Optofluidic transport and particle trapping using an all-dielectric quasi-BIC metasurface

Manipulating fluids by light at the nanoscale has been a long-sought-after goal for lab-on-a-chip applications. Plasmonic heating has been demonstrated to control microfluidic dynamics due to the enhanced and confined light absorption from the intrinsic losses of metals. Dielectrics, counterpart of metals, is used to avoid undesired thermal effects due to its negligible light absorption. Here, we report an innovative optofluidic system that leverages a quasi-BIC driven all-dielectric metasurface to achieve nanoscale control of temperature and fluid motion. Our experiments show that suspended particles down to 200 nanometers can be rapidly aggregated to the center of the illuminated metasurface with a velocity of tens of micrometers per second, and up to millimeter-scale particle transport is demonstrated. The strong electromagnetic field enhancement of the quasi-BIC resonance can facilitate increasing the flow velocity up to 3-times compared with the off-resonant situation. We also experimentally investigate the dynamics of particle aggregation with respect to laser wavelength and power. A physical model is presented to elucidate the phenomena and surfactants are added to the particle colloid to validate the model. Our study demonstrates the application of the recently emerged all-dielectric thermonanophotonics in dealing with functional liquids and opens new frontiers in harnessing non-plasmonic nanophotonics to manipulate microfluidic dynamics. Moreover, the synergistic effects of optofluidics and high-Q all-dielectric nanostructures can hold enormous potential in high-sensitivity biosensing applications

Scalable trapping of single nanosized extracellular vesicles using plasmonics

Heterogeneous nanoscale particles released by cells known as extracellular vesicles (EVs) are actively investigated for early disease detection1, monitoring2, and advanced therapeutics3. Due to their extremely small size, the stable trapping of nano-sized EVs using diffraction-limited optical tweezers4 has been met with challenges. Plasmon-enhanced optical trapping can confine light to the nanoscale to generate tight trapping potentials. Unfortunately, a long-standing challenge is that plasmonic tweezers have limited throughput and cannot provide rapid delivery and trapping of particles at plasmonic hotspots while precluding the intrinsic plasmon-induced photothermal heating effect at the same time. We report our original geometry-induced electrohydrodynamic tweezers (GET) that generate multiple electrohydrodynamic potentials for the parallelized transport and trapping of single EVs in parallel within seconds while enhancing the imaging of single trapped EVs. We show that the integration of nanoscale plasmonic cavities at the center of each GET trap results in the parallel placement of single EVs near plasmonic cavities enabling instantaneous plasmon-enhanced optical trapping upon laser illumination without any detrimental heating effect for the first time. These non-invasive scalable hybrid nanotweezers open new horizons for high-throughput tether-free plasmon-enhanced single EV trapping and spectroscopy. Other potential areas of impact include nanoplastics characterization, and scalable hybrid integration for quantum photonics.Comment: 21 pages, 5 figure

Merging toroidal dipole bound states in the continuum without up-down symmetry in Lieb lattice metasurfaces

The significance of bound states in the continuum (BICs) lies in their potential for theoretically infinite quality factors. However, their actual quality factors are limited by imperfections in fabrication, which lead to coupling with the radiation continuum. In this study, we present a novel approach to address this issue by introducing a merging BIC regime based on a Lieb lattice. By utilizing this approach, we effectively suppress the out-of-plane scattering loss, thereby enhancing the robustness of the structure against fabrication artifacts. Notably, unlike previous merging systems, our design does not rely on the up-down symmetry of metasurfaces. This characteristic grants more flexibility in applications that involve substrates and superstrates with different optical properties, such as microfluidic devices. Furthermore, we incorporate a lateral band gap mirror into the design to encapsulate the BIC structure. This mirror serves to suppress the in-plane radiation resulting from finite-size effects, leading to a remarkable ten-fold improvement in the quality factor. Consequently, our merged BIC metasurface, enclosed by the Lieb lattice photonic crystal mirror, achieves an exceptionally high-quality factor of 105 while maintaining a small footprint of 26.6X26.6 um. Our findings establish an appealing platform that capitalizes on the topological nature of BICs within compact structures. This platform holds great promise for various applications, including optical trapping, optofluidics, and high-sensitivity biodetection, opening up new possibilities in these fields

Towards rapid extracellular vesicles colorimetric detection using optofluidics-enhanced color-changing optical metasurface

Efficient transportation and delivery of analytes to the surface of optical sensors are crucial for overcoming limitations in diffusion-limited transport and analyte sensing. In this study, we propose a novel approach that combines metasurface optics with optofluidics-enabled active transport of extracellular vesicles (EVs). By leveraging this combination, we show that we can rapidly capture EVs and detect their adsorption through a color change generated by a specially designed optical metasurface that produces structural colors. Our results demonstrate that the integration of optofluidics and metasurface optics enables robust colorimetric read-out for EV concentrations as low as 107 EVs/ml, achieved within a short incubation time of two minutes, while using a CCD camera or naked eye for the read-out. This approach offers the potential for rapid sensing without the need for spectrometers and provides a short response time. Our findings suggest that the synergy between optofluidics and metasurface platforms can enhance the detection efficiency of low concentration bioparticle samples by overcoming the diffusion limits

Multiplexed long-range electrohydrodynamic transport and nano-optical trapping with cascaded bowtie photonic crystal cavities

Photonic crystal cavities have been widely studied for optical trapping due to their ability to generate high quality factor resonances. However, prior photonic crystal nanotweezers possess mode volumes significantly larger than those of plasmonic nanotweezers, which limit the gradient force. Additionally, they also suffer from low particle capture rates. In this paper, we propose a nanotweezer system based on a 1D bowtie photonic crystal nanobeam that achieves extreme mode confinement and an electromagnetic field enhancement factor of 68 times, while supporting a high-quality factor of 15,000 in water. Furthermore, by harnessing the localized heating of a water layer near the bowtie cavity region, combined with an applied alternating current electric field, our system provides long-range transport of particles with average velocities of 5 ${\mu}$m/s towards the bowtie cavities on demand. Once transported to the bowtie cavity region, our results show that a 20 nm quantum dot will be confined in a potential well with a depth of 35 ${k_B}$T. Thus, our approach effectively addresses the challenge of limited capture rate in photonic crystal nanotweezers for the first time. Finally, we present the concept of multiplexed long-range transport for hand-off of a single emitter from one cavity to the next by simply switching the wavelength of the input light. This novel multiplexed integrated particle trapping platform is expected to open new opportunities in directed assembly of nanoscale quantum emitters and ultrasensitive sensors for single particle spectroscopy.Comment: 11 pages, 4 figure

Single-peak and narrow-band mid-infrared thermal emitters driven by mirror-coupled plasmonic quasi-BIC metasurfaces

Wavelength-selective thermal emitters (WS-EMs) hold considerable appeal due to the scarcity of cost-effective, narrow-band sources in the mid-to-long-wave infrared spectrum. WS-EMs achieved via dielectric materials typically exhibit thermal emission peaks with high quality factors (Q factors), but their optical responses are prone to temperature fluctuations. Metallic EMs, on the other hand, show negligible drifts with temperature changes, but their Q factors usually hover around 10. In this study, we introduce and experimentally verify a novel EM grounded in plasmonic quasi-bound states in the continuum (BICs) within a mirror-coupled system. Our design numerically delivers an ultra-narrowband single peak with a Q factor of approximately 64, and near-unity absorptance that can be freely tuned within an expansive band of more than 10 {\mu}m. By introducing air slots symmetrically, the Q factor can be further augmented to around 100. Multipolar analysis and phase diagrams are presented to elucidate the operational principle. Importantly, our infrared spectral measurements affirm the remarkable resilience of our designs' resonance frequency in the face of temperature fluctuations over 300 degrees Celsius. Additionally, we develop an effective impedance model based on the optical nanoantenna theory to understand how further tuning of the emission properties is achieved through precise engineering of the slot. This research thus heralds the potential of applying plasmonic quasi-BICs in designing ultra-narrowband, temperature-stable thermal emitters in mid-infrared. Moreover, such a concept may be adaptable to other frequency ranges, such as near-infrared, Terahertz, and Gigahertz.Comment: 39 pages, 12 figure

Long-range and rapid transport of individual nano-objects by a hybrid electrothermoplasmonic nanotweezer

Plasmon-enhanced optical trapping is being actively studied to provide efficient manipulation of nanometre-sized objects. However, a long-standing issue with previously proposed solutions is how to controllably load the trap on-demand without relying on Brownian diffusion. Here, we show that the photo-induced heating of a nanoantenna in conjunction with an applied a.c. electric field can initiate rapid microscale fluid motion and particle transport with a velocity exceeding 10 μm s -1 , which is over two orders of magnitude faster than previously predicted. Our electrothermoplasmonic device enables on-demand long-range and rapid delivery of single nano-objects to specific plasmonic nanoantennas, where they can be trapped and even locked in place. We also present a physical model that elucidates the role of both heat-induced fluidic motion and plasmonic field enhancement in the plasmon-assisted optical trapping process. Finally, by applying a d.c. field or low-frequency a.c. field (below 10 Hz) while the particle is held in the trap by the gradient force, the trapped nano-objects can be immobilized into plasmonic hotspots, thereby providing the potential for effective low-power nanomanufacturing on-chip

Roadmap for Optical Tweezers 2023

Optical tweezers are tools made of light that enable contactless pushing, trapping, and manipulation of objects ranging from atoms to space light sails. Since the pioneering work by Arthur Ashkin in the 1970s, optical tweezers have evolved into sophisticated instruments and have been employed in a broad range of applications in life sciences, physics, and engineering. These include accurate force and torque measurement at the femtonewton level, microrheology of complex fluids, single micro- and nanoparticle spectroscopy, single-cell analysis, and statistical-physics experiments. This roadmap provides insights into current investigations involving optical forces and optical tweezers from their theoretical foundations to designs and setups. It also offers perspectives for applications to a wide range of research fields, from biophysics to space exploration