Manipulating small particles with laser beams has become an indispensable tool in diverse branches of modern science, ranging from physics to the biosciences and medicine.1 Using optical tweezers based on radiation pressure and with transparent liquid particles remains a leading approach in this field. However, there is growing demand for techniques that enable efficient trapping of absorbing aerosol particles,2 such as air contaminants and novel nanomaterials. The adverse effects of nanoparticles on human health are potentially significant and mostly unknown.
Figure 1. Trapping particles in air with counter-propagating optical vortices. (a) The scattered light from the agglomeration of carbon nanoparticles is clearly seen in the center. (b) Scanning-electron-microscopy image of a particle collected from the trap on a silicon wafer. The scale bar is 1μm. The image was taken by a Raith 150 electron-beam lithography system with 12.22× magnification, extra high tension (EHT) high voltage of 10kV, and an exposure working distance (WD) of 6mm.
One promising solution is to employ thermal forces to trap air particles, in particular the photophoretic force induced by a laser beam. Photophoresis is the phenomenon of particle motion in gases caused by local heating with light. When incident light heats a surface of a particle nonuniformly, the gas molecules rebound off the surface with different velocities, creating an integrated force on the particle. The main difficulty with thermal forces is the stability of trapping because absorbing particles are repelled from the laser beam's intensity maximum. We therefore developed a new trapping concept using counter-propagating, doughnut-shaped optical-vortex beams. It offers stable and robust trapping of absorbing particles in the open air.3,4 The approach's distinguishing feature is that particles are trapped at the beam's intensity minimum, thus involving minimal heating and intervention into the particle properties. This is important for direct studies of particle properties and air trapping of live cells.
We built a simple setup to conduct proof-of-principle experiments to trap light-absorbing particles in air using photophoretic force (see Figure 1). A linearly polarized Gaussian beam was transformed into a Laguerre-Gauss vortex beam by a fork-type hologram. The optical trap was formed by a standing wave of two counter-propagating vortex beams with opposite topological charges. This created a radially symmetric and azimuthally homogeneous intensity distribution in a large volume of the trap, which provided a solution to the stability problem.
We used clusters of carbon nanoparticles produced by a high-repetition-rate laser-ablation technique to illustrate trapping of nanoparticles in air.5,6 The nanoclusters were trapped and agglomerated in large chunks that could be collected from the trap for contents diagnosis. The size of the nanocluster aggregates varied from 0.1 to 10μm, depending on laser power.
The scheme offers two principal degrees of freedom for optical manipulation of trapped particles by changing the polarization and distance between the focal planes of two optical-vortex beams. This presents a unique possibility of using the vortex beam as an optical pipeline to transport absorbing particles over large distances in a gas.
Our approach provides a novel physical basis for previously unattainable optical manipulation. Selective trapping, guiding, and separation of airborne particles by noncontact optical means opens up diverse and rich practical opportunities for laser trapping of matter in gas, and allows simulating the processes studied in atmospheric and planetary sciences on laboratory scales. By measuring the trapping dynamics and threshold power, the method will allow access to physical characteristics of trapped particles with very small mass such as estimates of their thermal conductivity, optical absorption, and density.
The new approach will also be important in air-pollution and environmental-protection applications. Our challenges include developing photonic shielding and fencing for environmental protection in the nanotechnology industry and new methods of touch-free air transport of particles and small containers, which may hold dangerous substances or viruses and living cells.
Laser Physics Center and Nonlinear Physics Center
Research School of Physics and Engineering
Australian National University (ANU)
Vladlen Shvedov is a research fellow at ANU and assistant professor at the Taurida National University in Ukraine. His research interests focus on singular optics, optical vortices and their applications, optical micro- and nanostructures, and optical tweezers for multiple-trap systems.
Nonlinear Physics Center
Anton Desyatnikov graduated from Moscow Engineering Physics Institute and joined the Nonlinear Physics Center as an Australian Research Fellow after completing an Alexander von Humboldt fellowship at the University of Muenster, Germany. His research is in nonlinear photonics, spatial solitons, and nonlinear singular optics.
Laser Physics Center
Research School of Physics and Engineering
Andrei Rode is a senior fellow. His research interests are in short-pulse laser-matter interaction, laser-induced phase transitions and transient states of matter, ultrafast laser excitation of coherent phonons, laser-produced nanoclusters and their properties, laser ablation and deposition of nonlinear optical films for photonics applications, and related phenomena.
6. A. V. Rode, R. G. Elliman, E. G. Gamaly, A. I. Veinger, A. G. Christy, S. T. Hyde, B. Luther-Davies, Electronic and magnetic properties of carbon nanofoam produced by high-repetition-rate laser ablation, Appl. Surf. Sci. 197–198, pp. 644-649, 2002.