Measurements of Optical Trapping Efficiency for Micron-Sized Dielectric Particles in Various Surrounding Media

Abstract

DielectricThe maximum axial trapping efficiency Qmax has been measured in water, ethanol, butanol and octanol for polystyrene latex spheres having a diameter of 2-15 pm. The effects of the diameter of the particle as well as the refractive index and the viscosity of surrounding medium have been investigated. For particles of d~7 /Im, Qmax is almost independent 0L particle size, while it decreases as the diameters decrease to d~5 pm. It is also noted that Qmax is affected by the viscosity rather than the refractive index of the surrounding medium.Optical trapping is known to be a powerful tool which utilizes radiation pressure of a laser beam and remotely manipulates microscopic objects including microorganisms, Iiquid drops and biological cells. Since the threedimensional trapping using a single beam was first demonstrated by Ashkin et al. ,1) this technique has been applied in the fields of photochemistry,2,3) biology,4~7) and pattern formation of fine particles. 8,9) Recently the manipulation of metallic particles using a single beaml0,11) has been reported, and the enhancement of axial trapping force using a polarized annular beam was also proposed.12)The force exerted on a dielectric particle by the incident laser beam can be divided into two forces, the scattering force and the gradient force.1'10) The scattering force points in the direction of the pointing vector of the incident bearn and destabilizes the trapping. The gradient force is proportional to the gradient of light intensity and points to the high intensity region; it is this latter force that enables the optical trapping. When the incident Gaussian beam is focused tightly, the total force acting on a particle points in the neighborhood of the focus and it can be trapped and manipulated .For a small particle having a diameter d of less than approximately I /Im, Wright and Sonekl3) calculated the trapping force using an electromagnetic field model. In the size regime of d>> ~ , the trapping forces can be calculated by the ray optics model.14) Wright and his colleaguesl3,15) measured the axial and transverse trapping forces for a 1 pm silica particle and four kinds of polystyrene latex spheres 5-20 pm in diameter in water. The measured and the calculated values did not agree well,l0,15) however, and further theoretical and experimental investigations are required, especially for particles 1-10 ,um in diameter. 15) In this letter, we report the results of measurements of the axial trapping forces for five polystyrene latex spheres with a diameter of 2-15 /Im in four surrounding media -water, ethanol, butanol and octanol. The effects of the refractive index and the viscosity of the surrounding medium and the diarneter of the particle on the trapping efficiency are investigated. This is the first report on measurements of optical trapping efficiency in surrounding media other than water.The experimental setup used is schematically shown in Fig. 1. A TEMoo~mode Ar laser beam (Spinnaker Co., Ltd., model 1161-A11) at 488 nm was introduced into a micros...

Improvement of Laser Trapping Based Microprobe in Laser Shaded Condition

“…Therefore, the trapping efficiency is assessed by measuring the minimum trapping power necessary to trap the particle, while the upward component of the trapping force F Z is taken to be equivalent to the mass of the particle. This method is easy to measure, although there is a tendency to underestimate the trapping efficiency (9) . However, it is sufficient to evaluate the improvement from radial polarization.…”

Optical trapping of nanoparticles and microparticles by a Gaussian standing wave

“…The transfer of momentum between focused laser light and microparticles has been investigated in a number of experiments during the past 30 years. It has been proved that focusing laser light facilitates the trapping of particles whose sizes range from tens of nanometers to tens of micrometers that are immersed in water, 1 ethanol, 2 or air. 3 It also permits the trapping of living cells 4 and uncoiled DNA molecules 5 and the rotation of micro-objects.…”

Optical Manipulation. Theory of Optical Trapping Forces: A Review.

“…To date, both wave-optic7-13) and ray-optic2, [13][14][15] Experimentally, other factors can also impact the resulting forces1, 2,6,[12][13][14][19][20][21],including aberration22) and diffraction23) effects that exist in high numerical aperture focusing systems.…”