Crystal Growth, Characterization And Nonlinear Optical Frequency Conversion Investigation Of New Molybdenum Tellurites

Laser is widely used in military and civil fields owing to its high spatial and temporal coherence. Laser can be generated via solid-state lasers, dye lasers, gas lasers, etc.; however, the wavelengths of laser produced through this way are limited. One efficient way to obtain new laser is frequency conversion via second-order nonlinear optical (SONLO) crystals and stimulated Raman scattering (SRS) crystals. SONLO crystals can be divided into three categories:(ⅰ) Deep Ultraviolet and Ultraviolet crystals, which are used for frequency conversion below400nanometers;(ⅱ) Visible and near Infrared crystals, which are used between400nm and3μm;(iii) Mid infrared and far infrared NLO crystals, which are used in the range of3μm to20μm. Among the three types of SONLO crystals, the mid infrared crystals are less developed and are of current interest.For a SONLO crystal, the prerequisite is noncentrosymmetric (NCS) structure. How to design such compounds? One general strategy is to incorporate NCS building blocks into a compound. These building units include d0transition metal cations, and cations with stereochemically active lone pairs, both of which are in acentric coordination environments due to second-order Jahn-Teller (SOJT) effect. Since1998, a large amount of NCS compounds have been synthesized by P. S. Halasyamani, J. G. Mao and S. L. Pan et al. Many NCS compounds exhibit strong second-harmonic generation (SHG), suggesting that they are potential SONLO crystals. Our group (Tao group) reported bulk crystal growth and physical properties investigation of the monoclinic BaTeMo2O9(β-BTM) in2008. It has been demonstrated that β-BTM has broad transparency range (0.5-5μm), high effective NLO coefficient (3×KTP), and large electro-optic effect (3xKDP), indicating that β-BTM is a potential IR SONLO crystal and electro-optic crystal. Based on these achievements, we carried out further research on β-BTM, including crystal growth of higher quality and larger size single crystals, design and test of NLO frequency conversion devices. In the course of growing large dimensions of β-BTM single crystals, a new polymorph (α-BTM) with orthorhombic space group Pea21was discovered. The crystal structure, crystal growth, physical properties including thermal properties, optical properties and piezoelectric properties, and the composition-structure-property relationship were investigated in details.On the other hand, there are some shortcomings in β-BTM:(i) The dipole moment of NCS building blocks (MoO6and TeO4polyhedra) is not along the same direction, resulting relatively small net dipole moment and thus small longitude piezoelectric (-10.8pC/N) and nonlinear optical coefficients (4.57pm/V);(ⅱ) The specific heat and the thermal conductivity are not large enough, resulting in relatively small laser induced damage threshold and limited applications in high-output laser. To overcome the above-mentioned two disadvantages, two aspects of research were performed:(ⅰ) The compound Cs2TeMo3O12(CTM), substituting Ba atoms with Cs, has very large structural distortion because the direction of the dipole momemts of TeO3polyhedra is basically the same. We deduce that CTM crystal possesses physical properties comparable to β-BTM. In addition, CTM has higher symmetry, so it can be easily orientated and fabricated. Thus, exploration of the Cs2O-TeO2-MoO3ternary phase diagram, top-seeded solution crystal growth and physical properties investigation of CTM were carried out.(ii) To improve the thermal conductivity, the Ba atoms were substituted with Mg atoms. The ternary system MgO-TeO2-MoO3was explored, and single crystals of MgTeMoO6(MTM) were obtained for the first time. The crystal structure and physical properties were investigated in details.In this thesis, investigations on crystal growth, structure, physical properties, and nonlinear optical frequency conversion of several alkali/alkaline-earth metal molybdenum tellurites were carried out. Main contents and conclusions are as follows:Ⅰ. Synthesis, bulk growth, characterization of a-BTM crystals and the relationship between a-BTM and β-BTM Polycrystalline a-BTM was synthesized via the traditional solid-state reaction techniques. The reaction temperature was optimized to be580-590℃. SHG measurements using1064nm radiation show that α-BTM is type I phase-matchable, and the SHG response is limited to be0.2×KDP, remarkably smaller than that of β-BTM.The solubility of a-BTM and β-BTM in the flux system TeO2-MoO3(TeO2: MoO3=1.2:1) was measured. Results show that the saturation temperatures of both polymorphs in the same solution are almost the same, indicating that both polymorphs can be grown from the same solution. Transparent single crystals of a-BTM with dimensions of50x42x30mm3and β-BTM with dimensions of57×43x35mm3were obtained using the top-seeded solution growth (TSSG) method. The as-grown a-BTM crystals using [010]-and [001]-orientated seeds have comparable quality, with full width at half-maximum of rocking curve being16.55".The physical properties, including thermal properties, optical properties and piezoelectric properties, were investigated. The thermal expansion coefficients of a-BTM was measured to be αα=9.10×10-6K-1, αb=19.58×10-6K-1, and αc=11.94×10-6K-1. The thermal conductivity has very limited change with the increasing of temperature, and the conductivity at60℃is1.26,1.18,1.00W/(m·K) along the a-, b-, c-axis, respectively. α-BTM has very broad transparency range (380nm～5.53μn), large refractive indice and big birefringence (0.30at404.7nm), indicating that α-BTM may have important application as a birefringence crystal. a-BTM belongs to non-ferroelectric piezoelectric material. The longitude piezoelectric strain constant d33was measured to be0.3pC/N, much smaller than that of β-BTM (d22=-10.8pC/N). The elastic constants s11, s22and s33are16.70pm2/N,12.10pm2/N,13.88pm2/N, respectively. Additionally, the relationship between a-BTM and β-BTM was explored, and it has been found that under normal pressure (101.325kPa), neither polymorphs can transform into each other via temperature variation; however,β-BTM undergoes a phase transition to a-BTM in the presence of BaMoO4, providing a new insight to control polymorphism.Ⅱ. Bulk growth and characterization of CTM crystalsThe Cs2O-TeO2-MoO3ternary phase diagram was explored in details. It has been found that single phase CTM can be crystallized from the Cs2O-TeO2and TeO2-MoO3flux system. Millimeter-sized CTM single crystals were grown using platinum rod as a seed. Then, centimeter-sized CTM crystals were obtained using TSSG method using a-and c-axis seeds. The effects of growth conditions such as concentration, seed orientation, and cooling rates on crystal morphology and quality were investigated. The optimized growth conditions are as follows:a-axis orientated seed, flux TeO2-MoO3(TeO2:Mo03=3:2), concentration20-40mol%, cooling rates0.25-0.5℃C/d.The thermal properties of CTM were studied. CTM melts incongruent at494.95℃. The thermal expansion coefficients are αc=32.02×10-6K-1and αu=7.34×10-6K-1, αc/αa=4.4, indicating CTM has a large anisotropy. The specific heat was measured to be0.400J/(g·K) at22℃, linearly increasing to0.506J/(g-K) at440℃. The thermal conductivity are ka=1.86W/(m-K) and kc=0.76W/(m·K) at22℃, kc/ka=2.44. The conductivity decreases with the increasing of temperature.Optical measurements show that CTM possesses a wide transparency range (430nm-5.38μm), large birefringence (0.20at480nm), and its birefringence decreases to0.13at480nm with the increasing of wavelength. According to the Sellmeier equation, the phase matching angles are calculated. When the fundamental wavelength is1.064μm, the phase matching angle is42.7°. The SONLO coefficients of CTM were measured to be d32=6.8pm/V and d33=6.5pm/V using the Maker fringe techniques. When the fundamental wavelength is1.064μm, the effect SONLO cofficient of frequency doubling is calculated to be4.6pm/V, which is1.5times that of KTP. Taking into account the wide transparency range and its large SONLO effect, CTM is a promising mid-IR SONLO crystal.The complete sets of dielectric, elastic, and piezoelectric constants of CTM at room temperature were determined by means of the resonant techniques and impedance analysis. The longitude piezoelectric strain constant d33is on the order of20.3pC/N, which is8.8times that of α-SiO2and1.9times that of β-BTM, with the electromechanical coupling coefficients being36.6%. The piezoelectric voltage constant g33is calculated to be on the order of0.18Vm/N, indicating that CTM may find valuable use in sensor application. Moreover, temperature dependence of the electro-elastic coefficients was measured in the range of0-150℃, where the elastic constant S44was found to possess relatively low temperature coefficient (Ts44(1))=77X10-6/℃), and the variations of d33and k33were less than3.5%and1.0%, respectively.Ⅲ. Structure, crystal growth and characterization of MTM crystalsThe MgO-TeO2-MoO3ternary system was investigated, and MTM single crystals were obtained. For the first time, the crystal structure of MTM was solved. The UV-vis diffuse reflectance spectrum indicates a UV absorption edge near360nm. Infrared spectrum measurements show that MTM has a transmission window up to5.2μm. SHG measurements using1064nm radiation show that MTM is type I phase-matchable, and the SHG response is1.5times that of KTP. Considering the wide transparency range and strong SHG, MTM is a good mid-IR nonlinear optical material.IV. The composition-structure-property relationshipsThe distortion of NCS building blocks (MoO6, MoO4, TeO3, and TeO4) in α-BTM, β-BTM, CTM and MTM are calculated based on their structure parameters. The composition-structure-property relationships in α-BTM, β-BTM, CTM and MTM are analyzed.V. Nonlinear optical frequency conversion investigationThe Raman spectra of β-BTM, a-BTM and CTM were measured and analysed. The strongest Raman shift are around921.3cm-1,905.7cm-1,887.5cm-1for β-BTM, a-BTM and CTM, respectively, with corresponding intensity values and line-widths (45000,5.6cm-1),(25000,9.3cm-1) and (30000,6.3cm-1). Laser light with wavelengths of1178nm,1320nm,1500nm and589nm were obtained using the lst-order Stokes,2nd-order Stokes,3rd-order Stokes, and self-frequency-doubled Raman lasers, respectively, based on the fundamental wavelength of1064nm. These results lay a solid foundation for the applications of metal molybdenum tellurites.A SRS laser operating at1178nm (lst-order Stokes) with the bulk a-BTM single crystal was realized. The maximum output pulse energy of15.1mJ was obtained at the pump pulse energy of48mJ, corresponding to an optical-to-optical conversion efficiency of31.5%and a slope efficiency of39.6%. For β-BTM, the Raman resonator possesses a threshold of28MW/cm2at1064nm and a maximum output pulse energy of19.2mJ for the lst-order Stokes with an optical-to-optical conversion efficiency of48%and a slope efficiency of61.2%. The largest optical-to-optical conversion efficiency can reach50.4%at a pump energy of28.8mJ. Compared with a-BTM, β-BTM exhibits better Raman laser properties.The2nd-and3rd-Stokes dual-wavelength laser operation at1320and1500nm based on β-BTM crystals was demonstrated. Using an external resonator with a plane-plane configuration, the laser possesses a threshold of40MW/cm2at1064nm. The maximum output power of10.86mJ (2nd-Stokes) and9.06mJ (3rd-Stokes) were obtained at a pump power of60mJ using an output mirror with transmittance of44%at1320nm. The corresponding optical conversion efficiency from pump laser to the second-and third-Stokes laser is about18.1%and15.1%.The SRS and and SHG properties of β-BTM were investigated. The β-BTM crystal was cut along the type-Ⅱ SHG phase-matching direction for the lst-order Raman shift at1178nm to realize the SRS and SHG simultaneously. Pumped by a nanosecond1064nm laser source, a self-frequency-doubled BTM Raman laser operating at589nm has been demonstrated for the first time. At the pump pulse energy of48mJ, the maximum yellow laser output pulse energy of5.6mJ was obtained with an optical-to-optical conversion efficiency of11.7%.