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Optically Nonlinear Crystals

#A-538


Synthesis, Investigation and Growth of New Nonlinear Optical Crystals

Tech Area / Field

  • MAT-SYN/Materials Synthesis and Processing/Materials
  • PHY-OPL/Optics and Lasers/Physics

Status
3 Approved without Funding

Registration date
22.06.2000

Leading Institute
Yerevan State University, Armenia, Yerevan

Supporting institutes

  • A.I. Alikhanyan National Science Laboratory, Armenia, Yerevan\nScientific Technological Center of Organic and Pharmaceutical Chemistry / Molecular Structure Research Center, Armenia, Yerevan\nYerevan State Institute of Economy, Armenia, Yerevan

Collaborators

  • University of Central Florida / Center for Research and Education in Optics and Lasers (CREOL), USA, FL, Orlando\nNew Mexico Highlands University / Department of Chemistry, USA, NM, Las Vegas\nStanford University / SLAC / Center for Materials Research, USA, CA, Stanford\nUniversity of London / King's College London, UK, London

Project summary

Nonlinear optical crystals play a key role in the development of laser technology. Lately great progress in comprehension of mechanisms of nonlinear susceptibility in crystals has been achieved. In inorganic crystals the values of nonlinear optical (NLO) coefficients are determined by anharmonicity of chemical bonds and values of refractive indices. By this reason inorganic crystals have inherent limitations regarding possible values of NLO coefficients. In organic molecular crystals besides this mechanism the other mechanism is possible, the one connected with charge-transfer from donor groups to acceptor groups through the chain of conjugated bonds that can lead to very high NLO coefficients. But this is attained by considerable decrease of crystal transmittance range, especially in blue and violet ranges of spectrum. Quadratic NLO coefficient is not however a sole important parameter characterizing a crystal. Prospects of NLO crystals are assessed by the set of properties, such as conversion efficiency, transparency range, laser damage threshold, mechanical, thermal and chemical properties as well as technological effectiveness as regards both growth and crystal processing. Organic molecular crystals with high NLO coefficients usually do not possess the rest set of the required properties. This made the investigators search for the approaches (“semiorganic”, “inorganic-organic hybrid”) that would permit to use positive features of both organic and inorganic crystals. This purpose is met to the utmost by obtaining crystals of organic salts. Crystals of organic salts owing to their ionic nature can often be grown from aqueous solutions, have better mechanical properties as compared to organic molecular crystals, wider transparency range, higher laser resistance, and at the same time, like in case of organic compounds their properties can be purposefully controlled by the wide variation of the chemical composition. As a result, crystals of organic salts can possess optimal combination of properties characterizing an NLO crystal.

Perspective of search among salts of protonated amino acids became evident after revealing properties of L-arginine phosphate (LAP), L-Arg.H3PO4·H2O. LAP has a number of important advantages in comparison with the best crystals used for second harmonic generation (SHG) of Nd:YAG laser: KDP(KH2PO4), KTP(KTiOPO4), BBO(-BaB2O4) and LBO(LiB3O5). High laser resistance, efficiency, wide transparency range and possibility to grow crystals of large sizes and high optical quality have attracted specialists’ attention from the centers of laser fusion experiments to LAP as a possible replacement for KDP which is currently being used for these purposes. Along with research and perfection of LAP properties, the work on search for new more efficient crystals from the LAP family is being carried out in various research centers. Out of molecular crystals that have lately attracted great attention the stilbene derivatives should be noted, nonlinear susceptibility of which can achieve very large values. As an example we can mention MMONS (3-methyl-4-methoxy-41-nitrostilbene). Research of the relevant pyridine and pyrimidine derivatives and salts based on them is of great interest. As a striking example may serve crystal of styrylpyridinium (stilbazolium) salt DAST (4-dimethylamino-N-methyl-4-stilbazolium tosylate) possessing the record value of NLO coefficient, but strongly absorbing in the visible range of spectrum. Analysis of the available data shows that in this direction one can get crystals with rather high NLO coefficients and wider transparency range.

The main goal of the project proposed is revealing new more effective nonlinear optical crystals and elaboration of their growth technology. The search is planned to be carried out in three directions:

1. Salts of protonated amino acids, in particular of arginine,

2. Molecular compounds and salts of styrylpyridine,

3. Molecular compounds and salts of pyrimidine derivatives.

In theYerevan State University (YSU) work on search for new NLO crystals among amino acid salts is being successfully performed. New crystals of arginine, lysine and other amino acid salts have been obtained. It has been discovered that there exists the whole class of arginine salts of 1:2 composition (Arg.2H3PO4, Arg.2HIO3, Arg.2HNO3, Arg.2HF, Arg.2HCl.H2O, Arg.2HBr.H2O and others) and 10 new crystals have been revealed some of which are of obvious interest for further study and practical application. Two other directions are also being successfully developed in Armenia. In the Chemical Laboratory of the Yerevan Physics Institute the work on synthesis of new compounds, stilbene derivatives for creation of scintillation detectors of ionizing radiation based on polystyrene has being carried out for many years. In the Organic Synthesis Laboratory of the Yerevan State Institute of Economy the research on developing new methods of heterocyclic nitrogen-containing compounds synthesis (pyrimidines and others) for pharmaceutical purposes is being carried out. Recently the joint work of YSU with these institutes has been started aimed at revealing NLO compounds and the first extremely encouraging results have been gained. We are planning to synthesize about 50 new compounds. For greater purposefulness and synthesis efficiency, as well as for better understanding the composition-structure-properties linkage, we will carry out quantum-chemical calculations of molecular hyperpolarizability of the planned compounds. The compounds obtained will be identified and characterized by different physicochemical methods, including X-ray structure analysis, their NLO properties will also be examined by powder SHG. Based on these results the compounds that need quantitative measuring of NLO properties on crystals will be selected. For this purpose crystals of the selected compounds will be grown. Growth from solutions will be the main method, but depending on crystals inpidual properties, methods of growth from melt and vapor phase will be also used. For the same crystals piezoelectric and pyroelectric characteristics will be determined. Based on single crystal quantitative characteristics determination with regard to their growth peculiarities the best crystals will be selected.The optimal conditions for their growth will be determined, the ones that will serve as basis for developing industrial technology.

The special studies on experimental examination of dependence of molecular hyperpolarizability contribution to the macroscopic susceptibility on crystal symmetry will be carried out.

Modern, at times original methods will be used. Fulfillment of the project will result in revealing new efficient crystals, and elaboration of their growth technology that is of great applied significance, will contribute to the solution of economic problems of Armenia and transition to market-based economies responsive to civil needs. Close cooperation with collaborators will promote integration of specialists from Armenia into the international scientific community and redirect weapons specialists possessing knowledge and skills related to weapons to peaceful activities. Collaborators will participate in coordination of work on crystal growth (Prof. R.S. Feigelson, Stanford University, USA), studies of NLO quantitative characteristics (Prof. E.W. Van Stryland, CREOL, University of Central Florida, USA) and connection of crystal structure with the NLO properties (Prof. J.A.S. Smith, King’s College, UK). Some stages of work will be carried out in collaborators’ laboratories using the unique techniques developed by them. A guarantee of successful realization of the project proposed is high professional level of its participants: 80% of the key personnel are Ph.D., many are well-known specialists, and some are leaders in their field of science.


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