Surface Plasmon-Coupled Emission with Gold Films

Direct detection of virus-like particles using color images of plasmonic nanostructures

We propose a measurement method for sensitive and label-free detections of virus-like particles (VLPs) using color images of nanoplasmonic sensing chips. The nanoplasmonic chip consists of 5×5 gold nanoslit arrays and the gold surface is modified with specific antibodies for spike protein. The resonant wavelength of the 430-nm-period gold nanoslit arrays underwater environment is about 570 nm which falls between the green and red bands of the color CCD. The captured VLPs by the specific antibodies shift the plasmonic resonance of the gold nanoslits. It results in an increased brightness of green pixels and decreased brightness of red pixels. The image contrast signals of (green - red) / (red + green) show good linearity with the surface particle density. The experimental tests show the image contrast method can detect 100-nm polystyrene particles with a surface density smaller than 2 particles/µm². We demonstrate the application for direct detection of SARS-CoV-2 VLPs using a simple scanner platform. A detection limit smaller than 1 pg/mL with a detection time less than 30 minutes can be achieved.

Tailoring Resonant Energy Transfer Processes for Sustainable and Bio-Inspired Sensing

Dipole–Dipole interactions (DDI) constitute an effective mechanism by which two physical entities can interact with each other. DDI processes can occur in a resonance framework if the energies of the two dipoles are very close. In this case, an energy transfer can occur without the need to emit a photon, taking the name of Förster Resonance Energy Transfer (FRET). Given their large dependence on the distance and orientation between the two dipoles, as well as on the electromagnetic properties of the surrounding environment, DDIs are exceptional for sensing applications. There are two main ways to carry out FRET-based sensing: (i) enhancing or (ii) inhibiting it. Interaction with resonant environments such as plasmonic, optical cavities, and/or metamaterials promotes the former while acting on the distance between the FRET molecules favors the latter. In this review, we browse both the two ways, pointing the spotlight to the intrinsic interdisciplinarity these two sensing routes imply. We showcase FRET-based sensing mechanisms in a variety of contexts, from pH sensors to molecular structure measurements on a nano-metrical scale, with a particular accent on the central and still mostly overlooked role played between a nano-photonically structured environment and photoluminescent molecules.

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.

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.

Titanium nitride as an alternative and reusable plasmonic substrate for fluorescence coupling

The development of alternative plasmonic materials that can replace gold and silver is of long-standing interest in materials research. In this study, we have prepared and characterized thin films of TiN, an emerging plasmonic material, and examined its effectiveness for fluorescence coupling in metal-dielectric structures having TiN as the plasmonically active component. We have used a combination of experiment and reflectivity calculations to determine the nature and dispersion of the optical modes sustained by the metal-dielectric structures, which furthermore are adjustable by varying the thickness of the dielectric layer. Our results reveal that fluorophores placed on the TiN substrates can couple with the surface-plasmon mode and/or the waveguide modes supported by these structures, to provide polarized and directional emission over narrow angular ranges. The performance of TiN substrates for surface plasmon-coupled emission (SPCE) and waveguide-coupled emission (WGCE) is found to be comparable with conventional Au substrates. Importantly, the TiN thin films are reusable, which is certainly advantageous for their use in SPCE or WGCE-based fluorescence sensing applications.

Plasmon-Enhanced Nitrogen Vacancy-Rich Carbon Nitride Electrochemiluminescence Aptasensor for Highly Sensitive Detection of miRNA

The development of biosensors for biologically important substances with ultralow content such as microRNA is of great significance. Herein, a novel surface plasmon-enhanced electrogenerated chemiluminescence-based aptasensor was developed for ultrasensitive sensing of microRNA by using nitrogen vacancy-rich carbon nitride nanosheets as effective luminophores and gold nanoparticles as plasmonic sources. The introduction of nitrogen vacancies improved the electrochemiluminescence behavior due to improved conductance and electrogenerated chemiluminescence activity. The introduction of plasmonic gold nanoparticles increased the electrochemiluminescence signal intensity by more than eightfold. The developed surface plasmon-enhanced electrogenerated chemiluminescence aptasensor exhibited good selectivity, ultrasensitivity, excellent stability, and reproducibility for the determination of microRNA-133a, with a dynamic linear range of 1 aM to 100 pM and a limit of detection about 0.87 aM. Moreover, the surface plasmon-enhanced electrogenerated chemiluminescence sensor obtained a good recovery when detecting the content of microRNA in actual serum.

Surface Plasmon-Assisted Fluorescence Enhancing and Quenching: From Theory to Application

The integration of surface plasmon resonance and fluorescence yields a multiaspect improvement in surface fluorescence sensing and imaging, leading to a paradigm shift of surface plasmon-assisted fluorescence techniques, for example, surface plasmon enhanced field fluorescence spectroscopy, surface plasmon coupled emission (SPCE), and SPCE imaging. This Review aims to characterize the unique optical property with a common physical interpretation and diverse surface architecture-based measurements. The fundamental electromagnetic theory is employed to comprehensively unveil the fluorophore-surface plasmon interaction, and the associated surface-modification design is liberally highlighted to balance the surface plasmon-induced fluorescence-enhancement efforts and the surface plasmon-caused fluorescence-quenching effects. In particular, all types of surface structures, for example, silicon, carbon, protein, DNA, polymer, and multilayer, are systematically interrogated in terms of component, thickness, stiffness, and functionality. As a highly interdisciplinary and expanding field in physics, optics, chemistry, and surface chemistry, this Review could be of great interest to a broad readership, in particular, among physical chemists, analytical chemists, and in surface-based sensing and imaging studies.

Plasmon-enhanced quantum dots electrochemiluminescence aptasensor for selective and sensitive detection of cardiac troponin I

The development of highly sensitive electrochemiluminescence (ECL) immunosensors by using functional nanoparticles as signal amplifiers is a solution towards sensitive determination of many low concentration disease biomarkers. Herein, a sensitive aptamer-based, sandwich-type surface plasmon enhanced electrochemiluminescence (SPEECL) immunosensor was demonstrated for the detection of cardiac troponin I (cTnI), by means of aptamer conjugated CdS QDs and AuNPs as ECL luminophores and plasmon sources, respectively, in which Tro4 aptamer was used as a capture probe for cTnI and Tro6 aptamer as a detecting probe. The signal of the developed SPEECL system showed ~ 5-fold increment as compared to that of without AuNPs. Using this ECL platform for the detection of cTnI, a linear range and the limit of detection (LOD) were found to be 1 fg/mL - 10 ng/mL and 0.75 fg/mL, respectively.

Vectorial physical-optics modeling of Fourier microscopy systems in nanooptics

Fourier microscopy, which makes direct observation of the angular distribution possible, is widely used in the nanooptics community. The theory of such systems is typically based on ideal lenses. However, the real lenses in the typical complex lens systems have an impact on the image quality in the experiment. Therefore, it is desirable to have a model of the entire system, which is capable of predicting such phenomena, in order to conduct a preliminary detailed analysis of the setup before building it in the lab. In this work, we perform a vectorial physical-optics simulation of Fourier microscopy systems, which considers the real lenses; it also includes the nanostructure (e.g., photonic crystal). The systems are used to image the emission diagram of a single molecule as well as to analyze the angular-spectral property of a photonic crystal. We analyze various effects of the entire systems, e.g., Fresnel effects of the real lens surfaces, diffraction, polarization, chromatic aberration, and the effects of misalignment. We find that the above-mentioned effects have an influence on the final results, which should be taken into account when performing similar real-life experiments.

Extracting Electronic Transition Bands of Adsorbates from Molecule-Plasmon Excitation Coupling

The coupling between molecular electronic and particle plasmon excitations can result in various intriguing outcomes depending on how strongly or weakly the excitations couple to compete with their respective decay rates. In this work, using methylene blue and thionine dyes as model systems, we show that the electronic absorption band of resonant adsorbates can be determined with submonolayer sensitivity from the weak molecule-plasmon excitation coupling that results in the attenuation on the plasmonic absorption band. The extracted spectra are strongly similar to the absorption spectra of the corresponding molecules in solution, apart from the expected spectral red-shift and broadening. Interestingly, the adsorption isotherms determined on the basis of the magnitude of the attenuation correlate linearly with that determined from the adsorbate-induced plasmon resonance red-shift. The results demonstrate that in the weak coupling regimes the plasmon modes can be considered as an environment that supplies energy to and takes energy from the adsorbates.

Optical arrangement for surface plasmon-assisted directional enhanced Raman scattering spectroscopy

We present an optical arrangement for spectroscopy of enhanced Raman scattering assisted by surface plasmon resonance in continuous planar metallic films. Optical excitation of propagating surface plasmons (PSP) is aided by the hemispherical total internal reflectance prism in the Kretschmann geometry. In this geometry, the radiation produced by Raman scattering is directionally emitted inside the prism with the angular distribution in the shape of a hollow cone (the Kretschmann cone). The proposed configuration enables entire collection of the Kretschmann cone with the use of an elliptical mirror modified for enlarging the accessible angular range for both the incident beam and the scattered light. The spectroscopic performance of this arrangement was evaluated using the Rhodamine 6G dye as a surface enhanced Raman scattering (SERS) reporter. An evident difference in magnitudes of the enhancement factor for specific spectral lines as compared to SERS excitation by localized surface plasmon resonance (LSPR-SERS) was revealed. The origin of this difference is discussed in terms of expected distinctions between the PSP-assisted directional enhanced Raman scattering and the LSPR-SERS. Besides the spectroscopic applications, the proposed arrangement is also perfectly suited for simultaneous functioning as the SPR sensor. Integration of SERS spectroscopy with the SPR analysis shows promise as a platform for evolving an innovative analytical technique with enhanced potentialities in surface research, particularly in biochemical applications.

Chemiplasmonics for high-throughput biosensors

Background

The sensitivity of ELISA for biomarker detection can be significantly increased by integrating fluorescence with plasmonics. In surface-plasmon-coupled emission, the fluorophore emission is generally enhanced through the so-called physical mechanism due to an increase in the local electric field. Despite its fairly high enhancement factors, the use of surface-plasmon-coupled emission for high-throughput and point-of-care applications is still hampered due to the need for expensive focusing optics and spectrometers.

Methods

Here, we describe a new chemiplasmonic-sensing paradigm for enhanced emission through the molecular interactions between aromatic dyes and C₆₀ films on Ag substrates.

Results

A 20-fold enhancement in the emission from rhodamine B-labeled biomolecules can be readily elicited without quenching its red color emission. As a proof of concept, we demonstrate two model bioassays using: 1) the RhB–streptavidin and biotin complexes in which the dye was excited using an inexpensive laser pointer and the ensuing enhanced emission was recorded by a smartphone camera without the need for focusing optics and 2) high-throughput 96-well plate assay for a model antigen (rabbit immunoglobulin) that showed detection sensitivity as low as 6.6 pM.

Conclusion

Our results show clear evidence that chemiplasmonic sensors can be extended to detect biomarkers in a point-of-care setting through a smartphone in simple normal incidence geometry without the need for focusing optics. Furthermore, chemiplasmonic sensors also facilitate high-throughput screening of biomarkers in the conventional 96-well plate format with 10–20 times higher sensitivity.

Live-cell fluorescence imaging with extreme background suppression by plasmonic nanocoatings

Fluorescence microscopy allows specific and selective imaging of biological samples. Unfortunately, unspecific background due to auto-fluorescence, scattering, and non-ideal labeling efficiency often adversely affect imaging. Surface plasmon-coupled emission (SPCE) is known to selectively mediate fluorescence that spatially originates from regions close to the metal interface. However, SPCE combined with fluorescence imaging has not been widely successful so far, most likely due to its limited photon yield, which makes it tedious to identify the exact window of the application. As the strength of SPCE based imaging is its unique sectioning capabilities. We decided to identify its clear beneficial operational regime for biological settings by interrogating samples in the presence of ascending background levels. For fluorescent beads as well as live-cell imaging as examples, we show how to extend the imaging performance in extremely high photon background environments. In a common setup using plasmonic gold-coated coverslips using an objective-based total internal reflection fluorescence microscope (TIRF-M), we theoretically and experimentally characterize our fluoplasmonics (f-Pics) approach by providing general user guidance in choosing f-Pics over TIRF-M or classical wide-field (WF).

Reproducible Enhancement of Fluorescence by Bimetal Mediated Surface Plasmon Coupled Emission for Highly Sensitive Quantitative Diagnosis of Double-Stranded DNA

Plasmonic enhancement of fluorescence from SYBR Green I conjugated with a double-stranded DNA (dsDNA) amplicon is demonstrated on polymerase chain reaction (PCR) products. Theoretical computation leads to use of the bimetallic (Au 2 nm-Ag 50 nm) surface plasmons due to larger local fields (higher quality factors) than monometallic (Ag or Au) ones at both dye excitation and emission wavelengths simultaneously, optimizing fluorescence enhancement with surface plasmon coupled emission (SPCE). Two kinds of reverse Kretschmann configurations are used, which favor, in signal-to-noise ratio, a fluorescence assay that uses optically dense buffer such as blood plasma. The fluorescence enhancement (12.9 fold at maximum) with remarkably high reproducibility (coefficient of variation (CV) < 1%) is experimentally demonstrated. This facilitates credible quantitation of enhanced fluorescence, however unlikely to obtain by localized surface plasmons. The plasmon-induced optical gain of 46 dB due to SPCE-active dye molecules is also estimated. The fluorescence enhancement technologies with PCR enables LOD of the dsDNA template concentration of ≈400 fg µL^-1 (CV < 1%), the lowest ever reported in DNA fluorescence assay to date. SPCE also reduces photobleaching significantly. These technologies can be extended for a highly reproducible and sufficiently sensitive fluorescence assay with small volumes of analytes in multiplexed diagnostics.

Subwavelength resolution scanning diffracted-light microscopy using plasmonic ultra-thin condensers

We used a rotating slit placed at the back focal plane of the microscope's objective lens to scan the light diffracted by a plasmonic crystal, which had a period smaller than the resolution limit of the optical microscope. A set of images were collected at different orientations of the slit. A high-resolution image of the plasmonic crystal was obtained by processing the experimental images using a numerical Fourier ptychographic algorithm. Supporting simulations of the experiments are also presented.

Remote activation and detection of up-converted luminescence via surface plasmon polaritons propagating in a silver nanowire

In this paper, we demonstrate remote activation and detection of the 2-photon up-conversion luminescence via surface plasmon polaritons propagating in a long silver nanowire. The hybrid nanostructure was assembled by locally depositing a submicron droplet of nanocrystal-containing colloidal solution on one of the ends of the metallic nanowire. When - using a classic confocal microscope - the second end of the nanowire, without the nanocrystals, is illuminated with infrared laser light, we observe strong emission from the same end. Therefore, it indicates that surface plasmon polaritons activated with infrared light at the second end of the nanowire propagate along it and can excite nanocrystals in the droplet at the opposite end. Subsequently, the excited nanocrystals up-convert the energy and by launching surface plasmon polaritons can guide the up-converted luminescence back to the starting point. The emergence of this effect is much more pronounced for a laser polarized along the nanowire. The spectral and temporal character of this emission reveals strong interactions between surface plasmon polaritons and electronic states of the nanocrystals. The details of local and non-local aspects of the effects of remote excitation and guiding of energy in a silver nanowire are elucidated using a unique experimental setup, based on two microscope objectives for spatial separation and control of both excitation and emission beams.

Tailoring Optical Properties of a Large-Area Plasmonic Gold Nanoring Array Pattern

A new fabrication route, which combines nanosphere lithography with silicon-based clean-room microfabrication processes, has been developed to produce large-area long-range ordered gold nanoring array patterns in a controllable fashion. Both the experimentation and the finite-difference time-domain (FDTD) simulation show that the surface plasmon resonance peak (SPR) of the nanoring array pattern can be tuned systematically in a large spectral range by varying the geometry parameters such as the ring thickness, the ring height, the ringer outer diameter, and the gap between neighboring rings. For the Au nanoring arrays with a large gap in the absence of plasmon coupling between neighboring rings, the local electromagnetic (EM) field enhancement occurs at both the outer and inner surfaces of individual nanorings; and the periodicity of Au nanoring array has no any effect on the plasmonic properties. For the Au nanoring arrays with a small gap, plasmon coupling takes place between neighboring rings. As a result, the characteristic plasmonic band is split into two new peaks corresponding to a bonding SPR mode and an antibonding SPR mode. The local EM field enhancement becomes stronger with a decrease in the gap between neighboring rings, but the SPR peaks shift away. Therefore, to maximize the surface-enhanced Raman scattering signal, the geometry parameters of the Au nanoring array need to be tuned to balance the contributions from the resonance excitation (spectral overlap) and the local EM field enhancement.

Planar Double-Epsilon-Near-Zero Cavities for Spontaneous Emission and Purcell Effect Enhancement

The enhancement of the photophysical response of fluorophores is a crucial factor for photonic and optoelectronic technologies that involve fluorophores as gain media. Recent advances in the development of an extreme light propagation regime, called epsilon-near-zero (ENZ), provide a promising approach in this respect. In this work, we design metal/dielectric nanocavities to be resonant with the absorption and emission bands of the employed fluorophores. Using CsPbBr₃ perovskite nanocrystal films as light emitters, we study the spontaneous emission and decay rate enhancement induced by a specifically tailored double-epsilon-near-zero (double ENZ) structure. We experimentally demonstrate the existence of two ENZ wavelengths, by directly measuring their dielectric permittivity via ellipsometric analysis. The double ENZ nature of this plasmonic nanocavity has been exploited to achieve both surface plasmon enhanced absorption (SPEA) and surface plasmon coupled emission (SPCE), inducing a significant enhancement of both the spontaneous emission and the decay rate of the perovskite nanocrystal film that is placed on top of the nanocavity. Finally, we discuss the possibility of tailoring the two ENZ wavelengths of this structure within the visible spectrum simply by finely designing the thickness of the two dielectric layers, which enables resonance matching with a broad variety of dyes. Our device design is appealing for many practical applications, ranging from sensing to low threshold amplified spontaneous emission, since we achieve a strong PL enhancement with structures that allow for straightforward fluorophore deposition on a planar surface that keeps the fluorophores exposed and accessible.

Particle sensing with confined optical field enhanced fluorescence emission (Cofefe)

We describe the development and performance of a new type of optical sensor suitable for registering the binding/dissociation of nanoscopic particles near a gold sensing surface. The method shares similarities with surface plasmon resonance microscopy but uses a completely different optical signature for reading out binding events. This new optical read-out mechanism, which we call confined optical field enhanced fluorescence emission (Cofefe), uses pulsed surface plasmon polariton fields at the gold/liquid interface that give rise to confined optical fields upon binding of the target particle to the gold surface. The confined near-fields are sufficient to induce two-photon absorption in the gold sensor surface near the binding site. Subsequent radiative recombination of the electron-hole pairs in the gold produces fluorescence emission, which can be captured by a camera in the far-field. Bound nanoparticles show up as bright confined spots against a dark background on the camera. We show that the Cofefe sensor is capable of detecting gold and silicon nanoparticles, as well as polymer nanospheres and sub-μm lipid droplets in a label-free manner with average illumination powers of less than 10 μW/μm2.

Fluorescence emission difference with surface plasmon-coupled emission applied in confocal microscopy

We combined confocal surface plasmon coupled emission microscopy (C-SPCEM) together with fluorescence emission difference (FED) technique to pursuit super-resolution fluorescent image. Solid or hollow point spread function (PSF) for C-SPCEM is achieved with radially-polarized or circularly-polarized illumination. The reason why PSF can be manipulated by the polarization of illumination light is corroborated by the interaction of fluorescent emitter with vector focal field on the plasmonic substrate. After introduction of FED technique, PSF for C-SPECM can shrunk to around λ/4 in full-width half-maximum, which is unambiguously beyond Rayleigh's diffraction limit. The super-resolution capability of C-SPCEM with FED technique is experimentally demonstrated by imaging aggregated fluorescent beads with 150 nm in diameter.

Study of the momentum-resolved plasmonic field energy of Bloch-like surface plasmon polaritons from periodic nanohole array

The angular surface plasmon mediated fluorescence from a two-dimensional Au nanohole array has been studied by reflectivity spectroscopy and Fourier-space photoluminescence microscopy. By using the rate equation model and temporal coupled mode theory, we determine the momentum-dependent coupling rate of light emitters to (-1,0) Bloch-like surface plasmon polaritons (SPPs) in the first Brillouin zone. The rate increases gradually when the SPPs propagate away from the Γ-X direction and split into two at the Γ-M point where two coupled modes are formed. In addition, both the spectral density-of-states (SDOS) and the plasmonic field energy are found to govern the momentum dependence. We also examine the behavior of the field energy as a function of the SPP propagation direction and it agrees well with the finite-difference time-domain simulations, showing the energy plays a major role in controlling the angular emission intensity. Our results devise a new method in studying the momentum-dependent plasmonic field energy and they are expected to provide insight in directional emission from periodic arrays.

Spacer-controlled emission of randomly oriented fluorophores enhanced with surface plasmon-polaritons

In surface plasmon-polariton enhanced fluorescence, the use of spacers is simply understood to control the distance between the fluorescence dyes and metals to avoid quenching. However, the presence of a spacer layer over the metallic surface not only manipulates the quantum yield, but also affects the surface plasmon-polariton resonance, which in turn modifies the florescence excitation rate as well as the far-field radiation pattern of the emission. This study presents a systematic investigation on the spacer-controlled emission of randomly oriented emitters in the Kretschmann configuration, with the full leverage of the coupled transfer matrix, reciprocity and plane-wave decomposition methods. It demonstrates that the introduction of a spacer between the metal film and fluorescence dyes decreases the excitation rate. Furthermore, the excitation rate decreases more for spacers with a higher refractive index due to the reduction of the effective power that goes into the resonance excitation. Combining the excitation rate with the quantum yield and photon-collection efficiency, the detected fluorescence enhancement from either the medium side or substrate side is determined and optimized for the spacer thickness and material. It was found that the highest enhancement of a randomly oriented fluorophore's emission was generally achieved in detection from the substrate side with a low refractive index spacer (e.g. Teflon and SiO₂). In addition, the substrate-side measurements were thought to benefit from highly directional radiation and a more stable enhancement compared to the medium-side measurements. Our results clearly reveal physical insights into the spacer-controlled emission and provide concrete guidance in the design and measurement of fluorescence-based sensing and imaging systems.

Controlled surface plasmon enhanced fluorescence from 1D gold gratings via azimuth rotations

We demonstrate a method to maximise the fluorescence enhancement from a dye using gold coated diffraction gratings. Rotations about the azimuth provides a convenient approach to maximise the coupling between the grating and excitation source while achieving enhancements comparable to traditional optical configurations where the grating and in plane light vectors are parallel. This approach yields a 30 fold enhancement in the fluorescence signal over metal free substrates, while opening up the range of possible orientations and configurations suitable for fluorescence enhancement applications.

Directional fluorescence emission co-enhanced by localized and propagating surface plasmons for biosensing

We investigated the simultaneous excitation of localized surface plasmons (LSPs) and propagating surface plasmons (PSPs) on a thin metallic film with an array of nanoholes for the enhancement of fluorescence intensity in heterogeneous bioassays. Experiments supported by simulations reveal that the co-excitation of PSP and LSP modes on the nanohole array in a Kretschmann configuration allows for fluorescence enhancement of about 10(2) as compared to a flat Au surface irradiated off-resonance. Moreover, this fluorescence signal was about 3-fold higher on the substrate supporting both PSPs and LSPs than that on a flat surface where only PSPs were resonantly excited. Simulations also indicated the highly directional fluorescence emission as well as the high fluorescence collection efficiency on the nanohole array substrate. Our contribution attempts to de-convolute the origin of this enhancement and identify further ways to maximize the efficiency of surface plasmon-enhanced fluorescence spectroscopy for implementation in ultra-sensitive bioassays.

C60 as an active smart spacer material on silver thin film substrates for enhanced surface plasmon coupled emission

In this study, we present the use of C60 as an active spacer material on a silver (Ag) based surface plasmon coupled emission (SPCE) platform. In addition to its primary role of protecting the Ag thin film from oxidation, the incorporation of C60 facilitated the achievement of a 30-fold enhancement in the emission intensity of rhodamine B (RhB) fluorophore. The high signal yield was attributed to the unique π-π interactions between C60 thin films and RhB, which enabled efficient transfer of energy of RhB emission to Ag plasmon modes. Furthermore, minor variations in the C60 film thickness yielded large changes in the enhancement and angularity properties of the SPCE signal, which can be exploited for sensing applications. Finally, the low-cost fabrication process of the Ag-C60 thin film stacks render C60 based SPCE substrates ideal, for the economic and simplistic detection of analytes.

Purcell factor based understanding of enhancements in surface plasmon-coupled emission with DNA architectures

Tuning the Purcell factor with DNA architectures to realize >130-fold fluorescence enhancements in surface plasmon-coupled emission.

Beta-glucan quantification by fluorescence analysis using photonic crystals

In this study, 1DPhCs were utilized as a Bragg reflection mirror. Gold was deposited on 1DPhC films. 1DPhCs with Au were used for quantitative determination of beta-glucan.

Fluorescence emission difference with defocused surface plasmon-coupled emission microscopy

A novel fluorescence emission difference method is proposed to improve the lateral resolution of SPCEM without increasing instrument complexity. We discovered the profile of transverse PSF in SPCEM will dramatically change from a hollow spot to a solid spot, when the axial position of sample varies within one wavelength in the vicinity of the focal plane. The subtraction of an image whose PSF is hollow spot and an image with solid PSF will greatly enhance the resolution and contrast of SPCEM images. The mechanism of the distinctive PSF is demonstrated through basic optics theories, and the improvement of lateral resolution is verified by theoretical simulations and experimental results. It is believed that our method will stand out for its pleasant resolution enhancement and its instruments' simplicity to facilitate many biological cellular observations.

Metal slab superlens-negative refractive index versus inclined illumination: discussion

I describe experiments using a combination of an optical microscope and a plasmonic ultrathin condenser (UTC). The UTC's structure is similar to the original proposal of a silver slab superlens. I show that the observed improvement in image resolution is determined by the well-known equation describing the resolution obtainable when a condenser is included in the microscope setup. I argue that the described experiments indicate that the so-called metal slab superlens is better described as a novel ultrathin microscope condenser, and the observed improvement in resolution is due to the illumination of the object under observation with inclined light produced by the plasmonic UTC. Implications of this opinion for the development of an optical nanoscope based on plasmonic UTCs are presented.

Surface Plasmon-Coupled Directional Enhanced Raman Scattering by Means of the Reverse Kretschmann Configuration

Surface-enhanced Raman scattering (SERS) is a unique analytical technique that provides fingerprint spectra, yet facing the obstacle of low collection efficiency. In this study, we demonstrated a simple approach to measure surface plasmon-coupled directional enhanced Raman scattering by means of the reverse Kretschmann configuration (RK-SPCR). Highly directional and p-polarized Raman scattering of 4-aminothiophenol (4-ATP) was observed on a nanoparticle-on-film substrate at 46° through the prism coupler with a sharp angle distribution (full width at half-maximum of ∼3.3°). Because of the improved collection efficiency, the Raman scattering signal was enhanced 30-fold over the conventional SERS mode; this was consistent with finite-difference time-domain simulations. The effect of nanoparticles on the coupling efficiency of propagated surface plasmons was investigated. Possessing straightforward implementation and directional enhancement of Raman scattering, RK-SPCR is anticipated to simplify SERS instruments and to be broadly applicable to biochemical assays.

Resolution-enhanced surface plasmon-coupled emission microscopy

A novel fluorescence emission difference technique is proposed for further enhancements of the lateral resolution in surface plasmon-coupled emission microscopy (SPCEM). In the proposed method, the difference between the image with phase modulation by using a 0-2π vortex phase plate (VPP) along with a diaphragm and the original image obtained from SPCEM is used to estimate the spatial distribution of the analyzed sample. By optimizing the size of the diaphragm and the subtractive factor, the lateral resolution can be enhanced by about 20% and 33%, compared with that in SPCEM with a single 0-2π VPP and conventional wide-field fluorescence microscopy, respectively. Related simulation results are presented to verify the capability of the proposed method for improving lateral resolution and reducing imaging distortion. It is believed that the proposed method has potentials to improve the performance of SPCEM, thus facilitating biological observation and research.

When are surface plasmon polaritons excited in the Kretschmann-Raether configuration?

It is widely believed that the reflection minimum in a Kretschmann-Raether experiment results from direct coupling into surface plasmon polariton modes. Our experimental results provide a surprising discrepancy between the leakage radiation patterns of surface plasmon polaritons (SPPs) launched on a layered gold/germanium film compared to the K-R minimum, clearly challenging this belief. We provide definitive evidence that the reflectance dip in K-R experiments does not correlate with excitation of an SPP mode, but rather corresponds to a particular type of perfectly absorbing (PA) mode. Results from rigorous electrodynamics simulations show that the PA mode can only exist under external driving, whereas the SPP can exist in regions free from direct interaction with the driving field. These simulations show that it is possible to indirectly excite propagating SPPs guided by the reflectance minimum in a K-R experiment, but demonstrate the efficiency can be lower by more than a factor of 3. We find that optimal coupling into the SPP can be guided by the square magnitude of the Fresnel transmission amplitude.

Directional Emission from Metal-Dielectric-Metal Structures: Effect of Mixed Metal Layers, Dye Location and Dielectric Thickness

Metal-dielectric-metal (MDM) structures provide directional emission close to the surface normal, which offers opportunities for new design formats in fluorescence based applications. The directional emission arises due to near-field coupling of fluorophores with the optical modes present in the MDM substrate. Reflectivity simulations and dispersion diagrams provide a basic understanding of the mode profiles and the factors that affect the coupling efficiency and the spatial distribution of the coupled emission. This work reveals that the composition of the metal layers, the location of the dye in the MDM substrate and the dielectric thickness are important parameters that can be chosen to tune the color of the emission wavelength, the angle of observation, the angular divergence of the emission and the polarization of the emitted light. These features are valuable for displays and optical signage.

Two-photon luminescence and second harmonic generation from gold micro-plates

Micron-sized gold plates were prepared by reducing chloroauric acid with lemongrass extract. Their two-photon luminescence (TPL) and second harmonic generation (SHG) were investigated. The results show that the TPL and SHG intensity of gold plates is dependent on the wavelength and polarization of excitation laser. The TPL intensity of gold plates decreases with the increase of the excitation wavelength except for a small peak around 820-840 nm, while SHG intensity increases with the excitation wavelength redshift. In addition, it is found that the TPL intensity of the gold plate's edge is related with the angle between the edge orientation and the polarization direction of the excitation light. The TPL intensity increases with the angle increase from 0° to 90°.

Steering Fluorescence Emission with Metal-Dielectric-Metal Structures of Au, Ag and Al

Directional control over fluorescence emission is important for improving the sensitivity of fluorescence based techniques. In recent years, plasmonic and photonic structures have shown great promise in shaping the spectral and spatial distribution of fluorescence, which otherwise is typically isotropic in nature and independent of the observation direction. In this work we have explored the potential of metal-dielectric-metal (MDM) structures composed of Au, Ag or Al in steering the fluorescence emission from various probes emitting in the NIR, Visible or UV/blue region. We show that depending on the optical properties of the metal and the thickness of the dielectric layer, the emission from randomly oriented fluorophores embedded within the MDM substrate is transformed into beaming emission normal to the substrate. Agreement of the observed angular emission patterns with reflectivity calculations reveals that the directional emission is due to the coupling of the fluorescence with the electromagnetic modes supported by the MDM structure.

SPP tomography: a simple wide-field nanoscope

We explore the wide-field optical nanoimaging capabilities of the surface plasmon polariton (SPP) tomography technique. We show that nanofeatures with lateral dimensions smaller than λ/20 can be observed in the surface emission (SE) images of plasmonic crystals with a period of 300 nm. Moreover, as a proof-of-concept, we demonstrate that SPP tomography permits to resolve two single objects with a center-to-center separation of 200 nm and edge-to-edge separation as small as λ/7. We present a comprehensive discussion about the nanoimaging capabilities of the SPP tomography technique. In contrast to other optical subwavelength resolution techniques, in our approach for imaging nanosize features, enhanced evanescent waves are coupled to the far-field via leakage radiation associated with SPPs excited by near-field fluorescence; therefore wide-field images, which are not out-of-plane diffraction-limited, are formed directly in the microscope's camera. We also discuss additional imaging processing capabilities associated with the fact that SPP tomography SE images are formed by the microscope lenses through an analog tomography process.

Fundaments of optical far-field subwavelength resolution based on illumination with surface waves

We present a general discussion about the fundamental physical principles involved in a novel class of optical superlenses that permit to realize in the far-field direct non-scanning images with subwavelength resolution. Described superlenses are based in the illumination of the object under observation with surface waves excited by fluorescence, the enhanced transmission of fluorescence via coupling with surface waves, and the occurrence of far-field coherence-related fluorescence diffraction phenomena. A Fourier optics description of the image formation based on illumination with surface waves is presented, and several recent experimental realizations of this technique are discussed. Our theoretical approach explains why images with subwavelength resolution can be formed directly in the microscope camera, without involving scanning or numerical post-processing. While resolution of the order of λ/7 has been demonstrated using the described approach, we anticipate that deeper optical subwavelength resolution should be expected.

Imaging nanoscale features with plasmon-coupled leakage radiation far-field superlenses

Optical images from nano-scale features were obtained by collection of leakage radiation coupled to surface plasmon polaritons excited by near-field fluorescence. Plasmonic crystals with spatial periods as small as 190 nm and non-periodic features separated by 80 nm, corresponding to ~λ/7, were clearly visible in the real plane images using this far-field technique. We show that the leaked light from the investigated samples carries detailed information to the far-field which is not present in the images obtained with conventional optical microscopy.

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.

Silver cluster-biomolecule hybrids: from basics towards sensors

We focus on the functional role of small silver clusters in model hybrid systems involving peptides in the context of a new generation of nanostructured materials for biosensing. The optical properties of hybrids in the gas phase and at support will be addressed with the aim to bridge fundamental and application aspects. We show that extension and enhancement of absorption of peptides can be achieved by small silver clusters due to the interaction of intense intracluster excitations with the π-π* excitations of chromophoric aminoacids. Moreover, we demonstrate that the binding of a peptide to a supported silver cluster can be detected by the optical fingerprint. This illustrates that supported silver clusters can serve as building blocks for biosensing materials. Moreover, the clusters can be used simultaneously to immobilize biomolecules and to increase the sensitivity of detection, thus replacing the standard use of organic dyes and providing label-free detection. Complementary to that, we show that protected silver clusters containing a cluster core and a shell liganded by thiolates exhibit absorption properties with intense transitions in the visible regime which are also suitable for biosensing applications.

Feasibility of Using Bimetallic Plasmonic Nanostructures to Enhance the Intrinsic Emission of Biomolecules

Detection of the intrinsic fluorescence from proteins is important in bio-assays because it can potentially eliminate the labeling of external fluorophores to proteins. This is advantageous because using external fluorescent labels to tag biomolecules requires chemical modification and additional incubation and washing steps which can potentially perturb the native functionality of the biomolecules. Hence the external labeling steps add expense and complexity to bio-assays. In this paper, we investigate for the first time the feasibility of using bimetallic nanostructures made of silver (Ag) and aluminum (Al) to implement the metal enhanced fluorescence (MEF) phenomenon for enhancing the intrinsic emission of biomolecules in the ultra-violet (UV) spectral region. Fluorescence intensities and lifetimes of a tryptophan analogue N-acetyl-L-tryptophanamide (NATA) and a tyrosine analogue N-acetyl-L-tyrosinamide (NATA-tyr) were measured. Increase in fluorescence intensities of upto 10-fold and concurrent decrease in lifetimes for the amino acids were recorded in the presence of the bimetallic nanostructures when compared to quartz controls. We performed a model protein assay involving biotinylated bovine serum albumin (bt-BSA) and streptavidin on the bimetallic nanostructured substrate to investigate the distance dependent effects on the extent of MEF from the bimetallic nanostructures and found a maximum enhancement of over 15-fold for two layers of bt-BSA and streptavidin. We also used finite difference time domain (FDTD) calculations to explore how bimetallic nanostructures interact with plane waves and excited state fluorophores in the UV region and demonstrate that the bimetallic substrates are an effective platform for enhancing the intrinsic emission of proteins and other biomolecules.

Plasmonic antennas for directional sorting of fluorescence emission

Spontaneous emission of fluorescent molecules or quantum dots is radiated along all directions when emitters are diluted in a liquid solution, which severely limits the amount of collected light. Besides, the emission direction does not carry any useful information and cannot be used to sort different molecules. To go beyond these limits, optical antennas have been recently introduced as conceptual tools to control the radiation properties for nanoemitters fixed on a substrate. Despite intense recent research, controlling the luminescence directivity remains a challenge for emitters with random positions and orientations, which is a key for several biomolecular screening applications. Here, we present full directional control of the fluorescence emission from molecules in water solution by an optical antenna made of a nanoaperture surrounded by a periodic set of shallow grooves in a gold film. For each emission wavelength, the fluorescence beam can be directed along a specific direction with a given angular width, hereby realizing a micrometer-size dispersive antenna. We demonstrate the fluorescence beaming results from an interference phenomenon and provide physical optics guidelines to control the fluorescence directivity by tuning the groove-nanoaperture distance. This photon-sorting capability provides a new approach for high-sensitivity screening of molecular species in solution.

Laser-launched evanescent surface plasmon polariton field utilized as a direct coherent pumping source to generate emitted nonlinear four-wave mixing radiation

We develop a concept of surface plasmon polaritons (SPPs) based four-wave mixing (4WM), in which a laser-launched evanescent SPP field is utilized as a coherent pumping source to involve directly in a nonlinear 4WM process at the dielectric/metal interface. Conversion efficiency of the resulting 4WM radiation is expected to be dramatically increased due to the local-field enhancement effect. Feasibility of implementing this concept at the air/gold film and graphene flake/gold film interfaces is further examined by numerical simulations. The concept shows intriguing promise for applications in newly emerging nanophotonics, optoelectronics, and active plasmonics.