Tailoring Particles for Optical Trapping and Micromanipulation: An Overview

Abstract

Optical trapping and micromanipulation has developed from an interesting novelty to a powerful and widely used tool, with the capability to move or trap microscopic live biological specimens and measure forces on the order of piconewtons, typical of forces in microbiological systems. Despite this, the range of particles typically trapped or manipulated is quite small, and it is unusual to see applications involving objects other than biological specimens or homogeneous isotropic microspheres, typically polymer or silica. However, particles can be modifled or specially fabricated to expand the possible applications of optical tweezers. For example, while non-absorbing homogeneous isotropic spheres cannot be rotated, optically anisotropic spheres can, and can therefore function as microscopic torque sen- sors, extending the usual translational micromanipulation and force measurement to rotational manipulation and torque sensing. The development of such particles has led to applications in microscale metrology and biophysics, along with potential deployment of optically-driven micro- machines in lab-on-a-chip devices. We present an overview of our work on the tailoring of microparticles for versatile optical trap- ping and micromanipulation. This includes approaches based on controlled chemistry | nano- assembly | and optical microfabrication. Beginning with the production of anisotropic vaterite microspheres, we review some of the applications, and di-culties encountered along the way. Some of these di-culties can be overcome by coating of the vaterite microspheres. We also discuss the use of anti-re∞ection coating to allow strong trapping of high refractive index parti- cles. The alternative strategy of producing arbitrarily shaped polymer microstructures through two-photon photopolymerization is also discussed. This can be used to produce optically-driven microrotors or structurally anisotropic microspheres to replace vaterites for particular applica- tions.