PROCEEDINGS ARTICLE | August 20, 2009
Proc. SPIE. 7400, Optical Trapping and Optical Micromanipulation VI
KEYWORDS: Refractive index, Microfluidics, Photonic crystal fibers, Particles, Dielectrics, Free space optics, Integrated optics, Geometrical optics, Chromatography, Channel projecting optics
Optical chromatography is a powerful technique, capable of separating micron-sized particles within a fluid flow, based
on their intrinsic properties, including size, shape and refractive index. Briefly, particles in a fluid flow are subject to two
forces, the Stokes drag force due to the fluid and then an introduced optical force as supplied by a laser beam, acting in
opposite but collinear directions. According to the particle's intrinsic hydrodynamic and optical properties, equilibrium
positions may form where the two forces balance, which is highly dependent on the properties of the particle and as a
result provides a means for spatial separation in a sample mixture. Optical chromatography is a passive sorting
technique, where pre-tagging of the particles of interest is not required, allowing for non-discrete distributions to be
evaluated and/or separated. Firstly we review the current stage of optical chromatography. We present a new advance in
optical chromatography potentially enabling the unique beam delivery properties of photonic crystal fiber (PCF) to be
employed and integrated into microfluidic chips. Also, for the first time a finite element method is applied to the optical
field in the theoretical analysis of optical chromatography, which is found to be in excellent agreement with the current
ray optics model, even for particles much smaller than the optical wavelength. This will pave the way for the technique
to be extended into the nanoparticle regime.
