We present an all-optical tool for the massive transfer of airborne light-absorbing particles. A generated light sheet trap can be used as an “optical mill” for guiding particles via photophoretic forces. We show the possibility of transferring hundreds to thousands of trapped particles from one cuvette to another in a controllable manner. Two different types of particles were used for demonstration—nonspherical agglomerations of carbon nanoparticles and printer toner particles with a more regular shape. The proposed tool can be used for the transportation of light-absorbing particles, such as biological nano- and micro-objects, or for the touch-free sampling of airborne particles being measured.

Different types of airborne particles, including bacteria, fungal spores, plant pollen, and small fragments of plants and fungi, are potential carriers of various human or plant diseases.1,2 Optical tweezers based on the phenomenon of photophoresis, which is defined as the light-induced motion of microscopic particles in a gaseous medium,3 are unique nontouch instruments for the three-dimensional trapping of airborne particles. Many different approaches have been used for this purpose, including focused Gaussian beams,4 optical vortex beams,5,6 and optical “bottle” beams.7–12 It is well known that for air at room temperature, photophoretic forces can exceed the radiation pressure by several orders of magnitude,5 which makes it possible to transport and pinpoint the position of single light-absorbing particles over meter-scale distances.13,14 In addition, the high accuracy of positioning (less than 10 μm) of the trapped particles allows one to not only carry out spectroscopic analysis,12 but also realize a volumetric display with an image resolution of 1600 dpi.15 However, along with the identification and quantitative analysis of such individual particles, the average analysis of multiple particles is also crucial.16 

Using laser speckle fields is quite simple and reliable for trapping multiple particles. In 2010, Shvedov et al. showed that the laser radiation transmitted through a diffuser forms multiple optical bottle-beamlike traps that can be used for the simultaneous three-dimensional confinement of a few thousand microparticles in air.17 In addition, the ability to vary the shape of a cloud of trapped particles by changing the form of the diffuser's illumination has been demonstrated.18 A more controllable way of manipulating ensembles of light-absorbing particles in air is three-dimensional (3D) optical lattices that can be generated, for example, by overlapping coherent laser beams19 or using tapered-ring optical fields diffracted from circular apertures.20 In fact, in all these cases, the light-absorbing particles were trapped in areas of minimal intensity surrounded by high-intensity light regions that limit the movement of the particles. In 2012, researchers demonstrated a photophoretic trampoline21 based on a high-intensity light sheet that was generated using a cylindrical lens (CL) and off which light-absorbing liquid droplets could bounce. Depending on the power of the laser and the orientation of the light sheet, it is possible to control the droplets' trajectories. In this Letter, we propose using a light sheet trap generated by a cylindrical lens as a tool for the massive transfer of airborne light-absorbing particles. We call this tool an “optical mill.” The proposed photophoretic trap can be used not only for the massive trapping and positioning of light-absorbing particles but also for transferring hundreds to thousands of trapped particles from one cuvette to another, which has not previously been demonstrated.

A complex amplitude of a Gaussian beam diffracted on a cylindrical lens with a focal length f and transmission function of exp(ikξ2/2f) at distance z is described by a Fresnel transform,22 

U(x,y,z)=(ik2πz)exp[ik2z(x2+y2)]×E0(ξ,η)exp(ikξ22f)×exp[ik2z(ξ2+η2)ikz(ξx+ηy)]dξdη,
(1)

where (ξ,η) are the Cartesian coordinates in the plane of the cylindrical lens, (x, y) are the Cartesian coordinates at the distance of z, and k=2π/λ is the wavenumber of the laser radiation with a wavelength of λ. It is well known that the tilting of the cylindrical lens relative to the optical axis by angle α results in tilting of the generated intensity pattern by the same angle in the observation plane [see Fig. 1(a)]. As mentioned before, a light sheet distribution generated by a cylindrical lens can serve as a photophoretic trampoline for light-absorbing droplets:21 the photophoretic forces point from the high-intensity region and lead to continuous bouncing of the falling droplets. In this case, one can control the trajectories of the droplets by changing the inclination angle of the focused light sheet. However, the generated light sheets can be used not only for bouncing the falling airborne particles but also for trapping them, as in the case of a single Gaussian laser beam.4 Thus, different inclination angles of the generated light sheet give different directions and moduli of acting forces. For an estimated explanation of the behavior of light-absorbing particles trapped by the generated light sheet, let us consider the forces exerted on a single particle,23 

Fresult=Fg+FΔT+FΔα+FRP+Fc+Fb+Fdrag,
(2)

where Fg is the gravity, FΔT and FΔα are the photophoretic forces arising from different temperature distributions and surface accommodation coefficients around the trapped particle, respectively, FRP is the radiation pressure, Fc is the convection force, Fb is the buoyancy force, and Fdrag is the drag force, which is proportional to the linear velocity of the particle. For the trapping area inside a closed cuvette, there is no noticeable convection18 and the convection force is zero. The buoyancy force can also be ignored because the density of the particles in this study is three orders of magnitude greater than the density of air. In the case of a nonrotating light sheet, the linear velocity of the trapped particles is zero and the drag force is zero as well. For the explanation of the possibility of three-dimensional trapping of nonspherical particles, the trapping mechanism proposed by Rohatschek24,25 and adopted and modified by He et al.26 can be used [see Fig. 1(c)]. It is well known that for an irregular particle, the center of gravity does not overlap with the geometric center and the FΔα force points from the gravity center to the geometric center. For a particle trapped in the generated light-sheet trap, the direction of the FΔα force varies arbitrarily depending on the orientation of the particle at the first moments. The FΔT and Fg forces produce moments on the particles, which induce the rotation of the particle until the vector sum of the forces equals zero. When Fresult is zero, the particle is three-dimensionally trapped. In this way, some irregular particles can be trapped by a light sheet trap (see Movies 1 and 2 in the supplementary material, showing a set of particles trapped by the generated light sheet trap for two different inclination angles of a cylindrical lens, 0° and 90°). These particles can be trapped in different regions of the generated light sheet trap: some particles can be trapped at the interface between the laser beam and the air, while some particles can be trapped inside the light sheet trap [see Fig. 1(d)] (Multimedia view). The probability of trapping of particles at the interface is higher because to get into the region inside the light sheet, the particles should have the initial velocity that is enough to overcome the photophoretic force acting at the interface. However, in both cases, there is a point where Fresult is zero. However, rotating the cylindrical lens leads to the rotation of the generated light sheet and the trapped particles. The drag force is directed in the opposite direction to the direction of the motion of the particles [see Fig. 1(b)]. The increase in the inclination angle leads to the decrease in the vertical component of the photophoretic force FΔT. At a certain inclination angle, this leads to the release of some particles from the trapping region. Movie 3 in the supplementary material shows the changes in the position of the trapped particles when the generated light sheet trap rotates. The trajectory of the motion of these particles can be different depending on their size and shape—the particles can fall in the vertical direction or move along complex trajectories similar to the continuous bounces in the case of the photophoretic trampoline.

FIG. 1.

The light sheet generated by a cylindrical lens. (a) The transverse intensity distribution of the light sheet generated in the focal plane of a cylindrical lens tilted at different angles. (b) Illustration of possible trapping mechanisms involved in trapping using a light-sheet trap. (c) The force analysis of the three-dimensional trapping of a nonspherical irregular particle. Two different cases are shown: (I) the particle trapped at the interface between the light sheet trap and the air and (II) the particle trapped inside the light sheet trap. The direction of the laser is from left to right. The center of gravity is denoted as “c,” and the geometric center is denoted as “o.” θ1 and θ2 denote the angle between the FΔα force and the gravity Fg. (d) The experimental image of the four particles trapped in the different regions of the generated light sheet trap: the particles denoted as “1” and “2” are trapped at the interface between the light sheet trap and the air, and the particles “3” and “4” are trapped inside the light sheet trap. The dotted line shows the outline of the light sheet trap. Multimedia view: https://doi.org/10.1063/1.5125671.1

FIG. 1.

The light sheet generated by a cylindrical lens. (a) The transverse intensity distribution of the light sheet generated in the focal plane of a cylindrical lens tilted at different angles. (b) Illustration of possible trapping mechanisms involved in trapping using a light-sheet trap. (c) The force analysis of the three-dimensional trapping of a nonspherical irregular particle. Two different cases are shown: (I) the particle trapped at the interface between the light sheet trap and the air and (II) the particle trapped inside the light sheet trap. The direction of the laser is from left to right. The center of gravity is denoted as “c,” and the geometric center is denoted as “o.” θ1 and θ2 denote the angle between the FΔα force and the gravity Fg. (d) The experimental image of the four particles trapped in the different regions of the generated light sheet trap: the particles denoted as “1” and “2” are trapped at the interface between the light sheet trap and the air, and the particles “3” and “4” are trapped inside the light sheet trap. The dotted line shows the outline of the light sheet trap. Multimedia view: https://doi.org/10.1063/1.5125671.1

Close modal

The experimental study of the trapping of light-absorbing particles with a light sheet trap involved the optical setup shown in Fig. 2(a). A linearly polarized laser beam (λ = 457 nm; Pmax = 900 mW) was directed onto a cylindrical lens (CL) with a focal length of 100 mm. The generated light sheet intensity distribution was formed inside the cuvette C. Two video cameras, Cam1 and Cam2 (TOUPCAM UHCCD00800KPA, 1600 × 1200 pixels, 3.34 μm pixel size), were used for the observation of the laser light scattered from the trapped particles and collected with the help of lenses L1 and L2 with a focal length of 75 mm. In the experiments, we used two different types of light-absorbing particles with nonspherical shapes: agglomerations of carbon nanoparticles and printer toner particles. Figure 2(b) shows histograms of the particle area distributions for both types of particles. For the carbon particles, most of the particles have areas ranging from 10 to 30 μm2 (in total, approximately 24%). For the printer toner particles, most of the particles have areas ranging from 40 to 60 μm2 (in total, approximately 33%). In addition, it is clear that the carbon particles have more nonregular shapes than the printer toner particles. Figure 2(c) presents optical images of the trapped particles obtained for different inclination angles of the cylindrical lens CLC when the output laser power was 900 mW. These images represent only a part of the trapped particles captured using the video camera. These results agree with our estimated explanation: the larger inclination angles lead to smaller numbers of stably trapped particles. Figure 3 presents the dependence of the number of three-dimensionally trapped particles on the output laser power for both types of particles used. The frequency of the trapping of the printer toner particles with larger dimensions and weights is much less than that in the case of agglomerations of carbon nanoparticles. It is obvious that for both types of light-absorbing particles, a decrease in the output laser power leads to a decrease in the power density of the generated light sheet trap, which results in a decrease in the magnitudes of acting photophoretic forces and radiation pressure. While maintaining the magnitudes of other acting forces, this leads to a decrease in the stability of some trapped particles, especially particles with larger areas and larger weights. These particles leave the light sheet trapping region with velocities depending on acting forces. The velocities can be directed in the vertical direction (due to the gravity) or can have a velocity component directed in the horizontal direction [see Fig. 1(b)]. When the cylindrical lens forming the light sheet trap rotates, it causes part of the unstably trapped particles to move by inertia in the direction of the rotation of the generated trap. Thus, using such a laser manipulation technique, not only stably trapped particles are transported along with the generated trap but also particles flying out of the trap and moving by inertia (see Movie 4 in the supplementary material showing the movement of particles trapped by the generated inclined light sheet trap), as well as air-borne particles, which occur on the path of the rotating light sheet trap, being bounced from it (similar to the experiments with a photophoretic trampoline). This opens up the possibility of using the rotating light sheet trap as an optical mill for the massive transfer of airborne light-absorbing particles from one container to another. Previously, only the controllable transportation of single airborne particles from one glass cuvette to another using an especially designed optical bottle beam has been demonstrated with the help of optical bottle beams.4 

FIG. 2.

The experimental investigation of the trapping of different light-absorbing particles with a light sheet trap. (a) Experimental setup. (b) Area distribution of the two different types of light-absorbing particles used in the experiments: agglomerations of carbon nanoparticles and printer toner particles. The insets show the optical microscopy images of the particles. The scale bar is 10 μm. (c) Optical images of the trapped particles obtained for different inclination angles of the cylindrical lens (CL): side view of the laser light scattered from the trapped particles.

FIG. 2.

The experimental investigation of the trapping of different light-absorbing particles with a light sheet trap. (a) Experimental setup. (b) Area distribution of the two different types of light-absorbing particles used in the experiments: agglomerations of carbon nanoparticles and printer toner particles. The insets show the optical microscopy images of the particles. The scale bar is 10 μm. (c) Optical images of the trapped particles obtained for different inclination angles of the cylindrical lens (CL): side view of the laser light scattered from the trapped particles.

Close modal
FIG. 3.

Dependence of the number of stably trapped particles on the output laser power for different inclination angles of the cylindrical lens.

FIG. 3.

Dependence of the number of stably trapped particles on the output laser power for different inclination angles of the cylindrical lens.

Close modal

To realize the proposed optical mill for the massive transfer of particles, we extended the initial laser beam to stretch the generated light sheet trap in the direction transverse to the axis of propagation. The length of the generated light sheet trap in the transverse direction was approximately 2.7 cm [see Fig. 4(b)]. A glass cuvette with a cross section of 2.6 × 2.8 cm and a height of 7 cm divided into two independent cuvettes using a coverslip was used for the demonstrations [see Fig. 4(a)]. The output laser power was 900 mW. In these experiments, we rotated the cylindrical lens with an average frequency of approximately 0.1 Hz [see Fig. 4(c)]. Each 180-degree rotation of the lens results in the transportation of some particles, which can be optically observed from the scattered laser light. The estimated number of transported particles for one 180° rotation of the lens ranges from 500 to 1000 and decreases with each subsequent rotation due to a decrease in the number of particles in the first cuvette, which are in the trapping region [see Fig. 4(d) (Multimedia view) and Fig. 4(e) (Multimedia view)]. In order to exclude the influence of airflow and air convection, we also carried out these experiments with a nonrotating cylindrical lens that generated a static light sheet trap stretched in the horizontal direction. In this case, there was no particle transportation for both types of airborne particles.

FIG. 4.

Demonstration of the optical mill for the massive transfer of airborne light-absorbing particles. (a) A double cuvette used in the demonstrations. (b) The generated light sheet trap inside the cuvette. (c) A schematic illustrating the transfer of airborne particles floating in the first cuvette to the second one by the rotating light sheet trap. (d) Transportation of airborne carbon particles from one cuvette to another. (e) Transportation of airborne printer particles from one cuvette to another. The white dashed curves show the regions in the second cuvette with transferred particles. Multimedia views: https://doi.org/10.1063/1.5125671.2; https://doi.org/10.1063/1.5125671.3

FIG. 4.

Demonstration of the optical mill for the massive transfer of airborne light-absorbing particles. (a) A double cuvette used in the demonstrations. (b) The generated light sheet trap inside the cuvette. (c) A schematic illustrating the transfer of airborne particles floating in the first cuvette to the second one by the rotating light sheet trap. (d) Transportation of airborne carbon particles from one cuvette to another. (e) Transportation of airborne printer particles from one cuvette to another. The white dashed curves show the regions in the second cuvette with transferred particles. Multimedia views: https://doi.org/10.1063/1.5125671.2; https://doi.org/10.1063/1.5125671.3

Close modal

In conclusion, we investigated the photophoretic trapping and manipulation of airborne particles with the help of a light sheet trap generated by a cylindrical lens. Two different types of particles with different average sizes were used in the experiments. The effects of the inclination angle and output laser power on the trapping efficiency were demonstrated experimentally and explained theoretically. It was shown that the rotation of the lens leads to a decrease in the number of the stably trapped particles and an inertial movement of some of the unstably trapped particles in the direction of the rotation. This allowed us to propose the concept of an optical mill based on a rotating cylindrical lens—a tool for the massive transfer of airborne light-absorbing particles. The demonstrated technique made it possible to transport hundreds to thousands of airborne light-absorbing particles from one cuvette to another by simply rotating the cylindrical lens around the optical axis. This investigation opens up the possibility of implementing different all-optical devices for the touch-free sampling of multiple airborne particles inside containers for subsequent transportation and investigation or for transferring particles in applications involving bio-hazardous substances and materials.

See the supplementary material for the movies showing a set of particles trapped by the generated light sheet trap and movement of particles trapped by the generated inclined light sheet trap.

This work was financially supported by the Russian Science Foundation (Grant No. 19-72-00018).

1.
2.
C.
Wang
,
Y. L.
Pan
,
S. C.
Hill
, and
B.
Redding
,
J. Quant. Spectrosc. Radiat. Transfer
153
,
4
(
2015
).
3.
F.
Ehrenhaft
,
Ann. Phys.
56
,
81
(
1918
).
4.
Z.
Zhang
,
D.
Cannan
,
J.
Liu
,
P.
Zhang
,
D. N.
Christodoulides
, and
Z.
Chen
,
Opt. Express
20
,
16212
(
2012
).
5.
V. G.
Shvedov
,
A. S.
Desyatnikov
,
A. V.
Rode
,
W.
Krolikowski
, and
Y. S.
Kivshar
,
Opt. Express
17
,
5743
(
2009
).
6.
V. G.
Shvedov
,
A. S.
Desyatnikov
,
A. V.
Rode
,
Y. V.
Izdebskaya
,
W. Z.
Krolikowski
, and
Y. S.
Kivshar
,
Appl. Phys. A
100
,
327
331
(
2010
).
7.
V. G.
Shvedov
,
C.
Hnatovsky
,
A. V.
Rode
, and
W.
Krolikowski
,
Opt. Express
19
,
17350
(
2011
).
8.
P.
Zhang
,
Z.
Zhang
,
J.
Prakash
,
S.
Huang
,
D.
Hernandez
,
M.
Salazar
,
D. N.
Christodoulides
, and
Z.
Chen
,
Opt. Lett.
36
,
1491
(
2011
).
9.
A.
Turpin
,
V.
Shvedov
,
C.
Hnatovsky
,
Y. V.
Loiko
,
J.
Mompart
, and
W.
Krolikowski
,
Opt. Express
21
,
26335
(
2013
).
10.
A.
Lizana
,
H.
Zhang
,
A.
Turpin
,
A.
Van Eeckhout
,
F. A.
Torres-Ruiz
,
A.
Vargas
,
C.
Ramirez
,
F.
Pi
, and
J.
Campos
,
Sci. Rep.
8
,
11263
(
2018
).
11.
H.
Zhang
,
A.
Lizana
,
A.
Van Eeckhout
,
A.
Turpin
,
C.
Ramirez
,
C.
Iemmi
, and
J.
Campos
,
Appl. Sci
8
,
2310
(
2018
).
12.
Y. L.
Pan
,
S. C.
Hill
, and
M.
Coleman
,
Opt. Express
20
,
5325
(
2012
).
13.
V. G.
Shvedov
,
A. V.
Rode
,
Y. V.
Izdebskaya
,
A. S.
Desyatnikov
,
W.
Krolikowski
, and
Y. S.
Kivshar
,
Phys. Rev. Lett.
105
,
118103
(
2010
).
14.
J.
Lin
,
A. G.
Hart
, and
Y. Q.
Li
,
Appl. Phys. Lett.
106
,
171906
(
2015
).
15.
D. E.
Smalley
,
E.
Nygaard
,
K.
Squire
,
J.
Van Wagoner
,
J.
Rasmussen
,
S.
Gneiting
,
K.
Qaderi
,
J.
Goodsell
,
W.
Rogers
,
M.
Lindsey
,
K.
Costner
,
A.
Monk
,
M.
Pearson
,
B.
Haymore
, and
J.
Peatross
,
Nature
553
,
486
490
(
2018
).
16.
M.
Hanns
,
Analysis of Airborne Particles by Physical Methods
(
CRC Press
,
2017
).
17.
V. G.
Shvedov
,
A. V.
Rode
,
Y. V.
Izdebskaya
,
A. S.
Desyatnikov
,
W.
Krolikowski
, and
Y. S.
Kivshar
,
Opt. Express
18
,
3137
(
2010
).
18.
V. G.
Shvedov
,
A. V.
Rode
,
Y. V.
Izdebskaya
,
D.
Leykam
,
A. S.
Desyatnikov
,
W.
Krolikowski
, and
Y. S.
Kivshar
,
J. Opt.
12
,
124003
(
2010
).
19.
V. G.
Shvedov
,
C.
Hnatovsky
,
N.
Shostka
,
A. V.
Rode
, and
W.
Krolikowski
,
Opt. Lett.
37
,
1934
(
2012
).
20.
F.
Liu
,
Z.
Zhang
,
Y.
Wei
,
Q.
Zhang
,
T.
Cheng
, and
X.
Wu
,
Opt. Express
22
,
23716
(
2014
).
21.
M.
Esseling
,
P.
Rose
,
C.
Alpmann
, and
C.
Denz
,
Appl. Phys. Lett.
101
,
131115
(
2012
).
22.
V. V.
Kotlyar
,
A. A.
Kovalev
, and
A. P.
Porfirev
,
Appl. Opt.
56
,
4095
(
2017
).
23.
C.
Wang
,
Z.
Gong
,
Y. L.
Pan
, and
G.
Videen
,
Appl. Phys. Lett.
109
,
011905
(
2016
).
24.
H.
Rohatschek
,
J. Aerosol Sci.
16
,
29
(
1985
).
25.
H.
Rohatschek
,
J. Aerosol Sci.
27
,
467
(
1996
).
26.
B.
He
,
X.
Cheng
,
Y.
Zhan
,
Q.
Zhang
,
H.
Chen
,
Z.
Ren
,
C.
Niu
,
J.
Yao
,
T.
Jiao
, and
J.
Bai
,
Europhys. Lett.
126
,
64002
(
2019
).

Supplementary Material