Direct image of surface-plasmon-coupled emission by leakage radiation microscopy

Back focal plane imaging for light emission from a tunneling junction in a low-temperature ultrahigh-vacuum scanning tunneling microscope

We report the design and realization of the back focal plane (BFP) imaging for the light emission from a tunnel junction in a low-temperature ultrahigh-vacuum (UHV) scanning tunneling microscope (STM). To achieve the BFP imaging in a UHV environment, a compact "all-in-one" sample holder is designed and fabricated, which allows us to integrate the sample substrate with the photon collection units that include a hemisphere solid immersion lens and an aspherical collecting lens. Such a specially designed holder enables the characterization of light emission both within and beyond the critical angle and also facilitates the optical alignment inside a UHV chamber. To test the performance of the BFP imaging system, we first measure the photoluminescence from dye-doped polystyrene beads on a thin Ag film. A double-ring pattern is observed in the BFP image, arising from two kinds of emission channels: strong surface plasmon coupled emissions around the surface plasmon resonance angle and weak transmitted fluorescence maximized at the critical angle, respectively. Such an observation also helps to determine the emission angle for each image pixel in the BFP image and, more importantly, proves the feasibility of our BFP imaging system. Furthermore, as a proof-of-principle experiment, electrically driven plasmon emissions are used to demonstrate the capability of the constructed BFP imaging system for STM induced electroluminescence measurements. A single-ring pattern is obtained in the BFP image, which reveals the generation and detection of the leakage radiation from the surface plasmon propagating on the Ag surface. Further analyses of the BFP image provide valuable information on the emission angle of the leakage radiation, the orientation of the radiating dipole, and the plasmon wavevector. The UHV-BFP imaging technique demonstrated here opens new routes for future studies on the angular distributed emission and dipole orientation of individual quantum emitters in UHV.

Au-Ag Alloy Nanoshuttle Mediated Surface Plasmon Coupling for Enhanced Fluorescence Imaging

Surface plasmon-coupled emission (SPCE), a novel signal enhancement technology generated by the interactions between surface plasmons and excited fluorophores in close vicinity to metallic film, has shown excellent performance in bioimaging. Variable-angle nanoplasmonic fluorescence microscopy (VANFM), based on an SPCE imaging system, can selectively modulate the imaging depth by controlling the excitation angles. In order to further improve the imaging performance, Au-Ag alloy nanoshuttles were introduced into an Au substrate to mediate the plasmonic properties. Benefiting from the strong localized plasmon effect of the modified SPCE chip, better imaging brightness, signal-to-background ratio and axial resolution for imaging of the cell membrane region were obtained, which fully displays the imaging advantages of SPCE system. Meanwhile, the imaging signal obtained from the critical angle excitation mode was also amplified, which helps to acquire a more visible image of the cell both from near- and far-field in order to comprehensively investigate the cellular interactions.

Resonant dielectric multilayer with controlled absorption for enhanced total internal reflection fluorescence microscopy

Total internal reflection fluorescence microscopy (TIRF-M) is widely used in biological imaging. Evanescent waves, generated at the glass-sample interface, theoretically strongly improve the axial resolution down to a hundred of nanometers. However, objective based TIRF-M suffers from different limitations such as interference fringes and uneven illumination, mixing both propagating and evanescent waves, which degrade the image quality. In principle, uneven illumination could be avoided by increasing the excitation angle, but this results in a drastic loss of excitation power. We designed dedicated 1D photonic crystals in order to circumvent this power loss by directly acting on the intensity of the evanescent field at controlled incident angles. In this framework, we used dedicated resonant multi-dielectric stacks, supporting Bloch surface waves and resulting in large field enhancement when illuminated under the conditions of total internal reflection. Here, we present a numerical optimization of such resonant stacks by adapting the resulting resonance to the angular illumination conditions in TIRF-M and to the fluorescence collection constraints. We thus propose a dedicated resonant structure with a control of the absorption during thin film deposition. A first experimental demonstration illustrates the concept with a 3-fold fluorescence enhancement in agreement with the numerical predictions.

Stable directional emission in active optical waveguides shielding external environmental influences

The skillful confinement of light brought by the composite waveguide structure has shown great possibilities in the development of photonic devices. It has greatly expanded the application range of an on-chip system in dark-field imaging and confined the laser when containing an active medium. Here we experimentally proved a stable directional emission in an active waveguide composed of metal and photonic crystal, which is almost completely unaffected by the external environment and different from the common local light field that is seriously affected by the structure. When the refractive index of samples on the surface layer changes, it can ensure the constant emission intensity of the internal mode, while still retaining the external environmental sensitivity of the surface mode. It can also be used for imaging and sensing as a functional slide. This research of chip-based directional emission is very promising for various applications including quantitative detection of biological imaging, coupled emission intensity sensing, portable imaging equipment, and tunable micro lasers.

Geometric control over surface plasmon polariton out-coupling pathways in metal-insulator-metal tunnel junctions

Metal-insulator-metal tunnel junctions (MIM-TJs) can electrically excite surface plasmon polaritons (SPPs) well below the diffraction limit. When inelastically tunneling electrons traverse the tunnel barrier under applied external voltage, a highly confined cavity mode (MIM-SPP) is excited, which further out-couples from the MIM-TJ to photons and single-interface SPPs via multiple pathways. In this work we control the out-coupling pathways of the MIM-SPP mode by engineering the geometry of the MIM-TJ. We fabricated MIM-TJs with tunneling directions oriented vertical or lateral with respect to the directly integrated plasmonic strip waveguides. With control over the tunneling direction, preferential out-coupling of the MIM-SPP mode to SPPs or photons is achieved. Based on the wavevector distribution of the single-interface SPPs or photons in the far-field emission intensity obtained from back focal plane (BFP) imaging, we estimate the out-coupling efficiency of the MIM-SPP mode to multiple out-coupling pathways. We show that in the vertical-MIM-TJs the MIM-SPP mode preferentially out-couples to single-interface SPPs along the strip waveguides while in the lateral-MIM-TJs photon out-coupling to the far-field is more efficient.

Fluorescence Coupling to Internal Modes of 1D Photonic Crystals Characterized by Back Focal Plane Imaging

The coupling of fluorescence with surface electromagnetic modes, such as surface plasmons on thin metal films or Bloch surface waves (BSW) on truncated one-dimensional photonic crystals (1DPC), are presently utilized for many fluorescence-based applications. In addition to the surface wave, 1DPCs also support other electromagnetic modes that are confined within the 1DPC structure. These internal modes (IMs) have not received much attention for fluorescence coupling due to lack of spatial overlap of their electric fields with the surface bound fluorophores. However, our recent studies have indicated that the fluorescence coupling with IMs occurs quite efficiently. This observed internal mode-coupled emission (IMCE) is (similar to BSW-coupled emission) indeed wavelength dependent, directional and S-polarized. In this paper, we have carried out back-focal plane (BFP) imaging to reveal that the IMs of 1DPCs can couple with surface bound excited dye molecules, with or without a BSW mode presence. Depending on the emission wavelength, the coupling is observed with BSW and IMs or only IMs of the 1DPC structure. The experimental results are well matching with numerical simulations. The occurrence of IMCE regardless of the availability of BSWs removes the dependence on just the surface mode for obtaining coupled emission from 1DPCs. The observation of IMCE is expected to widen the scope of 1DPCs for surface-based fluorescence sensing and assays.

Surface plasmon-coupled emission imaging for biological applications

Fluorescence imaging technology has been extensively applied in chemical and biological research profiting from its high sensitivity and specificity. Much attention has been devoted to breaking the light diffraction-limited spatial resolution. However, it remains a great challenge to improve the axial resolution in a way that is accessible in general laboratories. Surface plasmon-coupled emission (SPCE), generated by the interactions between surface plasmons and excited fluorophores in close vicinity of the thin metal film, offers an opportunity for optical imaging with potential application in analysis of molecular and biological systems. Benefiting from the highly directional and distance-dependent properties, SPCE imaging (SPCEi) has displayed excellent performance in bioimaging with improved sensitivity and axial confinement. Herein, we give a brief overview of the development of SPCEi. We describe the unique optical characteristics and constructions of SPCEi systems and highlight recent advances in the use of SPCEi for biological applications. We hope this review provides readers with both the insights and future prospects of SPCEi as a new promising imaging platform for potentially widespread applications in biological research and medical diagnostics. Graphical abstract.

Coupling of Fluorophores in Single Nanoapertures to Tamm Plasmon Structures

Metal nanostructures (such as plasmonic antennas) have been widely demonstrated to be excellent devices for beaming and sorting the fluorescence emission. These effects rely on the constructive scattering or diffraction from different elements (such as metal corrugations or nanorings) of the nanostructures. However, subwavelength-size nanoholes, without nearby nanoscale features, results in an angularly dispersed emission from the distal surface. Herein, we demonstrate for the first time the emission redirection capabilities of a single isolated nanoaperture milled in a thick silver film deposited on a dielectric multilayer. Specifically, we show that a dye dissolved in ethanol filling in the nanoaperture can couple to Tamm Plasmon Polariton (TPP) modes of the structure. Due to the small in-plane wavevectors of the TPPs, the fluorescence from Tamm-coupled dyes within the nanoaperture is emitted normally to the sample surface, with a minimum angular width of about 12.54°. This kind of fluorescence manipulation has proven to be effective with various nanoaperture shapes, such as circles, squares, and triangles. Our work is also the first experimental demonstration of lateral coupling of fluorophores with TPPs in nanoholes, with potential applications in bioanalysis and biosciences.

Imaging optical fields below metal films and metal-dielectric waveguides by a scanning microscope

Laser scanning confocal fluorescence microscopy (LSCM) is now an important method for tissue and cell imaging when the samples are located on the surfaces of glass slides. In the past decade, there has been extensive development of nano-optical structures that display unique effects on incident and transmitted light, which will be used with novel configurations for medical and consumer products. For these applications, it is necessary to characterize the light distribution within short distances from the structures for efficient detection and elimination of bulky optical components. These devices will minimize or possibly eliminate the need for free-space light propagation outside of the device itself. We describe the use of the scanning function of a LSCM to obtain 3D images of the light intensities below the surface of nano-optical structures. More specifically, we image the spatial distributions inside the substrate of fluorescence emission coupled to waveguide modes after it leaks through thin metal films or dielectric-coated metal films. The observed spatial distribution were in general agreement with far-field calculations, but the scanning images also revealed light intensities at angles not observed with classical back focal plane imaging. Knowledge of the subsurface optical intensities will be crucial in the combination of nano-optical structures with rapidly evolving imaging detectors.

Radiative decay engineering 8: Coupled emission microscopy for lens-free high-throughput fluorescence detection

Fluorescence spectroscopy and imaging are now used throughout the biosciences. Fluorescence microscopes, spectrofluorometers, microwell plate readers and microarray imagers all use multiple optical components to collect, redirect and focus the emission onto single point or array imaging detectors. For almost all biological samples, except those with regular nanoscale features, emission occurs in all directions. With the exception of complex microscope objectives with large collection angles (NA ≤ 0.5), all these instruments collect only a small fraction of the total emission. Because of the increasing knowledge base on fluorophores within near-field (<200 nm) distances from plasmonic and photonic structures we can anticipate the development of compact devices in which the sample to be detected is located directly on solid state detectors such as CCDs or CMOS cameras. Near-field interactions of fluorophores with metallic or dielectric multi-layer structures (MLSs) can capture a large fraction of the total emission. Depending on the composition and dimensions of the MLSs, the spatial distribution of the sample emission results in distinct optical patterns on the detector surface. With either plain glass slides or MLSs the most commonly used front focal plane (FFP) images reveal the x-y spatial distribution of emission from the sample. Another approach, which is often used with two or three-dimensional nanostructures, is back focal plane (BFP) imaging. The BFP images reveal the angular distribution of the emission. The FFP and BFP images occur at certain distances from the sample which is determined by the details of the optical components. Obtaining these images requires multiple optical components and distances which are too large for the compact devices. For devices described in this paper, the images will be detected at a fixed distance between the sample and some arbitrary distance below the MLS which is determined by the geometry and thicknesses of the components. We refer to measurements at these locations as out-of-focal plane (OFP) imaging. Herein we describe a method to measure the optical fields at micron and multi-micron distances below the MLS, which will represent the images seen by an optically coupled array detector. The possibility of sub-surface optical images is illustrated using five different multi-layer structures. This is accomplished using an optical configuration which allows measurement at a front focal plane (FFP), back focal plane (BFP) or any OFP locations. Our OFP imaging method provides a link between the FFP images which reveals the surface distribution of fluorophores with the BFP images that reveal the angular distribution of emission. This linkage can be useful when examining structures which have nanoscale features due to fluorescence or leakage radiation from nanostructures.

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.

Direct imaging of isofrequency contours in photonic structures

The isofrequency contours of a photonic crystal are important for predicting and understanding exotic optical phenomena that are not apparent from high-symmetry band structure visualizations. We demonstrate a method to directly visualize the isofrequency contours of high-quality photonic crystal slabs that show quantitatively good agreement with numerical results throughout the visible spectrum. Our technique relies on resonance-enhanced photon scattering from generic fabrication disorder and surface roughness, so it can be applied to general photonic and plasmonic crystals or even quasi-crystals. We also present an analytical model of the scattering process, which explains the observation of isofrequency contours in our technique. Furthermore, the isofrequency contours provide information about the characteristics of the disorder and therefore serve as a feedback tool to improve fabrication processes.

High performance dual-mode surface plasmon coupled emission imaging apparatus integrating Kretschmann and reverse Kretschmann configurations for flexible measurements

A Kretschmann (KR) and reverse Kretschmann (RK) dual-mode surface plasmon coupled emission (SPCE) imaging apparatus based on prism coupling was built up. Highly directional and polarized fluorescence images for both RK and KR configurations were obtained. Besides, surface plasmon field-enhanced fluorescence and free space imaging can also be measured conveniently from this apparatus. Combining the high sensitivity of KR mode and the simplicity of RK mode, the multifunctional imaging system is flexible to provide different configurations for imaging applications. Compared to the free space imaging, SPCE imaging provides enhanced fluorescence, especially large enhancement up to about 50 fold in KR configuration. Additionally, the degree of evanescent field enhancement effect was easily estimated experimentally using the apparatus to compare the different imaging configurations. We believed that the dual-mode SPCE imaging apparatus will be useful in fundamental study of plasmon-controlled fluorescence and be a powerful tool for optical imaging, especially for microarray and biological applications.

Selectable Surface and Bulk Fluorescence Imaging with Plasmon-Coupled Waveguides

In this letter, we propose a new method for selective imaging of surface bound probes or simultaneous imaging of surface bound plus fluorescence from dye molecules in bulk water solution. The principle of this method relies on use of two optical modes with different mode distributions, filed decay lengths and polarization states that are sustaining in a plasmon waveguide. The two modes with different decay lengths couple with dye molecules of different regions, at different distances from the PCW-water interface. The emission from two different regions occur as two coupled emission rings with different polarizations and emitting angles in the back focal plane (BFP) images. By using an electric-driven liquid crystal in BFP imaging, we selectively imaged surface or surface plus bulk fluorescence. Accordingly two coupled emissions can be switched ON or OFF independently, that are for either surface or bulk fluorescence imaging. Our work provides a new method for fluorescence imaging or sensing just by using a planar multilayer film, which may be a useful for fluorescence-based techniques in chemistry, materials science, molecular biology, and medicine.

Directing fluorescence with plasmonic and photonic structures

Fluorescence technology pervades all areas of chemical and biological sciences. In recent years, it is being realized that traditional fluorescence can be enriched in many ways by harnessing the power of plasmonic or photonic structures that have remarkable abilities to mold the flow of optical energy. Conventional fluorescence is omnidirectional in nature, which makes it difficult to capture the entire emission. Suitably designed emission directivity can improve collection efficiency and is desirable for many fluorescence-based applications like sensing, imaging, single molecule spectroscopy, and optical communication. By incorporating fluorophores in plasmonic or photonic substrates, it is possible to tailor the optical environment surrounding the fluorophores and to modify the spatial distribution of emission. This promising approach works on the principle of near-field interaction of fluorescence with spectrally overlapping optical modes present in the substrates. In this Account, we present our studies on directional emission with different kinds of planar metallic, dielectric, and hybrid structures. In metal-dielectric substrates, the coupling of fluorescence with surface plasmons leads to directional surface-plasmon-coupled emission with characteristic dispersion and polarization properties. In one-dimensional photonic crystals (1DPC), fluorophores can interact with Bloch surface waves, giving rise to sharply directional Bloch surface wave-coupled emission. The interaction of fluorescence with Fabry-Pérot-like modes in metal-dielectric-metal substrates and with Tamm states in plasmonic-photonic hybrid substrates provides beaming emission normal to the substrate surface. These interesting features are explained in the context of reflectivity dispersion diagrams, which provide a complete picture of the mode profiles and the corresponding coupled emission patterns. Other than planar substrates, specially fabricated plasmonic nanoantennas also have tremendous potential in controlling and steering fluorescence beams. Some representative studies by other research groups with various nanoantenna structures are described. While there are complexities to near-field interactions of fluorescence with plasmonic and photonic structures, there are also many exciting possibilities. The routing of each emission wavelength along a specific direction with a given angular width and polarization will allow spatial and spectral multiplexing. Directional emission close to surface normal will be particularly useful for microscopy and array-based studies. Application-specific angular emission patterns can be obtained by varying the design parameters of the plasmonic/photonic substrates in a flexible manner. We anticipate that the ability to control the flow of emitted light in the nanoscale will lead to the development of a new generation of fluorescence-based assays, instrumentation, portable diagnostics, and emissive devices.

Effect of metal film thickness on Tamm plasmon-coupled emission

Tamm plasmons (TPs) are the result of trapping optical energy at the interface between a metal film and a one-dimensional photonic crystal. In contrast to surface plasmons, TPs display unique properties such as the ability to undergo direct optical excitation without the aid of prisms or gratings, being populated using both S- and P-polarized light, and importantly, they can be created with incident light normal to the surface. This latter property has recently been used to obtain Tamm plasmon-coupled emission (TPCE), which beams along a path directly perpendicular to the surface. In this paper the effects of metal film thickness on the TPCE are investigated using back focal plane (BFP) imaging and spectral resolutions. The observed experimental results are in agreement with the numerical simulations. The present work provides the basic understanding needed to design structures for TPCE, which in turn has potential applications in the fabrication of active materials for light emitting devices, fluorescence-based sensing, using microarrays, and imaging.

Back focal plane imaging of Tamm plasmons and their coupled emission

The unique optical properties of TPs - such as flexible wavevector matching conditions including in-plane wavevector within the light line, existing both S- and P-polarized TPs and ability of populating with KR and RK illuminations - facilitate them for direct optical excitation. The Tamm plasmon Coupled emission (TPCE) from a combined photonic-plasmonic structure sustaining both surface plasmons (SPs) and Tamm plasmons (TPs) is described. The sensitivity of TPCE to the emission wavelength and polarization is examined with back focal plane imaging and verified with the numerical calculations. The results reveal that the excited probe can couple with both TPs and SPs, resulting in SPCE and TPCE, respectively. The TPCE angle is strongly dependent on the wavelength allowing for spectral resolution using different observation angles. These Tamm structures provide a new tool to control the optical emission from dye molecules and have many potential applications in fluorescence based-sensing and imaging.

Extracting surface wave-coupled emission with subsurface dielectric gratings

Conventional surface plasmons (SPs) or Bloch surface waves (BSWs) have a wave vector exceeding that of light in vacuum, and, therefore, the surface plasmon-coupled emission (SPCE) or Bloch surface wave-coupled emission (BSWCE) cannot escape from the corresponding structures. With the aid of a high-refractive-index prism or an oil-immersion objective, the SPCE or BSWCE can be coupled into free space. But the large volumes of the prism and objective are certainly unfavorable for miniaturization of the optical systems or inconvenient for applications such as the optical displays. Here we experimentally demonstrate a new method to extract the SPCE or BSWCE with a subsurface dielectric grating. The experimental results verify that the chip-like substrate with two decorated sides can bring out the directional fluorescence emission in free space. The emitting direction and emitting patterns can be tuned by the period size and dimensionality of the gratings. Our work provides a new strategy to realize free-space directional fluorescence emission at a very low cost and compact configuration, which has potential applications in fluorescence-based sensing, imaging, light-emitting diodes, optical displays, and other near-field optical devices.

Back focal plane imaging of directional emission from dye molecules coupled to one-dimensional photonic crystals

Bloch surface waves (BSWs) on one-dimensional photonic crystals (1DPCs) have been used to beam the fluorescence emission from the dye molecules. All dielectric 1DPC displays its low propagating loss, narrow resonance and the absence of absorption or quenching. In this paper, back focal plane imaging reveals that in addition to the BSW mode, a guided mode and a cavity mode also exist in the 1DPC which all couple with the excited dye molecules. The appearance of these modes is sensitive to the wavelength of the fluorescence and alters the beaming effect by the 1DPC. Numerical simulations verify the existence of these modes which are consistent with the experimental results. Comparisons between the Bloch surface wave coupled emission and surface plasmon coupled emission are also presented for a clearer understanding of the multilayered film enabled directional emission.

Polymer-loaded propagating modes on a one-dimensional photonic crystal

We numerically and experimentally demonstrate that a polymer film-coated one-dimensional photonic crystal (1DPC) can sustain transverse electric (TE) polarized modes without the limit of guided layer's thickness. Our results indicate that two propagating modes are existing inside the polymer film, the first one is the TE polarized Bloch surface wave, and the second one is the TE polarized guided mode. Here in, the evolution of these two modes with change in the polymer film thickness is presented. Our numerical simulation results are in well-agreement with the experimental data obtained using back focal plane imaging.

Tamm plasmon- and surface plasmon-coupled emission from hybrid plasmonic-photonic structures

Photonic and plasmon-coupled emissions present new opportunities for control on light emission from fluorophores, and have many applications in the physical and biological sciences. The mechanism of and the influencing factors for the coupling between the fluorescent molecules and plasmon and/or photonic modes are active areas of research. In this paper, we describe a hybrid photonic-plasmonic structure that simultaneously contains two plasmon modes: surface plasmons (SPs) and Tamm plasmons (TPs), both of which can modulate fluorescence emission. Experimental results show that both SP-coupled emission (SPCE) and TP-coupled emission (TPCE) can be observed simultaneously with this hybrid structure. Due to the different resonant angles of the TP and SP modes, the TPCE and SPCE can be beamed in different directions and can be separated easily. Back focal plane images of the fluorescence emission show that the relative intensities of the SPCE and TPCE can be changed if the probes are at different locations inside the hybrid structure, which reveals the probe location-dependent different coupling strengths of the fluorescent molecules with SPs and TPs. The different coupling strengths are ascribed to the electric field distribution of the two modes in the structure. Here, we present an understanding of these factors influencing mode coupling with probes, which is vital for structure design for suitable applications in sensing and diagnostics.

Dark-field imaging by active polymer slab waveguide

A dark-field imaging technique taking advantage of the active polymer slab waveguide (APSW) is experimentally investigated. The dye molecules (Rhodamine 6G, Rh6G) are doped in the polymer film for the launching of surface waves on the APSW, such as the surface plasmon polaritons on the Ag-polymer-air interface, evanescent fields at the polymer-air interface by the total internal reflection, or the guided modes. The localized surface waves will not radiate into the far-field space directly. When the specimens are placed on the surface of the APSW, these surface waves will be scattered to the far-field region, which forms the dark-field image of the specimen. Experimental results show that usage of APSW leads to high-contrast dark-field images with the conventional optical microscope system. The polymer film involved in the proposed dark-field microscopy brings about the merits of reduced roughness, good stability, bio-compatibility, and shorter wavelength of the illumination light source.

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.

Surface-plasmon-coupled emission microscopy with a polarization converter

Although surface-plasmon-coupled emission-based fluorescence microscopy proves high sensitivity for surface imaging, its donut shape point spread function (PSF) leads to low optical resolution and inefficient signal collection. In this Letter, we experimentally demonstrate the feasibility of solving this problem by the use of a liquid-crystal plate, which could convert the polarization state of surface-plasmon-coupled fluorescence from radial to linear. After being focused by the collection lens, an Airy disk-like PSF of small size can be realized. Experimental results reveal that both the lateral resolution and the signal-to-noise ratio can be enhanced simultaneously.

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.

Surface plasmon-coupled emission: what can directional fluorescence bring to the analytical sciences?

Surface plasmon-coupled emission (SPCE) arose from the integration of fluorescence and plasmonics, two rapidly expanding research fields. SPCE is revealing novel phenomena and has potential applications in bioanalysis, medical diagnostics, drug discovery, and genomics. In SPCE, excited fluorophores couple with surface plasmons on a continuous thin metal film; plasmophores radiate into a higher-refractive index medium with a narrow angular distribution. Because of the directional emission, the sensitivity of this technique can be greatly improved with high collection efficiency. This review describes the unique features of SPCE. In particular, we focus on recent advances in SPCE-based analytical platforms and their applications in DNA sensing and the detection of other biomolecules and chemicals.

Surface plasmon leakage radiation microscopy at the diffraction limit

This paper describes the image formation process in optical leakage radiation microscopy of surface plasmon-polaritons with diffraction limited spatial resolution. The comparison of experimentally recorded images with simulations of point-like surface plasmon-polariton emitters allows for an assignment of the observed fringe patterns. A simple formula for the prediction of the fringe periodicity is presented and practically relevant effects of abberations in the imaging system are discussed.

Optimizing silver film for surface plasmon-coupled emission induced two-photon excited fluorescence imaging

In this study, the optimal condition of a silver (Ag) film deposited on a cover slip for surface plasmon-coupled emission (SPCE) induced two-photon excited fluorescence (TPEF) based on an objective-based, total internal reflection (TIR) microscope was investigated. According to the theoretical simulations of local electric field enhancement and fluorescence coupled emission efficiency, the thickness of the Ag film should be about 40 nm in order to maximize the TPEF collection efficiency by the objective. The deposited Ag film with a germanium seed layer on a cover slip exhibits additional improvement in surface smoothness by reducing variations in surface roughness to below 1.0 nm, thereby reduces local hot spots which degrade the image uniformity. Moreover, an Ag film with a 20 nm-thick SiO2 spacer not only prevents damage caused through interaction with the aqueous solution under high laser power irradiance, but also reduces the fluorescence quenching effect by the Ag film. By optimizing the Ag film thickness, surface smoothness, and a protective dielectric spacer, efficient TIR TPEF imaging can be achieved through SPCE.

Surface plasmon-coupled emission from shaped PMMA films doped with fluorescence molecules

Surface plasmon-coupled emission from shaped PMMA films doped with randomly oriented fluorescence molecules was investigated. Experimental results show that for different shapes, such as triangle or circular structures, the SPCE ring displays different intensity patterns. For a given shape, it was observed that the relative position and polarization of an incident laser spot on the shaped PMMA can be used to adjust the fluorescence intensity distribution of the SPCE ring. The proposed method enables controlling the fluorescence emission in azimuthal direction in addition to the radial angle controlled by common SPCE, which will further enhances the fluorescence collection efficiency and has applications in fluorescence sensing, imaging and so on.

Surface plasmon-enhanced and quenched two-photon excited fluorescence

This study investigated theoretically and experimentally that two-photon excited fluorescence is enhanced and quenched via surface plasmons (SPs) excited by total internal reflection with a silver film. The fluorescence intensity is fundamentally affected by the local electromagnetic field enhancement and the quantum yield change according to the surrounding structure and materials. By utilizing the Fresnel equation and classical dipole radiation modeling, local electric field enhancement, fluorescence quantum yield, and fluorescence emission coupling yield via SPs were theoretically analyzed at different dielectric spacer thicknesses between the fluorescence dye and the metal film. The fluorescence lifetime was also decreased substantially via the quenching effect. A two-photon excited total internal reflection fluorescence (TIRF) microscopy with a time-correlated single photon counting device has been developed to measure the fluorescence lifetimes, photostabilities, and enhancements. The experimental results demonstrate that the fluorescence lifetimes and the trend of the enhancements are consistent with the theoretical analysis. The maximum fluorescence enhancement factor in the surface plasmon-total internal reflection fluorescence (SP-TIRF) configuration can be increased up to 30 fold with a suitable thickness SiO(2) spacer. Also, to compromise for the fluorescence enhancement and the fluorophore photostability, we find that the SP-TIRF configuration with a 10 nm SiO(2) spacer can provide an enhanced and less photobleached fluorescent signal via the assistance of enhanced local electromagnetic field and quenched fluorescence lifetime, respectively.