A Review of Three-Dimensional Scanning Near-Field Optical Microscopy (3D-SNOM) and Its Applications in Nanoscale Light Management

Detection of surface enhanced Raman scattering active hotspot using near field scanning optical microscopy

Hotspots are high-intensity electromagnetic zones that form, for example, at the interstitials of plasmonic nanoaggregates, resulting in a considerable rise in the enhancement factor. However, it is inevitable to achieve specific nanometric geometry as well as a suitable technique to capture the details of hotspots. We report near-field surface-enhanced Raman scattering (SERS) spectroscopy of a well-defined gold nanoaggregate of a few nanoparticles adsorbed with a small number of target analytes. A spectrally and spatially resolved SERS measurement setup using an aperture near-field scanning optical microscope (a-NSOM) facilitated the direct observation of localized electromagnetic (EM) fields at the interstitials through SERS. Correlated optical image and corresponding nanometric geometry were captured through the home-built a-NSOM setup. Near-field SERS spectra were recorded at different sites of interest. It was evident that the interstitial positioned at the center of the tetramer provided the most intense Raman scattering, implying the possibility of a SERS-active hotspot therein. SERS bands of the spectrum of the Raman-active dye Rhodamine 6G recorded at the same hotspot coincided well with those reported so far. It was noteworthy that most of the SERS bands in such scenery got enhanced. Such direct observation with high spatial resolution is indispensable to understanding the origin of localized EM fields at "hotspots" and the EM enhancement factor in the SERS process. A finite-difference time-domain (FDTD) analysis was carried out to validate the results.

Step-Index (Semi-Immersed) Model for Photonic Nanojet and Experimental Characterization via Near-Field Optical Microscopy with Microcylinder

Experimental limitations such as design complexity and low optical throughput have prevented photonic nanojet (PNJ) and photonic hook (PH) measurements from demonstrating and characterizing the implementation of narrow intense electromagnetic beams generated from dielectric microelements with circular symmetry. Near-fields optical microscopy can mitigate these limitations and still present a capability of detecting a highly localized electromagnetic beam for applications in step-index media. Here we model a localized PNJ and PH formation in step-index media. We show that despite negligible refractive index contrast between the water (nwater=1.33) and silica microcylinder (∼1.1), a formation of PNJ and PH is observed with equivalent performance compared to that of silica microcylinder embedded in air (nair=1). This model features a practical fiber source and silica microcylinder as an auxiliary structure. Simultaneously, we performed experimental characterization of a photonic nanojet generated from an optical fiber and studied the resulting near-fields. Our electromagnetic simulation results are in good agreement with the experimental ones, demonstrating a full width at half maximum (FHWM) with a relative error of 0.64%. This system will make fiber-based nanojet realization and characterization accessible and practical for optics and laser engineering applications, super-resolution imaging, and nanolithography.

Selective triggering in-plane and out-of-plane dipolar modes of hexagonal Au nanoplate with the polarization of excitation beam

The dipolar responses of a single hexagonal Au nanoplate are investigated under the illuminations of linearly polarized beam and tightly focused radially polarized beam (RPB). It is found from the scattering spectra that the in-plane and out-of-plane electric dipole modes can be selectively triggered with a linearly polarized beam and tightly focused RPB, respectively. The features of these two dipolar modes are further confirmed in terms of electrical field and charge maps by the finite-difference time-domain simulation. Additionally, using the multipole expansion method, the existence of the out-of-plane dipole mode is further verified by the fact that thez-component of electric dipole response has a dominant contribution to the scattered power. Moreover, by combining the back focal plane imaging technique with the simulation, the appearance of in-plane and out-of-plane dipoles in the scattering pattern are clearly discerned. Our results provide an efficient method for selectively exciting the in-plane and out-of-plane dipolar modes of the nanoplate. We envision that the ease of tuning the dipolar momentum may facilitate the enhancement of the interaction between the plasmon and emitters at single-particle level.

Excitation and detection of acoustic phonons in nanoscale systems

Phonons play a key role in the physical properties of materials, and have long been a topic of study in physics. While the effects of phonons had historically been considered to be a hindrance, modern research has shown that phonons can be exploited due to their ability to couple to other excitations and consequently affect the thermal, dielectric, and electronic properties of solid state systems, greatly motivating the engineering of phononic structures. Advances in nanofabrication have allowed for structuring and phonon confinement even down to the nanoscale, drastically changing material properties. Despite developments in fabricating such nanoscale devices, the proper manipulation and characterization of phonons continues to be challenging. However, a fundamental understanding of these processes could enable the realization of key applications in diverse fields such as topological phononics, information technologies, sensing, and quantum electrodynamics, especially when integrated with existing electronic and photonic devices. Here, we highlight seven of the available methods for the excitation and detection of acoustic phonons and vibrations in solid materials, as well as advantages, disadvantages, and additional considerations related to their application. We then provide perspectives towards open challenges in nanophononics and how the additional understanding granted by these techniques could serve to enable the next generation of phononic technological applications.

Collection of propagating electromagnetic fields by uncoated probe

Understanding light-matter interaction at the nanoscale by observation of fine details of electromagnetic fields is achieved by bringing nanoscale probes into the nearfield of light sources, capturing information that is lost in the far field. Although metal coated probes are often used for nearfield microscopy, they strongly perturb the electromagnetic fields under study. Here, through experiment and simulation, we detail light collection by uncoated fiber probes, which minimize such perturbation. Second-harmonic light from intensely-irradiated sub-wavelength sub-surface features was imaged to avoid otherwise dominating fundamental light background, yielding clear nearfield details through a 50 nm aperture uncoated probe with ∼23 nm optical resolution. Simulations show how a metallic coating distorts optical nearfields and limits optical coupling into the probe in comparison to an uncoated probe.

Correlating structure, morphology and properties of metal nanostructures by combining single-particle optical spectroscopy and electron microscopy

The nanoscale morphology of metal nanostructures directly defines their optical, catalytic and electronic properties and even small morphological changes can cause significant property variations. On the one hand, this dependence allows for precisely tuning and exploring properties by shape engineering; next to advanced synthesis protocols, post-synthesis modification through tailored laser modification has become an emerging tool to do so. On the other hand, with this interconnection also comes the quest for detailed structure-property correlation and understanding of laser-induced reshaping processes on the individual nanostructure level beyond ensemble averages. With the development of single-particle (ultrafast) optical spectroscopy techniques and advanced electron microscopy such understanding can in principle be gained at the femtosecond temporal and atomic spatial scale, respectively. However, accessing both on the same individual nanostructure is far from straightforward as it requires the combination of optical spectroscopy and electron microscopy. In this Minireview, we highlight key studies from recent years that performed such correlative measurements on the same individual metal nanostructure either in a consecutive ex situ manner or in situ inside the electron microscope. We demonstrate that such a detailed correlation is critical for revealing the full picture of the structure-property relationship and the physics behind light-induced nanostructure modifications. We put emphasis on the advantages and disadvantages of each methodology as well as on the unique information that one can gain only by correlative studies performed on the same individual nanostructure and end with an outlook on possible further development of this field in the near future.

Nanoscale Imaging of Interstitial-dependent Optical Confinement through Near-Field Scanning Optical Microscopy

Exploitation of optical confinement in nanoscale unveils a wealth of information about the structure, optical, electronic, and chemical properties of the materials. However, realizing such confinement by optical microscopy and spectroscopic techniques have remained challenging due to fundamental formulation that is related to the diffraction theory of light. A state-of-art technique, known as near-field scanning optical microscopy (NSOM) has the ability to break such diffraction limitation, as the spatial resolution depends on the near-field probe diameter and the distance between the probe and the surface. A home-built apertured NSOM (a-NSOM) developed in the beginning of NSOM discovery facilitated to investigate N-particles nano-assemblies, where N is two or more. Through surface-sensitive spectroscopy such as surface-enhanced Raman scattering (SERS) and surface-enhanced two-photon-induced photoluminescence (TPI-PL), a correlated optometrology was revealed by taking snapshots of shear-force topography, SERS and TPI-PL simultaneously in single-channel and multi-channel detection system. Here in this "Personal Account" we have decorated near-field optical confinement observed by a-NSOM in three constructs; archetype dimer, nano-assembly of few nanoparticles and long-range two-dimensional (2D) nano-assembly. In the case of dimer, optical confinement was localized and interstitial-dependent whereas coalescence of nearby confinements was reported in few particles nanoaggregate. In the case of 2D nano-assembly, optical confinements were more complex because a nanoparticle was surrounded by six or more adjacent nanoparticles. FDTD simulation were carried out to support and validate the experimental observations. Such observations in nanoscale taking snapshots of nanometric topography and surface-sensitive spectroscopic signal not only inspire us to understand optical confinements in near-field, but also implement the concept in designing miniaturized and efficient system.

Scattering-type Scanning Near-Field Optical Microscopy of Polymer-Coated Gold Nanoparticles

Scattering-type scanning near-field optical microscopy (s-SNOM) has emerged over the past years as a powerful characterization tool that can probe important properties of advanced materials and biological samples in a label-free manner, with spatial resolutions lying in the nanoscale realm. In this work, we explore such usefulness in relationship with an interesting class of materials: polymer-coated gold nanoparticles (NPs). As thoroughly discussed in recent works, the interplay between the Au core and the polymeric shell has been found to be important in many applications devoted to biomedicine. We investigate bare Au NPs next to polystyrenesulfonate (PSS) and poly(diallyldimethylammonium chloride) (PDDA) coated ones under 532 nm laser excitation, an wavelength matching the surface plasmon band of the custom-synthesized nanoparticles. We observe consistent s-SNOM phase signals in the case of bare and shallow-coated Au NPs, whereas for thicker shell instances, these signals fade. For all investigated samples, the s-SNOM amplitude signals were found to be very weak, which may be related to reduced scattering efficiency due to absorption of the incident beam. We consider these observations important, as they may facilitate studies and applications in nanomedicine and nanotechnology where the precise positioning of polymer-coated Au NPs with nanoscale resolution is needed besides their dielectric function and related intrinsic optical properties, which are also quantitatively available with s-SNOM.

Preparation and characteristic analysis of nanofacula array

The development of nanofacula array is an effective methods to improve the performance of Near-field Scanning Optical Microscopy (NSOM) and achieve high-throughput array scanning. The nanofacula array is realized by preparing metal nanopore array through the "two etching-one development" method of double-layer resists and the negative lift-off process after metal film coating. The shading property of metal film plays important rules in nanofacula array fabrication. We investigate the shading coefficient of three kinds of metal films (gold–palladium alloy (Au/Pd), platinum (Pt), chromium (Cr)) under different coating times, and 3.5 min Au/Pd film is determined as the candidate of the nanofacula array fabrication for its lower thickness (about 23 nm) and higher shading coefficient (≥ 90%). The nanofacula array is obtained by irradiating with white light (central wavelength of 500 nm) through the metal nanopore array (250/450 nm pore diameter, 2 μm pore spacing and 7 μm group spacing). Moreover, the finite difference and time domain (FDTD) simulation proves that the combination of nanopore array and microlens array achieves high-energy focused nanofacula array, which shows a 3.2 times enhancement of electric field. It provides a new idea for NSOM to realize fast super-resolution focusing facula array.

Super-Resolution Imaging with Graphene

Super-resolution optical imaging is a consistent research hotspot for promoting studies in nanotechnology and biotechnology due to its capability of overcoming the diffraction limit, which is an intrinsic obstacle in pursuing higher resolution for conventional microscopy techniques. In the past few decades, a great number of techniques in this research domain have been theoretically proposed and experimentally demonstrated. Graphene, a special two-dimensional material, has become the most meritorious candidate and attracted incredible attention in high-resolution imaging domain due to its distinctive properties. In this article, the working principle of graphene-assisted imaging devices is summarized, and recent advances of super-resolution optical imaging based on graphene are reviewed for both near-field and far-field applications.

Advances in sperm analysis: techniques, discoveries and applications

Infertility affects one in six couples worldwide, and fertility continues to deteriorate globally, partly owing to a decline in semen quality. Sperm analysis has a central role in diagnosing and treating male factor infertility. Many emerging techniques, such as digital holography, super-resolution microscopy and next-generation sequencing, have been developed that enable improved analysis of sperm motility, morphology and genetics to help overcome limitations in accuracy and consistency, and improve sperm selection for infertility treatment. These techniques have also improved our understanding of fundamental sperm physiology by enabling discoveries in sperm behaviour and molecular structures. Further progress in sperm analysis and integrating these techniques into laboratories and clinics requires multidisciplinary collaboration, which will increase discovery and improve clinical outcomes.

Low-Background Tip-Enhanced Raman Spectroscopy Enabled by a Plasmon Thin-Film Waveguide Probe

Tip-enhanced Raman spectroscopy (TERS) is a nano-optical approach to extract spatially resolved chemical information with nanometer precision. However, in the case of direct-illumination TERS, which is often employed in commercial TERS instruments, strong fluorescence or far-field Raman signals from the illuminated areas may be excited as a background. They may overwhelm the near-field TERS signal and dramatically decrease the near-field to far-field signal contrast of TERS spectra. It is still challenging for TERS to study the surface of fluorescent materials or a bulk sample that cannot be placed on an Au/Ag substrate. In this study, we developed an indirect-illumination TERS probe that allows a laser to be focused on a flat interface of a thin-film waveguide located far away from the region generating the TERS signal. Surface plasmon polaritons are generated stably on the waveguide and eventually accumulated at the tip apex, thereby producing a spatially and energetically confined hotspot to ensure stable and high-resolution TERS measurements with a low background. With this thin-film waveguide probe, TERS spectra with obvious contrast from a diamond plate can be acquired. Furthermore, the TERS technique based on this probe exhibits excellent TERS signal stability, a long lifetime, and good spatial resolution. This technique is expected to have commercial potential and enable further popularization and development of TERS technology as a powerful analytical method.

In situ photoconductivity measurements of imidazole in optical fiber break-junctions

We developed a method based on the mechanically controllable break junction technique to investigate the electron transport properties of single molecular junctions upon fiber waveguided light. In our strategy, a metal-coated tapered optical fiber is fixed on a flexible substrate, and this tapered fiber serves as both the optical waveguide and metal electrodes after it breaks. For an imidazole bridged single-molecule junction, two probable conductance values below 1G0 are observed. The higher value shows an approximately 40% enhancement under illumination, while the lower one does not show distinguishable difference under illumination. Theoretical calculations reveal these two conductance values resulting from the imidazole monomer junction and the imidazole dimer junction linked via a hydrogen bond, respectively. In imidazole monomer junctions, the absorption of a single photon strongly shifts the transmission function resulting in optical-induced conductance enhancement. In contrast, the transmission function of imidazole dimer junctions remains at the same level in the bias window despite the light illumination. This work provides a robust experimental framework for studying the underlying mechanisms of photoconductivity in single-molecule junctions and offers tools for tuning the optoelectronic performance of single-molecule devices in situ.

Scanning Probe Microscopies: Imaging and Biomechanics in Reproductive Medicine Research

Basic and translational research in reproductive medicine can provide new insights with the application of scanning probe microscopies, such as atomic force microscopy (AFM) and scanning near-field optical microscopy (SNOM). These microscopies, which provide images with spatial resolution well beyond the optical resolution limit, enable users to achieve detailed descriptions of cell topography, inner cellular structure organization, and arrangements of single or cluster membrane proteins. A peculiar characteristic of AFM operating in force spectroscopy mode is its inherent ability to measure the interaction forces between single proteins or cells, and to quantify the mechanical properties (i.e., elasticity, viscoelasticity, and viscosity) of cells and tissues. The knowledge of the cell ultrastructure, the macromolecule organization, the protein dynamics, the investigation of biological interaction forces, and the quantification of biomechanical features can be essential clues for identifying the molecular mechanisms that govern responses in living cells. This review highlights the main findings achieved by the use of AFM and SNOM in assisted reproductive research, such as the description of gamete morphology; the quantification of mechanical properties of gametes; the role of forces in embryo development; the significance of investigating single-molecule interaction forces; the characterization of disorders of the reproductive system; and the visualization of molecular organization. New perspectives of analysis opened up by applying these techniques and the translational impacts on reproductive medicine are discussed.

How did correlative atomic force microscopy and super-resolution microscopy evolve in the quest for unravelling enigmas in biology?

With the invention of the Atomic Force Microscope (AFM) in 1986 and the subsequent developments in liquid imaging and cellular imaging it became possible to study the topography of cellular specimens under nearly physiological conditions with nanometric resolution. The application of AFM to biological research was further expanded with the technological advances in imaging modes where topographical data can be combined with nanomechanical measurements, offering the possibility to retrieve the biophysical properties of tissues, cells, fibrous components and biomolecules. Meanwhile, the quest for breaking the Abbe diffraction limit restricting microscopic resolution led to the development of super-resolution fluorescence microscopy techniques that brought the resolution of the light microscope comparable to the resolution obtained by AFM. The instrumental combination of AFM and optical microscopy techniques has evolved over the last decades from integration of AFM with bright-field and phase-contrast imaging techniques at first to correlative AFM and wide-field fluorescence systems and then further to the combination of AFM and fluorescence based super-resolution microscopy modalities. Motivated by the many developments made over the last decade, we provide here a review on AFM combined with super-resolution fluorescence microscopy techniques and how they can be applied for expanding our understanding of biological processes.

Universal Transfer Printing of Micelle-Templated Nanoparticles Using Plasma-Functionalized Graphene

Nanostructure incorporation into devices plays a key role in improving performance, yet processes for preparing two-dimensional (2D) arrays of colloidal nanoparticles tend not to be universally applicable, particularly for soft and oxygen-sensitive substrates for organic and perovskite-based electronics. Here, we show a method of transferring reverse micelle-deposited (RMD) nanoparticles (perovskite and metal oxide) on top of an organic layer, using a functionalized graphene carrier layer for transfer printing. As the technique can be applied universally to RMD nanoparticles, we used magnetic (γ-Fe₂O₃) and luminescent (methylammonium lead bromide (MAPbBr₃)) nanoparticles to validate the transfer-printing methodology. The strong photoluminescence from the MAPbBr₃ under UV illumination and high intrinsic field of the γ-Fe₂O₃ as measured by magnetic force microscopy (MFM), coupled with Raman measurements of the graphene layer, confirm that all components survive the transfer-printing process with little loss of properties. Such an approach to introducing uniform 2D arrays of nanoparticles onto sensitive substrates opens up new avenues to tune the device interfacial properties.

Nano illumination microscopy: a technique based on scanning with an array of individually addressable nanoLEDs

In lensless microscopy, spatial resolution is usually provided by the pixel density of current digital cameras, which are reaching a hard-to-surpass pixel size / resolution limit over 1 µm. As an alternative, the dependence of the resolving power can be moved from the detector to the light sources, offering a new kind of lensless microscopy setups. The use of continuously scaled-down Light-Emitting Diode (LED) arrays to scan the sample allows resolutions on order of the LED size, giving rise to compact and low-cost microscopes without mechanical scanners or optical accessories. In this paper, we present the operation principle of this new approach to lensless microscopy, with simulations that demonstrate the possibility to use it for super-resolution, as well as a first prototype. This proof-of-concept setup integrates an 8 × 8 array of LEDs, each 5 × 5 μm² pixel size and 10 μm pitch, and an optical detector. We characterize the system using Electron-Beam Lithography (EBL) pattern. Our prototype validates the imaging principle and opens the way to improve resolution by further miniaturizing the light sources.

Ultra-structural analysis of human spermatozoa by aperture scanning near-field optical microscopy

Scanning near-field optical microscopy (SNOM) represents a potential candidate for investigation of ultrastructure in human spermatozoa. It is a noninvasive optical technique that offers two main advantages: minimal sample preparation and simultaneous topographical and optical images acquisition with a spatial resolution beyond the diffraction limit. This enables the combination of surface characterization and information from the inner cellular organization in a single acquisition providing an immediate and comprehensive analysis of the cellular portions. In this work spermatozoa are immobilized on poly-L-lysine coated coverslips, fixed according to a standard protocol and imaged by aperture-SNOM in air. In the SNOM images, all peculiar sperm portions show well-resolved optical features, which exhibit good similarities with the structures revealed in transmission electron microscopy images, as compared with literature data. The optical features of anomalous spermatozoa are clearly different as respect with those observed for healthy ones. This analysis reveals the potentialities of SNOM and opens to its application to high-resolution analysis of sperm morphological alterations, which might be relevant in reproductive medicine.

Direct molecular-level near-field plasmon and temperature assessment in a single plasmonic hotspot

Tip-enhanced Raman spectroscopy (TERS) is currently widely recognized as an essential but still emergent technique for exploring the nanoscale. However, our lack of comprehension of crucial parameters still limits its potential as a user-friendly analytical tool. The tip's surface plasmon resonance, heating due to near-field temperature rise, and spatial resolution are undoubtedly three challenging experimental parameters to unravel. However, they are also the most fundamentally relevant parameters to explore, because they ultimately influence the state of the investigated molecule and consequently the probed signal. Here we propose a straightforward and purely experimental method to access quantitative information of the plasmon resonance and near-field temperature experienced exclusively by the molecules directly contributing to the TERS signal. The detailed near-field optical response, both at the molecular level and as a function of time, is evaluated using standard TERS experimental equipment by simultaneously probing the Stokes and anti-Stokes spectral intensities. Self-assembled 16-mercaptohexadodecanoic acid monolayers covalently bond to an ultra-flat gold surface were used as a demonstrator. Observation of blinking lines in the spectra also provides crucial information on the lateral resolution and indication of atomic-scale thermally induced morphological changes of the tip during the experiment. This study provides access to unprecedented molecular-level information on physical parameters that crucially affect experiments under TERS conditions. The study thereby improves the usability of TERS in day-to-day operation. The obtained information is of central importance for any experimental plasmonic investigation and for the application of TERS in the field of nanoscale thermometry.

Spatial characteristics of optical fields near a gold nanorod revealed by three-dimensional scanning near-field optical microscopy

We visualize plasmon mode patterns induced in a single gold nanorod by three-dimensional scanning near-field optical microscopy. From the near-field transmission imaging, we find that 3rd and 4th order plasmon modes are resonantly excited in the nanorod. We perform electromagnetic simulations based on the discrete dipole approximation method under focused Gaussian beam illumination and demonstrate that the observed near-field spectral and spatial features are well reproduced by the simulation. We also reveal from the three-dimensional near-field microscopy that the 4th order plasmon mode confines optical fields more tightly compared with the 3rd order mode. This result indicates that the even-order plasmon modes are promising for enhancing the light-matter interactions.

A fully automated dual-tip scanning near-field optical microscope for localized optical excitation and detection in the visible and near-infrared

Near-field optical microscopes with two independent tips for simultaneous excitation and detection can be essential tools for studying localized optical phenomena on the subwavelength scale. Here, we report on the implementation of a fully automated and robust dual-tip scanning near-field optical microscope (SNOM), in which the excitation tip is stationary, while the detection tip automatically scans the surrounding area. To monitor and control the distance between the two probes, mechanical interactions due to shear forces are used. We experimentally investigate suitable scan parameters and find that the automated dual-tip SNOM can operate stably for a wide range of parameters. To demonstrate the potential of the automated dual-tip SNOM, we characterize the propagation of surface plasmon polaritons on a gold film for visible and near-infrared wavelengths. The good agreement of the measurements with numerical simulations verifies the capability of the dual-tip SNOM for the near-field characterization of localized optical phenomena.

Optical plasmon nanostrip probe as an effective ultrashort pulse delivery system

In this paper, we analyze the ultrafast temporal and spectral responses of optical fields in tapered and metalized optical fibers (MOFs) and optical plasmon nanostrip probes (NPs). Computational experiment shows that output pulses of the NPs are virtually unchanged in shape and duration for input pulses with a duration of >1 fs and are not sensitive to changes in the parameters of the probe (such as convergence angle and taper length), while local enhancement of the electric field intensity reaches 300 times at the NP apex. Compared with the NPs, MOFs lead to significant output pulse distortions, even for input pulses with a duration of 10 fs. In addition, the temporal response at the MOF apex is critically sensitive to changes in MOF parameters and cannot provide any significant local enhancement of the electric field. These findings reveal the high potential of optical plasmon nanostrip probes as an ultrashort pulse delivery system to nanometer-size areas and indicate that its usage can be promising for a wide variety of techniques studying ultrafast processes in nanoscopic volumes.

Characterization of Overlapped Plasmon Modes in a Gold Hexagonal Plate Revealed by Three-Dimensional Near-Field Optical Microscopy

A detailed characterization of plasmon modes is important not only for a deeper understanding of plasmons but also for their practical applications. In this study, we investigated the three-dimensional near-field characteristics of high-order plasmon modes excited in a gold hexagonal nanoplate. From the near-field spectroscopic images, we found that both in-plane and out-of-plane plasmon modes observed near 900 nm were spectrally and spatially overlapped. We performed three-dimensional near-field measurement to reveal the optical characteristics of the overlapped modes in detail. We found that the steric near-field distribution near the nanoplate strongly depended on the plasmon mode, and the out-of-plane mode confines electromagnetic fields more tightly than the in-plane mode. We also found that the in-plane mode was dominantly visualized as the probe tip-sample distance increased. These findings demonstrate that the three-dimensional near-field technique enables selective visualization of a single plasmon mode even if multiple modes are spatially and spectrally overlapped.

Optical properties of molecular nanocrystals consisting of J-aggregates of anionic and cationic cyanine dyes

We report results of experimental studies of the photoabsorption, photoluminescent and photoelectric properties of a new type of multilayer molecular nanocrystals, consisting of highly ordered J-aggregates of one anionic and two cationic J-aggregates of cyanine dyes. In contrast to conventional J-aggregated dyes the multichromic nanocrystals synthesized in this work, are capable of efficient light absorption in three excitonic bands of the visible and near-IR spectral ranges. The spectral peak positions in the absorption bands can be controlled by appropriately selecting a set of dyes a molecular crystal is made of. Our investigations of the photoelectric properties of multichromic crystals have shown that each of them can potentially be used as a photosensitive layer of a photocell with photoconductivity in three peaks of excitonic absorption. The synthesized nanocrystals are attractive for the creation of thin-film organic photodetectors with a large photosensitive area and varied photoabsorption spectra, excitonic waveguides and for some other applications in organic and hybrid photonics and optoelectronics.

Nanoscale Optical Microscopy and Spectroscopy Using Near-Field Probes

Light-matter interactions can provide a wealth of detailed information about the structural, electronic, optical, and chemical properties of materials through various excitation and scattering processes that occur over different length, energy, and timescales. Unfortunately, the wavelike nature of light limits the achievable spatial resolution for interrogation and imaging of materials to roughly λ/2 because of diffraction. Scanning near-field optical microscopy (SNOM) breaks this diffraction limit by coupling light to nanostructures that are specifically designed to manipulate, enhance, and/or extract optical signals from very small regions of space. Progress in the SNOM field over the past 30 years has led to the development of many methods to optically characterize materials at lateral spatial resolutions well below 100 nm. We review these exciting developments and demonstrate how SNOM is truly extending optical imaging and spectroscopy to the nanoscale.

Plasmon response evaluation based on image-derived arbitrary nanostructures

The optical response of realistic 3D plasmonic substrates composed of randomly shaped particles of different size and interparticle distance distributions in addition to nanometer scale surface roughness is intrinsically challenging to simulate due to computational limitations. Here, we present a Finite Element Method (FEM)-based methodology that bridges in-depth theoretical investigations and experimental optical response of plasmonic substrates composed of such silver nanoparticles. Parametrized scanning electron microscopy (SEM) images of surface enhanced Raman spectroscopy (SERS) active substrate and tip-enhanced Raman spectroscopy (TERS) probes are used to simulate the far-and near-field optical response. Far-field calculations are consistent with experimental dark field spectra and charge distribution images reveal for the first time in arbitrary structures the contributions of interparticle hybridized modes such as sub-radiant and super-radiant modes that also locally organize as basic units for Fano resonances. Near-field simulations expose the spatial position-dependent impact of hybridization on field enhancement. Simulations of representative sections of TERS tips are shown to exhibit the same unexpected coupling modes. Near-field simulations suggest that these modes can contribute up to 50% of the amplitude of the plasmon resonance at the tip apex but, interestingly, have a small effect on its frequency in the visible range. The band position is shown to be extremely sensitive to particle nanoscale roughness, highlighting the necessity to preserve detailed information at both the largest and the smallest scales. To the best of our knowledge, no currently available method enables reaching such a detailed description of large scale realistic 3D plasmonic systems.

Detection of Intracellular Gold Nanoparticles: An Overview

Photothermal therapy (PTT) takes advantage of unique properties of gold nanoparticles (AuNPs) (nanospheres, nanoshells (AuNSs), nanorods (AuNRs)) to destroy cancer cells or tumor tissues. This is made possible thanks principally to both to the so-called near-infrared biological transparency window, characterized by wavelengths falling in the range 700–1100 nm, where light has its maximum depth of penetration in tissue, and to the efficiency of cellular uptake mechanisms of AuNPs. Consequently, the possible identification of intracellular AuNPs plays a key role for estimating the effectiveness of PTT treatments. Here, we review the recognized detection techniques of such intracellular probes with a special emphasis to the exploitation of near-infrared biological transparency window.