Evanescent-wave excitation of surface-enhanced Raman scattering substrates by an optical-fiber taper

Fiber Optic SERS Sensor with Silver Nanocubes Attached Based on Evanescent Wave for Detecting Pesticide Residues

Surface-enhanced Raman scattering (SERS) has great potential in the field of rapid detection of pesticide residues in food. In this paper, a fiber optic SERS sensor excited by evanescent waves was proposed for efficient detection of thiram. Silver nanocubes (Ag NCs) were prepared as SERS active substrates, which had much stronger electromagnetic field intensity than nanospheres under laser excitation due to much more "hot spots". By using the method of electrostatic adsorption and laser induction, Ag NCs were uniformly assembled at the fiber taper waist (FTW) for enhancing the Raman signal. Different from the traditional way of stimulation, evanescent wave excitation greatly increased the interaction area between the excitation and analyte, while reducing the damage of the excited light to the metal nanostructures. The methods proposed in this work have been successfully used to detect the pesticide residues of thiram and showed good detection performance. The detection limits for 4-Mercaptobenzoic acid (4-MBA) and thiram were determined to be 10^-9 and 10^-8 M, the corresponding enhancement factor could be 1.64 × 10⁵ and 6.38 × 10⁴. Low concentration of thiram was detected in the peels of tomatoes and cucumbers, indicating its feasibility in actual sample detection. The combination of evanescent waves and SERS provides a new direction for the application of SERS sensors, which had great application potential in the field of pesticide residue detection.

A nanotweezer system for evanescent wave excited surface enhanced Raman spectroscopy (SERS) of single nanoparticles

We experimentally demonstrate the integration of near-field optical tweezers with surface enhanced Raman scattering (SERS) spectroscopy by using the optical evanescent wave from a silicon nitride waveguide to trap single shell-isolated metallic nanoparticles (NPs) and simultaneously excite SERS signals of Raman reporter molecules adsorbed on the surface of the trapped metallic NPs. Both evanescent wave excited Stokes and anti-Stokes SERS spectra of waveguide trapped single silver (Ag) NPs were acquired, which were compared to their far-field SERS spectra. We investigated the trapping of bare and shell-isolated metallic NPs and determined that the addition of a shell to the metallic NPs minimized particle-induced laser damage to the waveguide, which allowed for the stable acquisition of the SERS spectra. This work realizes a new nanophotonic approach, which we refer to as near-field light scattering Raman (NLS-Raman), for simultaneous near-field optical trapping and SERS characterization of single metallic NPs.

Fast and cost-effective fabrication of large-area plasmonic transparent biosensor array

Surface enhanced Raman-based sensors are widely used for chemical and biological species analysis; but to date the high cost, long production time, hazardous, and toxic content as well as small sensing area and opacity are limiting their capabilities for widespread applications in the medical and environmental fields. We present a novel cost-effective method for fast laser-based fabrication of affordable large-area and transparent periodic arrays of ligand-free metallic nanoparticles, offering a maximum possibility for the adsorption/immobilization of molecules and labeling. Further, we demonstrate a remarkable detection limit in the picomolar range by means of Raman scattering, thus evidencing a superior signal-to-noise ratio compared to other sensor substrates. The high sensitivity performance along with a fast and cheap fabrication procedure of reusable large-area transparent plasmonic devices opens the route for direct, in situ multimodal optical analysis with broad applications in the biomedical/analytical fields.

Amplification of the signal intensity of fluorescence-based fiber-optic biosensors using a Fabry-Perot resonator structure

Fluorescent biosensors have been widely used in biomedical applications. To amplify the intensity of fluorescence signals, this study developed a novel structure for an evanescent wave fiber-optic biosensor by using a Fabry-Perot resonator structure. An excitation light was coupled into the optical fiber through a laser-drilled hole on the proximal end of the resonator. After entering the resonator, the excitation light was reflected back and forth inside the resonator, thereby amplifying the intensity of the light in the fiber. Subsequently, the light was used to excite the fluorescent molecules in the reactive region of the sensor. The experimental results showed that the biosensor signal was amplified eight-fold when the resonator reflector was formed using a 92% reflective coating. Furthermore, in a simulation, the biosensor signal could be amplified 20-fold by using a 99% reflector.

Laser-induced synthesis of Ag nanoparticles on the silanized surface of a fiber taper and applications as a SERS probe

The silanized fiber probe with the optimal laser-induced growth time exhibited very strong SERS activity and good measurement reproducibility.

Advantage of multi-mode sapphire optical fiber for evanescent-field SERS sensing

An unclad, multi-mode single crystal sapphire fiber was used as a platform, and immobilized colloidal Ag nanoparticles (NPs) were used as enabler, for evanescent-field fiber-optic sensing via surface-enhanced Raman scattering (SERS) of Rhodamine 6G (R6G) solution. The dependence of the measured Raman intensity on NP coverage density (to a maximum of 120  particles/μm²) as well as the coverage length (to a maximum of 6 cm) was investigated. We demonstrate the utility of SERS-active sapphire fibers for sensitive measurements (10⁻⁸ M R6G). We further reveal, with the aid of theoretical analysis, that multi-mode fiber offers a significant advantage compared to its single-mode counterpart because the former allows two orders of magnitude higher particle coverage density than the latter to maximize SERS benefit, while maintaining the dominance of Raman gain despite the competitive interplay of NP-induced absorption and scattering loss along the interaction path length.

Evanescent field characteristics of eccentric core optical fiber for distributed sensing

Fundamental core-mode cutoff and evanescent field are considered for an eccentric core optical fiber (ECOF). A method has been proposed to calculate the core-mode cutoff by solving the eigenvalue equations of an ECOF. Using conformal mapping, the asymmetric geometrical structure can be transformed into a simple, easily solved axisymmetric optical fiber with three layers. The variation of the fundamental core-mode cut-off frequency (V(c)) is also calculated with different eccentric distances, wavelengths, core radii, and coating refractive indices. The fractional power of evanescent fields for ECOF is also calculated with the eccentric distances and coating refractive indices. These calculations are necessary to design the structural parameters of an ECOF for long-distance, single-mode distributed evanescent field absorption sensors.

Miniature micro-wire based optical fiber-field access device

This paper presents an optical fiber-field access device suitable for use in different in-line fiber-optics' systems and fiber-based photonics' components. The proposed device utilizes a thin silica micro-wire positioned in-between two lead-in single mode fibers. The thin micro-wire acts as a waveguide that allows for low-loss interconnection between both lead-in fibers, while providing interaction between the guided optical field and the surrounding medium or other photonic structures. The field interaction strength, total loss, and phase matching conditions can be partially controlled by device-design. The presented all-fiber device is miniature in size and utilizes an all-silica construction. It has mechanical properties suitable for handling and packaging without the need for additional mechanical support or reinforcements. The proposed device was produced using a micromachining method that utilizes selective etching of a purposely-produced phosphorus pentoxide-doped optical fiber. This method is simple, compatible with batch processes, and has good high-volume manufacturing potential.

Characteristics of embedded-core hollow optical fiber

We propose a novel embedded-core hollow optical fiber composed of a central air hole, a semi-elliptical core, and an annular cladding. The fiber characteristics are investigated based on the finite element method (FEM), including mode properties, birefringence, confinement loss, evanescent field and bending loss. The results reveal that the embedded-core hollow optical fiber has a non-zero cut-off frequency for the fundamental mode. The birefringence of the hollow optical fiber is insensitive to the size of the central air hole and ultra-sensitive to the thickness of the cladding between the core and the air hole. Both thin cladding between the core and the air hole and small core ellipticity lead to high birefringence. An ultra-low birefringence fiber can be achieved by selecting a proper ellipticity of the core. The embedded-core hollow optical fiber holds a strong evanescent field due to special structure of thin cladding and therefore it is of importance for potential applications such as gas and biochemical sensors. The bending losses are measured experimentally. The bending loss strongly depends on bending orientations of the fiber. The proposed fiber can be used as polarization interference devices if the orientation angle of the fiber core is neither 0° nor 90°.