Low power density, high-efficiency reflective Raman system for polymer SERS substrates.

Surface-enhanced Raman spectroscopy (SERS) is a powerful measurement method in the chemical analysis field. It is much superior to bulk Raman owing to the enhancement of signal sensitivity from the SERS substrate. Nevertheless, the delicate SERS substrates are overpriced, which results in the difficulty of universal measurements. Accordingly, opting for a substrate made of polymer material based on the nanoimprint technique shows great potential for low-cost and high-performance SERS substrates. However, due to its low heat conductivity, the polymer's thermal properties may cause heat to concentrate on the incident spot and damage the nanostructures or analytes. In this article, we proposed a novel design of the Reflective Raman (RR) system to reduce the input power density and maintain high collection efficiency at the same time. The proposed RR system was directly compared with a traditional micro Raman (µ-Raman) system and demonstrated its outstanding performance for low damage threshold analytes and SERS substrates.

Development of an automated Raman system and use of principal component analysis to classify real and counterfeit liquors.

We developed an automated Raman measurement platform for the customized design of various solution containers. We used the software LabVIEW to integrate the entire automatic measurement process. By designing an intuitive human-machine interface, the user only needs to input a few setting parameters and can efficiently operate the machine in automation mode for an array of solutions containing real or counterfeit liquors such as kaoliang liquor, vodka, rum, gin, rice wine, ethanol, and methanol. In this study, data from various alcoholic beverage solutions were subjected to principal component analysis (PCA) to distinguish from the low-concentration counterfeit liquors (methanol <50 g L-1). Moreover, several brands of liquors with the same alcohol concentration were successfully classified into different groups based on a combination of Raman spectroscopy and PCA analysis.

A Review of Critical Issues in High-Speed Vat Photopolymerization.

Vat photopolymerization (VPP) is an effective additive manufacturing (AM) process known for its high dimensional accuracy and excellent surface finish. It employs vector scanning and mask projection techniques to cure photopolymer resin at a specific wavelength. Among the mask projection methods, digital light processing (DLP) and liquid crystal display (LCD) VPP have gained significant popularity in various industries. To upgrade DLP and LCC VPP into a high-speed process, increasing both the printing speed and projection area in terms of the volumetric print rate is crucial. However, challenges arise, such as the high separation force between the cured part and the interface and a longer resin refilling time. Additionally, the divergence of the light-emitting diode (LED) makes controlling the irradiance homogeneity of large-sized LCD panels difficult, while low transmission rates of near ultraviolet (NUV) impact the processing time of LCD VPP. Furthermore, limitations in light intensity and fixed pixel ratios of digital micromirror devices (DMDs) constrain the increase in the projection area of DLP VPP. This paper identifies these critical issues and provides detailed reviews of available solutions, aiming to guide future research towards developing a more productive and cost-effective high-speed VPP in terms of the high volumetric print rate.

Development of a quasi-collimated UV LED backlight for producing uniform and smooth 3D printing objects.

3D printing techniques have great potential in the direct fabrication of microfluidic and many kinds of molds, such as dental and jewelry models. However, the resolution, surface roughness, and critical dimension uniformity of 3D printing objects are still a challenge for improvement. In this article, we proposed a 405nm light emitting diode (LED) backlight module based on stacks of structured films, and the full width half maximum (FWHM) of the angular distribution of this module is reduced to less than ± 15°. Compared with the commercial lens array optical module, the ten points intensity uniformity of an 8.9" build area is improved from 56% to 80%. Moreover, we found that the surface roughness and the sharpness of the edge of the printing objects are also obviously improved by our novel quasi-collimated LED backlight module. These features give us a promising way for the application of microfluidics and micro-optics components in the future.

Rapid Biochemical Mixture Screening by Three-Dimensional Patterned Multifunctional Substrate with Ultra-Thin Layer Chromatography (UTLC) and Surface Enhanced Raman Scattering (SERS).

We present a three-dimensional patterned (3DP) multifunctional substrate with the functions of ultra-thin layer chromatography (UTLC) and surface enhanced Raman scattering (SERS), which simultaneously enables mixture separation, target localization and label-free detection. This multifunctional substrate is comprised of a 3DP silicon nanowires array (3DP-SiNWA), decorated with silver nano-dendrites (AgNDs) atop. The 3DP-SiNWA is fabricated by a facile photolithographic process and low-cost metal assisted chemical etching (MaCE) process. Then, the AgNDs are decorated onto 3DP-SiNWA by a wet chemical reduction process, obtaining 3DP-AgNDs@SiNWA multifunctional substrates. With various patterns designed on the substrates, the signal intensity could be maximized by the excellent confinement and concentrated effects of patterns. By using this 3DP-AgNDs@SiNWA substrate to scrutinize the mixture of two visible dyes, the individual target could be recognized and further boosted the Raman signal of target 15.42 times comparing to the un-patterned AgNDs@SiNWA substrate. Therefore, such a three-dimensional patterned multifunctional substrate empowers rapid mixture screening, and can be readily employed in practical applications for biochemical assays, food safety and other fields.

Near-Field Plasmonic Probe with Super Resolution and High Throughput and Signal-to-Noise Ratio.

Near-field scanning optical microscopy (NSOM) enables observation of light-matter interaction with a spatial resolution far below the diffraction limit without the need for a vacuum environment. However, modern NSOM techniques remain subject to a few fundamental restrictions. For example, concerning the aperture tip (a-tip), the throughput is extremely low, and the lateral resolution is poor; both are limited by the aperture size. Meanwhile, with regard to the scattering tip (s-tip), the signal-to-noise ratio (SNR) appears to be almost zero; consequently, one cannot directly use the measured data. In this work, we present a plasmonic tip (p-tip) developed by tailoring subwavelength annuli so as to couple internal radial illumination to surface plasmon polaritons (SPPs), resulting in an ultrastrong, superfocused spot. Our p-tip supports both a radial symmetric SPP excitation and a Fabry-Pérot resonance, and experimental results indicate an optical resolution of 10 nm, a topographic resolution of 10 nm, a throughput of 3.28%, and an outstanding SNR of up to 18.2 (nearly free of background). The demonstrated p-tip outperforms state-of-the-art NSOM tips and can be readily employed in near-field optics, nanolithography, tip-enhanced Raman spectroscopy, and other applications.

Three-Dimensional SERS Substrates Formed with Plasmonic Core-Satellite Nanostructures.

We demonstrate three-dimensional surface-enhanced Raman spectroscopy (SERS) substrates formed by accumulating plasmonic nanostructures that are synthesized using a DNA-assisted assembly method. We densely immobilize Au nanoparticles (AuNPs) on polymer beads to form core-satellite nanostructures for detecting molecules by SERS. The experimental parameters affecting the AuNP immobilization, including salt concentration and the number ratio of the AuNPs to the polymer beads, are tested to achieve a high density of the immobilized AuNPs. To create electromagnetic hot spots for sensitive SERS sensing, we add a Ag shell to the AuNPs to reduce the interparticle distance further, and we carefully adjust the thickness of the shell to optimize the SERS effects. In addition, to obtain sensitive and reproducible SERS results, instead of using the core-satellite nanostructures dispersed in solution directly, we prepare SERS substrates consisting of closely packed nanostructures by drying nanostructure-containing droplets on hydrophobic surfaces. The densely distributed small and well-controlled nanogaps on the accumulated nanostructures function as three-dimensional SERS hot spots. Our results show that the SERS spectra obtained using the substrates are much stronger and more reproducible than that obtained using the nanostructures dispersed in solution. Sensitive detection of melamine and sodium thiocyanate (NaSCN) are achieved using the SERS substrates.

A Low-cost, Highly-stable Surface Enhanced Raman Scattering Substrate by Si Nanowire Arrays Decorated with Au Nanoparticles and Au Backplate.

We present a facile and cost-effective manner to fabricate a highly sensitive and stable surface enhanced Raman scattering (SERS) substrate. First, a silicon nanowire array (SiNWA) is tailored by metal-assisted chemical etching (MaCE) method as a scaffold of the desired SERS substrate. Next, with an oblique angle deposition (OAD) method, optimized gold nanoparticles (AuNPs) are successfully decorated on the surface of the SiNWA. These AuNPs enable a strong localized electric field, providing abundant hot spots to intensify the Raman signals from the targeting molecules. By applying a well-established methodology, Taguchi method, which is invented for designing experiments, the optimized combination of parameters is obtained efficiently. The experimental results are also confirmed by finite-difference time-domain (FDTD) simulation calculations. Besides, a gold metal backplate (AuMBP) is applied to further enhancing the Raman signal intensity. Based on this developed SERS substrate, we demonstrated an enhancement factor (EF) of 1.78 × 106 and a coefficient of variation (CV) of 4.2%. Both EF and CV indicate a highly stable property and the optimized SERS substrate substantially outperform the commercial product. In the end, we also demonstrate a quantitative measurement on practical application of detecting malachite green (MG) with concentration from 10 nM to 100 µM.

Increasing the spectral shifts in LSPR biosensing using DNA-functionalized gold nanorods in a competitive assay format for the detection of interferon-γ.

We demonstrate an approach that utilizes DNA-functionalized gold nanorods (AuNRs) in an indirect competitive assay format to increase the spectra shift in localized surface plasmon resonance (LSPR) biosensing. We use interferon gamma (IFN-γ) as a model analyte to demonstrate the feasibility of our detection method. The LSPR chips with periodic gold nanodot arrays are fabricated using a thermal lithography process and are functionalized with IFN-γ aptamers for detection. The DNA-functionalized AuNRs and IFN-γ compete with each other to bind to the aptamers during detection, and the spectra shifts are mainly caused by the AuNRs rather than IFN-γ. When using our approach, the target molecules do not need to be captured by two capture ligands simultaneously during detection and thus do not require multiple binding sites. Both experiments and finite-difference time-domain (FDTD) simulations show that making the AuNRs as close to the chip surface as possible is very critical for increasing LSPR shifts, and the simulated results also show that the orientation of the AuNR affects the plasmon coupling between the gold nanodots on the chip surface and the nearby AuNRs. Although only the detection of IFN-γ is demonstrated in this study, we expect that the LSPR biosensing method can be applied to label-free detection of a variety of molecules as long as suitable aptamers are available.

Effects of the tip shape on the localized field enhancement and far field radiation pattern of the plasmonic inverted pyramidal nanostructures with the tips for surface-enhanced Raman scattering.

The plasmonic 2D W-shape and 3D inverted pyramidal nanostructures with and without the tips are studied. The effects of the tip height and tip tilt angle on the near field enhancement and far field radiation pattern are discussed in this paper. The localized hot spots are found around the pits and the radiation pattern can be affected by the tip structures. The inverted pyramidal nanostructures with and without the tips were fabricated and their reflection spectra and surface-enhanced Raman scattering (SERS) signals for the chemical molecules thiophenol were measured. The simulation according to the geometry parameters of the fabricated structures is demonstrated. We found that the SERS of our proposed structures with the tips can have stronger light field enhancements than the inverted pyramidal nanostructures without the tips, and the far field radiation pattern can be varied by changing the tip height and tip tilt angle. The study of surface plasmon modes and charge distributions can help the understanding of how to arrange the plasmonic structures to achieve high field enhancement and preferred far field radiation pattern. Our study can be useful for the design of the strong field enhancement SERS substrate with specific far field radiation properties. It can be also applied to the portable Raman detectors for in situ and remote measurements in specific applications.

Propagation characteristics of silver and tungsten subwavelength annular aperture generated sub-micron non-diffraction beams.

We examined the optical properties such as propagation modes, focal length, side lobes, etc. of metallic subwavelength annular apertures (SAA) and used finite-difference time-domain (FDTD) simulation to compare our experimental findings. Using two different metals, silver and tungsten, we examined the different optical transmission properties of the two metallic SAA structures. The far-field propagation of the silver SAA structure was found to be a type of quasi-Bessel beam when compared with a quasi-Bessel beam generated by a perfect axicon. The propagation characteristics of these two beams were found to match qualitatively. The far-field transmitted light generated by the silver SAA structure was found to possess a 390 nm sub-micron focal spot with a 24 microm depth of focus, which was much smaller than the focal spot generated by a perfect axicon. We also found that a silver SAA structure can generate a sub-micron quasi- Bessel beam that has a much lower far-field side-lobe when compared to that of non-diffraction beams generated by a tungsten SAA structure.

Effect of subwavelength annular aperture diameter on the nondiffracting region of generated Bessel beams.

A subwavelength annular aperture (SAA) made on metallic film and deposited on a glass substrate was fabricated by electron-beam lithography (EBL) and which was followed by a metal lift-off process to generate a long propagation range Bessel beam. We propose tuning the focal length and depth of focus (DOF) by changing the diameter of the SAA. We used finite-difference time domain (FDTD) simulations to verify our experimental data. We found that the position of the Bessel Beam focus spot (i.e. focal length) will be farther away from the SAA plane as the diameter of the SAA increases. In addition, the depth of focus (DOF) which is the length of the Bessel beam non-diffracting area, also increases as the diameter of the SAA expands.

Directional light beaming control by a subwavelength asymmetric surface structure.

We propose a direct experimental set-up to observe the directional beaming effect of surface plasmon. A single diffracted beam from an asymmetric-sided surface corrugation is demonstrated. A single subwavelength slit with an asymmetric structure was fabricated using a focused ion beam (FIB) onto a metal surface with a glass substrate. By means of surface plasmon (SP) diffraction, the directionality of the light can be changed by the period of the metallic gratings. We show corresponding numerical simulations achieved by a Rigorous Coupled-Wave Analysis (RCWA) method and a Finite-Difference Time-Domain (FDTD) method. The simulation results were in agreement with the experimental data.