On the use of optically trapped dust particles as micro-probes in process plasmas
In this paper we outline the progress in the development of an optical system for particle manipulation as a plasma diagnostics method. We demonstrate basic principles and preliminary experimental results for optical trapping of microparticles in a plasma. A counter-propagating laser beam was used t...
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Національний науковий центр «Харківський фізико-технічний інститут» НАН України
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irk-123456789-1092602016-11-23T03:02:16Z On the use of optically trapped dust particles as micro-probes in process plasmas Schneider, V. Kersten, H. Низкотемпературная плазма и плазменные технологии In this paper we outline the progress in the development of an optical system for particle manipulation as a plasma diagnostics method. We demonstrate basic principles and preliminary experimental results for optical trapping of microparticles in a plasma. A counter-propagating laser beam was used to trap particles in water as well as in an RF discharge. The experiments indicate that it is possible to manipulate particles, which are levitating in the plasma sheath, to obtain information on the sheath and plasma parameters. Oписывается прогресс в развитии манипуляционных частиц оптической системы, как метода плазменной диагностики. Описываются основные принципы и предварительные экспериментальные результаты при оптическом захвате. Для распространяющихся волн был использован лазерный луч, который улавливал частицы в воде, а также в РФ-разряде. Эксперименты показывают возможность манипулирования частицами, которые находятся в состоянии левитации в плазменной оболочке, чтобы получить информацию про саму оболочку и плазменные параметры. Описується прогрес у розвитку маніпуляційних частинок оптичної системи, як методу плазмової діагностики. Описані основні принципи і попередні експериментальні результати при оптичному захопленні мікрочастинок в плазмі. Лазерний пучок, що зустрічно поширювався, був використаний для захоплення частинок у воді, так само як і у РЧ-розряді. Експерименти показують можливість маніпулювання частинками, які знаходяться в стані левітації у плазмовій оболонці, щоб отримати інформацію про саму оболонку і параметри плазми. 2013 2013 Article On the use of optically trapped dust particles as micro-probes in process plasmas / V. Schneider, H. Kersten // Вопросы атомной науки и техники. — 2013. — № 1. — С. 164-167. — Бібліогр.: 15 назв. — англ. 1562-6016 PACS: 42.50.Wk, 52.27.Lw, 52.70.-m, 87.80.Cc http://dspace.nbuv.gov.ua/handle/123456789/109260 en Вопросы атомной науки и техники Національний науковий центр «Харківський фізико-технічний інститут» НАН України |
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Низкотемпературная плазма и плазменные технологии Низкотемпературная плазма и плазменные технологии |
| spellingShingle |
Низкотемпературная плазма и плазменные технологии Низкотемпературная плазма и плазменные технологии Schneider, V. Kersten, H. On the use of optically trapped dust particles as micro-probes in process plasmas Вопросы атомной науки и техники |
| description |
In this paper we outline the progress in the development of an optical system for particle manipulation as a plasma diagnostics method. We demonstrate basic principles and preliminary experimental results for optical trapping of microparticles in a plasma. A counter-propagating laser beam was used to trap particles in water as well as in an RF discharge. The experiments indicate that it is possible to manipulate particles, which are levitating in the plasma sheath, to obtain information on the sheath and plasma parameters. |
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Article |
| author |
Schneider, V. Kersten, H. |
| author_facet |
Schneider, V. Kersten, H. |
| author_sort |
Schneider, V. |
| title |
On the use of optically trapped dust particles as micro-probes in process plasmas |
| title_short |
On the use of optically trapped dust particles as micro-probes in process plasmas |
| title_full |
On the use of optically trapped dust particles as micro-probes in process plasmas |
| title_fullStr |
On the use of optically trapped dust particles as micro-probes in process plasmas |
| title_full_unstemmed |
On the use of optically trapped dust particles as micro-probes in process plasmas |
| title_sort |
on the use of optically trapped dust particles as micro-probes in process plasmas |
| publisher |
Національний науковий центр «Харківський фізико-технічний інститут» НАН України |
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2013 |
| topic_facet |
Низкотемпературная плазма и плазменные технологии |
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http://dspace.nbuv.gov.ua/handle/123456789/109260 |
| citation_txt |
On the use of optically trapped dust particles as micro-probes in process plasmas / V. Schneider, H. Kersten // Вопросы атомной науки и техники. — 2013. — № 1. — С. 164-167. — Бібліогр.: 15 назв. — англ. |
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164 ISSN 1562-6016. ВАНТ. 2013. №1(83)
ON THE USE OF OPTICALLY TRAPPED DUST PARTICLES
AS MICRO-PROBES IN PROCESS PLASMAS
V. Schneider, H. Kersten
Institute of Experimental and Applied Physics, Kiel University, Kiel, Germany
In this paper we outline the progress in the development of an optical system for particle manipulation as a plas-
ma diagnostics method. We demonstrate basic principles and preliminary experimental results for optical trapping of
microparticles in a plasma. A counter-propagating laser beam was used to trap particles in water as well as in an RF
discharge. The experiments indicate that it is possible to manipulate particles, which are levitating in the plasma
sheath, to obtain information on the sheath and plasma parameters.
PACS: 42.50.Wk, 52.27.Lw, 52.70.-m, 87.80.Cc
INTRODUCTION
The idea to use microscopic test particles as electro-
static or thermal probes, respectively, in complex plas-
mas has been consequently developed during the last
years [1-4]. Several experiments on the analysis of
plasma sheath properties, e.g. electric field measure-
ments or energy flux measurements, have been dis-
cussed [5, 6]. Due to the force balance of the particles in
the plasma sheath, however, one is often spatially re-
stricted and it is very difficult to change their position
without changing the external and internal plasma pa-
rameters. Recently, experiments have been performed
where the confined particles are affected by additional
centrifugal force [7] or by laser radiation [8].
In the present study for the first time a macroscopic
optical manipulation system for microparticles in plas-
ma has been realized, which is based on the principle of
laser tweezers [9]. The particles have been successfully
trapped in the focus of a split infrared laser beam whe-
reas the focus length was several tens of centimeters. By
vertical motion of the RF electrode the confined parti-
cles can be shifted to a certain extent through the sheath
in front of the electrode or into the plasma bulk. By this
non-invasive method it is possible to perform flexible
investigations without changing or disturbance of the
plasma and its conditions. The evaluation of the affected
force balance (in nN range) may yield information about
the potential and electric field at arbitrary positions in
the sheath.
1. BASICS OF OPTICAL TRAPPING
Already in the year 1619 J. Kepler assumed that the
light from the sun deviates a comets tail away from the
sun. The radiation pressure was deduced theoretically in
1873 by J. C. Maxwell and in 1876 by A. Bartoli. How-
ever, due to the small momentum of photons, the first
experimental proofs were performed at the beginning of
the 20th century. The technique of trapping and manipu-
lating small (nm-µm) particles by radiation pressure was
first shown by A. Ashkin in 1970 [10] going hand in
hand with the development of lasers producing intense
and coherent light.
Considering a transparent particle in a Gaussian la-
ser beam, we can apply ray optics, when the particle is
much bigger than the laser wavelength (d >> λ). Fig. 1
shows two situations in an unfocused beam. As light
carries momentum, which is conserved, due to reflec-
tions and refractions a net force acts on the bead.
The lateral component is the gradient force and the
scattering force is the component along the beam axis.
When the particle is in the center of the unfocused
beam, the lateral components compensate each other
and the total force points into the beam direction result-
ing in an acceleration of the particle due to scattering.
When the particle is displaced from the beam center, the
intensity distribution leads to a larger momentum trans-
fer from the light closer to the maximum intensity, re-
sulting in a net force toward the center of the laser.
However, the scattering component still pushes the
particle in transverse direction. This principle can be
already used to trap particles against gravity [10].
Fig. 1. Forces on a particle in an unfocused Gaussian
laser beam in the ray optics regime. The force due to
reflections always drags the particle in the beam direc-
tion. The lateral component of the total force is zero,
when the particle is in the center of the beam (left) and
is restoring the particle towards the maximum intensity,
when the particle is displaced (right)
A focused beam produces an axial gradient in the
intensity, which leads to an additional gradient force
along the beam axis (Fig. 2). This force is always di-
rected towards the focal point, where the intensity has
its highest value. Thus, the particle is located slightly
behind the focus, where the scattering force is compen-
sated by this axial gradient force. By moving the fo-
cused laser beam the particle can be manipulated with
so called optical tweezers, which are widely used in
biology, medicine or life sciences, for example, under
microscopes with short focal lengths in the millimeter
range and with high numerical apertures [11].
ISSN 1562-6016. ВАНТ. 2013. №1(83) 165
Fig. 2. Forces in a focused laser. The intensity distribu-
tion leads to an axial gradient force, which is directed
towards the focus and which compensates the scattering
force slightly behind the focal point
2. EXPERIMENTAL SETUP
The idea of the experiment is the manipulation of
particles in plasma without changing the internal or
external plasma parameters. For this purpose, dust parti-
cles are charged and confined in a capacitively coupled
asymmetric RF discharge (13.56 MHz) above the pow-
ered RF electrode which has a diameter of 100 mm. The
cylindrically shaped vacuum chamber (40 liter volume)
is equipped with several windows for diagnostics
(Fig. 3). The discharge is typically operated in argon at
a gas pressure of 10...100 Pa and at a power of
10…50 W. Usually the particles (MF, 10 μm in diame-
ter) are levitated in about 5 mm distance in front of the
electrode.
The optical trapping system in this experiment is based
on the counter-propagating principle [10, 12], where the
scattering forces of two laser beams compensate each
other and the gradient forces fix the particle in its trans-
versal position. In comparison to common laser tweez-
ers which are used in combination with microscopes,
this method is not restricted to high numerical apertures
and short focal lengths. Nevertheless, in our case, the
focus length is about 30 cm. Therefore, the requirements
for accuracy of adjustment, alignment and particle de-
tection in µm-range are very high.
The optical components used to trap particles are
shown in Fig. 3. The IR laser (1) at λ = 1070 nm is
mostly operated at 100 mW…1000 mW. After passing a
λ/2-plate (2) the laser beam is divided into two beams
(arms) by a polarizing beam splitter (3). Afterwards, the
beams are passing through beam expanders (4), 10 µm
pinholes (5), mirrors (6) and collimator lenses (7). Fi-
nally, the beams are focused by lenses (8) to the particle
position (9) in the plasma chamber. Once trapped, the
particle can be moved along the z-axis relative to the
plasma by moving the RF electrode (12) in z-direction
upwards or downwards, respectively. Thus, the particle
can be shifted to a certain extent through the sheath in
front of the electrode or into the plasma bulk. While
moving the electrode or while external forces are acting
on the particle, respectively, the particle position is
measured with a quadrant photo detector (11) by split-
ting the beam with another beam splitter (10). In addi-
tion to gravity and electrostatic field force in the sheath
on a charged microparticle, now the force due to the
optical confinement acts onto the particle. For small
vertical deviations of the particle from the beam axis
the force can be assumed as a spring force
with the stiffness of the trap. If forces – e.g. especially
the electrostatic field force on the charged particle due
to the E-field in the sheath – are acting on the particle
causing a change in position, one can determine this
force by these deviations and, thus, experimentally
determine the field strength in the sheath. By changing
the laser power (e.g. optical force) it is possible to re-
peat the measurements at different positions in the
sheath where stronger or weaker forces may occur.
Fig. 3. Schematic setup of the plasma chamber and the
laser trapping system used for the experiments
3. PRELIMINARY EXPERIMENTS
3.1. TRAPPING IN WATER
Due to the much higher damping of their motion
first experimental verifications of the trapping system
where performed with particles dispersed in water.
Fig. 4. Setup for particle trapping in a glass cuvette
filled with water. The cuvette is moveable in xyz-
direction. The laser power was less than 300 mW
A (1.5x1.5) mm2 squared glass cuvette was placed in
vertical direction into the focal plane of the two laser
beams on a xyz-translation stage (Fig. 4). The cuvette
was observed with a microscope camera. Two syringes
were used to pump the water with the particles through
the cuvette. After some adjusting procedures stable
trapping with 300 mW and less laser power was
achieved. Fig. 4 shows a trapped particle in the cuvette.
By moving the cuvette in z-direction the particle was
moved relative to it.
166 ISSN 1562-6016. ВАНТ. 2013. №1(83)
3.2. TRAPPING IN PLASMA
The use of the optical trapping system is much diffi-
cult to handle and to align under plasma conditions. Due
to the low pressure (~30 Pa) resulting in a weak damping
by the surrounding gas, the particles are much more un-
stable in their position in the plasma sheath as in water.
For easier handling of the particles an additional elec-
trode with a 6 mm slit was placed on the RF electrode
with the slit perpendicular to the beam axis. This align-
ment formed a shaped electrostatic potential, which
damped the transverse movement of the particles result-
ing in a 1D-particle chain confined in the plasma sheath
along the slit. Fig. 5 shows a top view of the electrode
with the particle chain in the plasma sheath. The bright
spot in the center is a particle in this chain optically
trapped by the laser beams. This was proved by electro-
statically moving the particle chain along the slit. The
trapped particle remained at the same position while the
other particles moved and passed around the fixed one.
Fig. 5. Setup for particle trapping in plasma. An addi-
tional electrode with a slit confines the particles in a 1D-
chain in the plasma sheath. The bright spot is an optically
trapped particle. Although the trapping laser is in the
NIR the CCD sensor is sensitive at this wavelength
The next step was to change the electrode in its ver-
tical position. For this, a particle was optically trapped
as described above and the electrode was lowered.
Fig. 6 shows this principle: One MF particle out of the
chain of confined particles is picked up by the laser
tweezers (Fig. 6 left) and the RF electrode is moved
downwards, e.g. the fixed particle is moved upwards in
the sheath (Fig. 6 right). It is still confined despite the
electric field force is different at the new equilibrium
position. When the particle suddenly escapes at a certain
position the force balance is not anymore fulfilled and
the external forces at this position can be estimated by
the maximum trapping force.
Due to the force balance a trapped and charged parti-
cle can be moved against the electric field force in the
sheath to higher positions above the electrode if the laser
power increases, see Fig. 7. The displacement is almost
linearly proportional to the optical force of the trap.
Fig. 7. Relative particle displacement of a particle from
its original position in the sheath by laser manipulation
in dependence on the applied laser power
CONCLUSIONS
An optical trapping system for microparticles based on a
two beam counter-propagating principle has been designed
and build. The “laser tweezers” is proposed to be a tool for
manipulating particles in plasma and its sheath as a suitable
diagnostic tool. This method is non-invasive referring to the
plasma and its parameters and it allows a long-term particle
manipulation. Compared to other experiments, where parti-
cles have also been manipulated by lasers [13-15], in this
experiment it is possible to move particles in both directions,
e.g. into the plasma sheath or into the plasma bulk, and over
longer distances and timescales.
The functionality of the trapping system was success-
fully demonstrated in water due to better damping of the
particle motion as well as in plasma at low pressure. By
moving the electrode and the plasma, respectively, relative
to the optically held particle it was possible to move the
particle through the sheath either into the direction of the
bulk or the electrode. Preliminary measurements showed
an almost linear dependence of the relative displacement of
the microparticle on the applied laser power.
The next step will be the installation of the quadrant
sensors with an increased position detection. The meas-
ured particle position in the trapping beams will be used
to determine the trap stiffness parameter and hence,
the external force acting on the microparticle.
Fig. 6. Side view of the electrode (yellow area) and the microparticles in the plasma sheath. Left: A particle (bright
spot with a green dot in the center) is taken from the particle chain (green line) by the laser tweezers. Right: By
lowering the RF electrode the particle is transferred to another position in the plasma sheath. Note that the rest
of the particles move with the electrode
ISSN 1562-6016. ВАНТ. 2013. №1(83) 167
ACKNOWLEDGEMENTS
This work was supported by the Deutsche For-
schungsgemeinschaft DFG in the framework of the
SFB-TR24 Greifswald Kiel, Project B4. We also would
like to thank the group of Prof. Pavel Zemánek from the
ISI of ASCR in Brno, Czech Republic, for the develop-
ment and construction of the optical trapping system.
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Article received 24.12.12
ИСПОЛЬЗОВАНИE ОПТИЧЕСКИ ЗАХВАЧЕННЫХ ЧАСТИЦ ПЫЛИ
КАК МИКРО-ЗОНДОВ В ПЛАЗМЕННОМ ПРОЦЕССЕ
V. Schneider, H. Kersten
Oписывается прогресс в развитии манипуляционных частиц оптической системы, как метода плазменной
диагностики. Описываются основные принципы и предварительные экспериментальные результаты при
оптическом захвате. Для распространяющихся волн был использован лазерный луч, который улавливал
частицы в воде, а также в РФ-разряде. Эксперименты показывают возможность манипулирования частица-
ми, которые находятся в состоянии левитации в плазменной оболочке, чтобы получить информацию про
саму оболочку и плазменные параметры.
ВИКОРИСТАННЯ ОПТИЧНО ЗАХОПЛЕНИХ ЧАСТИНОК ПИЛУ
ЯК МІКРО-ЗОНДІВ У ПЛАЗМОВОМУ ПРОЦЕСІ
V. Schneider, H. Kersten
Описується прогрес у розвитку маніпуляційних частинок оптичної системи, як методу плазмової діагнос-
тики. Описані основні принципи і попередні експериментальні результати при оптичному захопленні мікро-
частинок в плазмі. Лазерний пучок, що зустрічно поширювався, був використаний для захоплення частинок
у воді, так само як і у РЧ-розряді. Експерименти показують можливість маніпулювання частинками, які
знаходяться в стані левітації у плазмовій оболонці, щоб отримати інформацію про саму оболонку і парамет-
ри плазми.
|