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, 1999, 41, . 5 Synthesis, Characterization and Applications of Shaped Single Crystals William M. Yen Department of Physics and Astronomy University of Georgia Athens, GA 30602, USA In this paper we will review techniques for the growth of single crystal mostly of insulating material which are shaped in the growth process. In particular, we will focus on the growth of single crystal fibers of optically activated materials; the emphasis will be placed on the so-called Laser Heated Pedestal Growth (LHPG) method of pulling crystalline fibers. LHPG offers a number of logistical advantages which can be exploited as a tool for materials research. Progress in the synthesis of materials using LHPG is described, as are spectroscopic techniques which are employed in characterizing the optical and physical properties of the crystal fibers obtained through this method.

There has been interest on crystalline materials prepared The source rods are cut out of polycrystalline ceramics in fiber form for a sustained period of time; this is partially produced by mixing host and activator materials, sintering because single crystal fibers occurring naturally in the form and hot pressing the mixture into flat disks. Crystalline chips of whiskers often possess near ideal physical properties in and fibers can be used as source materials as well. The source rods we have employed at the University of Georgia crystallinity and in tensile strength. Earlier work addressed are typically 1 1 12 mm3 and our fiber diameters are in itself mostly to metallic materials, for example, in 1922 the 0.15 mm to 1.0 mm range; pulling speeds are typically von Gompers [1] was successful in pulling single crystal 0.12.5 mm/min.

metallic filaments directly from the melt. Later, in the 1950s, much of the work centered on the magnetic and mechanical properties of metallic whiskers [2]; however 1. Advantages of the LHPG Method the size and composition of these could not be controlled of Fiber Growth accurately.

One method which allows the growth of crystalline fibers Several advantages of LHPG have become apparent, not is the so-called Stepanov method in which the melts are only in the growth of fibers for applications but, more drawn through shapers and crystallization is made to occur importantly, as a general way to explore material synthesis after passage through a die [3]. Another method that enabled and the properties of crystal growth. Other practical us to grow single crystal fibers of the desired length and advantages of the LHPG method have also become apparent, diameter, in the proper crystallographic orientation and of as follows.

proper composition and doping is the Laser Heated Pedestal Growth (LHPG) method of fiber synthesis [4].

LHPG and the related float zone growth technique are micro-variants of the Czochralski growth method; the feed stoch used is generally in the shape of a rod and the melt is in the form of a self supporting bubble at the tip of the rod.

A number of heating sources have been used to produce the melt; the most common method by far has been laser heating with focused single or multiple beams. A seed is dipped into the melt and is wetted by in; as the seed is pulled out, surface tension of the molten materials forms a pedestal around the seed, hence the name of pedestal growth. The melt is kept in place solely by surface tension, hence, this fiber growth method does not require crucibles and eliminates one source of sample contamination. This type of container-less growth also permits the synthesis of materials with extremely high melting points. The laser heated version of pedestal growth (LHPG) is illustrated in Fig. 1 [5].

In a typical LHPG fiber pulling system, a stabilized COcw laser typically with an output of between 1575 W is used as a heating source. The usual focusing and turning optics for the beam are shown in Fig. 2 along with the pulling and feeding mechanisms. The fiber pulling assembly may be enclosed in a vacuum-tight chamber allowing growth in Figure 1. Schematic representation of the LHPG method for fiber controlled atmospheres [6]. growth showing the various regions involved in the growth.

Synthesis, Characterization and Applications of Shaped Single Crystals c) One of the most attractive features of the LHPG methods is the rapidity with which fibers can be grown by this method. With our pulling speeds, a fiber sample of length of 1 or 2 cm, sufficient for spectroscopic characterization, can be grown and characterized in a relatively short time [7].

The information feed back made possible by this time scale allows for the rapid re-adjustment of stock compositions and growth conditions for optimized materials. It is this feature method that makes this method such a powerful tool in the synthesis and engineering of crystalline materials in general.

d) Finally, for those of us interested in the optical spectroscopic properties of activated materials, the fiber configuration is ideal experimentally for conducting absorption, emission and other ancillary dynamical and static optical measurements.

We have been able to pull a great variety of oxide and fluoride crystal fibers doped with rare earth and transition metal ion activators in a great range of concentrations; a list of the materials we have been able to grow is shown in table.

2. Characterization of LHPG Materials Characterization of the fibers with regard to physical and Figure 2. Cross sectional view of the LHPG growth chamber mechanical properties can be done using the myriad of stanshowing details of the reflixicon and optical focusing system.

dard techniques such as X-ray diffractometry, crystal birefringence, microprobe analysis and optical microscopy [4].

We have been able to derive many characteristic of our a) The LHPG relies on surface tension to maintain the fiber samples by investigating the static and dynamic optical integrity of the melt and hence it is a method of growth properties of the fiber [9]. Static measurements obtained which does not require crucibles; nor does the enclosure under steady state conditions can be used to determine the containing the fiber growth region possesses walls heated to dopant concentrations, whether the concentration is uniform high temperatures as is the case in crystal growth furnaces.

along the length of the fiber and whether other species or Both crucible and furnace surfaces are generally understood active defect centers exist in the fiber. Dynamic measureto be the primary sources of unintentional contamination in ments entail the determination of radiative and non-radiative normal crystal growth, hence, it follows that the absence lifetimes and can yield information on the microscopic of these surfaces allows the growth of very pure crystal interactions between the active ions and their surroundings.

materials. The impurity levels found in LHPG fibers are Other optical determinations such as fluorescence lifetime solely determined by the purity of the starting materials of measurements can be used to provide a quick measure of the source rods.

the extent of self quenching or cross relaxation present and b) The source rod length as well as the melt volume in again allows for quick adjustments of the doping levels for LHPG are typically small, of the order of 10 mm and 1 mm3, optimized performance. Quite generally, the macroscopic respectively. The cost of the chemical compounds required optical properties and other physical properties of materials for the growth of single cristal fibers is, as a consequence, grown in fiber form through the LHPG method have been relatively small. Because of this, it is possible to grow fiber found to be identical to those in bulk materials [911].

crystals of materials which would be prohibitively expensive As an example of the type of measurements which can to grow by traditional methods, specially on a basis [7].

be conducted in the fiber configuration we mention tensile Further, it is also generally accepted that thermal gradients stress studies. Many piezo-spectroscopic studies have been within the melt container are responsible for introducing conducted in optically active insulating materials; in fact, stresses and other defects in bulk crystals, because of this both hydrostatic and uniaxial compressive stress studies LHPG pulled fibers can be made practically stress free.

of the behavior of the well known R lines of ruby have The small volume of the growth area also facilitates the established the shift of the R-lines as a function of applied introduction of external perturbations during synthesis of stress as a secondary pressure standard. Complementary the crystal. The application of an external field to the studies in which tensile or decompressive stresses are melt may influence the growing process by encouraging the applied have not been carried out because of experimental inclusion of domains or the formation of other stoichiometric difficulties encountered in stretching a bulk crystal; the fiber combinations [8]. geometry is in fact ideal for tensile stress studies [12]. Tensile 2 , 1999, 41, . 772 William M. Yen Material Grown with the UGA LHPG System Material Dopant Material Dopant Material Dopant Al2O3 Pure LiGa5O8 Co2+ Pr3+ Cr3+ Ni2+ Tb3+ Mg2+ Co2+ and Mg2+ Ti3+ Si4+ LiNbO3 Pure Tm3+ Ti4+ Er3+ V3+ Fe2+ and Cr4+ Er3+ and Al3+ Ca2+ and Mn4+ Co2+ and Si4+ Er3+ and Cr3+ Co2+ and Si4+ Cr3+ and Ti3+ LiYF4 Er3+ Fe2+ and Cr4+ Cr4+ and Si4+ Lu2O2 Pure Fe3+ and Nd3+ Mg2+ and Cr4+ Ce3+ Mg2+ and Mn4+ Mg2+ and Mn4+ Lu2SiO5 Ce3+ Mo4+ and Ca2+ Ti2+ and Si4+ MgAl2O4 Cr3+ Ti3+ and Nd3+ Mo3+ Ti3+ Ti2 and Si4+ BaTiO3 Eu3+ MgCaSiO4 Cr4+ Tm3+ and Ce3+ BaYF8 Er3+ Mg1.5Mn0.5SiO4 Cr3+ W4+ and Ca2+ Nd3+ Mg2SiO0 Cr3+ and Cr4+ YAlO3 Er3+ CaF2 Pb2+ Mn2SiO4 Pure Er3+ and Eu3+ Tb3+ NaLa(WO4)2 Er3+ YGAG Ca2+ and Cr4+ CaWO4 Er3+ Eu3+ YGG Mg2+ and Cr4+ Ti2+ Nd3+ YIG Pure CsB3O5 Pr3+ Er3+ and Yb3+ Y2O3 Ce3+ DyF3 Pure NaY(WO4)2 Eu3+ Dy3+ GdEuO3 Nd3+ NdF3 Pure Er3+ GGG Cr3+ PbMoO4 Pure Eu3+ Gd2O3 Pure RbMnF3 Pure Ho3+ Eu3+ Sc2O3 Er3+ Nd3+ Nd3+ Nd3+ Pr3+ GdScO3 Nd3+ Ti3+ Tb3+ Gd2SiO3 Ce3+ SrAl2O4 Cr4+ Tm3+ LaAlO3 Cr3+ and Eu3+ SrTiO3 Cr3+ Dy3+ and Tb3+ LaAl0.75Ga0.25O3 Tm3+ Eu3+ Pr3+ and Yb3+ LaAl0.5Ga0.5O3 Tm3+ Nd3+ Tm3+ and Yb3+ LaF3 Pure YAG Pure YScO3 Er3+ LaGaGeO7 Nd3+ Ca2+ Eu3+ La3Ga5SiO14 Pure Ce3+ Nd3+ La2O3 Pure Cr3+ Y2SiO5 Eu3+ Ce3+ Dy3+ YSAG Ca2+ and Cr4+ LiAl5O8 Pure Er3+ YSGG Mg2+ and Cr4+ Ni2+ Eu3+ YVO4 Er3+ Co2+ Fe2+ ZnGa2O4 Mn2+ LiCaAlF6 Cr3+ Mo4+ ZnSiO4 Cr4+ LiF Pure Nd3+ stress can readily be applied by simply attaching weight to 3. Phonon Spectroscopy in Single Fiber the fiber; the behavior of the R line as a function of tensile Geometry stress is shown in Fig. 3. The shifts are to the blue rather than to the red as is observed with uniaxial compressive Cristalline fibers can be pulled so that their diameters are stress and are linear up to a tensile stress of 6 kbar, defining comparable to the mean free path,, of high frequency, the yield point of the fiber. The tensile strength of the fiber non equilibrium (THz) phonons at low temperatures [15].

was determined to be 7.7 kbar; the quality of the materials as In other words, LHPG single crystal fiber can be mesoscopic measure by this parameter is comparable with those reported with respect to the characteristic dimensions of elementary by LaBelle and Mlavsky [13]. Torsional stress can also be excitations of the solid. These excitations include magnons, applied; the effects of torsional stress on the vibration Raman phonons and plasmons in insulators and conduction elecactive modes of sapphire fiber have been reported [14]. trons in the case of semiconductor fibers.

, 1999, 41, . Synthesis, Characterization and Applications of Shaped Single Crystals which are impacting an ever increasing number of technologies. This interest is not likely to diminish in the future and we foresee additional applications of these fibers in a variety of technologies.

5. Conclusions Though a variety of techniques are available for the growth of single crystal fibers, the LHPG method offers a number of advantages and is a cost efficient way for the synthesis of a large number of materials for fundamental material science studies and for a number of mechanical, electronic and optical applications.

References [1] E. von Gomperz. Z. Phys. 8, 184 (1922).

[2] C. Herring, J.K. Glat. Phys. Rev. 85, 1060 (1952).

[3] A.V. Stepanov. Soviet Phys. JETP 29, 339 (1959).

[4] Robert S. Feigelson. Growth of fiber crystals. In: Crystal Growth of Electronic Materials / Ed. by E. Kaldis. Elsevier Science Publishers, Amsterdam (1985). Chapter 11.

Figure 3. Tensile stress dependence of the blue shifts of the R-line [5] For a comprehensive review of LHPG and its applications see:

in a ruby fiber. Shifts arising from uniaxial stress are to the red W.M. Yen. Preparation of single crystal fibers. In: Insulating (dashed line). Tensile shifts are linear up to 6.0 kbar (yield point);

Materials for Optoelectronics: New Developments / Ed. by the tensile strength of the fiber is 7.7 kbar.

F. Agullo-Lopez. World Scientific, Singapore (1995). Ch. 2.

[6] M. Fejer, J. Nightingale, G. Magel, R.L. Byer. Rev. Sci. Inst.

55, 1791 (1984).

The transport properties and the dynamics of narrow [7] S.M. Jacobsen, B.M. Tissue, W.M. Yen. J. Phys. Chem. 96, 1547 (1992).

band, high frequency nonequilibrium phonons in crystalline [8] Kh.S. Bagdasarov. V.V. Ryabchenkov. Sol. Phys. Crystallogr.

fibers of ruby and of YAG : Pr3+ at low temperatures have 33, 394 (1988).

been investigated recently [16], as have been the narrow [9] W.M. Yen. Proc. SPIE 1033, 183 (1988).

band phonons 29 cm-1 phonons in a ruby fiber [17]. These [10] W. Jia, H. Liu, K. Lim, W.M. Yen. J. Lumin. 43, 323 (1989).

experiments have allowed us to investigate phonon-interface [11] W. Jia, L. Lu, B.M. Tissue, W.M. Yen. J. Cryst. Growth 109, interaction and the energy transport across boundaries as a 329 (1991).

function of the acoustic impedance encontered at the fiber [12] H. Liu, K.-S. Lim, W. Jia, E. Strauss, W.M. Yen, A.M. Buonboundaries.

cristiani, C.E. Byvik. Opt. Lett. 13, 931 (1988).

[13] H.E. LaBelle, Jr., A.L. Mlavsky. Nature 216, 574 (1980).

These initial results simply illustrate that the availability of [14] W. Jia, W.M. Yen. J. Raman Spectr. 20, 785 (1989).

LHPG fibers can open up whole new areas to investigation [15] A.A. Kaplyanskii, S.A. Basun. Multiple resonant scattering simply by providing us with good materials configured in a of the 29 cm-1 phonons in optically excited ruby. In:

useful geometry.

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