Photothermal trapping of dielectric particles by optical fiber-ring

Particle trapping with optical nanofibers: a review [Invited]

Optical trapping has proven to be an efficient method to control particles, including biological cells, single biological macromolecules, colloidal microparticles, and nanoparticles. Multiple types of particles have been successfully trapped, leading to various applications of optical tweezers ranging from biomedical through physics to material sciences. However, precise manipulation of particles with complex composition or of sizes down to nanometer-scales can be difficult with conventional optical tweezers, and an alternative manipulation tool is desirable. Optical nanofibers, that is, fibers with a waist diameter smaller than the propagating wavelength of light, are ideal candidates for optical manipulation due to their large evanescent field that extends beyond the fiber surface. They have the added advantages of being easily connected to a fibered experimental setup, being simple to fabricate, and providing strong electric field confinement and intense magnitude of evanescent fields at the nanofiber's surface. Many different particles have been trapped, rotated, transported, and assembled with such a system. This article reviews particle trapping using optical nanofibers and highlights some challenges and future potentials of this developing topic.

Manipulating nanoparticles based on a laser photothermal trap

A method of efficient directional optical manipulation of nanoparticles based on a laser photothermal trap is proposed, and the influence mechanism of external conditions on the photothermal trap is clarified. Through optical manipulation experiments and finite-element simulations, it is determined that the main cause of gold nanoparticle directional motion depends on the drag force. The laser power, boundary temperature, and thermal conductivity of the substrate at the bottom of the solution and liquid level essentially affect the intensity of the laser photothermal trap in the solution and then affect the directional movement and deposition speed of gold particles. The result shows the origin of the laser photothermal trap and the three-dimensional spatial velocity distribution of gold particles. It also clarifies the height boundary of photothermal effect onset, which clarifies the boundary between light force and photothermal effect. In addition, nanoplastics are manipulated successfully based on this theoretical study. In this study, the movement law of gold nanoparticles based on the photothermal effect is deeply analyzed through experiments and simulations, which is of significance to the theoretical study of the optical manipulation of nanoparticles using the photothermal effect.

Optical trapping and manipulation of massive particles based on spatial diffraction of a 45° tilted fiber Bragg grating

In this work, we proposed an optical trapping and manipulation technology based on spatial diffraction of 45° tilted fiber Bragg grating (TFBG). The length of the line-shape-facula of the TFBG diffraction light can be as large as tens of millimeters, which enables the TFBG trapping system control massive dielectric particles. We analyze the light distribution of the spatial diffraction by using the volume current method (VCM) and established a theoretical model to analyze the optical trapping force of TFBG based on the ray tracing method (RTM). Then, we designed several optical trapping schemes, with two-, three- and four-TFBGs respectively. Numeral simulation indicates that only the scheme with axisymmetric layout of TFBGs can achieve stable particle trapping. We comprehensively analyze the trapping force distribution of four- TFBG scheme with different influence factors. In addition, the rotation manipulation based on the two- and four- TFBGs schemes are also demonstrated. The proposed optical trapping technology open a new route for massive particles trapping and manipulation.

Microfiber-directed reversible assembly of Au nanoparticles for SERS detection of pollutants

Surface-enhanced Raman scattering (SERS) spectroscopy has attracted tremendous interest as a highly sensitive label-free tool to detect pollutants in aqueous environments. However, the high cost and poor reusability of conventional SERS substrates restrict their further applications in rapid and reproducible pollutant detection. Here, we report a reliable optical manipulation method to achieve rapid photothermal self-assembly of Au nanoparticles (AuNPs) in water within 30 s by a tapered optical fiber, which is utilized for highly sensitive SERS substrate preparation. The results show that the SERS substrate achieves low detection limits of 10^-9 mol/L with an enhancement factor (EF) of 10⁶ for chemical pollutants solutions, including thiram, pyrene, and rhodamine 6G. The SERS enhancement effect based on assembled AuNPs was more than 20 times that based on a gold colloid solution. As a result, the smart reversible assembly of AuNPs exhibits switchable plasmonic coupling for tuning SERS activity, which is promising for the application of SERS-based sensors and environmental pollutant detection.

Method of optical manipulation of gold nanoparticles for surface-enhanced Raman scattering in a microcavity

In this study, an optical manipulation and micro-surface-enhanced Raman scattering (microSERS) setup based on a microcavity was developed for efficient capture of gold nanoparticles using the photothermal effect. In addition, optical manipulation of gold nanoparticles and SERS signal detection were performed using only one laser. The results show that the SERS enhancement effect based on the microcavity was more than 20 times that based on a gold colloid solution. The laser power and velocity of nanoparticles exhibited a good linear relationship, and the velocity of nanoparticles decreased with decreasing radius r, which verifies the detriment of the radial thermophoresis in this study. This method can be used to quickly and efficiently drive metal nanoparticles and provides a promising approach for analysis of substances in the fields of chemistry and biology.

Fiber Optofluidic Technology Based on Optical Force and Photothermal Effects

Optofluidics is an exciting new area of study resulting from the fusion of microfluidics and photonics. It broadens the application and extends the functionality of microfluidics and has been extensively investigated in biocontrol, molecular diagnosis, material synthesis, and drug delivery. When light interacts with a microfluidic system, optical force and/or photothermal effects may occur due to the strong interaction between light and liquid. Such opto-physical effects can be used for optical manipulation and sensing due to their unique advantages over conventional microfluidics and photonics, including their simple fabrication process, flexible manipulation capability, compact configuration, and low cost. In this review, we summarize the latest progress in fiber optofluidic (FOF) technology based on optical force and photothermal effects in manipulation and sensing applications. Optical force can be used for optofluidic manipulation and sensing in two categories: stable single optical traps and stable combined optical traps. The photothermal effect can be applied to optofluidics based on two major structures: optical microfibers and optical fiber tips. The advantages and disadvantages of each FOF technology are also discussed.

Peculiarities of control of erythrocytes moving in an evanescent field

An investigation of the influence of an evanescent wave on the dynamics of motion of erythrocytes in blood plasma is presented. Computer simulation of erythrocytes moving in an evanescent field and experimental demonstration of the forecasted motion substantiate the possibility for control of position of red blood cells in a solution. The range of velocities of transversal motion of erythrocytes due to the action of the optical force of the generated evanescent field is determined as a function of the angle of illumination of a cell by a linearly polarized wave with the azimuth of polarization 45 deg.

Higher-order micro-fiber modes for Escherichia coli manipulation using a tapered seven-core fiber

Optical manipulation using optical micro- and nano-fibers has shown potential for controlling bacterial activities such as E. coli trapping, propelling, and binding. Most of these manipulations have been performed using the propagation of the fundamental mode through the fiber. However, along the maximum mode-intensity axis, the higher-order modes have longer evanescent field extensions and larger field amplitudes at the fiber waist than the fundamental mode, opening up new possibilities for manipulating E. coli bacteria. In this work, a compact seven-core fiber (SCF)-based micro-fiber/optical tweezers was demonstrated for trapping, propelling, and rotating E. coli bacteria using the excitation of higher-order modes. The diameter of the SCF taper was 4 µm at the taper waist, which was much larger than that of previous nano-fiber tweezers. The laser wavelength was tunable from 1500 nm to 1600 nm, simultaneously causing photophoretic force, gradient force, and scattering force. This work provides a new opportunity for better understanding optical manipulation using higher-order modes at the single-cell level.

Thermal gradient induced tweezers for the manipulation of particles and cells

Optical tweezers are a well-established tool for manipulating small objects. However, their integration with microfluidic devices often requires an objective lens. More importantly, trapping of non-transparent or optically sensitive targets is particularly challenging for optical tweezers. Here, for the first time, we present a photon-free trapping technique based on electro-thermally induced forces. We demonstrate that thermal-gradient-induced thermophoresis and thermal convection can lead to trapping of polystyrene spheres and live cells. While the subject of thermophoresis, particularly in the micro- and nano-scale, still remains to be fully explored, our experimental results have provided a reasonable explanation for the trapping effect. The so-called thermal tweezers, which can be readily fabricated by femtosecond laser writing, operate with low input power density and are highly versatile in terms of device configuration, thus rendering high potential for integration with microfluidic devices as well as lab-on-a-chip systems.

Metal-insulator-metal waveguides for particle trapping and separation

Optical particle trapping and separation are essential techniques in the fields of biology and chemistry. In many applications, it is important to identify passive separation techniques that only rely on intrinsic forces in a system with a fixed device geometry. We present a dual-waveguide sorter that utilizes the loss of metal-insulator-metal (MIM) waveguides for completely passive particle trapping and separation and is created using a unique angle sidewall deposition process. Our experiments show that an inner Au-Si3N4-Au waveguide is able to trap particles within the propagation distance of its dominant modes and release the particles into an outer Au-H2O-Au waveguide. The outer waveguide then propels the particles and separates them by size. The separation results are accurately modeled by a first-principles, analytical model.

Optically controlled circling of particles with a particle-decorated fiber probe

Optically controlled particle circling using a particle-decorated fiber probe was demonstrated based on the temperature gradient force and thermal convection.

Optofluidic realization and retaining of cell-cell contact using an abrupt tapered optical fibre

Studies reveal that there exists much interaction and communication between bacterial cells, with parts of these social behaviors depending on cell–cell contacts. The cell–cell contact has proved to be crucial for determining various biochemical processes. However, for cell culture with relatively low cell concentration, it is difficult to precisely control and retain the contact of a small group of cells. Particularly, the retaining of cell–cell contact is difficult when flows occur in the medium. Here, we report an optofluidic method for realization and retaining of Escherichia coli cell–cell contact in a microfluidic channel using an abrupt tapered optical fibre. The contact process is based on launching a 980-nm wavelength laser into the fibre, E. coli cells were trapped onto the fibre tip one after another, retaining cell–cell contact and forming a highly organized cell chain. The formed chains further show the ability as bio-optical waveguides.

Microbe removal using a micrometre-sized optical fiber

The growing global shortage of fresh water has lead to the need for technological innovations for water purification and reuse. The removal of pathogenic microbes from urban, laboratory or industrial wastewater is one of the most challenging and critical issues due to the potential risk of microbe outbreaks. In addition, microbe removal in human blood or tissues has also inspired novel techniques for extracting and collecting different cells in fluidic channels or vessels. Recently, efficient removal of microbes from flowing water running under gravity feed has been achieved using filters in nanotubes or nanofibers. Here we report a highly efficient removal of microbes from flowing water in a fluidic channel using a reusable micrometre-sized optical fiber. Our technique is based on photophoresis of the microbes induced by the radiation of 1.55 μm wavelength injected into the fiber. Yeast cell suspensions, as a sample of microbe-contaminated water, are flown through a fluidic channel and the suspended cells are collected by the photophoretic forces, leading to a consistent accumulation of the yeast cells. The experiments indicate that a removal efficiency of 99.9% can be obtained when the flow velocity of the suspensions is less than the peak photophoretic velocity of the yeast cells.

Particle separation in fluidic flow by optical fiber

We report a separation of two different size particles in fluidic flow by an optical fiber. With a light of 1.55 μm launched into the fiber, particles in stationary water were massively trapped and assembled around the fiber by a negative photophoretic force. By introducing a fluidic flow, the assembled particles were separated into two different downstream positions according to their sizes by the negative photophoretic force and the dragging force acted on the particles. The intensity distribution of light leaked from the fiber and the asymmetry factor of energy distribution have been analysed as crucial factors in this separation. Poly(methyl methacrylate) particles (5-/10-μm diameter), SiO(2) particles (2.08-/5.65-μm diameter), and SiO(2) particles (2.08-μm diameter) mixed with yeast cells were used to demonstrate the effectiveness of the separation. The separation mechanism has also been numerical simulated and theoretical interpreted.

Massive photothermal trapping and migration of particles by a tapered optical fiber

A simple but highly efficient method for particles or bacteria trapping and removal from water is of great importance for local water purification, particularly, for sanitation. Here, we report a massive photothermal trapping and migration of dielectric particles (SiO2, 2.08-µm diameter) in water by using a tapered optical fiber (3.1-µm diameter for taper). With a laser beam of 1.55 µm (170 mW) injected into the fiber, particles moved towards the position, which is about 380 µm away from the tip of the fiber, and assembled at a 290 µm × 100 µm spindle-shaped region. The highest assembly speed of particles is 22.1 ind./s and the highest moving velocity is 20.5 µm/s, which were induced by both negative photophoresis and temperature gradient. The number of assembled particles can reach 10,150 in 15 minutes. With a move of the fiber, the assembled particles will also migrate. We found that, when the fiber was moved 172 µm away from its original location, almost all of the assembled 10,150 particles were migrated to a new location in 140 s with a distance of 172 µm from their original location.

Photothermal delivery of microscopic objects via convection flows induced by laser beam from fiber tip

We report a photothermal delivery of microscopic objects based on convection flows at the surface of water. The convection flows were induced by photothermal effect through a laser beam of 1.55 μm wavelength from a fiber tip. A 206 μm diameter oil drop was delivered forward and backward by changing the laser beam at a power of 28.5-40 mW. In addition, the delivery has been further demonstrated with a cluster of carbon and red blood cells at the laser powers of 14 and 20 mW, respectively.

Targeted delivery and controllable release of nanoparticles using a defect-decorated optical nanofiber

Targeted drug delivery and controllable release are particularly beneficial in medical therapy. This work provides a demonstration of nanoparticles targeted delivery and controllable release using a defect-decorated optical nanofiber (NF). By using the NF, polystyrene particles (PSs) (713-nm diameter) suspended in water were successfully trapped, then delivered along the NF at an average velocity of 4.8 µm/s with the assistance of a laser beam of 980-nm wavelength at an optical power of 39 mW, and finally, assembled at the defect. Subsequently, by turning off the optical power, 90% of the assembled PSs can be released in 30 s. This method would be useful in targeted drug delivery and controllable release, and provide potential applications in targeted therapy.