light from the trimers. Overall, we showed an exemplary quantitative and extensive study of the coupled plasmonic modes from nanoparticles, giving a simple but clear insight.

Dielectric nanostructures with high refractive index and low optical loss have attracted considerable attention as an alternative to plasmonic nanostructures. We experimentally demonstrated to control the visible electromagnetic resonances of Si-based core-shell nanostructures by thermally varying the core-shell ratio. We also found a Fano resonance which was generated by the interference between the electric and magnetic dipole moments excited in the core-shell nanostructures. The all-dielectric nanostructures realized low energy loss and high electromagnetic field enhancement comparable with that exhibited by plasmonic nanostructures. These unique optical properties would enable us to demonstrate effective field-enhanced spectroscopy and imaging with low heat generation.

The excitation of surface phonon-polariton (SPhP) modes in polar materials using scattering type near-field optical microscopy (s-SNOM) has recently become an area of interest because of its potential for application as naturally occurring meta-materials and in low-loss energy transfer. Within this area, hexagonal boron nitride (h-BN) and boron nitride nanotubes (BNNTs) are the primary structures under investigation. Here we present pump-probe continuous wave (CW) scattering-type scanning near-field optical microscopy (s-SNOM) - a novel two color pump-probe infrared technique which uses two continuous wave tunable light sources and is based on s-SNOM. The technique allows us to spatially resolve coupling of the longitudinal optical and surface phonon polariton modes in BNNTs. However, no similar coupling is observed in two-dimensional h-BN crystals.

A grand challenge in nanoscience is to correlate structure or morphology of individual nano-sized objects with their photo-physical properties. An early example have been measurements of the emission spectra and polarization of single semiconductor quantum dots as well as their crystallographic structure by a combination of confocal fluorescence microscopy and transmission electron microscopy.[1] Recently, the simultaneous use of confocal fluorescence and atomic force microscopy (AFM) has allowed for correlating the morphology/conformation of individual nanoparticle oligomers or molecules with their photo-physics.[2, 3] In particular, we have employed the tip of an AFM cantilever to apply compressive stress to single molecules adsorbed on a surface and follow the effect of the impact on the electronic states of the molecule by fluorescence spectroscopy.[3] Quantum mechanical calculations corroborate that the spectral changes induced by the localized force can be associated to transitions among the different possible conformers of the adsorbed molecule.

Electron energy-loss spectroscopy (EELS) has become an experimental method of choice for the investigation of localized surface plasmon resonances, allowing the simultaneous mapping of the associated field distributions and their resonant energies with a nanoscale spatial resolution. The experimental observations have been well-supported by numerical models based on the computation of the Lorentz force acting on the impinging electrons by the scattered field. However, in this framework, the influence of the intrinsic properties of the plasmonic nanostructures studied with the electron energy-loss (EEL) measurements is somehow hidden in the global response. To overcome this limitation, we propose to go beyond this standard, and well-established, electron perspective and instead to interpret the EELS data using directly the intrinsic properties of the nanostructures, without regard to the force acting on the electron. The proposed method is particularly well-suited for the description of coupled plasmonic systems, because the role played by each individual nanoparticle in the observed EEL spectrum can be clearly disentangled, enabling a more subtle understanding of the underlying physical processes. As examples, we consider different plasmonic geometries in order to emphasize the benefits of this new conceptual approach for interpreting experimental EELS data. In particular, we use it to describe results from samples made by traditional thin film patterning and by arranging colloidal nanostructures.

This study presents the synthesis, SERS properties in three dimensions, and an application of 3D symmetric nanoporous silver microparticles. The particles are synthesized by purely chemical process: controlled precipitation of AgCl to acquire highly symmetric AgCl microparticle, followed by in-place to convert AgCl into nanoporous silver. The particles display highly predictable SERS enhancement pattern in three dimensions, which resembles particle shape and retains symmetry. The highly regular enhancement pattern allows an application in the study of inhomogeneity in two-layer polymer system, by improving spatial resolution in Z axis.

2D TMD and perovskite materials have attracted intensive research interests due to their unique electrooptical properties. Detailed understanding structural information and layer-layer interaction will help greatly in tailoring their properties for applications. We use Raman and PL spectroscopic/imaging techniques to study few-layer 2D TMD samples and perovskites. Our results show that layer-layer coupling and stacking sequence significantly affect spin-orbit coupling in 2D samples. High pressure study and ab initio calculations are used to elucidate the 4000 times PL enhancement in CH3NH3PbBr3. The proposed indirect to direct band gap transition is further confirmed using time resolved PL measurements.

Despite often illustrated as a perfect two-dimensional sheet, real graphene sample is not always flat. Nanostructures can be occurred on graphene sheet, especially for epitaxial graphene. The nanostructures alter the electrical and mechanical properties of graphene. This is crucial for epitaxial graphene since its main potential is in the electronics and optics application. This study investigates nanostructures on epitaxial graphene by tip-enhanced Raman spectroscopy, which is a technique that can provide Raman spectra with great spatial resolution, exceeding the diffraction limit of light. The results suggest that the compressive strain on nanoridges is weaker compared to neighbor flat area, supporting the ‘ridge as compressive strain relaxation’ mechanism. TERS measurement of nanoridges on epitaxial graphene microisland also indicates that the ‘Si vapor trapping’ mechanism for ridge formation is unlikely to occur.

Two-dimensional transitional metal dichalcogenide materials (2D TMD) provide new paradigm for the construction of novel devices based on heterostructures. The weak Van de Waals force between layers allows much easier growth and integration different 2D materials together to form devices with novel functionalities and applications. 2D TMD materials have attracted intense study in the past few years. Nonetheless, the optical and electronic structures of 2D materials often show strong stacking-dependent properties. For example, stacking order in MoS2 strongly affects the spin-orbital coupling which in turn determines the polarization of the light emitted. Detailed understanding of the inter-layer interaction will help greatly in tailoring the properties of 2D materials for applications. We have extensively used Raman/PL spectroscopy and imaging in the study of nano-materials and nano-devices, which provide critical information such as electronic structure, optical property, phonon structure, as well as defects, doping and stacking sequence. In this talk, we use Raman and PL techniques to study few-layer MoS2 samples. They show clear correlation with layer-thickness and stacking order. Our ab initio calculations reveal that difference in the electronic structures mainly arises from competition between spin-orbit coupling and interlayer coupling in different structural configurations.