Tunable Three-Dimensional Plasmonic Arrays for Large Near-Infrared Fluorescence Enhancement

Metal-enhanced fluorescence (MEF), resulting from the near-field interaction of fluorophores with metallic nanostructures, has emerged as a powerful tool for dramatically improving the performance of fluorescence-based biomedical applications. Allowing for lower autofluorescence and minimal photoinduced damage, the development of multifunctional and multiplexed MEF platforms in the near-infrared (NIR) windows is particularly desirable. Here, a low-cost fabrication method based on nanosphere lithography is applied to produce tunable three-dimensional (3D) gold (Au) nanohole-disc arrays (Au-NHDAs). The arrays consist of nanoscale glass pillars atop nanoholes in a Au thin film: the top surfaces of the pillars are Au-covered (effectively nanodiscs), and small Au nanoparticles (nanodots) are located on the sidewalls of the pillars. This 3D hole-disc (and possibly nanodot) construct is critical to the properties of the device. The versatility of our approach is illustrated through the production of uniform and highly reproducible Au-NHDAs with controlled structural properties and tunable optical features in the NIR windows. Au-NHDAs allow for a very large NIR fluorescence enhancement (more than 400 times), which is attributed to the 3D plasmonic structure of the arrays that allows strong surface plasmon polariton and localized surface plasmon resonance coupling through glass nanogaps. By considering arrays with the same resonance peak and the same nanodisc separation distance, we show that the enhancement factor varies with nanodisc diameter. Using computational electromagnetic modeling, the electric field enhancement at 790 nm was calculated to provide insights into excitation enhancement, which occurs due to an increase in the intensity of the electric field. Fluorescence lifetime measurements indicate that the total fluorescence enhancement may depend on controlling excitation enhancement and therefore the array morphology. Our findings provide important insights into the mechanism of MEF from 3D plasmonic arrays and establish a low-cost versatile approach that could pave the way for novel NIR-MEF bioapplications.

Surface Lattice Plasmon Resonances by Direct In Situ Substrate Growth of Gold Nanoparticles in Ordered Arrays

Precise arrangements of plasmonic nanoparticles on substrates are important for designing optoelectronics, sensors and metamaterials with rational electronic, optical and magnetic properties. Bottom-up synthesis offers unmatched control over morphology and optical response of individual plasmonic building blocks. Usually, the incorporation of nanoparticles made by bottom-up wet chemistry starts from batch synthesis of colloids, which requires time-consuming and hard-to-scale steps like ligand exchange and self-assembly. Herein, an unconventional bottom-up wet-chemical synthetic approach for producing gold nanoparticle ordered arrays is developed. Water-processable hydroxypropyl cellulose stencils facilitate the patterning of a reductant chemical ink on which nanoparticle growth selectively occurs. Arrays exhibiting lattice plasmon resonances in the visible region and near infrared (quality factors of >20) are produced following a rapid synthetic step (<10 min), all without cleanroom fabrication, specialized equipment, or self-assembly, constituting a major step forward in establishing in situ growth approaches. Further, the technical capabilities of this method through modulation of the particle size, shape, and array spacings directly on the substrate are demonstrated. Ultimately, establishing a fundamental understanding of in situ growth has the potential to inform the fabrication of plasmonic materials; opening the door for in situ growth fabrication of waveguides, lasing platforms, and plasmonic sensors.

Enantiomeric discrimination by chiral electromagnetic resonance enhancement

Circularly polarized light interacts preferentially with the biomolecules to generate spectral fingerprints reflecting their primary and secondary structure in the ultraviolet region of the electromagnetic spectrum. The spectral features can be transferred to the visible and near-infrared regions by coupling the biomolecules with plasmonic assemblies made of noble metals. Nanoscale gold tetrahelices were used to detect the presence of chiral objects that are 40 times smaller in size by using plane-polarized light of 550 nm wavelength. The emergence of chiral hotspots in the gaps between 80 nm long tetrahelices differentiate between weakly scattering S- vs R-molecules with optical constants similar to that of organic solvents. Simulations map the spatial distribution of the scattered field to reveal enantiomeric discrimination with selectivity up to 0.54.

A CW-Pumped Dye Nanolaser With an Optimized Nanocavity

Plasmonic nanolasers, which are promising coherent light sources for integrated photonics, super-resolution imaging, and ultrasensitive sensing applications, face the challenge of high thresholds due to inherent losses in plasmonic nanocavities. While considerable efforts have been made to improve the Q factor, typically quantified by the full width at half maximum (FWHM), intensity (ΔI) is another critical feature of plasmonic resonance. However, the combined influence of both the Q factor and resonance intensity of a plasmonic nanocavity on nanolasing threshold has not been sufficiently explored, as experimentally controlling one variable while keeping the other constant is challenging. Here, an ultra-low threshold plasmonic nanolaser is demonstrated by systematically optimizing the plasmonic nanocavity. By carefully controlling both the FWHMs and resonance intensities of the plasmonic resonators, a record low threshold of 2.6 µJ cm-2 for a dye-based nanolaser is achieved at room temperature- an order of magnitude lower than previous records. In addition, nanolasing under continuous-wave (CW) excitation is reported at room temperature gained by the same dye molecule. The results provide new insights into the design of high-performance plasmonic nanolasers and offer a promising path toward realizing applications of nanoscale coherent light sources.

High-performance plasmonic lab-on-fiber sensing system constructed by universal polymer assisted transfer technique

Plasmonic lab-on-fiber (LOF) system has become an emerging sensing platform for the realization of miniaturized and portable plasmonic sensors. Herein, a facile and efficient polymer assisted transfer technique was reported for the preparation of plasmonic LOF systems. The proposed plasmonic LOF system was constructed through transferring plasmonic arrays to the end surface of optical fibers using polylactic acid as the sacrificial layer. The morphology of the transferred plasmonic arrays maintains excellent consistency with the original arrays. Importantly, the as-prepared plasmonic LOF system also possesses outstanding sensing performance in refractive index sensing and quantitative label-free biosensing applications. Additionally, the proposed polymer assisted transfer technique shows broad universality for various plasmonic arrays. Together with the above features, it is believed that the polymer assisted transfer technique will show great potential for the application of future plasmonic LOF systems.

Silver Nanocube Epitaxy via Nanogap-Induced Electrostatics

Silver nanostructures are highly valued in nanophotonic devices due to their appealing plasmonic properties and affordability relative to gold. Yet, fabricating high-quality, monocrystalline silver nanostructures, with full control over the shape, is challenging. A mild, liquid-phase method for the epitaxial welding of adjacent monocrystalline silver nanocubes in reductant-free conditions is introduced to prevent the formation of detrimental nuclei on the surface that can degrade the nanostructures' optical qualities. The mechanism is thoroughly investigated and it is found that the nanocubes themselves can act as reducing agents, promoting growth preferentially into the gap as a result of electrostatic interactions. By controlling experimental parameters such as temperature, pH, and the introduction of capping agents, a balance between nanocube epitaxy and shape retention is achieved. Finally, by applying this procedure to nanoparticle assembled in predefined meta-atoms, the feasibility of creating intricate silver nanostructures, that are monocrystalline as verified by transmission electron microscopy (TEM), is demonstrated. This advancement paves the way for bottom-up fabrication of optical metasurfaces that can be swiftly integrated in devices.