The using possibility of the vortex light beam with topological charges 2 and 5 for the rotation of the micromechanical element (microturbine) was considered. The results of the full-scale experiment on the optical rotation of the special form microturbine were presented. We evaluated the force moment, acting the microturbine.
The light beam with the orbital angular moment can be formed by the diffractive
optical elements for a defined shape micromechanical component to optimize the process
of the light energy transformation into mechanical energy. Such beams are widely
used to rotate microscopic objects with the size of 0.1-10 microns, which is confirmed
by the numerous publications. For example, the [
However, the number of the researches on the use of light vortex beams with
microscopic objects in a rarefied medium (air or vacuum), is not enough, due to the
significant technical difficulties during the experiments realization. Therefore, it is
important to develop a system where the light beams and micromechanical elements will
be designed to each other and are optimized for the operation in air or vacuum. But
with the use of the diffractive optical elements we are able to realize the system where
the operating micromechanical element will be illuminated by the light beam with the
orbital angular moment, which will be formed to fit the shape of the microscopic
objects, so that the rotation moment transmission can occur due to the light pressure as a
result of the light reflection from the surface. As the result, it is possible to minimize
the light absorption by the microscopic object, and consequently its thermal effect.
One the way to minimize the thermal effect is the distribution of the light beam
intensity by the maximum area. Such vortex beams exist, for example, Bessel beams [
To rotate the microturbine, it was decided to use one of the beams represented in the
research [
The Figure 1 shows the phase function of the DOE (Fig. 1a), and formed intensity
distribution (Fig. 1b) and the beam phase (Fig. 1c).
Fig. 1. Phase function of the DOE (a), the intensity distribution in the far field of diffraction
(b), the phase of the beam (c)
Initially, this beam is assumed to rotate the microturbines from an opaque material,
however, this beam enables to rotate the set of the polystyrene micropatricles, and as
will be shown below, the beam successfully rotates the transparent microturbines.
To minimize thermal effect on the microturbine it is necessary to form the light beam
in the way that the intensity of the beam has been distributed over the maximum area
and the beam size was approximately comparable to the size of the microturbine (Fig.
2). Also, for efficient transmission of the rotation moment from the vortex beam to
the microturbine the following requirement should be satisfied: the intension, acting
the microturbine blade, is on the same level, according to its position, it is achieved
on the basis of the intension distribution in the polar angle [
R IS ( ) I (r, ) dr const (1) 0 where I (r, φ) is the intensity of the vortex beam in polar coordinates centered on the beam axis, Is is the total intensity of the beam at the current narrow sector. Fig. 2. The phase function of the DOE (a), the beam intensity distribution in the far field of the diffraction (b), the phase of the beam in the far field of the diffraction (c).
The requirement is satisfied for the beam shown in the Figure 2b. The Figure 3 shows the Is distribution according to the polar angle.
The microscopic objects of the special form (microturbines) were designed in order to
realize the rotation experiment. The circular laser work station (CLWS-2014) was
used for this purpose. The optical setup, which scheme shown in the Figure 2, was
used to form the complicated vortex beams and to focus them on the microturbines.
The laser with a wavelength of 532 nm and a maximum power of 4 W was used as the
radiation source. The laser beam input is realized from below to reduce the friction
which is increased when the laser beam is focused on the top and presses the particles
to the substrate by the light pressure. The beam was increased to a diameter of about
2.3 mm by the collimator, and then was limited to 2 mm by the iris aperture to match
the size of the beam to the size of the DOE. The microlens 16x were used to focus the
beam and to form the operation area image. The microturbines were placed in a cell
with water and salt was added to increase its density. Because the density of the resist
material whereof the microturbine was produced is 1200 kg/m3, that is much higher
than the water density, the salt addition reduces the microturbine weight and
consequently decreases friction.
The operation area was illuminated with white light from a standard illuminator. The
recoding was carried out by the CCD camera and the spectral filter was placed in
front of it. The Figure 3 shows the stages of the microturbine rotation.
Fig. 5. The stages of the polymer microturbine rotation with the interval of 2 s in the beam with
the topological charges 5, 2, 5
The Figure 5 shows that the rotation of the microturbine with the diameter of 100 μm
was realized by the focused light beam. We calculate the rotating speed of the
microturbine and the moment of the friction, acted it, using the first and the last image. We
find the coordinates of the blade in position before and after rotation. To calculate the
friction moment we use the formula for the moment of friction acting the wheel of the
terminating radius[
The rotating speed of the microturbine was 0.056 c-1, and the moment of the rotating intensity acting it is 2.87 fmN m . This enables the evaluation of the total intensity exerted on the microturbine by the beam – it is about 100 pN.
The weight of the turbine is decreased with adding salt, which is resulted in microturbine rotation. However, there are some difficulties: before each intake of the drop it is necessary to heat the solution in the water bath for several minutes, but even in this Information Technology and Nanotechnology (ITNT-2016) 29 case the salt is not completely dissolved - figures show salt crystals, which are the obstacle for the microturbine rotation. Therefore, another method based on the highfrequency vibration was used to minimize the friction. The special device for the creation of the high-frequency mechanical vibrations was added to the setup. The experiment was realized using light traps, designed for the microturbine rotation (Fig. 2b). The microturbine rotation speed at high-frequency vibrations is that exceeds the speed of microturbine rotation in salt brine using a beam, shown in the Figure 1 b. It proves the effectiveness of the proposed method to the experiment realization.
The complex vortex beam with topological charges 2 and 5, in spite of the noncircular structure that allows the microturbine, that matches in the form with its wavefront, to be rotated. It is necessary to reduce the weight of the microturbines by increasing the fluid density for the successful experiments with such large objects as the microturbine (diameter 100 μm). The intensity moment acting the microturbine was about 3 fmN, which is quite a high value within the parameters of the optical circuit used in the experiment. We proved the perspective of the vibration use in the rotation experiments.