More about the network
The Network started its activities in crystal growth and research according to the plan accepted on the launching meeting in Lyon, July 1994. A small workshop was organised on 12-13 September in Osnabrück with participants from Kiev, Jerevan/Ashtarak, Budapest and about 30 local specialists to discuss the situation in the growth and investigation of stoichiometric LiNbO3. A fully international workshop with about 40 participants from 14 institutions and 7 countries, including half of the Coordination Committee of the Network, was held on 27-31 March in Lisbon on "LiNbO3 and related oxide crystals; nuclear physics applications". This meeting also made a critical selection of measurement methods suitable for standard characterisation of the composition and defect structure of lithium niobate which is crucial for further progress in non-linear and integrated optics applications. The recommendations of the meeting will be published in the near future.
Up to now 18 research visits have been supported by the Network. Some of the discussion of persons involved or interested in the Network occurred during international conferences
Progress in the Network was based until now on high quality cry
- In the case of LiNbO3 stoichiometric or nearly stoichiometric samples have been produced (I) in Budapest supplying Osnabrück, Lisbon, Lyon/Metz, Parma, Kiev, Kent, Giessen and Sussex; (ii) in Ashtarak, supplying Osnabrück, Giessen and Metz; (iii) in Madrid supplying also Sussex and Lisbon, and (iv) in Moscow also supplying Osnabrück.
- various bismuth oxides were produced (I) in Madrid supplying also Sussex, and (ii) in Budapest supplying also Parma, Madrid and Sussex
- BaTiO3 has been grown in Osnabrück, and in Dijon supplying Metz.
Some crystals, partly of high priority, have been grown only in Budapest:
- borates for Parma, Lyon/Metz and Giessen,
- bismuth tellurite for Parma, Storrs, Oklahoma, Mexico and Metz,
- ZnWO4 samples for Storrs and Giessen and TeO2 for Storrrs.
- Comparison, improvement and standardisation of procedures is one of the main concerns of the Network.
Out of the results of numerous physical investigations on these and closely related crystals up to now more than 50 papers have been prepared for publication. Outstanding results include the remarkable changes in bulk properties and/or dopant incorporation found for LiNbO3 as a result of adding potassium or some other defect-healing ions to the melt, and also the results obtained in implanted waveguiding specimens.
News/current status
Staggering advances have been made in the production of crystals during the last 50 years leading, for example, in the field of semiconductors from simple diodes to highly specialised, miniaturised integrated circuits resulting in the computer revolution. However, solid state technology has great potential for further advances if light, instead of electrical signals, is used for data processing. Optical data transfer through transparent silicon oxide fibres is already much more efficient than electronic data transfer through conventional cables. The other stages of optical data processing have not yet reached this level of industrial maturity, although the required materials, mostly oxide crystals, are already under development.
These crystals have the potential to switch and filter light and to code the message by modulating the properties of the optical signal such as intensity. Some of them are also capable of storing and handling incoming optical information in the form of holograms and of producing an output in the form of optical, electrical or even acoustical signals. For example, simple associative thinking processes have already been modelled by holographic processes in oxide crystals with photorefractive capabilities.
The basic problem is to find crystals with the required properties and produce them in a reliable and reproducible way. As the example of the semiconductor gallium arsenide has shown, this is not an easy task. GaAs was hailed in the 1980s as the future material for high-tech applications that would end the supremacy of silicon technology. Yet it is still restricted to niche optics and laser applications where high volume manufacture is not required. This is due to difficulties understanding and controlling the defect structures including compositional variations and antisite defects in the crystal lattice, a problem not encountered with the simple one-element structure of silicon. Note that controlled defects in crystal structures are in fact desirable, because they lead to many of the required physical properties. Given such problems with GaAs, it is not surprising that applications using much more complex crystals like high temperature oxide superconductors are still in their infancy.
This Network focuses on a few oxide crystals for which these problems appear to be resolvable in the near future, and which have efficient non-linear and/or photorefractive properties, often related to electro-optic and acousto-optic couplings. These crystals are capable of generating higher harmonics of incoming light (e.g. converting infrared radiation to light with doubled frequency in the visible region) and of storing enormous quantities of information as holograms. With the addition of laser active dopants they can be used as detectors and converters for visual, acoustic, mechanic or thermal signals, or as gas sensors, which are further ingredients of future intelligent all-optical data processing systems. Some of these crystals also have scintillator capabilities useful for radiation detectors.
The Network is achieving progress in the field by fostering the exchange of information and expertise between academic and commercial centres, with a limited role in coordinating activities in the chosen field. One of these activities is the preparation of crystals, including choosing the composition, dopants, crystal growth methods and thermal treatments etc. The other principal activity is the development of standard experimental characterisation and less standard theoretical modelling methods. Understanding the defect structure and the resulting phenomena is a prerequisite for controlled and optimised production of high-tech crystals, including efficient defect engineering.
Key materials now at different stages of investigation within t
- Lithium niobate, a multipurpose ferroelectric crystal that is a prospective material for integrated optics. As a result of coordinated research important progress has been achieved in the growth of crystals with the exact 1:1 lithium-niobium ratio and also in crystal characterisation both in the bulk and wave-guiding geometries.
- Borates, a new class of recently synthesised materials having superior higher harmonic generation features that operate even in the ultraviolet up to a wavelength of 200 nm. A European manufacturing source is now available due to development in the Network.
- Bismuth tellurite, a new photorefractive material, large single crystals of which have recently grown in Budapest leading to discoveries concerning the photorefractive and spectroscopic properties.
- Bismuth oxides of the sillenite group, which are good candidates for real-time optical processing because of their high sensitivity and fast response. Defect studies in the Network are in progress.
- Bismuth germanate, a fast response scintillator used for example in large elementary particle accelerators. Advances in luminescence and spectroscopic work have been made.
- Barium titanate and related perovskites for electro-optic and photorefractive applications for which complete scenarios of charge transfer processes have been identified.
- Progress has also been made in respect of other oxide crystals such as the scintillator zinc tungstate and the leading acousto-optic crystal paratellurite as well.
