Hongxiang Lei

Growth of well-arrayed ZnO nanorods on thinned silica fiber and application for humidity sensing

Thinned silica fibers were fabricated by drawing conventional single mode silica fiber through flame heated method and well-arrayed ZnO nanorods were grown on the thinned silica fibers by a hydrothermal method. The structure enables efficient light coupling between the fiber and the nanorods. With the unique property of high surface to volume ratio of one-dimensional ZnO nanorods, light coupled to nanorods array enhances the optical interaction between the device and the ambient environment. Sensitive humidity sensor was demonstrated by launching laser into ZnO nanorod-covered fibers. Theoretical and experimental results are presented.

Photothermal trapping of dielectric particles by optical fiber-ring.

The removal of dielectric particles and bacteria from water is an extremely important global issue, particularly, for drinking and sanitation. This work provides a demonstration of optical purification of water using an optical fiber-ring. The size of particles suspended in water for trapping is 2.08 μm in diameter and the wavelength of light used for inducing photothermal effect is 1.55 μm with a power of 97 mW. The fiber, 6 μm in diameter, was formed to a racket-shaped ring with a minimum diameter of 167 μm and a maximum one of 350 μm. Experiment indicates that the particles moved toward the ring with the highest velocity of 4.2 μm/s and are trapped/assembled in the center of the ring once the laser beam of 1.55-μm wavelength was launched into the fiber. With a moving of the fiber-ring, the trapped/assembled particles were moved and the water can be purified by removal of the particles.

Trapping and Detection of Nanoparticles and Cells Using a Parallel Photonic Nanojet Array

In advanced nanoscience, there is a strong desire to trap and detect nanoscale objects with high-throughput, single-nanoparticle resolution and high selectivity. Although emerging optical methods have enabled the selective trapping and detection of multiple micrometer-sized objects, it remains a great challenge to extend this functionality to the nanoscale. Here, we report an approach to trap and detect nanoparticles and subwavelength cells at low optical power using a parallel photonic nanojet array produced by assembling microlenses on an optical fiber probe. Benefiting from the subwavelength confinement of the photonic nanojets, tens to hundreds of nanotraps were formed in three dimensions. Backscattering signals were detected in real time with single-nanoparticle resolution and enhancement factors of 10(3)-10(4). Selective trapping of nanoparticles and cells from a particle mixture or human blood solution was demonstrated using the nanojet array. The developed nanojet array is potentially a powerful tool for nanoparticle assembly, biosensing, single-cell analysis, and optical sorting.

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.

Manipulation and detection of single nanoparticles and biomolecules by a photonic nanojet

Optical methods to manipulate and detect nanoscale objects are highly desired in both nanomaterials and molecular biology fields. Optical tweezers have been used to manipulate objects that range in size from a few hundred nanometres to several micrometres. The emergence of near-field methods that overcome the diffraction limit has enabled the manipulation of objects below 100 nm. A highly free manipulation with signal-enhanced real-time detection, however, remains a challenge for single sub-100-nm nanoparticles or biomolecules. Here we show an approach that uses a photonic nanojet to perform the manipulation and detection of single sub-100-nm objects. With the photonic nanojet generated by a dielectric microlens bound to an optical fibre probe, three-dimensional manipulations were achieved for a single 85-nm fluorescent polystyrene nanoparticle as well as for a plasmid DNA molecule. Backscattering and fluorescent signals were detected with the enhancement factors up to ∼103 and ∼30, respectively. The demonstrated approach provides a potentially powerful tool for nanostructure assembly, biosensing and single-biomolecule studies.

Bidirectional optical transportation and controllable positioning of nanoparticles using an optical nanofiber

This work provides a technique allowing bidirectional optical transportation and controllable positioning of nanoparticles using two counter-propagating laser beams at a wavelength of 980 nm in an optical nanofiber. With the assistance of an evanescent wave at the fiber surface, particles suspended in water were trapped onto the fiber by a gradient force and then transported along the fiber by a scattering force. By changing the difference between the input laser powers coupled into two ends of the fiber with ΔP = -10 to 10 mW, the magnitude and direction of the scattering force that acted on the particles were changed, and thus the transportation direction and velocity of the particles were controlled. According to these properties, the bidirectional optical transportation of the particles along the fiber can be realized by coupling different laser powers into the two ends of the fiber (ΔP≠ 0 mW). At the same time, the transported particles can be controllably positioned on the fiber by coupling the same laser powers into the two ends of the fiber (ΔP = 0 mW). The relationship between the transportation velocity of the particles and the input optical power difference was investigated. Experiments were conducted with a 910 nm diameter fiber and 713 nm diameter polystyrene (PS) particle suspensions to demonstrate the effectiveness of this method. The experimental results were interpreted by numerical simulation and theoretical analysis.

Controllable orientation of single silver nanowire using two fiber probes

We report a strategy for realizing precise orientation of single silver nanowire using two fiber probes. By launching a laser of 980 nm wavelength into the two fibers, single silver nanowire with a diameter of 600 nm and a length of 6.5 μm suspended in water was trapped and rotated by optical torque resulting from its interaction with optical fields outputted from the fiber probes. Angular orientation of the nanowire was controlled by varying the relative distance between the two fiber probes. The angular stiffness, which refers to the stability of orientation, was estimated to be on the order of 10−19 J/rad2·mW. The experiments were interpreted by theoretical analysis.

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.

Backward transport of nanoparticles in fluidic flow.

We demonstrate a backward transport of polystyrene (PS) particles (713-nm in diameter) in a pressure-driven fluidic flow using an optical fiber with a diameter of 710 nm. When a light of 980-nm wavelength was launched into the fiber in the opposite direction of the flow, the PS particles near the fiber were attracted onto the fiber and transported along the propagation direction of the light. The relationship between the velocity of the transported PS particles and the velocity of the flow at different input optical powers was investigated. Numerical analyses on both the optical field and the fluid field were carried out. The particle-size dependence of backward transport capability has also been investigated.

Optofluidic extraction of particles using a sub-microfiber

This work provides an optofluidic particle extraction in a microfluidic channel using a multiple-defect decorated sub-microfiber by injecting a light of 532-nm-wavelength into it. Numerical analysis shows that effective extraction was resulted from the difference between the gradient force exerted on the target 3-μm-diameter polystyrene (PS) particles and the 700-nm diameter PS particles. Further experiments show that 3-μm diameter PS particles in the mixed solution can be trapped around the defects of the sub-microfiber, with the extraction efficiency reaching its maximum of 92% at the fluid velocity of 10 μm/s and the laser power of 0.6 mW.