Literature survey and theoretical aspects of nonlinear optics, crystal growth and lattice imperfections

The extensive survey of the importance of nonlinear optical single crystals in view of their photonic applications in Information & Technology for high density data storage, processing and transmission has been given. The historical importance of single crystals and their growth has been carried out. The fundamentals of the second harmonic generation and phase-matching have been discussed. Theoretical aspects of nucleation and crystal growth have been provided and the single crystal growth methods have been categorized. A discussion about the imperfections/ defects in single crystals has been carried out. Chapter – 1 Literature Survey & Theory ~ 2 ~ 1.1 LITERATURE SURVEY Due to high-speed and ease of production of photons (light), the area of photonics has become an active field of research in view of the modern society’s demand for improved telecommunications, data storage, retrieving, processing and transmission. The design of devices that utilize photons instead of electrons for the transmission of information has created a need for new materials with unique nonlinear optical (NLO) materials (Williams, 1984). The nonlinear optics is the field which includes all phenomena in which optical parameters of materials are changed with the interaction of intense coherent source of light. The NLO phenomena have lead to the enhancement in understanding of light-matter interactions. The search for new molecular materials with NLO properties is currently the subject of considerable importance investigations due to their potential applications in photonic devices (Prasad & Williams, 1991). The invention of laser has created a revolution for research and development in the area of photonics and the investigations on NLO properties enhanced and widened the horizon of application of lasers. The drawback of most of the laser materials is their inability to generate the radiation in a wide spectral region, such as, for some applications the appropriate laser materials exist but due to thermal properties of source materials the emitted power is restricted. The nonlinear optics makes it possible to transfer the energy from wavelength to another and hence provide a solution for the radiation sources in the spectra range of radiation (Hannes et al., 2010). The history of nonlinear optics began early in 1875 with the observation of electric field induced change in the refractive index of CS2 by Kerr (Kerr, 1875). In 1893, Pockel made similar kind of observations in quartz crystals (Pockels, 1893). However, the full realization of these theories could be realized with the discovery of laser having the intense beams (electric field), in 1960. Throughout the 1960’s, Bloembergen and coworkers contributed significantly to the field of nonlinear optics (Bloembergen, 1965). In early 1970’s, Davydov and co-workers (Davydov et al., 1970) have demonstrated the strong second-order NLO effects in donor-acceptor substituted aromatic compounds. The finding of third order NLO effects in polydiacetylenes in 1976 by Sauteret and coworkers and the advent of optical fiber in Chapter – 1 Literature Survey & Theory ~ 3 ~ telecommunication in late 1970’s given a boost up and broadened the horizon of research for NLO properties of organic materials (Sauteret et al., 1976; Rustagi & Ducuing, 1974). The organic, polymeric and composite materials have emerged with interesting NLO properties (Chemla & Zyss, 1987; Flytzanis, 1986; Hann, 1989; Orenstein, 1987; Williams, 1983; Marder, 1991). Today, the research on NLO materials has been extended as interdisciplinary frontier of science and technology and assimilated with the disciplines of Chemistry, Physics, Optical Engineering and Materials science because of the applicability of NLO materials to many braches of information and image processing technologies. The NLO single crystals therefore can be employed to generate the sources of different wavelengths for which the lasers are not available. The output radiation beam from a nonlinear optical device has the similar properties as those of a laser source, hence can be directly employed as laser source. For certain applications it is required to have highly powerful laser radiation (such as inertial confinement fusion research) (De Yoreo et al., 2002). However, instead of attempting to scale up the laser itself it is simpler to amplify a well performing laser in a second stage. Thus the overall process is split into two steps where the generator will focus only on the generation of high quality signal, while the amplifier specifies in boosting the signal without adding too much noise. The nonlinear optical crystals are suggested to employ to amplify the coherent radiation. The invention of holography and nonlinear optics in the nonlinear dielectric materials has revolutionized the photonics for advanced technology of information (Gabor, 1948; Franken et al., 1961; Armstrong et al., 1962; Bloembergen, 1965). The crystals are the most stable state of matter and admired the mankind, right from the pre-historic era of human civilization, ‘Lord Rama offered an ornament made with diamonds to his most faithful and beloved warrior, Hanuman, on the occasion of great victory over Ravana’. These have acquired an auspicious place for the spiritual and scientific development of human life. Mother Earth has abundance wealth of naturally crystallized materials like, quartz, diamond, ruby crystals, etc., they are not only aesthetically pleasing but also of great importance to industry and economy growth. The art of crystallization extends far back in the past and antedates Chapter – 1 Literature Survey & Theory ~ 4 ~ considerably the written history of man. The crystallization of salt from sea water by evaporation was practiced at many places in the prehistoric time and has been considered as one of the oldest technical methods of transforming materials. The alchemist Geber, whose papers are dated in the 12 th or 13 th century (cf. Darmstaedter, 1922), described the preparation and purification of various materials by recrystallization as well as by sublimation and distillation. In the middle of the 16 th century, Birriguccio (1540) recorded the leeching and recrystallization of saltpeter and the Saxonion scientist Agricola (1556) in his famous extensive work ‘De re Metallica’ gave instructions on how to produce various salts, alums and vitrols by using strings for seeding [Feigelson, 2004]. In the following century, the word ‘crystal’ came into use as modern name; originally, Homer had used the expression ‘crystallos’ for ice crystals. The modern scientific development of crystallography started in 17 th century. In 18 th century, Westfeld (1767), Bergman (1773), Haüy (1782) given the idea of smallest possible unit a ‘molecule integrante’, by repetition of which whole crystal is built up and in 19 th century Weiss (1804) considered the crystal to be as anisotropic continua; he derived the crystal system (1815) and discovered the law of rational intercepts (1816) and zone law (1820) [Feigelson, 2004]. Miller (1839) and Bravais (1849) have proposed the idea of Miller’s indices and lattice types. Laue, Friedrich, Knipping (1912) and Bragg (1913) gave the X-ray diffraction law and crystal structure determination. Our quest for knowledge and technological advancement has made us to grow synthetic crystals as an alternative to the scarcely available natural crystals. The ideal crystal is defined as the infinite lattice of atoms arranged in particular pattern, which repeat in all three dimensions with repeated distances (lattice spacings), with all physical and electrical properties anisotropic in nature (Dryburgh, 1986). The real single crystals are finite in size but consist long-range order and also defects. The orders and defects result in the crystals with unique properties (Brandle, 1979; Brice, 1986). Single crystal technology is the mother technology of almost all the recent technologies of the modern science. In the modern world, the large-scale use of these crystals has been brought about mainly by the demands of solid state physics of Chapter – 1 Literature Survey & Theory ~ 5 ~ materials for research and developments. A variety of crystals are needed to meet some very important gaps the conventional production engineering. Several kinds of single crystals find application for the development of technologies such as: laser, semiconductor, high and low energy particle physics, nuclear fusion, medical diagnostic, display, thermal imaging, etc. (Bailey, 1991; Nalwa & Miyata, 1996). To satisfy the need of science, technology and ornamental purposes, the artificial crystals are being produced. The ability to grow high quality crystals has become an essential criterion for the competitiveness of nations. In the recent decade, NLO crystals have shown a promising role in photonics for high density optical data storage, quantum storage of photonic entanglement, cascaded photon-pair generation, acousto-optic interaction phenomenon applications (Lee et al., 2000; Christoph et al., 2011; Hannes et al., 2010; Mark Haw, 2003; Lounis, & Moerner, 2000; McPherson et al., 2002). The NLO crystal research is strongly motivated and based on inorganic materials such as LiNbO3, GaAs, InP, etc., with good mechanical stability and sufficiently large NLO coefficients. A series of inorganic NLO crystals have been investigated and reviewed (Nikogosyan, 2005). Lithium niobate (LiNbO3) is an excellent NLO material with various pronounced physical properties. It has been particularly fruitful in the optical regime, where many effects have been found in LiNbO3 and devices introduced using it as a host. Its photorefraction property has been considerably important for applications such as holographic volume data storage, optical image and signal processing, phase conjugation, real time interferometry, beam deflection and novelty filters (Gunter & Huignard, 1988, 1989). The structure and density of intrinsic (non-stoichiometric) defects substantially influence the optical properties, including the photorefractive effects (Anghert et al., 1972; Furukawa, 1992; Malovichko et al., 1993; Arizmendi et al., 1991). LiNbO3 have been efficiently used for the optical parametric amplification applications (Giordmaine & Miller, 1965; Kingston, 1962; Malcolm & Majid, 1999; Ebrahimzadeh & Dunn, 2000). Waveguides, optical dispersion, acoustic resonators, transducers, acousto-optic, ultrasonic waves (100 MHz-10 GHz), surface acoustic waves (SAW), etc. applications of lithium niobate have been successfully performed (Wong, 2002). Due to the anisotropic nature of single crystals, the properties strongly Chapter – 1 Literature Survey & Theory ~ 6 ~ depend on the specimen grain boundaries, dislocations, impurities and inclusions, also influenced by the electric/magnetic domain boundaries in ferroelectric/ferromagnetic media, LiNbO3 is of ferroelectric class. However, the organic materials exhibited the larger NLO efficiencies due to their purely electronic response, and therefore promise to meet future requirements for ultrahigh bandwidth photonic devices (Eaton, 1991; Zyss, 1994; Günter, 2000; Dalton, 2002, Jazbinsek, 2008; Williams et al., 1984; Williams et al., 1983; Marder, et al., 1989). Due to the immense important of NLO crystals, it is useful to synthesis new NLO materials, grow in single crystals form and study their structural, physical, thermal and optical properties. The active π-bonds in organic materials make them flexible to engineer their structure, which drag the attention of researchers to a great extent. Therefor the high NLO effect of organic materials could have been combined with the high mechanical and thermal stability of inorganic materials to form the semi-organic NLO materials. In the recent decades extensive research has been carried out on such materials. The semiorganic materials exhibited the large nonlinearity, high resistance to laser damage, low angular sensitivity and good mechanical hardness (Marcy et al., 1992; Velsko, 1990; Xing et al., 1987). The complexes of urea and urea analogs, such as tris(thiourea)zinc sulphate (ZTS), bis(thiourea)zinc chloride (ZTC), bis(thiourea)cadmium chloride (BTCC), etc. have been explored for NLO applications (Newman et al., 1990). A series of thiocyanates bimetallic complexes found to exhibit the efficient second harmonic generation at short wavelengths (Nakamoto, 1986). In parallel to the invention of new NLO materials it is also equally important to improve the structural and physical properties of crystals for the tailor made applications, by using suitable dopants and addition of functional groups to the available materials. Growth of NLO single crystals of bulk size of good quality using the suitable technique is of great importance from the device application point of view. As defects affect the properties of crystals and hence the efficiency of device, therefore, it becomes of stringent importance to investigate the grown crystals for structural defects (point defects, dislocations and their agglomerates, structural grain boundaries), which arise during their growth or post growth treatments. Chapter – 1 Literature Survey & Theory ~ 7 ~ 1.2 THEORETICAL ASPECTS OF NONLINEAR OPTICS 1.2.1 Nonlinear optics In the pre-laser era the optical properties of the materials were considered to be linear independent of the intensity of radiation. The basis for this conclusion for this conclusion is that the filed strengths of the conventional light sources before the advent of lasers were much smaller than the field strengths of atomic and inter-atomic fields. The latter are of the order of 10 7 10 10 V/cm whereas the former would not exceed 10 3 V/cm. The light sources with such a low intensity are not able to affect the atomic fields to the extent of changing the optical parameters. The laser radiation with high degree of coherence made it possible to attain the high spatial concentration of light power. It is now possible to generate 1 MW pulses, lasting a few tenths of nanosecond, using moderately powerful lasers. The energy current density in a beam of cross-section 1 mm 2 of such a laser is JE ≈ 10 6 MW/m 2 , which corresponds to peak electric field strength E ≈ 3 × 10 7 V/m, due to coherence, the beam can be focused to an area A ≈ λ 2 . If the laser wavelength is assumed to be 1 μm, then A = 10 -12 m 2 and hence JE ≈ 10 18 MW/m 2 giving E ≈ 3 × 10 10 V/m, which is within the range of atomic fields. Under such high fields, the relationship between the electric polarization P and the field strength E ceases to be linear and some interesting nonlinear effects come to the fore. 1.2.2 Harmonic generation Just after the invention of lasers, the nonlinear properties in the optical medium have been strikingly demonstrated by the harmonic generation of light observed for the first time by Franken and his coworkers in 1961 (Franken et al., 1961). They have observed ultraviolet light (λ = 3472 Å) at twice the frequency of a ruby laser (λ = 6493 Å), when the light was made to traverse a quartz crystal. This experiment attracted the widespread attention and marked the beginning of the experimental and theoretical investigation of nonlinear optical properties. The theoretical aspect of nonlinear optical behavior of the dielectric medium is illustrated as follows. Chapter – 1 Literature Survey & Theory ~ 8 ~ When a dielectric medium is placed in an electric field it gets polarized. Each molecule acts as a dipole, with a dipole moment Pi which results in the dipole moment vector per unit volume P, given by   i i P P , here summation is over the dipoles per unit volume. The orientation effect of the external field on the molecular dipoles depends both on the properties of the medium and strength of the field. Hence, P can be expressed as a function of field E;