We demonstrate laser manipulation of light-absorbing particles in air using a spatial light modulator. By controlling the three-dimensional distribution of the generated optical field, we demonstrate how to control the position of individual trapped particles dynamically and independently. The transfer of a trapped particle from one optical trap to another is shown.
Various primary biological aerosol particles, which include bacteria, fungal spores,
plant pollens, small fragments of plants or fungi, and secondary biological aerosol
particles, such as the particles formed by an ozone-initiated polymerization of
terpenes are potential carriers of a number of human or plants diseases (e.g., inhalation
anthrax) [
In order to solve the problem of retaining particles, optical tweezers based on
photophoretic forces can be used [
Thus, the combination of a spatial light modulator (for generation of multiple light
traps) and a spectroscopic measurement device (for spectral measurements) allows
parallel analysis of many aerosols trapped in the test volume. Using low-power lasers
and compact hyperspectral cameras [
Design method
In the case of a positive photophoresis, light-absorbing air-borne particles move away
from the light source, and in the case of negative photophoresis – move towards the
light source [
Laser trapping the light-absorbing particles in the air with a Hermite-Gaussian
TEM10 laser beam has been demonstrated previously [
The "superposition method prisms and lenses," technique can be used to generate an
array of light traps [
Experimental investigation of laser trapping using a spatial light modulator In order to generate an arbitrary configuration of light traps, a spatial light modulator PLUTO VIS was utilised. The experimental optical setup is shown in Fig. 2. The output laser beam of solid-state laser (λ= 532 nm, with a maximum output power of 1500 mW) was expanded with a telescope composed of a micro-objective, MO1 (8×, NA=0.2), and a lens, L1 (f1 = 350 mm) to illuminate the SLM. Then, lenses L2 and L3 with focal lengths f2 = 350 mm and f3 = 150 mm, respectively, formed an image of the plane conjugated to the SLM display in the focal plane of the micro-objective MO2 (16×, NA=0.3) An airborne light-absorbing particle in a cuvette C was trapped in the area of the focal point of a micro-objective. Observation of the particle trapping was possible due to the scattered light recorded by the video camera, Cam (TOUPCAM UHCCD00800KPA, 1024×768 pixels). The particles were imaged through a microobjective, MO3 (8×, NA=0.2).
In the experiments, the agglomeration of carbon particles was studied, the transverse
dimensions of which were in a range from a few to tens of micrometers. These
particles have a non-spherical shape, and therefore ideally suited for experiments of this
kind that has been shown in previous experiments [
Figure 3 shows the experimental results of how the position of each of the trapped
particles using a spatial light modulator can be controlled dynamically. Changing the
phase transmission function calculated in accordance with Eq. (1), we can change the
position of the formed intensity peaks independently of each other. Thus, in these
experiments, we move one of the formed three light traps along the z axis. The
particles trapped in the vicinity of the trap move behind them. The velocity the trapped
particle is 1.8±0.1μm/s. Thus, SLMs may be used to dynamically control the position
of trapped air-borne light-absorbing micro-particles, which is analogous to the optical
manipulation of dielectric micro-objects in a liquid [
Fig. 2. Experimental optical setup: L is a solid-state laser, MO1 is a micro-objective (8×, NA = 0.2), MO2 is a micro-objective (8×, NA = 0.2), L1, L2, and L3 are the lenses with focal lengths f1 = 250 mm, f2 = 350 mm, and f3 = 150 mm, respectively, MO2 is a microobjective (16×, NA = 0.3), SLM is a spatial light modulator (PLUTO Spatial Light Modulator, 1920 × 1080 pixel resolution), CAM is a video camera (LOMO TC-1000), C is a cuvette with air-borne light-absorbing particles Fig. 3. Experiment on moving particles trapped in the various traps, independently of one another. Black arrow marked the moving particles Fig. 4. Sequential transfer the trapped particles from one light trap to another light trap. Dotted outlines indicate "offline trap" solid contours - "online trap", the dotted arrow indicates the direction of movement of the corresponding traps Figure 4 shows the motion of particles that are consistently transferred from one generated trap to another, and then to a third generated trap. To do this, we generated three light traps and one of them held particles (trap #1). The trap #2 was generated next to the trap #1 (see Fig. 4, frame 2). Then we "turned" the trap #1 “off”. After that we "turned" trap #1 “off”, particles were captured in the trap #2. After that, we moved the particle using the light trap #2. Similar operations were carried out to transfer particles from the light trap #2 into the light trap #3 (see Fig. 4, frames 4-6) and then to transfer particles from the light trap #3 into the light trap #1 (see Fig. 4, frames 79).
Figure 5 shows the motion of particles trapped by a generated light trap. In this
experiment, we generated two light traps (#1 and #2 in Fig. 5). The distance between these
two light traps is about 30 μm. First, the particle was trapped by light trap #1. At that
moment, we started to decrease the energy used for the generating the light trap #1. At
the same time, energy used in generating light trap #2 was not changed. When the
energy of light trap #1 became equal to 50% of the energy of light trap #2, the trapped
particle left trap #1 and was trapped in the light trap #2. With decreasing energy of the
generated laser trap, photophoretic forces also decrease. After some time, gravity
acting on the trapped particle becomes greater than the photophoretic forces, and the
particle leaves the trapping region. The dimensions of trapping region of the
generated traps are generally determined by the depth of focus [