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

In situ and sensitive monitoring of configuration-switching involved dynamic adsorption by surface plasmon-coupled directional enhanced Raman scattering

Surface adsorption studies play a crucial role in numerous fields from surface catalysis to molecular separation. However, investigation on adsorption mechanisms has been restricted to limited analytes and approaches, which calls for an in situ and sensitive surface analysis technique capable of revealing the mechanisms as well as discriminating different adsorbates and their geometry at different adsorption stages. In this study, we employed surface plasmon-coupled directional enhanced Raman scattering (SPCR), a novel technique developed by coupling surface plasmon-coupled emission with SERS, to study conformation-switching involved dynamic adsorption with background suppression and improved sensitivity (nearly 30-fold). We obtained the isotherms for a conformation-changing Raman model analyte, malachite green. An S-type Langmuir model was fitted from the time-resolved SPCR signals sensitively and without any interference from the bulk solution. The reorientation of the analyte from a predominantly parallel configuration to a perpendicular one was captured by the dramatic increase in the intensity ratios of the adsorption-related peaks to the adsorption-unrelated peak. We believe that this new sensitive and selective SPCR technique will be a promising tool for surface adsorption kinetics analysis.

The evolution of total internal reflection Raman spectroscopy for the chemical characterization of thin films and interfaces

Total internal reflection (TIR) optical spectroscopies have been widely used for decades as non-destructive and surface-sensitive measurements of thin films and interfaces. Under TIR conditions, an evanescent wave propagates into the sample layer within a region approximately 50 nm to 2 μm from the interface, which limits the spatial extent of the optical signal. The most common TIR optical spectroscopies are fluorescence (i.e., TIRF) and infrared spectroscopy (i.e., attenuated total reflection infrared). Despite the first report of TIR Raman spectroscopy appearing in 1973, this method has not received the same attention to date. While TIR Raman methods can provide chemical specific information, it has been outshined in many respects by surface-enhanced Raman spectroscopy (SERS). TIR Raman spectroscopy, however, is garnering more interest for analyzing the chemical and physical properties of thin polymer films, self-assembled monolayers (SAMs), multilayered systems, and adsorption at an interface. Herein, we discuss the early experimental and computational work that laid the foundation for recent developments in the use of TIR Raman techniques. Recent applications of TIR Raman spectroscopy as well as modern TIR Raman instruments capable of measuring monolayer-sensitive vibrational modes on smooth metallic surfaces are also discussed. The use of TIR Raman spectroscopy has been on a rise and will continue to push the limits for chemical specific interfacial and thin film measurements. Graphical abstract Total internal reflection (TIR) Raman spectroscopy can extract the chemical and physical information from thin films and 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.

Influence of the dielectric substrate on the effective optical constants of silver plasmonic films

The effective optical properties of plasmonic thin films can be used to model the far-field response of nanostructured materials to an incident electromagnetic field. In the present work, optically thin nanostructured silver (Ag) plasmonic films were fabricated on transparent dielectric substrates of soda-lime glass, sapphire, and fused silica using oblique angle deposition. The influence of the underlying dielectric substrate on the effective optical properties of the nanostructured layer was investigated by an ellipsometric-optical model based on Mueller matrix ellipsometry. The wavelength-dependent uniaxial optical responses of the nanostructured Ag films fabricated on sapphire were modeled with three Gaussian and one Tanguy oscillator, representing key optical phenomena over the range from 300 to 1000 nm. In comparison with the same Ag films on glass, the results confirm that the effective optical properties cannot be considered in isolation from the substrate. As expected, the extinction peak associated with the localized surface plasmon resonance was redshifted by approximately 220 nm per unit of the substrate refractive index. Importantly, it was found that the direction of incidence also influences the film behavior, with a substantial redshift in the extinction peak for light directed through the dielectric compared to free-space illumination. This property can have a significant effect on the far-field performance of these films.

Mapping the weak plasmonic transverse field by a dielectric-nanoparticle-on-film structure with ultra-high precision

Highly confined electromagnetic fields play a significant role in modern nano-optics, among which surface plasmon polaritons (SPPs) are outstanding because of their subwavelength and enhancement nature. While many state-of-the-art methods have been proposed to uncover the field distribution of SPPs, it still faces challenge to map the weak transverse field component (the field tangential to the interface) of SPPs with high contrast and precision. We propose a direct imaging technique, which employs a dielectric-nanoparticle-on-metal-film (DNP-MF) structure as a near-field probe, to overcome this difficulty. The angular distribution of the scattering radiation from the structure is strongly polarization dependent. By extracting the scattering signals that are mainly induced by the horizontal polarization, the imaging of the weak plasmonic transverse field with high precision can be achieved. The mappings of SPPs distributions excited by various vector beams were performed in experiment, which accord excellent with theory. This technique provides a new approach for near-field imaging with high contrast and reliability, which is expected to be valuable for studying the vectorial features of SPPs such as transverse spin, spin-orbit interactions, etc.

Experimental analysis of waveguide-coupled surface-plasmon-polariton cone properties

Experimental data for waveguide-coupled surface-plasmon-polariton (SPP) cones generated from dielectric waveguides is presented. The results demonstrate a simpler route to collect plasmon waveguide resonance (i.e., PWR) data. In the reverse-Kretschmann configuration (illumination from the sample side) and Kretschmann configuration (illumination from the prism side), all the waveguide modes are excited simultaneously with p- or s-polarized incident light, which permits rapid acquisition of PWR data without the need to scan the incident angle or wavelength, in the former configuration. The concentric SPP cone properties depend on the thickness and index of refraction of the waveguide. The angular intensity pattern of the cone is well-matched to simulation results in the reverse-Kretschmann configuration, and is found to be dependent on the polarization of the incident light and the polarization of the waveguide mode. In the Kretschmann geometry, all waveguide-coupled SPP cones are measured at incident angles that produce attenuated light reflectivity. In addition, the enhanced electric field produced under total internal reflection allows high signal-to-noise ratio multimodal spectroscopies (e.g., Raman scattering, luminescence) to measure the chemical content of the waveguide film, which traditionally is not measured with PWR.

Combined measurement of directional Raman scattering and surface-plasmon-polariton cone from adsorbates on smooth planar gold surfaces

Directional-surface-plasmon-coupled Raman scattering (directional RS) has the combined benefits of surface plasmon resonance and Raman spectroscopy, and provides the ability to measure adsorption and monolayer-sensitive chemical information. Directional RS is performed by optically coupling a 50 nm gold film to a Weierstrass prism in the Kretschmann configuration and scanning the angle of the incident laser under total internal reflection. The collected parameters on the prism side of the interface include a full surface-plasmon-polariton cone and the full Raman signal radiating from the cone as a function of incident angle. An instrument for performing directional RS and a quantitative study of the instrumental parameters are herein reported. To test the sensitivity and quantify the instrument parameters, self-assembled monolayers and 10 to 100 nm polymer films are studied. The signals are found to be well-modeled by two calculated angle-dependent parameters: three-dimensional finite-difference time-domain calculations of the electric field generated in the sample layer and projected to the far-field, and Fresnel calculations of the reflected light intensity. This is the first report of the quantitative study of the full surface-plasmon-polariton cone intensity, cone diameter, and directional Raman signal as a function of incident angle. We propose that directional RS is a viable alternative to surface plasmon resonance when added chemical information is beneficial.

Surface-enhanced Raman spectroscopy: bottlenecks and future directions

This feature article discusses developmental bottleneck issues in surface Raman spectroscopy in its early stages and surface-enhanced Raman spectroscopy (SERS) in the past four decades and future perspectives.

Integrated plasmon-enhanced Raman scattering (iPERS) spectroscopy

A new strategy named integrated plasmon-enhanced Raman scattering (iPERS) spectroscopy that features a configuration of evanescent field excitation and inverted collection is presented, which well unites the local field enhancement and far field emission, couples localized and propagating surface plasmons, integrates the SERS substrates and Raman spectrometers via a self-designed aplanatic solid immersion lens. A metallic nanoparticle-on-a film (NOF) system was adopted in this configuration because it improves the amplification of the incidence light field in near field by 10 orders of magnitude due to the simultaneous excitation of quadrupolar and dipolar resonance modes. This iPERS allows for higher excitation efficiency to probed molecules and full collection of the directional-radiation Raman scattering signal in an inverted way, which exhibits a practical possibility to monitor plasmonic photocatalytic reactions in nanoscale and a bright future on interfacial reaction studies.

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.