Towards integrated optical chromatography using photonic crystal fiber

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.