|
Optical trapping using focused laser beams has been proven extremely useful for contact-less manipulation of a variety of small objects, including colloidal plasmonic nanoparticles [1]. We found that single-crystal gold nanorods with side-lengths of the order 160 nm could be easily trapped and manipulated in 2D by laser tweezers inside thin liquid cells compatible with standard optical microscopy [2]. The nanorods could be driven to rotate at frequencies above 40 kHz (2.5x106 r.p.m.) by applying circularly polarized laser light with power as low as a few mW. The driving torque, caused by transfer of photon angular momentum (spin), is dominated by plasmonic resonant scattering rather than absorption, which drastically reduces laser-heating effects and allows for sensitive control of the optomechanical properties through the particles nanoscale morphology. Single nanorods could be operated as rotational nanomotors and kept spinning for hours. By varying the applied laser power, the surface temperature of the rotating nanorods could be varied from close to room temperature to well above the boiling point of water. This provides a simple means of thermal control over chemical or conformational reactions of adsorbed molecules on the particles, which can be read out via either changes in the rotation frequency or rotational Brownian fluctuations [3].
References
[1] A. Lehmuskero, P. Johansson, H. Rubinsztein-Dunlop, L. Tong, M. Käll, Laser trapping of colloidal metal nanoparticles, ACS Nano 9, 3453-3469 (2015)
[2] L. Shao, Y. Zhong-Jian, D. Andrén, P. Johansson, M. Käll, Gold nanorod rotary motors driven by resonant light scattering, ACS Nano 9, 12542-12451 (2015).
[3] F. Hajizadeh, L. Shao, D. Andrén, P. Johansson, H. Rubinsztein-Dunlop, M. Käll, Brownian fluctuations of an optically rotated nanorod, Optica 4, 746-751 (2017).
|
