Dielectric particle, optical trapping

A. Ashkin, J. M. Dziedzic, J. E. Bjorkholm, and S. Chu, Observation of a single-beam gradient force optical trap for dielectric particles. Optics Letters 11 p. 288-290 (1986). 

The LS theory was applied to the localization of a Brownian particle in a three-dimensional optical trap [89] a transparent dielectric spherical silica particle of diameter 0.6 pm suspended in a liquid [88]. The particle moves at random within the potential well created with a gradient three-dimensional optical trap—a technique widely used in biophysical studies. The potential was modulated by a biharmonic force. By changing the phase shift between the two harmonics it was possible to localize the particle in one of the wells in very good quantitative agreement with the predictions based on the LS. 

The optical trapping method uses a highly focused laser beam to trap and manipulate particles of interest in a medium (illustrated in Figure 3). The laser is focused on a dielectric particle (e.g., a silica microscopic bead), the refractive index of which is higher than the suspension medium. This produces a light pressure (or gradient force), which moves the particle towards the focal point of the beam, that is, the beam waist (Lim et al., 2006). 

Figure 3 Optical trapping of a dielectric particle (simplified from Lim et al., 2006).

In 1986, Ashkin and coworkers reported on the first successful single-beam gradient-force trap, or laser tweezers, for dielectric particles [21]. They were able to trap particles of glass, silica, and polystyrene (PS) in the range from 25 nm to 10 pm in water. Optical trapping techniques have since been integrated to a range of different... 

Around 1970 it was found that laser radiation forces can be used to trap and manipulate small dielectric particles [83]. A laser beam can push a particle towards the centre of the beam, provided the particle has a higher refractive index than the surrounding medium. Thus, optical tweezers allow to pick up and manipulate... 

Ashkin A, Dziedzic JM, Bjorkholm JE, Chu S (1986) Observation of a Single-Beam Gradient Eorce Optical Trap for Dielectric Particles. Opt Lett 11 288-290... 

Ashkin and Dziedzic (1987) gave an impressive demonstration of the optical trapping of dielectric particies in experiments with a biological particle (tobacco mosaic virus) in water by means of a single-beam gradient trap formed by an argon laser 0.1-0.3 W in power. The rodlike tobacco mosaic virus is 3000 nm long and 200 nm in diameter and has a refractive index of 1.57. In this experiment, Ashkin and Dziedzic... 

Raman probes. SERS can then be performed on optically induced aggregates of the trapped particles. Alternatively, metal nanoparticles can also be attached on micron-sized dielectric beads, which are much easier to trap. Raman probes can be adsorbed on the surface of the metal nanoparticles. In addition, combined with other techniques, such as microfluidics, the applicability of optical tweezers for SERS can be even more expanded. 

Here the authors consider the possibility of inferring such statistical characteristics from the spectral features of probe photons or particles that are scattered by the density fluctuations of trapped atoms, notably in optical lattices, in two hitherto unexplored scenarios, (a) The probe is weakly (perturbatively) scattered by the local atomic density corresponding to the random occupancy of different lattice sites, (b) The probe is multiply scattered by an arbitrary (possibly unknown a priori) multi-atom distribution in the lattice. The highlight of the analysis, which is based on this random matrix approach, is the prediction of a semicircular spectral lineshape of the probe scattering in the large-fluctuation limit of trapped atomic ensembles. Thus far, the only known case of quasi-semicircular lineshapes in optical scattering has been predicted [Akulin 1993] and experimentally verified [Ngo 1994] in dielectric microspheres with randomly distributed internal scatterers. 

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