Keywords:optical manipulation, glass capillary, optofluidics
Light can provide an efficient and contactless way to detect and separate micro- and nano-objects in liquid colloidal solution with different optical properties. Depending for instance on its wavelength and polarization, light can be used to apply object-dependent optical forces based on radiation pressure. This effect can be used to achieve optical sorting of nanoparticles. However, optical manipulation based on radiation pressure usually requires a large light intensity, especially for handling nanoparticles.In this work, we investigate the use of tapered glass capillaries as an original optofluidic system for optical chromatography. Tapered glass capillaries with micrometer- or nanometer-scale inner and outer diameters can be fabricated using a standard optical fiber pulling device. Once filled with a small amount of colloidal solution (low-concentration suspension of nanoparticles in water), a tapered glass capillary suspended in the air acts as a partially-liquid-core waveguide capable of highly focusing the laser beam on a few-millimeter-long distance. Finite-Element Method numerical simulations (COMSOL) have been conducted in order to evaluate the optical properties of such a system. The modal dispersion of the waveguide is discussed as a function of the laser wavelength and the dimensions of the tapered capillary. We also evaluate the optical forces acting on a nanoparticle located inside the capillary as a function of its size, refractive index and absorption properties. Since the large overlap between the laser beam and the colloidal solution inside the tapered capillary allows for a large enhancement of light-matter interactions, the use of tapered glass capillaries could lead to the practical optical sorting of sub-100nm-sized nanoparticles depending on their optical properties.