Plasmas containing chemically active
species meet several applications, e.g. the oxygen atom containing
plasmas can be used for plasma based sterilization (bacteria
inactivation and removal of biological contaminations from surfaces),
as well as in synthesis of metal-oxide nanowires, oxide films
deposition, functionalization of polymers, removal of organic
impurities, selective etching of composites, wool treatment and surface
activation; while the plasmas reach in CH radicals are applicable for
biocompatible thin film deposition. In the application processes
the synergetic effect between different species, such as chemically
active species, ions and/or UV radiation can be observed. Thus each
application requires different group of active species, which can be
obtained by using different gas mixtures.
In case of reactive discharges, the
reactive species created in the discharge can interact with the
electrodes placed in the gas volume, thus making the operation of the
discharge unstable. Therefore the electrodeless discharges constitute
one of the most stable plasma sources. The new generation of
electrodeless plasmas generated with high frequency electromagnetic
waves, are those sustained with electromagnetic surface waves. With a
non-ionizing surface wave, that propagates along the interface between
the plasma column and its surrounding dielectric tube inhomogeneous
large volume plasmas or long plasma columns can be sustained. The long
plasma column discharges generated with surface waves are characterized
by an unusual flexibility in operating conditions, depending on the tube
radius (from ≈0.5 mm to 150 mm) discharges can be produced at gas
pressures as low as some 10−5Torr up to a few times
atmospheric pressure. Several applications – due to different reasons,
such as too high gas temperature in the active discharge zone, or the
presence of high energy ions that can modify the surfaces –apply the
flowing afterglow intead of the active discharge region. If the
discharge is generated in flowing gas the active species created in the
plasma can be transported by the gas flow to a remote reactor, which
can have considerably larger diameters as the discharge tube, e.g.
60×30×28 cm3 comparing to the 6 mm diameter 10 cm long tube.
At low pressures depending on the application either the smaller volume
early-afterglow (that develops downstream the discharge in the same
tube) or the larger volume post-discharge (late-afterglow) can be used,
due to their different composition. At atmospheric pressure the plasma
jet achieved downstream the discharge can be applied.
Postdischarge experimental setup
Since every application calls for
different species or group of species, each application requires
different plasma composition. The detailed description of the
discharges and their afterglows under different discharge conditions
gives the possibility of choosing the optimal discharge system for the
given application. The knowledge of the species densities along the
system and their distribution in the large volume reactor can help in
the understanding of the role of different species and reactions in the
application process. During the last years we have investigated several
discharge systems applicable for plasma based sterilization, surface
treatment and nanostructuring, by determining the species densities
distributions along the system. These applications generally make use
of the N and O-atoms, Ar+ ions and UV photons (originating from the Ar resonant state atoms and excited NO(A) and NO(B) molecules) present in plasma.
1. C. D. Pintassilgo, K. Kutasi, J. Loureiro:
Modelling of a low pressure N2-O2 discharge and post-discharge
reactor for plasma sterilization, Plasma Sources Sci. and Technol 16
S115 (2007) [2.120]
2. K. Kutasi and J. Loureiro: Role of the wall
reactor material on the species density distributions in an N2-O2
post-discharge for plasma sterilization, J.Phys.D : Appl. Phys. 40 5612
(2007) [2.2]
3. K. Kutasi, B. Saoudi, C. D. Pintassilgo, J.
Loureiro, M. Moisan: “Modelling the low-pressure N2-O2 plasma afterglow
to determine the kinetic mechanisms controlling the UV emission
intensity and its spatial distribution for achieving an efficient
sterilization process”, Plasma Processes and Polymers 5 840 (2008)
[2.921]
4. K. Kutasi, C. D. Pintassilgo, J. Loureiro:” An
overview of modelling of low-pressure post-discharge systems used for
plasma sterilization”, 2nd Int. Workshop on Non-equilibrium Processes
in Plasmas and Environmental Science, Journal of Physics: Conference
Series 162 (2009) 012008
5. V. Guerra, K. Kutasi, P. A. Sá “O2(a1 ∆g )
production in flowing Ar-O2 surface-wave microwave discharges: possible
use for oxygen-iodine laser excitation” Applied Physics Letters 96
071503 (2010) [3.554]
6. K. Kutasi, V. Guerra, P Sá “Theoretical insight
into Ar-O2 surface-wave microwave discharges” J. Phys. D: Appl. Phys.
43 175201 (2010) [2.083]
7. K. Kutasi, V. Guerra, P Sá “Active species
downstream an Ar-O2 surface-wave microwave discharge for biomedicine,
surface treatment and nanostructuring” Plasma Sources Sci. Technol 20
(2011) [2.384]
8. K. Kutasi “Composition of a plasma generated from
N2 -O2 by an Ar ion jet in a low pressure reactor” J. Phys. D: Appl.
Phys. 43 055201 (2010) [2.083]
9. K. Kutasi “Modelling of NO destruction in a low
pressure reactor by an Ar plasma jet: species abundances in the
reactor” J. Phys. D: Appl. Phys 44 105202 (2011) [2.083]
Multispectral "Expanded Beam" ellipsometry
Ellipsometry - one of the most
powerful optical inspection tool of semiconductor industry - determines
angle of incidence dependent polarization data of light reflection, so
generally it works by parallel light beams with a well defined angle of
incidence, therefore the contemporary ellipsomeretes contains laser
light sources.
A fundamentally different new method from the usual ellipsometric
techniques was developed in our laboratory in cooperation with the
Research Institute for Technical Physics (MFA).
The target is illuminated by non-collimated, almost diffuse, “expanded
beam” light in our case, providing a lot of beams with diverse angles
of incidence at every point of the sample. The precise
“angle-selection” is made at the detector side, by a pinhole camera.
The pinhole, as an “angle-filter,” selects only one single light beam
from every sample point directions, and it works as an unusual kind of
imaging ellipsometry.
Adding multicolour supplements, it provides spectral (a few laser or
LED wavelengths on a 2D image, or a full spectrum along a line)
information from rapid measurements of many points on a large (several
dm2) area.
Actually our precision is suitable for detecting sub-nanometer
thickness and a refractive index change of 0.01. The increased with
orders of magnitude speed of the measuring system makes it suitable for
using even on production lines. We are working now on the realization
of this „on-line” system.
Imaging multispectral ellipsometer.
1. HORVÁTH, Zoltán György; JUHÁSZ, György; FRIED, Miklós; MAJOR, Csaba;
PETRIK, Péter: Imaging optical device with a pinhole camera Pub. No.:
WO/2008/142468; International Application No.: PCT/
HU2008/000058; Publication Date: 27.11.2008; Priority Data: P
0700366 23.05.2007 HU.
2. C. Major, G. Juhasz, Z. Horvath, O. Polgar, M. Fried: Wide angle
beam
ellipsometry for extremely large samples, Phys. Stat. Sol. C 5
1077-1080 (2008)
3. G. Juhász, Z. Horváth, C. Major, P. Petrik, O. Polgar, M. Fried,
Non-collimated beam ellipsometry Phys. Stat. Sol. C 5 1081-1084.
(2008)
4. C. Major, G. Juhasz, P. Petrik, Z. Horvath, O. Polgar, M. Fried:
Application of wide angle beam spectroscopic ellipsometry for quality
control in solar cell productionVacuum 84, 119. (2009)
5. M. Fried, G. Juhász, C. Major, P. Petrik, O. Polgár, Z. Horváth, A.
Nutsch: Expanded beam (macro-imaging) ellipsometry Thin Film
Solids, 519, 2730-2736 (2011)
Theory of
nonlinear processes in strong laser fields
■ Interaction of strong electromagnetic fields with matter.
When one or more components of the
electromagnetic fields (these may mean a laser field or other, static
or slowly-varying fields, like a microwave or terahertz radiation), are
strongly interacting with matter, then the response may be highly
non-linear [1, 2]. This means that the response (like the induced
atomic polarization, photoelectric current, or the strength of the
outgoing radiation) is a non-linear function of the incoming laser
intensity and phase, thus simple proportionality laws are not
satisfied. In case of oscillating (alternating) fields this leads to
the appearance of high-order harmonics or multiple side-bands in the
spectrum of the signals, with respect to the fundamental frequency of
the excitation. For the theoretical description of such processed
studied by us, even the high-order perturbation theory loses its
validity (moreover, becomes in fact meaningless), and effective
non-perturbative methods are needed. We have worked out several such
theoretical approaches (which are in fact exact with respect to the
strong laser field, even at relativistic intensities), and applied
them, for instance, to describe high-intensity Compton and Thomson
scattering, induced multiphoton Bremsstrahlung [3, 4, 5], nonlinear
photoeffect, plasmon- (or other concentrated near-field)-induced
electron emission and high-harmonic generation [ 1, 2, 6, 7, 8]. One of
the interesting conclusions of our earlier investigations was that the
broad spectra can be manipulated to a large extent, if the excitation
is bichromatic, having a rational frequency ratio of the components. By
changing the relative phase of these components, a coherent control [9]
of the signal characteristics can be managed. By means of similar
theoretical methods, we have been able to keep track of
carrier-envelope phase difference effects in the nonlinear signals
induced by ultrashort laser pulses [1, 2], and the phase relations
playing a key role in generation of attosecond light pulses and
attosecond electron pulses [10]. In our recent theoretical research,
perhaps the most promising unique element (in comparison with standard
non-perturbative semiclassical methods which have been widely used so
far) is that we bring in the methods of genuine quantum optics, and go
beyond the (non-perturbative) semiclassical description, by considering
the strong radiation field as quantized. On the basis of this approach,
we have been already able to show that quantum entanglement [11, 12]
may play a considerable role in determining higher-order correlations
in the extreme signals. Besides, our earlier and recent results on
(gauge-invariant and relativistic) Wigner functions [13, 14] may also
be applied in a natural way in the theoretical investigation of the
above-mentioned strong-field phenomena.
■ Fundamental questions of wave-particle duality.
In the course of investigating
extreme light and electron signals one often encounters several
fundamental questions which have a close connection with the dual
nature of light or massive elementary particles. As an excellent
illustration for that, the so-called optical tunneling effect offers
itself, which shows up in the nonlinear photoeffect at solid surfaces
or in ionization of atoms. At relatively moderate intensities the
electron spectrum consists of discrete, equally spaced peaks, and this
is naturally explained by the absorptions of photons (i.e. absorption
of an integer number of elementary excitations). On the other hand, in
the tunneling regime, the appearance of the current can be adequately
be interpreted as a result of “breaking down the binding potential
barrier by the electric field of the laser”, in such a way that the
electron escapes by tunneling through the barrier within one optical
cycle. It is clear that in this case we encounter with the “two faces”
of both the electron and of the light. From the laser light discrete
quanta are absorbed, but, on the other hand, it also behaves like a
wave having a continuous amplitude. Similarly, the electron (according
to its wave character) “flows through” the potential barrier, which is
not penetrable for an associated classical particle, under the same
circumstances. Besides of all that, the outgoing current produces shot
noise stemming from the granular nature of the distribution of the
elementary charges. Thus, in investigating such phenomena one
necessarily encounters with the problem of wave-particle duality and
several still unsolved interpretational questions. Such questions
appear very sharply, in particular, in the context of the quantal phase
relations and coherence properties of the extreme signals under
discussion. We have realized that the historical analysis of the birth
of quantum physics (e.g. in connection of the black-body radiation and
the discovery of Planck’s constant [15]) really gives us some useful
hints for correctly answer these kind of questions. Our results
concerning the physical understanding and the mathematical description
of the problem of „unlimited divisibily” of the random energy of
radiation [16] may perhaps be useful in physical information science.
Along the lines of these investigations, we have also been searching
for possible interrelations („passages”) between quantum coherence
theory (the method of quantized amplitudes) and classical probability
theory, and arrived at quite satifactory results concerning Hanbury and
Twiss type correlations for both bosons and fermions [17]. In the frame
of the generally accepted formalism based on Hilbert spaces, we have
studied the question of quantum phase uncertainties (correct
definitions of the phase and angle operators) of ultrashort light
pulses [1, 10, 11]. We are convinced that this aspect of the ongoing
research, concerning in particular the synchronization of extreme light
signals for attosecond pulse generation, may be of practical importance
in the near future.
1. Varró S; Intensity effects and carrier-envelope phase difference
effects in nonlinear laser-matter interactions. in F. J. Duarte
(Editor); Laser Pulse Phenomena and Applications, Chapter 12; pp.
243-266 (InTech, Rijeka, 2010)
2. Varró S; Linear and nonlinear absolute phase effects in interactions
of ulrashort laser pulses with a metal nano-layer or with a thin plasma
layer.
Laser and Particle Beams; 25, 379-390 (2007); arXiv: physics/0610266
[plasm-ph]
3. Bergou J and Varró S; Nonlinear scattering processes in the presence
of a quantised radiation field: I. Non-relativistic treatment; II.
Relativistic treatment.
Journal of Physics A: Math. Gen.; 14, 1469-1482; 2305-2315 (1981)
4. Varró S and Ehlotzky F; Generalized coherent states for electrons in
external fields and application to potential scattering.
Physical Review A; 36, 497-509 (1987)
5. Varró S and Ehlotzky F; Thomson scattering in strong external fields.
Zetschrift für Physik D; 22, 619-628 (1992)
6. Varró S and Ehlotzky F; High-order multiphoton ionization at metal
surfaces by laser fields of moderate power.
Physical Review A; 57, 663-666 (1998)
7. Varró S and Ehlotzky F; Higher harmonic generation at metal
surfaces by powerful femtosecond laser pulses.
Physical Review A; 54, 3245-3249 (1996)
8. Varró S, Kroó N, Farkas Gy and Dombi P; Spontaneous emission of
radiation by metallic electrons in the presence of
electromagnetic fields of surface plasmon oscillations.
Journal of Modern Optics; 57, 80-90 (2010); arXiv: 0903.0074
[physics.optics]
9. Varró S and Ehlotzky F; Potential scattering of electrons in a
bichromatic laser field of frequencies w and 2w or w and 3w.
Optics Communications 99,177-184 (1993)
10. Varró S and Farkas Gy; Attosecond electron pulses from interference
of above-threshold de Broglie waves.
Laser and Particle Beams; 26, 9-19 (2008); arXiv: 0705.0212v1
[physics.plasm-ph]
11. Varró S : Entangled photon-electron states and the number-phase
minimum uncertainty states of the photon field.
New Journal of Physics; 10, 053028 (35pp) (2008); arXiv:
0712.3849v1 [quant-ph]
12. Varró S; Entangled states and entropy remnants of a photon-electron
system.
Physica Scripta; T140, 014038/1-8 (2010); arXiv: 0712.3849
[quant-ph]
13. Varró S and Javanainen J; Gauge-invariant relativistic Wigner
functions.
Journal of Optics B: Quantum and Semiclassical Optics; 5,
S402-S406 (2003)
14. Dahl J P, Varró S, Wolf A, Schleich W P; Wigner functions of
s-waves.
Physical Review; A 75, 052107 (21 pages) (2007)
15. Varró S; Einstein’s fluctuation formula. A historical overview.
Fluctuation and Noise Letters; 6, R11-R46 (2006); arXiv:
quant-ph/0611023
16. Varró S; Irreducible decomposition of Gaussian distributions and
the spectrum of black-body radiation.
Physica Scripta; 75, 160-169 (2007); arXiv: quant-ph/0610184
17.Varró S; The role of self-coherence in correlations of bosons and
fermions in linear counting experiments. Notes on the wave-particle
duality;
Fortschritte der Physik; 59, 296-324 (2011); arXiv: 1004.2975v2
[quant-ph]
