Mid-Infrared Optical Force Chromatography of Microspheres Containing Siloxane Bonds

Recent interest in particle sorting using optical forces has grown due to its ability to separate micro- and nanomaterials based on their optical properties. Here, we present a mid-infrared optical force manipulation technique that enables precise sorting of microspheres based on their molecular vibrational properties using a mid-infrared quantum cascade laser. Utilizing the optical pushing force driven by a 9.3 μm mid-infrared evanescent field generated on a prism through total internal reflection, a variety of microspheres, including those composed of Si-O-Si bonds, can be separated in accordance with their absorbance values at 9.3 μm. The experimental results are in good agreement with the optical force calculations using finite-difference time-domain simulation. Thus, each microsphere's displacement and velocity can be predicted from the absorbance value; conversely, the optical properties (e.g., absorbance and complex refractive index in the mid-infrared region) of individual microspheres can be estimated by monitoring their velocity.