Surface plasmon resonance imaging of cell-substrate contacts with radially polarized beams

Optical generation and continuous transformation of plasmonic skyrmions

Topological quasiparticles, including skyrmions and merons, are topological textures with sophisticated vectorial structures that can be used for high-density information storage, precision metrology, position sensing, etc. Here, we realized the optical generation and continuous transformation of plasmonic field skyrmions. We generated the isolated Néel-type skyrmion using surface plasmon polaritons (SPPs) excited by a focused structured light on a silver film. We used a square and a hexagonal aperture for symmetry constraints and successfully generated the meron lattice and the skyrmion lattice. We unveiled the mechanism of topological texture generation and transformation and optimized the distribution of skyrmion and meron topologies. We further demonstrated the continuous transformation among the isolated skyrmion, the meron lattice, and the skyrmion lattice using well-designed circular-fourfold, circular-sixfold, and fourfold-sixfold symmetry apertures, respectively. This work can open up a pathway for the generation and transformation of skyrmion and meron topologies, which is expected to facilitate new applications in optical information storage and encoding.

Recent advances in surface plasmon resonance imaging and biological applications

Surface Plasmon Resonance Imaging (SPRI) is a robust technique for visualizing refractive index changes, which enables researchers to observe interactions between nanoscale objects in an imaging manner. In the past period, scholars have been attracted by the Prism-Coupled and Non-prism Coupled configurations of SPRI and have published numerous experimental results. This review describes the principle of SPRI and discusses recent developments in Prism-Coupled and Non-prism Coupled SPRI techniques in detail, respectively. And then, major advances in biological applications of SPRI are reviewed, including four sub-fields (cells, viruses, bacteria, exosomes, and biomolecules). The purpose is to briefly summarize the recent advances of SPRI and provide an outlook on the development of SPRI in various fields.

Circular nanocavity substrate-assisted plasmonic tip for its enhancement in nanofocusing and optical trapping

Plasmonic tip nanofocusing has widely been applied in tip-enhanced Raman spectroscopy, optical trapping, nonlinear optics, and super-resolution imaging due to its capability of high local field enhancement. In this work, a substrate with a circular nanocavity is proposed to enhance the nanofocusing and optical trapping characteristics of the plasmonic tip. Under axial illumination of a tightly focused radial polarized beam, the circular nanohole etched on a metallic substrate can form a nanocavity to induce an interference effect and further enhance the electric field intensity. When a plasmonic tip is placed closely above such a substrate, the electric field intensity of the gap-plasmon mode can further be improved, which is 10 folds stronger than that of the conventional gap-plasmon mode. Further analysis reveals that the enhanced gap-plasmon mode can significantly strengthen the optical force exerted on a nanoparticle and stably trap a 4-nm-diameter dielectric nanoparticle. Our proposed method can improve the performance of tip-enhanced spectroscopy, plasmonic tweezers and extend their applications. We anticipate that our methods allow simultaneously manipulating and characterizing single nanoparticles in-situ.

Plasmonic tweezers: for nanoscale optical trapping and beyond

Optical tweezers and associated manipulation tools in the far field have had a major impact on scientific and engineering research by offering precise manipulation of small objects. More recently, the possibility of performing manipulation with surface plasmons has opened opportunities not feasible with conventional far-field optical methods. The use of surface plasmon techniques enables excitation of hotspots much smaller than the free-space wavelength; with this confinement, the plasmonic field facilitates trapping of various nanostructures and materials with higher precision. The successful manipulation of small particles has fostered numerous and expanding applications. In this paper, we review the principles of and developments in plasmonic tweezers techniques, including both nanostructure-assisted platforms and structureless systems. Construction methods and evaluation criteria of the techniques are presented, aiming to provide a guide for the design and optimization of the systems. The most common novel applications of plasmonic tweezers, namely, sorting and transport, sensing and imaging, and especially those in a biological context, are critically discussed. Finally, we consider the future of the development and new potential applications of this technique and discuss prospects for its impact on science.

Refractive index of biological tissues: Review, measurement techniques, and applications

Refractive index (RI) is a characteristic optical variable that controls the propagation of light in the medium (e.g., biological tissues). Basic research with the aim to investigate the RI of biological tissues is of paramount importance for biomedical optics and associated applications. Herein, we reviewed and summarized the RI data of biological tissues and the associated insights. Different techniques for the measurement of RI of biological tissues are also discussed. Moreover, several examples of the RI applications from basic research, clinics and optics industry are outlined. This study may provide a comprehensive reference for RI data of biological tissues for the biomedical research and beyond.

High-performance imaging of cell-substrate contacts using refractive index quantification microscopy

Non-invasive imaging of living cells is an advanced technique that is widely used in the life sciences and medical research. We demonstrate a refractive index quantification microscopy (RIQM) that enables label-free studies of glioma cell-substrate contacts involving cell adhesion molecules and the extracellular matrix. This microscopy takes advantage of the smallest available spot created when an azimuthally polarized perfect optical vortex beam (POV) is tightly focused with a first-order spiral phase, which results in a relatively high imaging resolution among biosensors. A high refractive index (RI) resolution enables the RI distribution within neuronal cells to be monitored. The microscopy shows excellent capability for recognizing cellular structures and activities, demonstrating great potential in biological sensing and live-cell kinetic imaging.

Cell refractive index: Models, insights, applications and future perspectives

Cell refractive index (RI) is an intrinsic optical parameter that governs the propagation of light (i.e., scattering and absorption) in the cell matrix. The RI of cell is sensitively correlated with its mass distribution and thereby has the capability to provide important insights for diverse biological models. Herein, we review the cell refractive index and the fundamental models for measurement of cell RI, summarize the published RI data of cell and cell organelles and discuss the associated insights. Illustrative applications of cell RI in cell biology are also outlined. Finally, future research trends and applications of cell RI, including novel imaging techniques, reshaping flow cytometry and microfluidic platforms for single cell manipulation are discussed. The rapid technological advances in optical imaging integrated with microfluidic regime seems to enable deeper understanding of subcellular dynamics with high spatio-temporal resolution in real time.

Platinum substrate for surface plasmon microscopy at small angles

Platinum is reported as the main component of the substrate in surface plasmon microscopy of the metal-dielectric interface for small-angle measurements. In the absence of a narrow dip in the angular spectrum of platinum, the refractive index of the dielectric medium or the thickness of a deposited layer is proven deducible from the observed sharp peak, close to the critical angle. The sensitivities of refractive index and thickness measurements using platinum are compared with that of a gold surface plasmon resonance chip. Furthermore, the thickness of a structured layer of (3-Aminopropyl)triethoxysilane on the platinum substrate is measured to be 0.7 nm, demonstrating the high sensitivity of the technique.

Lab-on-a-Chip Technologies for the Single Cell Level: Separation, Analysis, and Diagnostics

In the last three decades, microfluidics and its applications have been on an exponential rise, including approaches to isolate rare cells and diagnose diseases on the single-cell level. The techniques mentioned herein have already had significant impacts in our lives, from in-the-field diagnosis of disease and parasitic infections, through home fertility tests, to uncovering the interactions between SARS-CoV-2 and their host cells. This review gives an overview of the field in general and the most notable developments of the last five years, in three parts: 1. What can we detect? 2. Which detection technologies are used in which setting? 3. How do these techniques work? Finally, this review discusses potentials, shortfalls, and an outlook on future developments, especially in respect to the funding landscape and the field-application of these chips.

Graphene-Based Confocal Refractive Index Microscopy for Label-Free Differentiation of Living Epithelial and Mesenchymal Cells

Label-free imaging and investigation of living cells are significant for many biomedical studies. It has been challenging to detect the epithelial-mesenchymal transition of cells in situ without affecting cellular activity. Here, we present a common-path differential confocal microscope based on the polarization-sensitive absorption of graphene to realize high-performance refractive index imaging and differentiation of living colorectal cancer cells (HCT116) with large detecting depth (1.29 μm), excellent refractive index resolution (2.86 × 10-5 RIU), and high spatial resolution (727 nm) simultaneously. Compared with epithelial (parental HCT116) cells, mesenchymal (paclitaxel-resistant HCT116) cells manifest generally lower refractive index values through the refractive index statistics, which is due to the stronger migration ability and weaker surface adherence of mesenchymal cells. The graphene-based microscopy provides an effective label-free approach to high-resolution imaging and study of living cell kinetics, and we expect it to be widely used in the research fields of pathology, tumorigenesis, and chemotherapy.

Enhanced surface plasmon microscopy based on multi-channel spatial light switching for label-free neuronal imaging

In this paper, we have investigated multi-channel switching of light incidence in multiple directions to improve image clarity in surface plasmon microscopy (SPM) for robust and consistent imaging performance regardless of the pattern geometry and shape. Multi-channel light switching in SPM allows significant reduction of adverse scattering effects by surface plasmon (SP). For proof of concept, an eight-channel spatially switched SPM (ssSPM) system has been set up. The results with reference objects including square arrays and Siemens stars experimentally confirm much improved images with ssSPM by reducing the artifacts of SP scattering significantly. On a quantitative basis, contrast analysis preformed with square arrays shows image contrast enhanced by more than three times over conventional SPM. Three image reconstruction algorithms were evaluated for optimal image acquisition. It is suggested that averaging combined with minimum-filtering produces the highest resolution. ssSPM was applied to label-free imaging of primary neuron cultures and shown to present enhanced images with clarity far better than conventional SPM.

Refractive index sensing and imaging based on polarization-sensitive graphene

Graphene exhibits extraordinary opto-electronic properties due to its unique dynamic conductivity, bringing great value in optical sensing, surface plasmon modulation and photonic devices. Based on the polarization-sensitive absorption of graphene working at near infrared to ultraviolet wavelengths, we theoretically investigate the refractive index sensing and imaging mechanism under oblique and tight focusing incidences of light respectively. We demonstrate that such graphene-based methods can provide ultrahigh refractive index resolution (∼2.09×10^-8 RIU) for label-free sensing, and high transverse spatial resolution (∼200 nm) and large longitudinal detecting length (∼750 nm) for imaging under 532 nm incident wavelength. The proposed methods could potentially guide future researches in graphene optical detection, non-invasive biological sensing and imaging, and other applications.

Characterizing Large-Scale Receptor Clustering on the Single Cell Level: A Comparative Plasmon Coupling and Fluorescence Superresolution Microscopy Study

Spatial clustering of cell membrane receptors has been indicated to play a regulatory role in signal initiation, and the distribution of receptors on the cell surface may represent a potential biomarker. To realize its potential for diagnostic purposes, scalable assays capable of mapping spatial receptor heterogeneity with high throughput are needed. In this work, we use gold nanoparticle (NP) labels with an average diameter of 72.17 ± 2.16 nm as bright markers for large-scale epidermal growth factor receptor (EGFR) clustering in hyperspectral plasmon coupling microscopy and compare the obtained clustering maps with those obtained through fluorescence superresolution microscopy (direct stochastic optical reconstruction microscopy, dSTORM). Our dSTORM experiments reveal average EGFR cluster sizes of 172 ± 99 and 150 ± 90 nm for MDA-MB-468 and HeLa, respectively. The cluster sizes decrease after EGFR activation. Hyperspectral imaging of the NP labels shows that differences in the EGFR cluster sizes are accompanied by differences in the average separations between electromagnetically coupled NPs. Because of the distance dependence of plasmon coupling, changes in the average interparticle separation result in significant spectral shifts. For the experimental conditions investigated in this work, hyperspectral plasmon coupling microscopy of NP labels identified the same trends in large-scale EGFR clustering as dSTORM, but the NP imaging approach provided the information in a fraction of the time. Both dSTORM and hyperspectral plasmon coupling microscopy confirm the cortical actin network as one structural component that determines the average size of EGFR clusters.

Common-path surface plasmon interferometer with radial polarization

We present a common-path surface plasmon interferometer with radial polarization. We show how the V(z) effect, the output of the microscope versus defocus z, can be derived utilizing a radially polarized illumination and a virtual annulus. The measurement of the V(z) effect gives a strong signature of the surface plasmon propagation, which is functionally related to the material properties. We discuss the advantages of using radial polarization compared to linear polarization.

Demonstration of a terahertz pure vector beam by tailoring geometric phase

We demonstrate the creation of a vector beam by tailoring geometric phase of left- and right- circularly polarized beams. Such a vector beam with a uniform phase has not been demonstrated before because a vortex phase remains in the beam. We focus on vortex phase cancellation to generate vector beams in terahertz regions, and measure the geometric phase of the beam and its spatial distribution of polarization. We conduct proof-of-principle experiments for producing a vector beam with radial polarization and uniform phase at 0.36 THz. We determine the vortex phase of the vector beam to be below 4%, thus highlighting the extendibility and availability of the proposed concept to the super broadband spectral region from ultraviolet to terahertz. The extended range of our proposed techniques could lead to breakthroughs in the fields of microscopy, chiral nano-materials, and quantum information science.

Surface plasmon microscopy by spatial light switching for label-free imaging with enhanced resolution

In this Letter, we describe spatially switched surface plasmon microscopy (ssSPM) based on two-channel momentum sampling. The performance evaluated with periodic nanowires in comparison with conventional SPM and bright-field microscopy shows that the resolution of ssSPM is enhanced by almost 15 times over conventional SPM. ssSPM provides an extremely simple way to attain diffraction limit in SPM and to go beyond for super-resolution in label-free microscopy techniques.

Real-time subcellular imaging based on graphene biosensors

Non-invasive living cell microscopy in real time is essential for a wide variety of biomedical research. Here, we present a subcellular refractive index imaging technique for living cells based on a graphene biosensor system. Owing to the optical reflectivity differences of graphene to s- and p-polarizations, a 45° generalized-cylindrical-vector-polarized laser beam is employed to demodulate the reflected cylindrical vector beam for differential detecting. Benefitting from the vector beam-enabled common-path graphene biosensor, the imaging spatial resolution and refractive index sensitivity are noticeably improved. Subcellular refractive index mapping of live human colonic cancer cells is perfectly achieved without inducing any cell damage. Furthermore, real-time monitoring of an individual cell is also performed with the disassembly of the cell nucleolus clearly observed. This technique would be a promising tool for the study of living cell morphology, kinetics, and pathology, and for other biomedical research.

Out-of-Focal Plane Imaging by Leakage Radiation Microscopy

Leakage radiation microscopy (LRM) is used to investigate the optical properties of surfaces. The front-focal plane (FFP) image with LRM reveals structural features on the surfaces. Back-focal plane (BFP) image with LRM reveals the angular distribution of the radiation. Herein we experimentally demonstrate that the out-of-focal plane (OFP) images present a link between the FFP and BFP images and provide optical information that cannot be resolved by either FFP or BFP images. The OFP image provides a linkage between the spatial location of the emission and the angular distribution from the same location, and thus information about the film's discontinuity, nonuniformity or variable thickness can be uncovered. The use of OFP imaging will extend the scope and applications of the LRM and coupled emission imaging which are powerful tools in nanophotonics and high throughput fluorescence screening.

Cell refractive index for cell biology and disease diagnosis: past, present and future

Cell refractive index is a key biophysical parameter, which has been extensively studied. It is correlated with other cell biophysical properties including mechanical, electrical and optical properties, and not only represents the intracellular mass and concentration of a cell, but also provides important insight for various biological models. Measurement techniques developed earlier only measure the effective refractive index of a cell or a cell suspension, providing only limited information on cell refractive index and hence hindering its in-depth analysis and correlation. Recently, the emergence of microfluidic, photonic and imaging technologies has enabled the manipulation of a single cell and the 3D refractive index of a single cell down to sub-micron resolution, providing powerful tools to study cells based on refractive index. In this review, we provide an overview of cell refractive index models and measurement techniques including microfluidic chip-based techniques for the last 50 years, present the applications and significance of cell refractive index in cell biology, hematology, and pathology, and discuss future research trends in the field, including 3D imaging methods, integration with microfluidics and potential applications in new and breakthrough research areas.

Time-lapse scanning surface plasmon microscopy of living adherent cells with a radially polarized beam

We report on a fibered high-resolution scanning surface plasmon microscope for long term imaging of living adherent cells. The coupling of a high numerical aperture objective lens and a fibered heterodyne interferometer enhances both the sensitivity and the long term stability of this microscope, allowing for time-lapse recording over several days. The diffraction limit is reached with a radially polarized illumination beam. Adherence and motility of living C2C12 myoblast cells are followed for 50 h, revealing that the dynamics of these cells change after 10 h. This plasmon enhanced evanescent wave microscopy is particularly suited for investigating cell adhesion, since it can not only be performed without staining of the sample but it can also capture in real time the exchange of extracellular matrix elements between the substrate and the cells.

Resonant evanescent complex fields on dielectric multilayers

Complex light fields, including evanescent Bessel beams, can be generated at dielectric interfaces by means of oil-immersion optics operating in total internal reflection conditions. Here we report on the observation of evanescent complex fields produced on a dielectric multilayer through the interference of surface modes resonantly sustained by the multilayer itself. The coupling to surface modes is attained by modifying the wavefront of an incident laser beam in such a way that the resulting intensity distribution in k-space matches the dispersion of the surface mode. The phase of surface modes can be further controlled, and two-dimensional vortex beams can also be produced according to the same working principle.

Planar Photonic Crystal Biosensor for Quantitative Label-Free Cell Attachment Microscopy

In this study, a planar-surface photonic crystal (PC) biosensor for quantitative, kinetic, label-free imaging of cell-surface interactions is demonstrated. The planar biosensor surface eliminates external stimuli to the cells caused by substrate topography to more accurately reflect smooth surface environment encountered by many cell types in vitro. Here, a fabrication approach that combines nanoreplica molding and a horizontal dipping process is used to planarize the surface of the PC biosensor. The planar PC biosensor maintains a high detection sensitivity that enables the monitoring of live cell-substrate interactions with spatial resolution sufficient for observing intracellular attachment strength gradients and the extensions of filopodia from the cell body. The evolution of cell morphology during the attachment and spreading process of 3T3 fibroblast cells is compared between planar and grating-structured PC biosensors. The planar surface effectively eliminates the directionally biased cellular attachment behaviors that are observed on the grating-structured surface. This work represents an important step forward in the development of label-free techniques for observing cellular processes without unintended external environmental modulation.

Characteristic investigation of scanning surface plasmon microscopy for nucleotide functionalized nanoarray

A calculation based on surface plasmon coupling condition and Maxwell-Garnett equation was performed for predicting the coupling angle shift and thin film thickness in scanning surface plasmon microscopy (SSPM). The refractive index sensitivity and lateral resolution of an SSPM system was also investigated. The limit of detection of angle shift was 0.01°, the limit of quantification of angle shift was 0.03°, and the sensitivity was around 0.12° shift per nm ZnO film when the film thickness was less than 22.6 nm. Two partially connected Au nano-discs with a center-to-center distance of 1.1 μm could be identified as two peaks. The system was applied to image nanostructure defects and a virus-probe functionalized nanoarray. We expect the potential application in nanobiosensors with further optimization in the future.

Living-cell imaging using a photonic crystal nanolaser array

We proposed and demonstrated a label-free imaging method for living cells using a GaInAsP H0-type photonic crystal nanolaser array. We integrated 441 nanolasers in an arrayed configuration and achieved photopumped lasing with a 100% yield. Then, we attached HeLa cells on it, measured the wavelengths of all nanolasers and used them as pixel information. We acquired cell images, which partially corresponds to optical micrographs and probably reflects the attachment condition of the cells. We improved the refractive index resolution from ~10(-2) to 2 × 10(-3) by incorporating a nanoslot into each nanolaser and compensating the nonuniformity of each index sensitivity.

Focused plasmonic trapping of metallic particles

Scattering forces in focused light beams push away metallic particles. Thus, trapping metallic particles with conventional optical tweezers, especially those of Mie particle size, is difficult. Here we investigate a mechanism by which metallic particles are attracted and trapped by plasmonic tweezers when surface plasmons are excited and focused by a radially polarized beam in a high-numerical-aperture microscopic configuration. This contrasts the repulsion exerted in optical tweezers with the same configuration. We believe that different types of forces exerted on particles are responsible for this contrary trapping behaviour. Further, trapping with plasmonic tweezers is found not to be due to a gradient force balancing an opposing scattering force but results from the sum of both gradient and scattering forces acting in the same direction established by the strong coupling between the metallic particle and the highly focused plasmonic field. Theoretical analysis and simulations yield good agreement with experimental results.

Focused light beams can be used as optical tweezers for manipulating small dielectric particles, but they normally repel metallic ones. By exploiting surface plasmons excited by a radially polarized beam, Min et al. show that it is possible to trap metallic particles with diameters up to 2.2 μm.

Single-nanoparticle near-infrared surface plasmon resonance microscopy for real-time measurements of DNA hybridization adsorption

A novel 814 nm near-infrared surface plasmon resonance (SPR) microscope is used for the real-time detection of the sequence-selective hybridization adsorption of single DNA-functionalized gold nanoparticles. The objective-coupled, high numerical aperture SPR microscope is capable of imaging in situ the adsorption of single polystyrene and gold particles with diameters ranging from 450 to 20 nm onto a 90 μm × 70 μm area of a gold thin film with a time resolution of approximately 1-3 s. Initial real-time SPR imaging (SPRI) measurements were performed to detect the accumulation of 40 nm gold nanoparticles for 10 min onto a gold thin film functionalized with a 100% complementary DNA surface at concentrations from 5 pM to 100 fM by counting individual particle binding events. A 100% noncomplementary DNA surface exhibited virtually no nanoparticle adsorption. In contrast, in a second set of SPRI measurements, two component complementary/noncomplementary mixed DNA monolayers that contained a very small percentage of complementary sequences ranging from 0.1 to 0.001%, showed both permanent and transient hybridization adsorption of the gold nanoparticles that could be tracked both temporally and spatially with the SPR microscope. These experiments demonstrate that SPR imaging measurements of single biofunctionalized nanoparticles can be incorporated into bioaffinity biosensing methods at subpicomolar concentrations.

Mapping plasmonic near-field profiles and interferences by surface-enhanced Raman scattering

Mapping near-field profiles and dynamics of surface plasmon polaritons is crucial for understanding their fundamental optical properties and designing miniaturized photonic devices. This requires a spatial resolution on the sub-wavelength scale because the effective polariton wavelength is shorter than free-space excitation wavelengths. Here by combining total internal reflection excitation with surface-enhanced Raman scattering imaging, we mapped at the sub-wavelength scale the spatial distribution of the dominant perpendicular component of surface plasmon fields in a metal nanoparticle-film system through spectrally selective and polarization-resolved excitation of the vertical gap mode. The lateral field-extension at the junction, which is determined by the gap-mode volume, is small enough to distinguish a spot size ~0.355λ0 generated by a focused radially polarized beam with high reproducibility. The same excitation and imaging schemes are also used to trace near-field nano-focusing and interferences of surface plasmon polaritons created by a variety of plasmon lenses.

Vectorial optical field generator for the creation of arbitrarily complex fields

Generation of vectorial optical fields with complex spatial distribution in the cross section is of great interest in areas where exotic optical fields are desired, including particle manipulation, optical nanofabrication, beam shaping and optical imaging. In this work, a vectorial optical field generator capable of creating arbitrarily complex beam cross section is designed, built and tested. Based on two reflective phase-only liquid crystal spatial light modulators, this generator is capable of controlling all the parameters of the spatial distributions of an optical field, including the phase, amplitude and polarization (ellipticity and orientation) on a pixel-by-pixel basis. Various optical fields containing phase, amplitude and/or polarization modulations are successfully generated and tested using Stokes parameter measurement to demonstrate the capability and versatility of this optical field generator.

Quantitative reflection imaging of fixed Aplysia californica pedal ganglion neurons on nanostructured plasmonic crystals

Studies of the interactions between cells and surrounding environment including cell culture surfaces and their responses to distinct chemical and physical cues are essential to understanding the regulation of cell growth, migration, and differentiation. In this work, we demonstrate the capability of a label-free optical imaging technique-surface plasmon resonance (SPR)-to quantitatively investigate the relative thickness of complex biomolecular structures using a nanoimprinted plasmonic crystal and laboratory microscope. Polyelectrolyte films of different thicknesses deposited by layer-by-layer assembly served as the model system to calibrate the reflection contrast response originating from SPRs. The calibrated SPR system allows quantitative analysis of the thicknesses of the interface formed between the cell culture substrate and cellular membrane regions of fixed Aplysia californica pedal ganglion neurons. Bandpass filters were used to isolate spectral regions of reflected light with distinctive image contrast changes. Combining of the data from images acquired using different bandpass filters leads to increase image contrast and sensitivity to topological differences in interface thicknesses. This SPR-based imaging technique is restricted in measurable thickness range (∼100-200 nm) due to the limited plasmonic sensing volume, but we complement this technique with an interferometric analysis method. Described here simple reflection imaging techniques show promise as quantitative methods for analyzing surface thicknesses at nanometer scale over large areas in real-time and in physicochemical diverse environments.

High-resolution simultaneous microscopy of refractive index and fluorescent intensity distributions by using localized surface plasmons

We propose a localized surface plasmon microscope that provides simultaneous imaging of refractive index and fluorescent intensity distributions. We show experimental images of fluorescent and transparent particles under circular pupil illumination to confirm simultaneous high-resolution imaging. Furthermore, we investigate applicability of annular pupil illumination employing two axicons to improve energy efficiency in the fluorescent imaging and find that a brighter image is obtainable by maintaining high spatial resolution for both imaging modes.

Launching plasmonic Bloch waves with excited dye molecules

In this paper, we will demonstrate that excited dye molecules can be used to launch the plasmonic Bloch waves (PBWs) propagating at multi-metal-dielectric interfaces. The properties of the PBWs, such as wavevectors, propagating bands, the interface and grating period effect, were characterized by a leakage radiation microscope. Theoretical simulations were also carried out to reveal the properties of the PBWs and were consistent with the experimental results. What is more, experimental results reveal an interesting phenomenon: the PBWs launched by the excited dye molecules present different optical behaviors from those launched by far-field laser beams through attenuated total reflection. The mechanism of this difference was analyzed based on the energy conversion between the optical near-field and far-field. Our work provides a new way to launch the PBWs. Further, the coupling between the dye molecules and PBWs also demonstrates a new method to manipulate the fluorescence emission from random to controllable.

A dynamic plasmonic manipulation technique assisted by phase modulation of an incident optical vortex beam

A novel phase modulation method for dynamic manipulation of surface plasmon polaritons (SPPs) with a phase engineered optical vortex (OV) beam illuminating on nanoslits is experimentally demonstrated. Because of the unique helical phase carried by an OV beam, dynamic control of SPP multiple focusing and standing wave generation is realized by changing the OV beam's topological charge constituent with the help of a liquid-crystal spatial light modulator. Measurement of SPP distributions with near-field scanning optical microscopy showed an excellent agreement with numerical predictions. The proposed phase modulation technique for manipulating SPPs features has seemingly dynamic and reconfigurable advantages, with profound potential for development of SPP coupling, routing, multiplexing and high-resolution imaging devices on plasmonic chips.

Focused cylindrical vector beam assisted microscopic pSPR biosensor with an ultra wide dynamic range

A novel phase-sensitive surface plasmon resonance (pSPR) biosensor based on differential phase measurement between two cylindrical vector beams, namely radially polarized and azmuthally polarized beams, is proposed and studied in an inverted microscope. Different from a fixed angle or a relatively small angular range for SPR excitation in the attenuated total reflection (ATR) configuration, the signal beam focused by a total internal reflection fluorescence microscopic objective contains the entire angular range from 0 to the maximum angle given by the numerical aperture, leading to a dynamic range of 0.41 RIU which is over seven times wider than the best result of the ATR pSPR sensor. Moreover, with the technique of differential phase measurement between radial and azimuthal polarizations employed in our configuration, high sensitivity of ±9.05×10(-8) refractive index unit/1 deg can simultaneously be achieved in principle. The proposed technique maintains the unique advantages in terms of securing high imaging resolution and sensitivity with an ultra-wide dynamic range simultaneously.

Surface plasmon microscopy: resolution, sensitivity and crosstalk

This paper develops the theoretical framework to understand the capability of the interferometric surface plasmon microscope to quantify sample properties in a confined region. We use rigorous diffraction theory to quantify the ability of the system to measure local properties and eliminate crosstalk from adjacent regions. We argue that the interferometric system in the defocused condition defines the measured point of excitation and reradiation of the surface plasmons; which greatly improves localisation. We also present results for the noninterferometric microscope, which confirm that the interferometric based system can perform quantitative measurements over smaller regions.

Plasmonic rainbow rings induced by white radial polarization

This Letter presents a scheme to embed both angular/spectral surface plasmon resonance (SPR) in a unique far-field rainbow feature by tightly focusing (effective NA=1.45) a polychromatic radially polarized beam on an Au (20 nm)/SiO2 (500 nm)/Au (20 nm) sandwich structure. Without the need for angular or spectral scanning, the virtual spectral probe snapshots a wide operation range (n=1-1.42; λ=400-700 nm) of SPR excitation in a locally nanosized region. Combined with the high-speed spectral analysis, a proof-of-concept scenario was given by monitoring the NaCl liquid concentration change in real time. The proposed scheme will certainly has a promising impact on the development of objective-based SPR sensor and biometric studies due to its rapidity and versatility.

Ultra-high enhancement of the field concentration in split ring resonators by azimuthally polarized excitation

We study the field enhancement and resonance frequencies in split-ring resonators (SRR) illuminated by azimuthally polarized light. We find that compared to linearly polarized illumination, the azimuthally polarized illumination increase the intensity enhancement by more than an order of magnitude. We attribute the increase in the intensity enhancement to the improved overlap between the SRR geometry and the direction of the electric field vector at each point. In addition, we present and explore a method to tune the resonance frequency of the SRR (for azimuthal polarization) by introducing more gaps in the structure. This approach allows for simple and straightforward tuning of the resonance frequency with small impact on the intensity enhancement. The impact of the design parameters on the intensity enhancement under azimuthally polarized illumination is also studied in details, exhibiting clear differences compared to the case of linear polarized illumination.

Improved Sensitivity of Localized Surface Plasmon Resonance Transducers Using Reflection Measurements

The refractive index sensitivity (RIS) of a localized surface plasmon resonance (LSPR) transducer is one of the key parameters determining its effectiveness in sensing applications. LSPR spectra of nanoparticulate gold films, including Au island films prepared by evaporation on glass and annealing as well as immobilized Au nanoparticle (NP) films, were measured in the transmission and reflection modes. It is shown that the RIS, measured as the wavelength shift in solvents with varying refractive index (RI), is significantly higher in reflection measurements.

Light microscopy with doughnut modes: a concept to detect, characterize, and manipulate individual nanoobjects

Higher order laser modes, mainly called doughnut modes (DMs) have use in many different branches of research, such as, bio-imaging, material science, single-molecule microscopy, and spectroscopy. The main reason of their increasing importance is that recently, the techniques to generate well-defined DMs have been refined or rediscovered. Although their potential is still not fully utilized, their specifically polarized field distribution gives rise to a wide field of applications. They are contributing to complete our fundamental knowledge of the optical properties of single emitting species, such as molecules, nanoparticles, or quantum dots, offering insight into the three-dimensional dipole or particle orientation in space. The perfect zero intensity in the focus center qualifies some DMs for stimulated emission depletion (STED) microscopy. For the same reason, they have been suggested for trapping and tweezing applications.

Amplitude and phase images of cellular structures with a scanning surface plasmon microscope

Imaging cellular internal structure at nanometer scale axial resolution with non invasive microscopy techniques has been a major technical challenge since the nineties. We propose here a complement to fluorescence based microscopies with no need of staining the biological samples, based on a Scanning Surface Plasmon Microscope (SSPM). We describe the advantages of this microscope, namely the possibility of both amplitude and phase imaging and, due to evanescent field enhancement by the surface plasmon resonance, a very high resolution in Z scanning (Z being the axis normal to the sample). We show for fibroblast cells (IMR90) that SSPM offers an enhanced detection of index gradient regions, and we conclude it is very well suited to discriminate regions of variable density in biological media such as cell compartments, nucleus, nucleoli and membranes.

Enhanced detection of fluorescent nanospheres using two-channel radially polarized surface plasmon microscopy

We detected single dye-stained latex nanospheres as small as 20 nm using a two-detection channel modified surface plasmon microscope. We found that a radially polarized incident beam leading to excitation of well-focused surface plasmons induces both fluorescence and elastic linear scattering from the spheres. The two complementary emitted signals were detected in parallel by the two separated channels, leading to well-colocalized images. We obtained high spatial resolutions for both channels down to 250 nm in the lateral direction and 300 nm along the longitudinal axis. We believe this multimodal microscope can be useful to track nano objects and to compensate for intermittent fluorescence, thanks to a permanently activated parallel scattering detection channel.

Vectorial fiber laser using intracavity axial birefringence

In this paper we investigate the polarization properties of a fiber laser with an intracavity c-cut calcite crystal that is capable of producing reconfigurable vectorial output modes. Vectorial modes with radial, azimuthal and generalized cylindrical vector polarizations can be generated by translating one lens within the laser cavity. Detailed studies of the mode polarization evolution show that the modes inside the laser cavity can be spatially homogeneously polarized in one section of the cavity while being spatially inhomogeneously polarized in another section of the cavity, which opens the opportunities for many potential new fiber laser design possibilities and applications. Furthermore, more complicated vectorial vortex output modes are also observed by purposefully introducing angular misalignments.

Imaging live cell membranes via surface plasmon-enhanced fluorescence and phase microscopy

This paper demonstrates the first combination for wide-field surface plasmon (SP) phase microscopy and SP-enhanced fluorescence microscopy to image living cells' contacts on the surface of a bio-substrate simultaneously. The phase microscopy with a phase-shift interferometry and common-path optical setup can provide high-sensitivity phase information in long-term stability. Simultaneously, the fluorescence microscopy with the enhancement of a local electromagnetic field can supply bright fluorescent images. The combined microscope imposes a high numerical aperture objective upon the excitation of surface plasmon through a silver film with a thickness of 30 nm. The developed SP microscope is successfully applied to the real-time bright observation of the transfected fluorescence of living cells localized near the cell membrane on the bio-substrate and the high-sensitivity phase image of the cell-substrate contacts at the same time.

Use of radially polarized beams in three-dimensional photonic crystal fabrication with the two-photon polymerization method

Radially polarized ultrafast laser beams are used in the fabrication of three-dimensional photonic crystals with the two-photon polymerization technique in organic-inorganic hybrid materials. It has been found that when a radially polarized beam is employed, the lateral size of the fabricated polymer rods is decreased by 27.5% from 138 to 100 nm under a threshold fabrication condition, leading to a 17.35% reduction in the filling ratio of the photonic crystal. A comparison of the stop gaps between radially polarized and linearly polarized beam illumination shows a higher suppression ratio in transmission and a wider wavelength range in the former case owing to the favorable tuning of the filling ratio of the three-dimensional photonic crystals.

Radial polarization induced surface plasmon virtual probe for two-photon fluorescence microscopy

Surface plasmons excited by a focused femtosecond radially polarized beam on a metal surface form a standing wave pattern with a sharp peak that can be used as a "virtual probe" for surface plasmon microscopy. The rotational symmetry of radially polarized light effectively provides the TM polarization required for coupling to the surface plasmons while the short pulse nature of the probe allows for nonlinear processes to be studied.