Laser trapping gets tighter grip on larger particles

Using light to trap particles as small as atoms has been popular with researchers for some time, a technique that enables optical cooling and trapping, as recognized by the 1997 Nobel Prize in physics. Now trapping of bigger particles has moved up on the agenda, with potential applications in biological systems research or nanotechnology – although grabbing larger pieces is more challenging because light generates forces in the piconewton (pN = 10^—12 N) range only.

Researchers from Israel’s Pixer Technology Ltd., now part of Carl Zeiss SMT, and from St. Petersburg State University have reported a technique that traps bubbles in water with forces much stronger than those known previously (Tech. Phys. Lett., 2009, Vol. 35, No. 3, p. 282). They used a laser emitting femtosecond pulses at a 100-kHz repetition rate to generate and hold a gas bubble in a firm location in the focus of the beam.

Optical trapping uses a strongly focused laser beam, and the effect can be explained by the dielectric particles being dragged by the field gradient into the strongest electrical field in the focus. However, if the particles are large (compared with the wavelength of the light), ray optics can be used to understand or describe this; i.e., each ray of light experiences refraction as it enters and exits a dielectric bead in the focused beam. This change of direction comes with a momentum change, which, according to Newton’s law, provides an equal but opposite momentum change to the particle, kicking it toward the center of the beam. However, if the particle is exactly in the middle of the focused Gaussian beam, all rays are refracted symmetrically, leading to no net lateral force.

Once particles are trapped, the holding forces can be measured; e.g., by moving the trapped particle around and measuring the maximum velocity until it becomes detached. The researchers say that their gas bubble in water was held with ~200 pN, at least an order of magnitude more than previously known. They attribute this phenomenon to the continuous flux of short but intense laser pulses, which heat the gas inside the bubble, increasing its size and creating equilibrium between the heat absorbed by the surrounding water and the power from the next pulse – a strong returning force if this heat source is displaced and approaches the bubble wall.

More recently, a group at Hokkaido University in Sapporo published work using a near-infrared (1064 nm) laser beam to trap and manipulate small amino acid molecules in aqueous arginine solutions. The researchers reported that a particlelike assembly of objects grew gradually in the focal point during laser irradiation (J. Phys. Chem. C, online, Dec. 23, 2009).

Using confocal Raman microspectroscopy, they confirmed that a molecular assembly of arginine had formed, generating trapped clusters for other amino acids, including glycine, proline, serine and alanine. This revealed that the optical manipulation technique can be extensively applied not only to that of large-size biological structures but also to their manufacture.