Microwave discharges at the surfaces of dielectrics

 

 

Microwave discharges at the surfaces of dielectrics

 

Abstract

Electrodeless pulsed microwave discharges occurring at the surface of insulating crystals in the vacuum are being studied both experimentally and theoretically. Investigations of nonlinear physical phenomena developing in the near-surface layer of insulating crystals (excited by pulsed microwave discharges) are in progress. A classification of different types of electrodeless pulsed microwave discharges is under development.

Of special interest are those physical processes in the near-surface layer of insulating crystals which bring about the accumulation of high concentrations of radiation-induced point defects, the arising of luminescence of induced color centers, the appearance of induced electrical conductivity, the stimulation of strong absorbtion of microwave power, the occurring of both a contracted discharge and an electrodeless breakdown of crystals in a field of microwaves.

Characteristics of planar optical waveguides created at the surface of insulating crystals (previously colored by microwave discharges) will be under investigation both experimentally and theoretically. On the basis of the created waveguides a number of new devices important for integrated optics and fiber optics will be developed. The most important devices are miniature lasers and optical amplifiers which will be frequency-tuned in the near infrared spectral range.

 

Basic and applied scientific problems at a solution which the project is aimed. Urgency and priority of studies.

  1. Study of correlations between the secondary-electron emission and electrical, energetic, and optical rocesses developing in the near-surface layer of insulating crystals excited by electrodeless pulsed microwave discharges.
  2. Study of mechanisms of physical processes initiating pulsed microwave breakdown of insulating crystals.
  3. Classification of different types of electrodeless pulsed microwave discharges developing at the surface of insulating crystals.
  4. Creation of integrated-optics and fiber-optics miniature lasers and optical amplifiers (frequency-tuned in the near infrared spectral range) at the surface of insulating crystals containing color centers induced by pulsed microwave discharges.
  5. The problems 1, 2 and 3 are very important not only for plasma physics of gas discharges but also for plasma physics of solids, for secondary-electron-emission electronics of dielectrics, for optics and spectroscopy of crystals, for physics of interaction of radiation with solids and etc. The results obtained by us are beyond the scope of above-mentioned traditional areas of physics. New relationships between the areas are discovered, these relationships appear only under high-intensive excitation of crystals by microwave discharges.

The obtained results conform to world scientific standards and have a priority nature. These results are published in prestige scientific journals (Sov. Phys.- J. Tech. Phys. Letters, Sov. Phys. - J. Exp. Teor. Phys. (JETP) Letters, Physica Status Solidi) as well as in proceedings of large international conferences (ICPIG XXI; ICPIG XXII; Strong Microwaves in Plasmas 1990, 1996). 

 

Main scientific topics within the framework of formulated problems. 

Basic studies:

  1. Study of the processes of excitation and maintenance of different types of electrodeless microwave discharges at the surface of insulating crystals in a field of pulsed microwave radiation;
  2. Study of mechanisms and kinetics of formation and decay processes of short-lived electronic excitations (generated by pulsed microwave discharges in the near-surface layer of insulating crystals), which bring about the accumulation of high concentrations of induced defects, the arising of luminescence of induced color centers, the appearance of induced electrical conductivity, the stimulation of strong absorbtion of microwave power, the occurring of both a contracted discharge and an electrodeless breakdown of crystals in a field of microwave radiation;
  3. Study of the secondary-electron-emission, electrical, absorptional and optical processes developing in the near-surface layer of insulating crystals excited by pulsed microwave discharges;
  4. Formulation of qualitative and quantitative criteria for the characterization of different types of electrodeless pulsed microwave discharges developing at the surface of insulating crystals;
  5. Determination of conditions for transformation of one type of electrodeless pulsed microwave discharges to other;
  6. Classification of different types of electrodeless pulsed microwave discharges (developing on the surface of insulating crystals) on the basis of formulated criteria and studied properties of microwave discharges.

Applied studies:

Study of microwave discharges at the surface of insulating crystals and other materials from the point of view of raising an electric strength of output windows of microwave devices and waveguides during powerful microwave radiation propagates through the windows;
Creation and study of planar optical waveguides on the surface of insulating crystals (previously colored by microwave discharges);
Creation of miniature integrated-optics and fiber-optics lasers and optical amplifiers (frequency-tuned in the near infrared spectral range) at the surface of insulating crystals containing color centers induced by pulsed microwave discharges;
It is possible also to perform the investigations on the interaction of microwaves with solids for solving the urgent problems of processing and modification of insulators and semiconductors:

- creation of sensitive optical storage media at the surface of insulating crystals previously colored by  microwave discharges,
- coloration of jewels by use of microwave discharges,
- sputtering of different insulating targets by use of microwave discharges,
- deposition of insulating films on the surface of different materials by use of microwave discharges,
- sintering of different powdered mixtures for making the new types of ceramics with use a microwave radiation,
- amorphization and modification of near-surface layer of semiconductors and metals with use microwave  discharges.

 

Investigations of the following phenomena taking place in plasma-flare microwave discharges (developing at the surface of insulators) were performed:
- excitation of langmuir waves in a plasma resonance region,
- acceleration of electrons due to a self-breaking of strong langmuir waves,
- acceleration of ions due to a potential jump in a plasma resonance region,
- transformation of microwave power to the power of quasistationary electric current in the plasma.
A new type of breakdown was observed at the surface of solids - microwave breakdown of insulating crystals initiated by a secondary-electron-emission microwave discharge. The conditions of excitation of the secondary-electron-emission discharge and the mechanisms for transformation of the one to the plasma-flare microwave discharge were determined
It was established that microwave breakdown of insulating crystals is characterized by threshold values of both power and duration of single pulses of microwave radiation.
It was found that microwave breakdown of insulating crystals is accompanied by a pronounced absorbtion of microwave energy, an intense flash of light, and a formation of contracted discharge and breakdown channel at the surface of crystals.
The velocity of formation of the contracted discharge was determined. Evaluations of values of both an electrical conductivity and an electron density in the breakdown channel at the surface of crystals were performed.
It is shown that the further evolution of the contracted discharge is accompanied by evaporation, dissociation, ionization, and explosive fly to bits of the material in the breakdown channel. In such a way the plasma-flare microwave discharge is formed.
It was found that the characteristic duration of luminescence growth/decay (for LiF, NaCl, KCl, CsJ crystals excited by pulsed microwave discharges at room temperature) is about 1-2 microseconds. 
Optical-absorption spectra of LiF crystals previously colored by microwave discharges were measured. The induced F, F2, F3-, F3, F3+, N centers stable at room temperature were observed.
It is shown that a density of induced aggregate F2 and F3+ color centers is about a density of induced F centers. This fact testifies a high-intensive excitation of LiF crystals by microwave discharges. 
Luminescence spectra of LiF crystals (previously colored by microwave discharges) excited by laser were obtained. Optical characteristics of F2 and F3+ centers in LiF crystals were measured. Similarity of investigated here color centers with those produced in gamma-irradiated LiF crystals was obtained.
It was measured (at room temperature) a luminescence spectrum of short-lived F2 and F3+ color centers temporaly induced in uncolored LiF crystals by pulsed microwave discharges at the pre-breakdown stage of evolution. The spectrum is very close to the one that was measured for LiF crystals (previously colored by microwave discharges and containing F2 è F3+ centers) excited by laser.
The explanations of origin and further evolution of pulsed microwave discharges were done on the basis of the idea of high-intensive excitation of electronic subsystem of solids. A relaxation of the excitation in the near-surface layer of insulating crystals leads to a creation of short-lived color centers with high density. A recombination of induced color centers brings about the microwave breakdown of the crystals.
The following original techniques for investigation of processes developing in the crystals and in the plasma-flare microwave discharges have been proposed and applied in experiments:
- the photoelasticity technique for studying of strong acoustic waves in the bulk of crystals excited by pulsed microwave discharges;
- the techniques for resonance-laser-absorbtion and resonance-laser-fluorescence measurements of probe laser beams for studying of movement of Na (or Cs) atoms emitted from the surface of NaCl (or CsJ) crystals as a result of microwave breakdown.
These techniques as applied to study of pulsed microwave discharges developing at the surface of insulating crystals are unique.
Methods for selective formation of induced color centers (stable at room temperature) in the near-surface layer of insulating crystals, excited by microwave discharges, were developed.
Methods for creation of an optically-dense layer on the surface of insulating crystals during coloring by microwave discharges were developed.
Processes of excitation and maintenance of electrodeless microwave discharges on a surface of dielectric crystals were studied. In the process, radiation defects (color centers) are created in the near surface layers of the crystals. Kinetics of accumulation and relaxation of metastable and stable defects in the crystals, excited by microwave discharges, was studied. It was shown, that a high density of defects in the crystals considerably changes the characteristics of microwave discharges, developing on the surface of dielectric crystals. For example, when compared to uncolored crystals, the excitation of microwave discharges on the surface of colored lithium fluoride (LiF) crystals (containing induced stable F2 è F3+ color centers with high density ~1x(1020-1021) ñì-3) is accompanied by considerable decrease of electron current from the discharge and luminescence intensity of induced F2 è F3+ color centers. It was found that under this conditions the luminescence kinetics of LiF crystals (excited by microwave discharges) changes substantially: in the bands of light emission of induced F2 è F3+ color centers (in the wavelength regions 670±30 nm and 540±30 NM) there appear, together with slow components of luminescence ~ 1 microsecond (typical for uncolored crystals), fast components (typical for colored crystals) with characteristic intensity rise and fall times ~ 0.1 microsecond.
The physical mechanisms and dynamics of formation of induced electrical conductivity in the near-surface layer of dielectric crystals, excited by electrodeless microwave discharges, were studied. It was shown that microsecond microwave discharge at the pre-breakdown stage of development gives rise to a high-intensive excitation of the surface layer of the crystals. It was found that excitation of the surface layer of LiF crystals by low-energy electrons from a microwave discharge with a characteristic electron energy of 1 keV, electron density 1x1010 cm-3, and microwave discharge pulse duration 1 microsecond is characterized by a specific input energy density ~ 500 J/cm3. In the process, the excitation is accompanied by creation of short-lived aggregate F2 è F3+ color centers with high density ~1x(1020-1021) ñì-3 in the surface layer of LiF crystals. These values are several orders of magnitude higher than the corresponding ones when LiF crystals are excited with high-current pulsed electron beams, x rays or gamma rays. It is proposed the physical model of transformation of secondary-electron-emission microwave discharge to plasma-flare microwave discharge as a result of formation of contracted discharge and arising electrodeless microwave breakdown of dielectric crystals.

 

References.

[1] G.M.Batanov, V.A.Ivanov, I.A.Kossyi, K.F.Sergeichev. Large-Amplitude Plasma Waves and Particle Acceleration  in the Plasma Corona of a Microwave Discharge. Sov. Phys. - J. Plasma Physics, 1986. Vol. 12. No. 5. Pp. 317-325.

[2] G.M.Batanov, V.A.Ivanov. Plasma-Flare Conversion of the Energy of Microwaves in the Decimeter Band into the Energy of Quasi-Stationary Electric Current . /In: "Generation of Nonlinear Waves and Quasistationary Currents in Plasma", Ed. by L.M.Kovrizhnykh. New-York: Nova Science Publishers, Inc., 1992. 227 p.(Proc. of the Institute of General Physics, Academy of Sciences of the USSR. Moscow: Nauka, 1988. Vol. 16. Pp. 46-79 [in Russian]).

[3] G.M.Batanov, V.A.Ivanov, M.E.Konyzhev et al. Generation of High Potentials in the Plasma by the Interaction with Intense Microwave Radiation. /Proc. Of the International Workshop on Strong Microwaves in Plasmas. Suzdal (USSR), September 17-22, 1990.

[4] V.A.Ivanov, M.E.Konyzhev et al. Generation of High Potentials and Fast Electron Diagnostic in Microwave Produced Plasma Flare. /Proc. Of the XX-th International Conference on Phenomena in Ionized Gases. Contributed 
Papers. Vol. 5. Pp. 1091-1092. Pisa (Italy), July 8-12, 1991.

[5] G.M.Batanov, V.A.Ivanov, M.E.Konyzhev, V.A.Konyushkin, and S.B.Mirov. Coloration of LiF Single-Crystals by Surface Microwave Discharges. /Proc. Of the International School of Plasma Physics (ISPP-13 "Piero Caldirola"). 
Industrial Application of Plasma Physics, Ed. By G.Bonizzoni, W.Hooke and E.Sindoni. SIF, Bologna 1993. Pp. 521-525.

[6] G.M.Batanov, V.A.Ivanov, M.E.Konyzhev, V.A.Konyushkin, and S.B.Mirov. Microwave Discharge Method for Formation of Optically-Dense Submicron- Thickness Layers with High Concentrations of Color Centers on the Surfaces of Alkali-Halide Crystals /Proceedings I. XXI International Conference on Phenomena in Ionised Gases. Pp. 37-38. Sept. 19-24, 1993. Rurh-University, Bochum, Germany.

[7] G.M.Batanov, V.A.Ivanov, M.E.Konyzhev, V.A.Konyushkin, and S.B.Mirov. Creation of an Optically Dense Layer on the Surface of a Lithium Fluoride Crystal During Coloring in a Microwave Discharge. Sov. Phys. - J. Tech. Phys. Lett., 1993. Vol. 19. No. 11. Pp. 653-654.

[8] G.M.Batanov, V.A.Ivanov, M.E.Konyzhev. Microwave Breakdown of Ionic Crystals Initiated by a  Secondary-Emission Discharge. Sov. Phys. - J. Exper. Theor. Phys. Lett. (JETP Lett.), 1994. Vol. 59. No. 10. Pp. 
690-694.

[9] G.M.Batanov, V.A.Ivanov, M.E.Konyzhev. Microwave Breakdown on the Surface of Ionic Crystals in Vacuum.  XXII Conference on Phenomena in Ionized Gases. Vol.IV. /Editors: K.H.Becker, W.E.Carr, E.E.Kunhardt. Hoboken, New Jersey, USA, 1995. Pp. 143-144.

[10] G.M.Batanov, V.A.Ivanov, M.E.Konyzhev. Luminescence of short-lived color centers in LiF crystals excited by secondary-electron emission microwave discharge. /In book: Strong Microwaves in Plasmas. Vol.1. Ed. By A.G.Litvak. Nizhny Novgorod: Institute of Applied Physics, 1997. Pp. 401-406.

[11] V.V.Ter-Mikirtychev, T.Tsuboi, M.E.Konyzhev, V.P.Danilov. Spectroscopic characteristics of color centers  produced in a LiF crystal surface layer by microwave discharge. Phys. Stat. Solidi (b), 1996. Vol. 196. No. 1. Pp.  269-274.

[12] G.M.Batanov, V.A.Ivanov, M.E.Konyzhev, A.A.Letunov. Luminescence of short-lived color centers induced in LiF crystals by a pulsed microwave discharge. Sov. Phys. - J. Exper. Theor. Phys. Lett. (JETP Lett.), 1997. Vol. 66. No 3. Pp. 170-174.

 

Available installation and scientific equipment.

1. The experimental installation consists of the following main components: 

pulsed magnetron (microwave power is up to 5 ÌW, oscillation frequency of microwaves is 2 GHz, pulse duration ranges from 1 to 50 ms), 
waveguide for delivery of microwave power from the magnetron to the vacuum chamber, 
titanium pumps for pumping down the chamber to a high vacuum, 
high-voltage rectifier unit, 
pulsed modulator unit, 
power sources.

2. Scientific equipment consists of the following main components: 

directional waveguide couplers for measurements of power of the incident to the discharge and reflected from the discharge pulsed microwave radiation,
optical techniques for measurements of spectral and kinetic characteristics of luminescence and optical-absorbtion of insulating crystals excited by pulsed microwave discharges, 
optical techniques for measurements of both luminescence and optical-absorbtion spectra of insulating crystals previously colored by pulsed microwave discharges, 
multigrid electrostatic analyzers of energy of charge particles for measurements of currents of electrons and ions from the region of microwave discharges, photoelasticity technique for studying of strong acoustic waves in the bulk of transparent insulating crystals excited by pulsed microwave discharges; 
optical techniques for resonance-laser-absorbtion and resonance-laser-fluorescence measurements of probe laser beams for studying of movement of NA (or Cs) atoms emitted from the surface of NaCl (or CsJ) crystals as a result of microwave breakdown.
integrated-optics technique for study of characteristics of planar optical waveguides created at the surface of insulating crystals previously colored by microwave discharges.
  1. Study of electron current ejected from the region of microwave discharges (developing at the surface of LiF crystals) at the different stages of discharge evolution.Study of dynamical and spectral characteristics of luminescence of LiF crystals excited by pulsed microwave discharges at different stages of evolution.
  2. Study of dynamical and spectral characteristics of optical absorbtion of LiF crystals excited by pulsed microwave discharges at different stages of evolution.
  3. Determination of characteristic values of life time and density of short-living color centers generated in the near-surface layer of LiF crystals excited by pulsed microwave discharges at different stages of evolution.
  4. Study of a relationship between the density of indused color centers, stable at room temperature, and the evolution of microwave discharges at the surface of LiF crystals.
  5. Study of an evolution of a density of Cs atoms emitted from the surface of CsJ crystals exited by microwave discharges.
  6. Study with use an electron microscope technique of erosion areas produced at the surface of LiF crystals as a result of interaction of pulsed microwave discharges with the crystals.
  7. Study of characteristics of planar optical waveguides created at the surface of LiF crystals (previously colored by microwave discharges) with use of integrated-optics and fiber-optics techniques.
  8. Creation of integrated-optics and fiber-optics miniature lasers and optical amplifiers (frequency-tuned in the near infrared spectral range) at the surface of LiF crystals containing color centers induced by pulsed microwave discharges.

If required, not only LiF crystals but also another insulating crystals will be used for studies.

 


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