Plasmonic Optical Tweezers based on Nanostructures: fundamentals, advances and prospects

The ability of metallic nanostructures to confine light at the sub-wavelength scale enables new perspectives and opportunities in the field of nanotechnology. Making use of this unique advantage, nano-optical trapping techniques have been developed to tackle new challenges in a wide range of areas from biology to quantum optics. In this work, starting from basic theories, we present a review of research progress in near-field optical manipulation techniques based on metallic nanostructures, with an emphasis on some of the most promising advances in molecular technology, such as the precise control of single-biomolecules. We also provide an overview of possible future research directions of nano-manipulation techniques.Comment: 19 page

Plasmonic and metamaterial biosensors: A game-changer for virus detection

One of the most important processes in the fight against current and future pandemics is the rapid diagnosis and initiation of treatment of viruses in humans. In these times, the development of high-sensitivity tests and diagnostic kits is an important research area. Plasmonic platforms, which control light in subwavelength volumes, have opened up exciting prospects for biosensing applications. Their significant sensitivity and selectivity allow for the non-invasive and rapid detection of viruses. In particular, plasmonic-assisted virus detection platforms can be achieved by various approaches, including propagating surface and localized plasmon resonances, as well as surface-enhanced Raman spectroscopy. In this review, we discuss both the fundamental principles governing a plasmonic biosensor and prospects for achieving improved sensor performance. We highlight several nanostructure schemes to combat virus-related diseases. We also examine technological limitations and challenges of plasmonic-based biosensing, such as reducing the overall cost and handling of complex biological samples. Finally, we provide a future prospective for opportunities to improve plasmonic-based approaches to increase their impact on global health issues.Comment: 1

Enabling self-induced back-action trapping of gold nanoparticles in metamaterial plasmonic tweezers

The pursuit for efficient nanoparticle trapping with low powers has led to optical tweezers technology moving from the conventional free-space configuration to advanced plasmonic tweezers systems. However, trapping nanoparticles smaller than 10 nm still remains a challenge even for plasmonic tweezers. Proper nanocavity design and excitation has given rise to the self-induced back-action (SIBA) effect offering enhanced trapping stiffness with decreased laser power. In this work, we investigate the SIBA effect in metamaterial tweezers and its synergy with the exhibited Fano resonance. We demonstrate stable trapping of 20 nm gold particles for on-resonant and off-resonant conditions with experimental trap stiffnesses as high as 4.18 fN/(nm*mW/$\mu$m$^2$ and very low excitation intensity of about 1 mW/$\mu$m$^2$. Simulations reveal the existence of two different groups of hotspots per unit cell of the metamaterial array. The two hotspots exhibit tunable trap stiffnesses and this is a unique feature of these systems. It can allow for sorting of particles and biological molecules based on their size, shape, and refractive index.Comment: 27 pages, 10 figure

In-situ analysis of small microplastics in coastal surface water samples of the subtropical island of Okinawa, Japan

Marine plastic debris is widely recognized as a global environmental issue. Sun-micron plastic particles, with an upper size limit of 20 um, have been identified as having the highest potential for causing damage to marine ecosystems. Having accurate methods for quantifying the abundance of such particles in a natural environment is essential for defining the extent of the problem they pose. Using an optical micro-Raman tweezers setup, we have identified the composition of particles trapped in marine aggregates collected from the coastal surface waters around the subtropical island of Okinawa. Chemical composition analysis at the single-particle level indicates dominance by low-density polyethylene, which accounted for 75% of the total sub-micron plastics analyzed. Our results show the occurrence of plastics at all test sites, with the highest concentration in areas with high human activities. The average, smallest sub-micron plastics size is (2.53 +/- 0.85)um for polystyrene. We also observed additional Raman peaks on the plastics spectrum with decreasing debris size which could be related to structural modification due to weathering or embedding in organic matter. By single-particle level sub-micron plastics identification, we can begin to understand their dispersion in the ocean and define their toxicity and impacts on marine biodiversity and food chain.Comment: 9 page

Fano-Resonant, Asymmetric, Metamaterial-Assisted Tweezers for Single Nanoparticle Trapping

Plasmonic nanostructures can overcome Abbe's diffraction limit to generate strong gradient fields, enabling efficient optical trapping of nano-sized particles. However, it remains challenging to achieve stable trapping with low incident laser intensity. Here, we demonstrate a Fano resonance-assisted plasmonic optical tweezers (FAPOT), for single nanoparticle trapping in an array of asymmetrical split nano-apertures, milled on a 50 nm gold thin film. Stable trapping is achieved by tuning the trapping wavelength and varying the incident trapping laser intensity. A very large normalized trap stiffness of 8.65 fN/nm/mW for 20 nm polystyrene particles at a near-resonance trapping wavelength of 930 nm was achieved. We show that trap stiffness on resonance is enhanced by a factor of 63 compared to off-resonance conditions. This can be attributed to the ultra-small mode volume, which enables large near-field strengths and a cavity Purcell effect contribution. These results should facilitate strong trapping with low incident trapping laser intensity, thereby providing new options for studying transition paths of single molecules, such as proteins, DNA, or viruses.Comment: 28 pages, 7 figure

Dynamic multiple nanoparticle trapping using metamaterial plasmonic tweezers

Optical manipulation has attracted remarkable interest owing to its versatile and noninvasive nature. However, conventional optical trapping remains inefficient in the nanoscopic world. The emergence of plasmonics in recent years has brought a revolutionary change in overcoming limitations due to diffraction and the requirements for high trapping laser powers. Among the near-field optical trapping cavity-based systems, Fano-resonant optical tweezers have a robust trapping capability. In this work, we experimentally demonstrate sequential trapping of 20nm particles through the use of metamaterial plasmonic optical tweezers. We investigate the multiple trapping via trap stiffness measurements for various trapping configurations at low and high incident laser intensities. Our plasmonic configuration could be used as a light-driven nanoscale sorting device under low laser excitation. Our results provide an alternative approach to trap multiple nanoparticles at distinct hotspots, enabling ways to control mass transport on the nanoscale

Analysis of small microplastics in coastal surface water samples of the subtropical island of Okinawa, Japan

Marine plastic debris is widely recognized as a global environmental issue. Small microplastic particles, with an upper size limit of 20 mu m, have been identified as having the highest potential for causing damage to marine ecosystems. Having accurate methods for quantifying the abundance of such particles in a natural environment is essential for defining the extent of the problem they pose. Using an optical micro-Raman tweezers setup, we have identified the composition of particles trapped in marine aggregates collected from the coastal surface waters around the subtropical island of Okinawa. Chemical composition analysis at the single-particle level indicates dominance by low-density polyethylene, which accounted for 75% of the small microplastics analysed. The smallest microplastics identified were (2.53 ± 0.85) mu m polystyrene. Our results show the occurrence of plastics at all test sites, with the highest concentration in areas with high human activities. We also observed additional Raman peaks on the plastics spectrum with decreasing debris size which could be related to structural modification due to weathering or embedding in organic matter. By identifying small microplastics at the single-particle level, we obtain some indication on their dispersion in the ocean which could be useful for future studies on their potential impact on marine biodiversity

Asymmetric split-ring plasmonic nanostructures for optical sensing of Escherichia coli

Strategies for in-liquid micro-organism detection are crucial for the clinical and pharmaceutical industries. While Raman spectroscopy is a promising label-free technique for micro-organism detection, it remains challenging due to the weak bacterial Raman signals. In this work, we exploit the unique electromagnetic properties of metamaterials to identify bacterial components in liquid using an array of Fano-resonant metamolecules. This Fano-enhanced Raman scattering (FERS) platform is designed to exhibit a Fano resonance close to the protein amide group fingerprint around 6030 nm. Raman signatures of Escherichia coli were recorded at several locations on the metamaterial under off-resonance laser excitation at 530 nm, where the photodamage effect is minimized. As the sizes of the Escherichia coli are comparable to the micro-gaps, i.e 0.41 {\mu}m, of the metamaterials, its local immobilisation leads to an increase in the Raman sensitivity. We also observed that the time-dependent FERS signal related to bacterial amide peaks increased during the bacteria's mid-exponential phase while it decreased during the stationary phase. This work provides a new set of opportunities for developing ultrasensitive FERS platforms suitable for large-scale applications and could be particularly useful for diagnostics and environmental studies at off-resonance excitation.Comment: 15 pages, 5 figure