The authors present a power-efficient large-scale lensless optical traps on a chip (OTOCs) as an optofluidic element for optical sorting of microparticles. Based on the well-known Talbot self-imaging effect in the Fresnel region, the OTOC makes use of a two-dimensional microfabricated chessboardlike structure to create an optical lattice near its emergent plane. Simultaneous trapping of hundreds of microparticles in a regular array (>200×200μm2) is proved experimentally without adopting an external optical projection lens configuration. Furthermore, the authors demonstrate experimental results for large-scale sorting of microparticles by sizes using the OTOC.
We theoretically investigate the optical properties in one-dimensional graded soft photonic crystals (1D GSPCs). The proposed structure is constituted of the stacked ferrofluids layer and the dielectric layer. Due to the supermagnetic response of the ferromagnetic nanoparticles, they will align in a line under the influence of the initiated magnetic field, thereby modulating the refractive index of the ferrofluids layer. By resorting to the transfer matrix method, the dispersion relation, transmittance and reflectance in 1D GSPCs were calculated. Numerical results show that a broad photonic band gap appears in such systems, which can even be broadened by increasing the volume fraction of ferromagnetic nanoparticles. Moreover, perfect transmittance of our proposed structure can be realized with an increased number of ferrofluid layers. In comparison with conventional PCs materials, 1D GSPCs composed of liquid material offer a very flexible route to implementation, which can be widely used in the application of optical filters, waveguides, reflectors and so on.
The third-order nonlinear optical properties of two novel fullerene derivatives under picosecond laser excitation have been investigated by the Z-scan technique. The experimental results reveal that all the derivatives have nonlinear absorption coefficient under excitation of 532 nm pulses and a high third-order nonlinear refraction index under excitation of 1064 nm pulses. The molecular second hyperpolarizabilities have been obtained from our experimental results.
A composite microlens array (MLA) with two cascaded guiding axes has been fabricated to achieve a large lateral separation of an object with different refractive indices or sizes. The MLA projects a composite pattern formed by its focal spots into a microchamber for optical sorting in a microscopic system. This approach enables passive, high power, efficient, and continuous microfluidic sorting without requiring complicated optical assembly. Separation of particles with different refractive indices to a lateral angle of 40° is experimentally demonstrated with moderate laser power.Optical tweezers that were invented by Ashkin et al. 1 are able to trap and manipulate colloidal particles or biological cells with a gradient force generated by a highly focused laser beam. In the development of optical trapping technology itself, it has been matured for applications ranging from simple trapping and manipulation to more sophisticated areas such as optical vortices, 2 optical actuator, 3 three-dimensional (3D) optical tweezers, 4 laser guidance, 5 creation of two-dimensional (2D)/3D colloidal crystals, and structures. 6 Among the applications, optical actuator and optical sorting are two key components that might give rise to all optical laboratory-on-a-chip and optofluidic devices. An all-optical sorting was reported that was based on an optical lattice created by multibeam interference particles that can be sorted by sizes or by refractive indices. 7 Unlike those approaches using fluorescence as labels for sorting, 8,9 the all-optical sorting method makes use of the natural physical attributes of particles, such as size or refractive index as a label for sorting; hence, it has the advantages of being continuous, noninteractive, and minimally invasive. In addition to the multi-interference, computer generated holograms 10,11 and phase contrast 12 are also used to generate optical lattice for sorting.The principle of all-optical sorting lies in the fact that the trajectories of particles may deviate when they pass through an optical potential landscape, and the trajectories of the particles are strongly dependent on their sizes and refractive indices. If relevant parameters and conditions are selected properly, the group of particles mixed with different sizes or refractive indices will be separated laterally. The lateral angle of the separation is a key
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