Anisotropic Laser Center Stabilization
New Method of Creation of Active Elements of Tunable Lasers on Color Centers by Combined Action of External Fields on Ionic Crystals, and the Investigation of Their Optical-Generation Properties
Tech Area / Field
- PHY-OPL/Optics and Lasers/Physics
- PHY-SSP/Solid State Physics/Physics
3 Approved without Funding
Tbilisi State University / Institute of Physics (Ge), Georgia, Tbilisi
- Duke University / The Fitzpatrick Center, USA, NC, Durham\nUniversity of Alabama at Birmingham, USA, AL, Birmingham
Project summaryThe objective of the Project is development and practical realization of stable laser centers creation method on the basis of the effect (detected by Project participants) – stabilization of F2+-centres in nominally pure LiF crystals through combined influence of external fields (radiation, mechanical stress, electromagnetic field, etc.).
Colored alkali-halide crystals (AHC), particularly LiF with F2+-centres, are widely used for creation of tunable lasers on color centers (CC). The mentioned centers, together with the other CC, provide wide range of generation frequency tuning – from 0.7 to 4 micron and they are out of competition as organic dyes work only up to ~1 micron and the other sources with tuning in a near infrared spectral region are extremely bulky.
In spite of the unique features of laser on F2+-centers, in many cases, their generation can be suppressed by parasitic processes. They involve, first of all, spontaneous decaying of centers at room temperature (in LiF half-decay time 12 h) and, consequently, generation on the crystals with such centers, due to their low thermal stability, requires cooling down to quite low temperatures thus complicating the laser itself and working on it. Low thermal stability of F2+-centers also represents serious obstacle for application of AHC with such CC for the laser radiation control facility – passive Q-switch and apodized apertures.
The main role in increasing of F2+-centers thermal stability belongs to impurity ions of bivalent metal and/or hydroxyl decay products, which, together with generation stable electron traps and additional anionic vacancies, are stimulating appearance of various disturbing defects in the neighborhood of F2+-centers. Thus, ionizing irradiation of crystals with quite high concentration of hydroxyl ions is one of the means of obtaining stable at room temperature F2+-centers. At large doses of irradiation, achieving almost full radiolysis of OH- ions, there can be obtained significant concentration F2+-like centers stabilized by oxygen ions (F2+O2- complexes). However, there should be kept in mind that large concentration of impurity so as large doses of irradiation lead to unwanted effects, worsening basic generation characteristics of crystals and their optical stability.
Thus, basic problems in the field of creation of active laser elements on CC, first of all, are related to searching of new methods for obtaining in different matrices CC, having significant photo- and thermal stability as well as high enough optical parameters, determining their generation possibilities.
The authors of the Project, in several scientific articles, have found new method for obtaining stable F2+-CC in nominally pure LiF crystals (there are patent and scientific publications). The method is based on mechanical influence on a crystal in the process of its irradiation with small doses of γ-rays. This method does not require preliminary doping of a crystal by means of stabilizing impurity ions. Combined influence of radiation and mechanical stress (in yield stress region of sample) increases as stability of LiF crystals against powerful laser radiation, so thermal stability of active CC by more than 130oC.
Creation of active laser elements with optimal characteristics is possible through two ways. The first is improvement of features of already existing materials (this was mentioned above), the second – searching of a new laser material – of more perspective crystal system. In the form of such matrix there can be considered magnesium oxide (as in the form of a single crystal, so in ceramics). Stability of elementary and aggregate CC in MgO in comparison with speaks in favor of such choice. On the basis of the given matrix there are already being developed active laser elements with impurity centers (Ni, Co, Cr, etc.) for a near infrared spectral region. However, structural defects of irradiated crystals of magnesium oxide are not so far considered in this role. With this purpose, as the laser centers there will be investigated F-aggregate centers for creation and accumulation of which there will be used the new method (developed by the authors of the Project) for creation of stable CC. Besides, the method may turn out useful also in the case, when the laser centers are represented in MgO by impurity ions (Ni, Co, Cr). Preliminary experiments carried out by participants of the Project have shown that irradiation of MgO single crystals in stressed condition may promote to purposeful changing of optical characteristics of these media.
The new method for creation and stabilization of laser CC provides several advantages:
- There is eliminated necessity of special introduction of impurity ions in initial crystal;
- Working concentrations of stable F2+ CC are achieved at rather low doses of ionizing radiation;
- Combined influence of external fields increases optical stability of crystals.
In the result of Project there will be:
- Investigated optical and generation characteristics of a new type of F2+ CC (in LiF) and the ways of optimization of these characteristics;
- Investigated MgO crystals (with structural defects of F-aggregate type centers created by new method) as the active elements of laser radiation.
- Investigated influence of combined effect of mechanical stress and low-intensity gamma and roentgen radiation on generation characteristics of MgO single crystals with impurity ions (Ni, Cr).
- Developed the methodology of creation of stable laser CC as a result of producing in single crystals of dislocation assembly with definite configuration.
In spectral region of generation of lasers on charged color centers, particularly on F2+-like CC, there are located many absorption lines (especially, overtones and component frequencies) of many compounds polluting the atmosphere. Consequently, with the help of lasers on the color centers it is possible to excite concrete atom and molecular electron transition and, thus, use the methods of resonance scattering and differential absorption of the atmosphere remote probing, in resolving the problem of CO2 among them. Therefore, creation of corresponding lidar systems on the basis of such lasers will undoubtedly play significant role in resolving important ecological problems. Moreover, the lasers on the color centers have shown their availability for express-diagnostics of ecological state of water areas. Traditional methods do not allow execution of operative mapping and stratification of distribution of toxic substances and bio-mass on large water areas in real time. Also important is that field condition requires creation of compact portable multiple-frequency lidar which would use only solid state laser. In such cases, lasers on CC do not have alternatives in the required range of wavelengths. The lidars of such type can be installed not only stationary in the regions of ecological hazard, but they can be also located on different carriers: ships, planes, helicopters.
In the process of experiments there will be used modern methods for studying optical and generation characteristics of the investigated ionic crystals. Combined influence on the investigated crystals will consist in:
- irradiation of crystals in free and stressed state;
- influence on preliminarily radiationally colored crystals of:
- ultraviolet radiation and electric field;
- ultraviolet radiation and shock wave;
- ultraviolet radiation and mechanical stress.
The subject of the Project and the wide range of problems allow to solve simultaneously several objectives and tasks, specified by ISTC as priorities:
- The Project enables leading specialists, engaged in the sphere of military developments, to direct their knowledge and experience to peaceful purposes;
- International cooperation of participants of the Project with the leading foreign specialists including foreign collaborators will assist in their integration in the International scientific community;
- The investigations being offered in the limits of the Project are directed to development of simpler and more reliable method of creation and stabilization of laser CC in crystals and improvement of their generation characteristics. Industrial implementation of the investigation results will allow to provide national ecological services with effective means of analysis of the medium status.
The Project will be executed by the scientists and specialists of the E. Andronikashvili Institute of Physics of the Georgian Academy of Sciences. In the works of the Project active participation will execute also the leading scientists of former Tbilisi branch “Luch” SRI of Aviation Systems (further SRIAS "Skhivi"), specializing earlier in the sphere of missile technologies (equipment for guidance and management of missiles, including optical and television systems of guidance, aiming and targeting systems of homing guidance), significant part of which are on unpaid leaves of long duration.
The International Science and Technology Center (ISTC) is an intergovernmental organization connecting scientists from Kazakhstan, Armenia, Tajikistan, Kyrgyzstan, and Georgia with their peers and research organizations in the EU, Japan, Republic of Korea, Norway and the United States.
ISTC facilitates international science projects and assists the global scientific and business community to source and engage with CIS and Georgian institutes that develop or possess an excellence of scientific know-how.