Bessel beam optical tweezers for manipulating superparamagnetic beads

Abstract

We propose a Bessel beam optical tweezers setup that can stably trap superparamagnetic beads. The trap stiffness measured is practically independent of the radius of the Bessel beam and of the bead height (distance from the coverlip of the sample chamber), indicating that the beads can be trapped with high accuracy within a wide range of such parameters. On the other hand, the trap stiffness exhibits the expected linear increase with the laser power, despite the non-negligible absorption coefficient of the superparamagnetic beads. A geometrical optics model that considers spherical aberration and light absorption by the beads was used to predict the optical forces and trap stiffness, showing excellent agreement with the experimental data. We believe the results presented here advance the field of optical trapping manipulation of absorbing magnetic particles, and future applications will involve, for example, the design of new hybrid optomagnetic tweezers.

Use of Organic Semiconductors as Handles for Optical Tweezers Experiments: Trapping and Manipulating Polyaniline (PANI) Microparticles

“…Furthermore, for the lowest powers used the experimental data is slight higher than the theoretical prediction, and the opposite behavior occurs for the highest powers used. Such a type of behavior was previously verified for other absorbing beads (although for higher laser powers) and strongly suggests that the absorption coefficient of PANI beads is intensity-dependent, as in the case of traditional semiconductors such as germanium or silicon. , In other words, increasing laser intensity increases the rate of generation of free charge carriers in semiconductors, which in turn increases the absorption coefficient by the free carrier (FC) absorption mechanism (α FC ). , Thus, the deviation of the experimental data from the linearity can be understood by assuming that for powers less than ∼3.5 mW the absorption coefficient is slightly lower than 2300 cm –1 (and therefore the experimental κ is higher than that predicted by the GO model); while for laser powers higher than 3.5 mW the absorption coefficient is slightly higher than 2300 cm –1 (and therefore the experimental κ is smaller than predicted by the GO model).…”

“…The trap stiffness κ was measured by analyzing the Brownian fluctuations of the bead position in the optical potential for 1 min, using a CMOS camera (Basler ac1920-155uc). The technical details of our analysis process were described elsewhere …”

“…The technical details of our analysis process were described elsewhere. 32 In Figure 3, panel b, we show the experimental trap stiffness (squares) measured as a function of the laser power at the focus (PF) for the two axes (x and y) transverse to the laser propagation direction (z). Such measurements were performed using a PANI bead with a radius a = (3.17 ± 0.02) μm and at a height of 10 μm above the surface of the bottom coverslip of the sample chamber.…”

“…Furthermore, for the lowest powers used the experimental data is slight higher than the theoretical prediction, and the opposite behavior occurs for the highest powers used. Such a type of behavior was previously verified for other absorbing beads (although for higher laser powers) 32 and strongly suggests that the absorption coefficient of PANI beads is intensity-dependent, as in the case of traditional semiconductors such as germanium or silicon. 38,39 In other words, increasing laser intensity increases the rate of generation of free charge carriers in semiconductors, which in turn increases the absorption coefficient by the free carrier (FC) absorption mechanism (α FC ).…”

Optical Force and Torque on a Graphene-Coated Gold Nanosphere by a Vector Bessel Beam

“…To overcome the diffraction, some novel manipulation techniques based on nondiffracting beams [ 1 , 2 , 3 , 4 ] have been developed. Bessel beams [ 5 , 6 , 7 , 8 ], a typical nondiffracting beam, can simultaneously trap and manipulate many particles in multiple planes because of their unique properties of nondiffraction and self-healing. In addition, by adjusting the beam parameters including half-cone angle, beam order, and polarization, Bessel beams can exert pulling force [ 9 , 10 , 11 , 12 , 13 , 14 , 15 , 16 , 17 , 18 , 19 , 20 , 21 , 22 , 23 , 24 , 25 , 26 , 27 , 28 , 29 , 30 , 31 , 32 , 33 , 34 , 35 , 36 ] and negative optical torque [ 3 , 37 , 38 , 39 , 40 , 41 ] on particles.…”