Single-Crystal Rutile TiO₂ Nanocylinders are Highly Effective Transducers of Optical Force and Torque

Abstract

Optical trapping of (sub)micron-sized particles is broadly employed in nanoscience and engineering. The materials commonly employed for these particles, however, have physical properties that limit the transfer of linear or angular momentum (or both). This reduces the magnitude of forces and torques, and the spatiotemporal resolution, achievable in linear and angular traps. Here, we overcome these limitations through the use of single-crystal rutile TiO2, which has an exceptionally large optical birefringence, a high index of refraction, good chemical stability, and is amenable to geometric control at the nanoscale. We show that rutile TiO2 nanocylinders form powerful joint force and torque transducers in aqueous environments by using only moderate laser powers to apply nN·nm torques at kHz rotational frequencies to tightly trapped particles. In doing so, we demonstrate how rutile TiO2 nanocylinders outperform other materials and offer unprecedented opportunities to expand the control of optical force and torque at the nanoscale.

Initiating revolutions for optical manipulation: the origins and applications of rotational dynamics of trapped particles

“…The main feature of these particles is that the synthesis procedure allows them to be doped with rare-earth ions that provide them with special characteristics such as the ability to control [83,150] and measure [152] temperature. Other types of dielectric particles, such as those made of rutile TiO 2 [153] or Hg 2 Cl 2 (Calomel) [101], present higher refractive indices which enhance the applied optical forces and torque.…”

Angular Trapping of Spherical Janus Particles

“…[ 27 ] To expand the application of angular trapping to a broader community, developing new angular trapping methodologies is of critical importance. [ 27 ] Here, we demonstrate a novel angular trapping method using chemically prepared spherical Janus particles. This novel method can be easily implemented in an optical trapping system and the rotation of the Janus particle can be directly visualized and measured using a complementary metal–oxide–semiconductor (CMOS) camera.…”

“…[8][9][10][11][12] Many attempts have been made to realize angular trapping of particles. [13,14] Amongst these efforts, angular trapping has been successfully implemented using birefringent particles, including calcite fragment, [8] vaterite microsphere, [10,[15][16][17][18][19] and fabricated quartz cylinders, [9,[20][21][22][23][24][25][26][27] leading to the controlled rotation of such particles in optical tweezers. Due to its chemical stability and easy surface func tionalization, nanofabricated quartz cylinderbased angular trapping has been successfully used in single molecule biophysics studies.…”

“…[9,12,[21][22][23][24][25][26] Despite its suc cess, the use of quartz cylinderbased angular trapping remains limited, largely due to the challenges in the nanofabrication of quartz cylinders as well as the low birefringence of quartz, which leads to a low transfer angular momentum efficiency. [27] To expand the application of angular trapping to a broader com munity, developing new angular trapping methodologies is of critical importance. [27] Here, we demonstrate a novel angular trapping method using chemically prepared spherical Janus particles.…”

Validity of cylindrical approximation for spherical birefringent microparticles in rotational optical tweezers

“…People have used many tools to rotate particles in the mesoscopic domain, namely optical tweezers [1,2], magnetic tweezers [3], rotation of micropipettes [4], optically induced forces [5], lever action rotation [6], active motion of particles [7] and so on. Among these, the rotation induced by the optical tweezers preferentially relies on use of birefringent probes which can then be addressed by polarized light [1,2,[8][9][10].…”