Nanoscale Optical Trapping: A Review

Abstract

The First TrapIn 1969, a "back-of-the-envelope" calculation inspired Ashkin to conduct a simple experiment and determine whether it was feasible to use light radiation pressure to accelerate objects to practical speeds. Photons carry momentum hν/c (with h, ν and c being the Planck's constant, the frequency of the photon and the speed of light, respectively). If light from a source with power P shines on a mirror, P/hν photons hit the surface every second and transfer a total momentum of (2P/hν)(hν/c) = 2P/c onto it. A perfectly reflecting mirror should therefore-due to conservation-acquire an equal momentum in the same direction light propagates.Based on this crude calculation, Ashkin predicted that a light source of power P = 1 W, would produce a force (on an ideally reflecting mirror) of ≈10 nN-indeed small in absolute terms. [3] Nevertheless, if a laser beam is used as the light source and is focused on a spot of ≈1 µm 2 to hit a particle ≈1 µm in diame ter, the resulting force does become relevant. Assuming the particle is perfectly reflective and has a density of 1 g cm −3 , the calculated acceleration is ≈10 9 cm s −2 , i.e., roughly 10 6 times the acceleration of gravity. In the experiment, Ashkin conducted to test his hypotheses, [2] a cw argon laser (wavelength λ = 514.5 nm, waist radius w 0 = 6.2 µm at the focal point) was employed to accelerate latex spheres (diameter 0.59, 1.31, and 2.68 µm), which were freely suspended in water, in a glass chamber. With just milliwatts of laser power, Ashkin observed that the particles were pushed in the direction of the mildly focused Gaussian laser beam, with values for the acceleration consistent with his rough predictions. Interestingly, he also observed an unanticipated phenomenon. Particles located in the fringes of the beam were drawn toward the beam axiswhere the light intensity is the highest-before being accelerated and pushed with ≈µm s −1 speeds toward the back of the chamber. They would disperse by Brownian motion away from the beam axis once the laser was switched off, yet they would be drawn again toward the center of the beam upon turning the laser back on-as the radiation pressure had a transverse component to the force, as well as the predicted longitudinal one.The origin of both the transversal and longitudinal force is usually understood by considering two distinct regimes, depending on the relative size of the particles to the wavelength of the laser beam: the geometrical (ray optics) regime and the Rayleigh (dipole approximation) regime.Optical trapping is the craft of manipulating objects with light. Decades after its first inception in 1970, the technique has become a powerful tool for ultracold-atom physics and manipulation of micron-sized particles. Yet, optical trapping of objects at the intermediate-nanoscale-range is still beyond full grasp. This matters because the nanometric realm is where several promising advances, from mastering single-molecule experiments in biology, to fabricating hybrid devices for nanoelectronics and photonics, a...

From Far-Field to Near-Field Micro- and Nanoparticle Optical Trapping

“…The bottom panel shows trapping using counter-propagating beams with the same characteristics so that the scattering forces cancel each other to achieve a stable trap. Figure reproduced with permission from [17].…”

“…It is also convenient to calculate the time-averaged total force since it is the one that is observable (electromagnetic fields oscillate on the order of ∼ 10 15 Hz which is very fast). Rigorous calculations have been done in [17,20,22]. According to these works, the resulting, time-averaged force acting on a dipole is given by…”

Passive laser irradiation as a tool for optical catalysis

“…In absorption or scattering processes it is the delivery of momentum that produces the motion of particles, whereas in a potential energy or force it is an associated positional or angle-dependence which, as a secondary effect, yields particle motion. Putting inter-particle interactions aside for a moment, the most well-known of the latter optical forces is the gradient force, which constitutes part of the total optical tweezer effect 17 . At the photon level the gradient force occurs through a forward Rayleigh scattering mechanism, which can again be viewed as a dynamic Stark shift.…”

“…Forward Rayleigh scattering mediated by two or more particles leads to optical binding forces: a mutual interparticle interaction energy. The off-resonance processes of trapping and binding are extremely well utilized in the field of optical nanomanipulation 4 . In this paper we study distinctly different applications: the passive effects of an off-resonance or detuned laser grouped under the banner of 'optical catalysis'.…”

Kramers-Heisenberg dispersion formula for scattering of twisted light

“…The study of scattered light has enabled the determination of structures of biologically and clinically important macromolecules [2,3], nanoparticle characterization [4], and biomedical imaging [5,6], to name but a few examples. The mechanism of light scattering itself is responsible for the gradient force in optical tweezers as well as optical binding, the two most pivotal techniques in optical trapping and manipulation [7,8]. Scattering of light in the optical regime is dominated by Rayleigh (elastic) and Raman (inelastic) scattering processes.…”