Crystal Database Description

You are browsing the "Crystal" database (DB) containing information about properties of acousto-optical, electro-optical and nonlinear-optical substances (see descriptions of substances properties). These substances are widely used in modern optoelectronics and laser technology as working bodies of optical modulators, deflectors, gates, filters, frequency multipliers and other converters of light beam parameters (especially laser radiation). At present the number of potentially promising for applications substances of this type exceeds a hundred, while the study of their properties is at a very different level. Due to the need to improve the efficiency of materials, an in-depth study of their properties is carried out, work on synthesis of new substances with improved characteristics is carried out. As a result of such research, the number of promising substances for applications increases, data on their properties are continuously supplemented. In this connection, the collection, critical evaluation, and systematization of information available in the literature on the corresponding properties of acousto-, electro-, and nonlinear-optical substances are also of particular importance. In particular, complete information on the properties is necessary for a reasonable choice of materials for various kinds of optoelectronic devices, optimization of their operating modes, for timely introduction of new materials with improved characteristics, for reduction of duplication of research works. From time to time reference books and monographs containing the relevant information are published [1-16], but the information given in them quickly becomes outdated. The latest data on different materials are scattered, as a rule, over numerous journal publications, which complicates their use. The problem of taking into account new data is solved by creating computerized, regularly updated databases of crystal properties of different classes. The present DB on properties of acousto-, electro-, and nonlinear-optical substances belongs to them [17-20]. Combining information on these three classes of materials in a single database is due to the proximity of physical properties, compositions, and applications of the corresponding substances.

Physical basis of acousto-, electro-, and nonlinear-optical materials

The functioning of the three considered classes of materials is based on the use of acousto-optical, electro-optical and nonlinear optical effects, respectively. Consideration of their essence is important to highlight and analyze the main parameters that determine the effectiveness of materials. The acousto-optical effect is a phenomenon of diffraction, refraction, reflection or scattering of light on inhomogeneities of the medium, caused by elastic deformations during the passage of an acoustic wave in it. Occurring periodic alternations of medium inhomogeneities "work" as a diffraction lattice, which is used in acousto-optical devices for light diffraction on acoustic waves. The intensity of the diffracted light wave is proportional to the parameter M2 = n6p2/ρV3, called the acousto-optic quality factor. Its value is determined by the material properties - the refractive index n, the corresponding element of the elastic-optical tensor pij, the density ρ, the speed of sound V. This parameter is one of the most important for the overall assessment of the effectiveness of the acousto-optical material. The quality factor M1 = n7p2/ρV, M3 = n7p2/ρV2 and M4 = n6p2V/ρ3, which characterize the performance of an acousto-optic device in cases where the maximum ultrasonic front length, conversion width and power density are limited, respectively, are also used less frequently. The formulas determining M1, М2, M3 and M4 include components of the tensor pij and vector V, so their values depend significantly on the direction of propagation and polarization of sound and light waves. The quality factor М2 is usually measured in relation to the quality factor of fused quartz, found in light diffraction on a longitudinal ultrasonic wave, when the light wave is polarized perpendicular to the wave vector of the acoustic wave (p12, VL). In this case, for fused quartz M2(SiO2 melt) = 1.56*10-18 с3/g. The common elements of all acousto-optical devices are piezoelectric transducers that generate an acoustic wave in the material at frequencies ranging from a few megahertz to the gigahertz range. Therefore, piezoelectric materials are usually considered together with acousto-optical materials. The electro-optical effect is a change in the refractive index of a substance under the action of an electric field. Materials whose refractive indices change significantly under the action of the field are called electro-optical. There are two types of the electro-optical effect: 1) linear (Pockels effect) - changes in the refractive index depend linearly on the field strength (Δ(1/n2)ij = rijkEk); 2) quadratic (Kerr effect) - changes in the refractive index are proportional to the square of the field strength (Δ(1/n2)ij = RijklEkEl). The first occurs only in noncentrosymmetric crystals, the second is purely characteristic of centrosymmetric substances. Because of the linearity and low operating voltage, the Pockels effect is more commonly used in engineering. The most important parameters characterizing the linear electro-optical effect are the electro-optical coefficient tensor rijk, and the half-wave control voltage Uλ/2, which causes a π shift in the phase of the light wave. Nonlinear optical effects occur in crystals when they are irradiated by light of high intensity (with Е>105 V/cm), when in addition to the polarization of matter proportional to the strength of the applied field, there are noticeable nonlinear contributions to polarization proportional to higher degrees of field strength - Pi/εo = αijEj + 2dijkEjEk + χijklEjEkEl +.... These contributions cause such phenomena as generation of the second harmonic of laser radiation, addition and subtraction of frequencies of two radiations, shift of them by a certain value, parametric emission, etc. To obtain a high doubling coefficient it is necessary to have sufficiently high nonlinear optical coefficients dijk optically transparent crystal, phase matching and coincidence of phase velocities in the emission of the fundamental frequency and higher harmonics, which is achieved by using crystals with double ray refraction. It should be noted that the linear electro-optical effect and the second harmonic generation effect are only possible in noncentrosymmetric crystals; they are most pronounced in ferroelectric and related crystals. The acousto-optic effect is possible in both noncentrosymmetric and centrosymmetric crystals; the increased lability of structures of segnelecrons and segmented elastics near the Curie point contributes to a decrease in the control actions and, accordingly, to a decrease in the energy consumption.

Principles of selecting substances that are promising for applications

At present, there is a fairly wide range of substances that, to varying degrees, exhibit the above effects and are therefore, in principle, suitable for applications. The list of substances recommended by different authors for applications is much narrower. As a review of the literature shows, the vast majority of the most interesting and significant works in the field of acoustooptics, electrooptics and nonlinear optics have been performed for a relatively narrow range of materials. Even fewer materials are used in industrial developments, their list contains no more than ~50 names. The reason for the narrowness of the range of industrial materials is that they must meet a number of diverse requirements in terms of their performance characteristics, manufacturability and economic parameters. The main parameters of an acousto-optical material that determine its efficiency include data on the transparency of the crystal, the attenuation of elastic waves in it, the acousto-optic Q-value and its refractive index, the elastic-optical coefficients, density, velocities of elastic wave propagation in the crystal. Thus, the attenuation of elastic waves in the material limits the upper frequencies of most acousto-optic devices to ~300 MHz. To assess the efficiency of a piezoelectric in the mode of conversion of electrical signals into elastic waves, the Gutin coefficient diμ/sλμ (diμ and sλμ are elements of the piezomoduli and elastic stiffness tensors, respectively) and the electromechanical coupling factor k are used. For these purposes "strong" piezoelectrics with k > 10 % are used. When selecting electro-optical materials, the "half-wave" voltage Uλ/2 and the electro-optical efficiency coefficient (εUλ/22)-1 are mainly used as quality criteria; data on electro-optical constants, refractive indices, crystal transparency area, dielectric characteristics are also taken into account. When evaluating nonlinear optical materials, data on nonlinear optical coefficients, Miller tensor components, refractive indices and optical transparency region of the crystal, possibility to achieve phase synchronism condition in it (nω = n2ω) are taken into account. In addition to the abovementioned basic requirements for acoustic and optical properties, the material must also meet a number of additional requirements. These are specific requirements related to the specific purpose of the instrument or device in which the material is used and the conditions of its operation. They include, in particular, such requirements common to all crystals:

Taking all requirements into account and optimizing their combinations noticeably narrows the range of potentially interesting substances for applications. In particular, the optimization of some factors can often markedly degrade others. For example, it is noted that substances with a high acousto-optic quality coefficient tend to have large acoustic losses; the applicability of many promising substances is severely limited by difficulties in obtaining them in the form of single crystals. Thus, for applications, materials can be chosen that are far from the first place in the main acousto-optical, electro-optical or nonlinear-optical parameters. Among solids, the most convenient for practical use as acoustooptical materials are the following: Chalcogenide and telluric glasses, fused and crystalline quartz, crystals PbMoO4, TeO2, LiMO3 with M = Nb, Ta, Tl3MX4 with M = V, As, Ta and X = S, Se; and as ultrasound sources - compounds like Pb(Ti,Zr)O3, ZnO, LiNbO3. Significant linear electro-optical effect is exhibited by crystals of KDP family (KH2PO4, KD2PO4, CsD2AsPO4 etc.), LiMO3 with M = Nb, Ta, Bi12XO20 (X = Ge, Si, Ti), K3Li2Nb5O15, Ba2NaNb5O15, (Ba,Sr)Nb2O6, KTiOPO4, ZnTe, GaAs, CuCl, (NH4)2C2H4*H2O. The most promising nonlinear optical materials include the following single crystals: LiNbO3, LiTaO3, KNbO3, KDP, ADP, DADP, Ba2NaNb5O15, K2Li3Nb5O15, (Ba,Sr)Nb2O6, MNbB2O6 - M = K, Rb, Tl, LiB3O5, Li2B4O7, Ca4GdO(BO4)3, β-BaB2O4, LaBGeO5, MTiOXO4 (M = K, Rb, X = P, As), NdAl3(PO4)3. As can be seen from the above list, oxide materials are the most common in modern optoelectronic technology, which is due, in particular, to their high mechanical and electrical strength, resistance to environmental influences, low dielectric losses.

Database composition and structure

Одной из главных целей проводимых нами работ является сбор и экспертная оценка данных о веществах с особыми акустооптическими, электрооптическими и нелинейно-оптическими свойствами. Одна из задач, поставленная разработчиками базы данных, была связана с объединением в единой информационной системе данных о всех свойствах, необходимых для использования веществ в акустооптических, электрооптических и нелинейно-оптических устройствах. Существенно, что информация базы данных собрана и оценена российскими специалистами, разрабатывающими и использующими материалы, относящиеся к этим классам веществ. Учитывая темпы развития данной предметной области, было решено использовать базу данных в качестве хранилища информации. Применение новых информационных технологий позволяет постоянно обновлять данные по свойствам широко применяемых в практике кристаллов, а также постоянно пополнять базу данных информацией о малоизученных, недавно синтезированных, но перспективных кристаллах с целью их использования в приборах акустооптики, электрооптики и нелинейной оптики. Обновление данных производится ежемесячно. Однако разработчики не претендуют на всеохватывающую полноту информации. Основу БД составляют таблицы, содержащие численные данные экспериментальных исследований выделенных свойств рассматриваемых акустооптических, электрооптических и нелинейно-оптических кристаллов, в число которых, наряду с известными, включаются и новые, потенциально перспективные кристаллы. В основу подхода к выбору свойств веществ, информация о которых заносится в БД, положено стремление к наиболее полному охвату тех свойств, которые позволяют охарактеризовать вещество в плане перспективности его практического использования в соответствующих приборах и устройствах. В результате такого анализа выделены следующие характеристики, численные данные, о которых приводятся в БД: - химический состав кристалла; - температуры фазовых переходов (в том числе температуры плавления) и точки Кюри; - полиморфизм, симметрия, размеры элементарной ячейки; - плотность, твердость, растворимость в разных растворителях; - тепловое расширение, теплоемкость, теплопроводность; - скорость распространения и затухания упругих волн в кристаллах; - показатели преломления и коэффициенты Селмейера; - области прозрачности кристаллов; - тензоры, описывающие упругие (cij, sij), пьезоэлектрические (dij, eij, hij, gij), пьезо- и упругооптические (πij, pij), диэлектрические (εij, tgδij), нелинейно-оптические (dij, δij), электрооптические (rij, gij) свойства кристаллов; - коэффициенты электромеханической связи (kij). Численные параметры, характеризующие указанные свойства, приводятся по каждому из кристаллов в 30 таблицах, в которых содержатся также данные о погрешностях и методах измерений приведенных значений. Всем значениям свойств и характеристик кристалла соответствует номер литературной ссылки, из которой взято это значение. Эти ссылки приводятся в отдельной таблице и включают в себя номер ссылки, название статьи (монографии, справочника), фамилии и инициалы авторов, выходные данные литературного источника. Табличная информация о каждом веществе дополнена аналитическим обзором, в котором кратко описана технология получения монокристаллов, возможные области их применения, не охваченные жесткой структурой БД данных особые свойства кристаллов, по возможности, дана экспертная оценка приводимых в базе данных сведений. Для расширения возможностей БД создан и используется комплекс программ, позволяющий обработать рисунки, приведенные в оригинальных статьях или монографиях, хранить эту информацию в виде отдельных файлов и воспроизводить ее по запросу пользователя. Потенциальными пользователями разработанного БД являются научные работники, инженеры и студенты, занимающиеся проблемами разработки и использования материалов для акустооптики, радиоэлектроники, электрооптики, нелинейной оптики, акустоэлектроники и т.п. Разработчики базы данных будут признательны за замечания и дополнения. В разработке базы данных принимали участие следующие специалисты:

One of the main goals of our work is the collection and expert evaluation of data on substances with special acousto-optical, electro-optical, and nonlinear-optical properties. One of the tasks set by the developers of the database was to combine in a single information system the data on all the properties necessary for the use of substances in acoustooptical, electro-optical and nonlinear-optical devices. It is essential that information of the database is collected and evaluated by Russian specialists who develop and use materials related to these classes of substances. Taking into account the rate of development of this subject area, it was decided to use the database as an information storehouse. Application of new information technologies allows to constantly update the data on properties of crystals widely used in practice, and also to constantly replenish the database with information on poorly studied, recently synthesized, but promising crystals for the purpose of their use in devices of acoustooptics, electrooptics and nonlinear optics. The data are updated monthly. However, the developers do not claim to be exhaustive. The database is based on tables containing numerical data of experimental studies of selected properties of the considered acousto-optical, electro-optical and nonlinear optical crystals, which include new potentially promising crystals along with the known ones. The approach to the selection of properties of substances, information about which is included in the database, is based on striving for the most complete coverage of those properties that allow to characterize the substance in terms of the prospects of its practical use in the relevant devices and devices. As a result of such analysis the following characteristics were singled out, numerical data on which are given in DB: Numerical parameters characterizing the mentioned properties are given for each crystal in 30 tables that also contain data on errors and methods of measurement of the given values. All values of properties and characteristics of crystal correspond to the number of literature reference from which this value is taken. These references are given in a separate table and include reference number, name of article (monograph, reference book), surnames and initials of authors, output data of literary source. Tabular information about each substance is supplemented by analytical overviews that briefly describe technology of single crystals production, possible fields of their application, special properties of crystals not covered by rigid structure of the database, and, if possible, expert evaluation of information given in the database. To extend the capabilities of the database, a set of programs has been created and used to process figures given in original articles or monographs, store this information as separate files and reproduce it at the user's request. Potential users of the developed database are scientists, engineers and students involved in problems of development and use of materials for acoustooptics, radioelectronics, electrooptics, nonlinear optics, acoustoelectronics, etc. Comments and additions would be appreciated from the developers of the database. The following specialists participated in the development of the database:

Literature

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