by Vladimir SHIRYAEV, Dr. Sc. (Chem.), leading research assistant of the Institute of Chemistry of High-Purity Substances named after G. Devyatykh (Nizhni Novgorod)

High-purity substances are a basis for many sections of modern materials science. They form a basis for materials required for new industry sectors such as micro- and nanoelectronics, fiber and power optics, optoelectronics. Since 1988 the Devyatykh Institute of Chemistry of High-Purity Substances is a leading center for production and analysis of such substances in our country. It develops science-intensive optical materials and functional units on their basis in cooperation with the Moscow Scientific Center for Fiber Optics and the largest laboratories in the world. The research findings of the institute have been marked with the RF Government Award in the field of science and technology for 1998, diplomas and medals of national and international exhibitions. In 2014 the institute joined the European Consortium for development and creation of infrared fiber lasers and coherent sources of emission.

Fiber-optic communication facilities based on quartz glass lightguides became an integral part of our life. But in a spectral wavelength range above 2.5 µm these facilities are nontransparent. Therefore specialists follow the line of a wider mastery of different ranges of the optical spectrum including the infrared and even terahertz regions.

Medium infrared radiation range of 3-25 µm wavelength bears information on presence and temperature of hot bodies and is a convenient form of energy for processing of materials and biological tissues. It provides an opportunity for application of infrared fiber light-guides in laser surgery and chemical technology. Vibration frequencies of different chemical bonds and functional groups are also in the medium infrared region. Therefore, application of optic systems of this spectrum allows making of a remote quality and quantity analysis of gases, vapors and liquids and also the environmental control.

Chalcogenide and fluoride glasses compare favorably with medium infrared range materials designed for production of optic lightguides. The first of them, most commonly encountered in the last 30 years, are divided conventionally into three main families, i.e. sulphide, selenide and telluride groups. As, P, Si, Tl, Ge act as glass-forming cations in these families. Antimony, gallium, bismuth and some other elements serve as a grid modifier and stabilizer. Anion-based compounds such as sulfo-selenides and selenic tellurides are also well-known. Changed properties of chalcogenide glasses also change introduction of alloying additive. For example, selenium atoms are replaced partially with tellurium or halogen atoms (I, Cl, Br) for reduction of multi-background absorption. Gallium, indium or iodine is introduced into a glass matrix for improvement of rare-earth element solubility.

The potential advantages of such elements for application in fiber optics are as follows: transparence and small optic losses in a medium infrared region, low photon energy, slight tendency to crystallization, high value of a nonlinear refraction index, chemical stability and possibility of receiving lightguides of different length. Therefore optical devices made of chalcogenide glasses are promising for transmission of a powerful laser emission of a medium infrared range, thermal monitoring, analytical remote spectroscopy and creation of new elements and systems of nonlinear and integral optics such as infrared fiber-optic lasers, amplifiers, high-speed switches, frequency convertors and supercontinuum generators (coherent electromagnetic emission with an extra-wide spectrum).

Chalcogenide glasses are known since the 19th century. Good transmission in auripigment (mineral of a sulphide class with the chemical formula As₂S₃) was first described in 1870. But only after the repeated discovery in 1950 by the American chemist Rester Freriks of As₂S₃ glass and production of the first specimens of optical lightguides they were considered materials for fiber optics. They displayed good transparency in a medium infrared region. But optical losses in the first lightguides remained high, therefore Freriks was first to note an admixture sensitive feature of chalcogenide glasses.

From the mid-1950s this subject became a research issue of numerous laboratories in all developed countries. The research boom was connected first of all with semiconductor properties of this material.

A substantial contribution to studies of different compounds of chalcogenide glasses and their optical properties was made in the 1950s-1960s by workers of the loffe Physico-Technical Institute and the Vavilov State Optical Institute (both in Leningrad). Leadership of the Russian scientists in this field was indubitable. At that time specialists developed methods of chalcogen cleaning from metal impurities and studied the production methods and basic physical and chemical properties of a majority of glass-forming compounds.

The achievement of optical losses in quartz lightguides at a level of 1-5 dB/km by Corning Glass Company (USA) early in the 1970s gave an impetus to rapid development of fiber-optic communication systems on their basis and active search for other substances of high purity for waveguides with new promising properties required in telecommunication and optoelectronics. For example, in 1974 in the glass and ceramics laboratory of the University of Rennes (France) Professor Jacques Luc and his team developed a new class of materials, i.e. fluoride glasses which consist of chemical compounds of zirconium, barium, aluminum, sodium and other elements. Their emission band extended from ultraviolet to medium infrared regions, and solubility of rare-earth elements opened a way for development of fiber lasers and amplifiers. According to Russian and American physicists the theoretical estimates of optical losses in chalcogenide and fluoride lightguides were by two orders below than in quartz-based ones, and they fell on the medium infrared region.

At that time, early in the 1980s, there was an increased interest in studies of such glasses for production of light-guides with the lowest feasible optical losses. Development of the first promising materials started in laboratories of the USA, France, Japan, Germany, Czechia and many national institutes. The methods of glass synthesis and cleaning were not developed yet, therefore industrial elements were used as basic substances. But such lightguides contained a great amount of limiting impurities (oxygen, hydrogen, carbon), which resulted in substantial optical losses, i.e. above 500dB/km.

At the end of the 1970s the problem of chalcogenide glass production and devices based on such glasses was handled by Professor (today Academician) Mikhail Churbanov's team at the USSR AS Institute of Chemistry (today the RAS Institute of Chemistry of High-Purity Substances, Nizhni Novgorod). His research workers managed to achieve high results in fine cleaning of chalcogens (1965-1980) and in technology of quartz fiber lightguides with small optical losses (1972-1989). Further studies of these glasses as a material for fiber optics was initiated by academicians Grigory Devyatykh, Alexander Prokhorov and Yevgeny Dianov*; the latter headed the Scientific Center for Fiber Optics in

* See: A. Prokhorov. Ye. Dianov. "Fiber Optics: Problems and Prospects", Science in Russia, No. 1, 2001.-Ed.

1994. Since then our institute works in a close cooperation with this institution known in our country and abroad for a pioneer development of a technology of special fiber lightguides, lasers and amplifiers*.

By the 1980s the Nizhni Novgorod specialists studied thoroughly the cleaning methods of chalcogens (S, Se and Te), therefore for synthesis of proper glasses they used not commercial but high-purity basic elements. But by the 1990s they already mastered the methods of their production based on arsenic sulphide and selenide, which opened a way for creation of lightguides with minimum optical losses, i.e. 23-100 dB/km, and later, in 2008, even 12 dB/km in the medium infrared region. Our laboratory came out to leading positions as far as purity of produced glasses and a level of optical losses in

*See: Ye. Dianov, "On the Way to Peta Era", Science in Russia, No. 3, 2014.-Ed.

lightguides are concerned. Therefore, the work of Russian scientists drew more and more attention of foreign researchers.

I got acquainted with chalcogenide glasses in 1981 when 1 was preparing a course paper and a degree work. As a student of the Lobachevsky State University (city of Gorky) 1 implemented them at the laboratory of chemistry of high-purity oxygen-free glasses of the Institute of Chemistry of High-Purity Substances under guidance of Professor Mikhail Churbanov, and in 1985 1 got a regular work there. At the laboratory one group was engaged in the work with chalcogenide glasses, and another group-in fluoride glasses and lightguides. The institute was visited by leading scientists in fiber optics such as Jacques Luc and Jean-Luc Adam from France, James Harrington, Eeshwar Aggarwal and Jasbinder Sangera from the USA, Angela Seddon from Great Britain and

Minimal losses in hyaline arsenic sulfide.

Philip Russel from Germany. The foreign colleagues were greatly impressed by our advances in cleaning of basic components and production of high-purity samples of glasses and lightguides with small optical losses.

Early in the 1990s the interest in fluoride glass devices declined in the world as the maximum possible results in reduction of their relaying losses were achieved. Therefore, the research area of our laboratory and most of foreign centers was focused on chalcogenide glasses which had great opportunities for emission transfer of medium infrared range waves. They opened up new horizons in development of fiber optics and optoelectronics, in creation of laser devices and amplifiers on their basis.

Our foreign contacts were also strengthening. We used to meet our colleagues at international conferences and visited laboratories in France, Brazil, Great Britain and other countries where we carried out joint experiments. Foreign specialists always took a sustained interest in our research work. The national technologies of receiving high-purity substances and also chalcogenide glasses and optical lightguides were of interest to institutes and universities of the USA, Great Britain, France, Canada, Germany, China, Brazil, South Korea and Australia. We established long-standing partnership relations with a number of foreign scientific groups. For example, in 1995 our laboratory entered into contract with the Chinese Institute of Semi-Conductor Materials from the city of Tianjin for production and studies of chalcogenide lightguides. For two years the Chinese specialists worked at our institute. During this time we gained a unique

experience in scientific and informal communication.

For more than 20 years Astrium and A.R.T. Photonics companies from Germany are our regular customers. They use chalcogenide lightguides for production of cables and regular cable strands for systems of infrared emission and image transfer.

For many years we maintain good relations with the glass and ceramics laboratory of the University of Rennes (France). It is one of the leading organizations in the world established in the 1970s by the fellow of the French Academy of Sciences Jacques Luc for creation and study of infrared glasses and glass-ceramic materials. Here in 1974 fluoride glasses were created and later, in 1986, also chalcogenide glasses transparent in the spectral range of 1-25 µm. The laboratory equipped with qualified personnel and a wide range of technological and analytical equipment makes a search for and all-embracing studies of new materials for fiber optics, synthesizes high-purity chalcogenide glasses and glass-ceramics, carries on drawing out of lightguides by the

"small bar-pipe" method* and studies their optical, mechanical, thermal and luminescent properties.

Under the joint research program with the University of Rennes for creation and studies of fluoride and chalcogenide glasses, which started back in the 1980s, our institute entered into a number of long-term international projects. Their main objectives are as follows: studies of crystallization kinetics of fluoride and chalcogenide glasses, production of chalcogenide lightguides based on As-Se-Te and Ge-Sb-S glasses with small optical losses, creation on their basis of remote chemical sensors for an analysis and control of the composition of different biological and technical objects and media, and also development of single-mode lightguides with a transmission band 2-16 µm for an interference telescope. It is intended to be used within the framework of the DARWIN project launched by the European Space Agency designed for search of life on other planets. In the course of the Russian-French cooperation interesting and important results were obtained and more than a dozen of articles and two monographs on photon glasses were published.

* The idea of the "small bar-pipe" method lies in the fact that a bar made of core glass is put in a pipe made of casing glass. During the process of drawing out the bar glass fuses together with the pipe glass thus forming a fiber consisting of a core and a casing.-Ed.

In cooperation with the infrared glass laboratory headed by Professor Angela Seddon from the University of Nottingham (Great Britain) we are creating fundamentals and methods of obtaining high-purity materials alloyed with rare-earth ions for fiber lasers and studying nonlinear and luminescent properties of chalcogenide glasses. A group of scientists from the University of Nottingham visited our institute, and I had a great pleasure to visit them twice as an invited professor. The British laboratory is developing chalcogenide lightguides for sensors and lasers of the medium infrared range. Here they are developing methods of getting chalcogenide glasses alloyed and non-alloyed with rare-earth ions and make preforms for double-layer and microstructured lightguides using the extrusion method (through a forming device). In the course of mutual work we obtained new original results and infrared materials with the assigned functional properties and published several papers. The chalcogenide cone-shaped lightguides developed at our institute were used as end pieces in a scanning infrared microscope with synchrotron radiation, which increased substantially its resolution.

It should be noted that the main advantage of international cooperation is its striving to gain results and practical application of the obtained materials in different systems of the industrial scale. It implies first of all instruments for medical examinations, environmental control, determination of a chemical composition of substances and also other devices using infrared emission.

Lately specialists of Germany and Canada take an increased interest in development of terahertz lightguides based on chalcogenide glasses. The frequency spectrum of terahertz emission is located between the infrared and superhigh-frequency ranges in the wavelength region of 3-0.03 µm. In contradistinction to X-ray radiation, the said emission does not harm a human organism, therefore with the development of terahertz spectroscopy it has become widely used in some sectors of national economy and everyday life, for example, in safety systems for scanning of luggage and people, noncontact inspection and in medical tomography. Waveguides in the form of hollow capillaries or microstructures made of sapphire, metal or plastic are used for channeled transfer of terahertz emission. Application of chalcogenide glasses as a structural material for production of such lightguides is a promising business. Cooperation in this field with German and Canadian scientists will make it possible to create optical devices with a wide transmission spectrum, i.e. from 5 to 200 µm.