Remote SERS detection at a 10-m scale using silica fiber SERS probes coupled with a convolutional neural network.

A silica fiber surface-enhanced Raman scattering (SERS) probe provides a practical way for remote SERS detection of analytes, but it faces the major bottleneck that the relatively large Raman background of silica fiber itself greatly limits the remote detection sensitivity and distance. In this article, we developed a convolutional neural network (CNN)-based deep learning algorithm to effectively remove the Raman background of silica fiber itself and thus significantly improved the remote detection capability of the silica fiber SERS probes. The CNN model was constructed based on a U-Net architecture and instead of concatenating, the residual connection was adopted to fully leverage the features of both the shallow and deep layers. After training, this CNN model presented an excellent background removal capacity and thus improved the detection sensitivity by an order of magnitude compared with the conventional reference spectrum method (RSM). By combining the CNN algorithm and the highly sensitive fiber SERS probes fabricated by the laser-induced evaporation self-assembly method, a limit of detection (LOD) as low as 10-8 M for Rh6G solution was achieved with a long detection distance of 10 m. To the best of our knowledge, this is the first report of remote SERS detection at a 10-m scale with fiber SERS probes. As the proposed remote detection system with silica fiber SERS probes was very simple and low cost, this work may find important applications in hazardous detection, contaminant monitoring, and other remote spectroscopic detection in biomedicine and environmental sciences.

Highly sensitive SERS detection in a non-volatile liquid-phase system with nanocluster-patterned optical fiber SERS probes.

The use of surface-enhanced Raman scattering (SERS) spectroscopy for the detection of substances in non-volatile systems, such as edible oil and biological cells, is an important issue in the fields of food safety and biomedicine. However, traditional dry-state SERS detection with planar SERS substrates is not suitable for highly sensitive and rapid SERS detection in non-volatile liquid-phase systems. In this paper, we take contaminant in edible oil as an example and propose an in situ SERS detection method for non-volatile complex liquid-phase systems with high-performance optical fiber SERS probes. Au-nanorod clusters are successfully prepared on optical fiber facet by a laboratory-developed laser-induced dynamic dip-coating method, and relatively high detection sensitivity (LOD of 2.4 × 10-6 mol/L for Sudan red and 3.6 × 10-7 mol/L for thiram in sunflower oil) and good reproducibility (RSD less than 10%) are achieved with a portable Raman spectrometer and short spectral integration time of 10 s even in complex edible oil systems. Additionally, the recovery rate experiment indicates the reliability and capability of this method for quantitative detection applications. This work provides a new insight for highly sensitive and rapid SERS detection in non-volatile liquid-phase systems with optical fiber SERS probes and may find important practical applications in food safety and biomedicine.

Compact all-fiber thermo-optic modulator based on a Michelson interferometer coated with NaNdF4 nanoparticles.

We propose and investigate an all-fiber thermo-optic modulator based on a side-polished twin-core fiber (TCF) Michelson interferometer (MI) coated with NaNdF4 nanoparticles. The MI was fabricated by tapering the splicing point between the TCF and a single mode fiber (SMF). A short suspended core fiber (SCF) is spliced to one core of the TCF to introduce a fixed optical phase difference (OPD). The side-polished core is coated with photo-thermal material NaNdF4. Owing to the ohmic heating of NaNdF4 nanoparticles under 808 nm pump laser, the effective refractive index of the polished core is changed, resulting in a phase shift of the MI. The MI has a significant modulation phase shift with 2.9 π near the wavelength of 1260 nm and can obtain an optical switching with a rise (fall) time of 152 (50) ms. The proposed device will have a great application potential in optical modulators due to compact structure and strong robustness.

High extinction ratio D-shaped fiber polarizers coated by a double graphene/PMMA stack.

We demonstrate theoretically and experimentally a high extinction ratio and compact size TE-pass polarizer made by a D-shaped fiber coated with a double graphene/PMMA stack. The light propagating in the core of the fiber can be efficiently coupled into the graphene sheet thanks to the giant enhancement of the modal evanescent field associated with the high refractive index graphene/PMMA cladding. The strong interaction between the light and graphene produces a large attenuation difference between modes with orthogonal polarizations, resulting in an improved extinction ratio and a reduced insertion loss due to the device compactness. A double graphene/PMMA stack coated polarizer with an extinction ratio of up to 36 dB and an insertion loss of 5 dB has been achieved when the device length is only 2.5 mm. The double graphene/PMMA stack has proved to be significantly better than single graphene/PMMA stack and bilayer graphene/PMMA structures, providing a polarizer with maximum extinction ratio of 44 dB for a length of 4 mm. The achieved results indicate that the proposed high extinction ratio polarizer is a promising candidate for novel in-fiber graphene-based devices.

Generation of ultra-wideband achromatic Airy plasmons on a graphene surface.

Tunable ultra-wideband achromatic plasmonic Airy beams are demonstrated on graphene surfaces. Surface plasmonic polaritons are excited using diffractive gratings. The phase and amplitude of plasmonic waves on the graphene surface are determined by the relative position between the grating arrays and the duty ratio of the grating unit cell, respectively. The transverse acceleration and nondiffraction properties of plasmonic waves are observed. The achromatic Airy plasmons with identical acceleration trajectory at different excited frequencies can be achieved by tuning dynamically the Fermi energy of graphene without reoptimizing the grating structures. The proposed devices may find applications in photonics integrations and surface optical manipulation.

All-solid microstructured fibers with double cross linear arrays.

An all-solid microstructured fiber containing a pure silica core and the double cross linear arrays of high refractive index dielectric rods imbedded in the silica serving as the cladding is proposed. The bandgap and confinement loss (CL) are numerically investigated. The fiber has a lowest loss of 3.37 dB/km that can be achieved in the first-order photonic bandgap (PBG) and the bandwidth with transmission loss less than <0.05 dB/m is up to 550 nm. The effects of refractive index difference and geometric structure are discussed on the CL in the first-order PBG. The minimal attenuation can be reduced to 0.14 dB/km by choosing appropriate parameters. The bending property is also presented.