| Laser has greatly promoted the development of related scientific research fields because of its high energy density,monochromaticity,directionality,coherence and so on.Laser modulation is the key technology in the field of laser research,in which frequency,time domain and spatial modulation are important to broaden laser application bands and improve laser performance.And the optoelectronic functional crystal is the vital material basis.Molybdates have been attracted extensively attentions as laser host crystals and Raman crystals because of its abundant chemical and physical properties.However,molybdate crystals are less involved in nonlinear optical and acousto-optic applications.Over the past decade,our research group has introduced elements such as Te4+into traditional molybdate materials to modulate the nonlinear optical properties.A series of tellurium-containing tungsten/molybdate crystals have been successfully grown by top-seeded solution growth method,and they have widely explored in laser modulation such as acousto-optic,polarizer and so on.However,the poor thermal conductivity limits their applications in high-power laser.In this thesis,the double alkali metallic high damage threshold crystal LiNa5Mo9O30 was selected as the research topic,and large sized crystal with high quality has been grown by optimization of growth parameters.The basic physical properties of LiNa5Mo9O30 have been systematically characterized;and their applications in laser modulation have been explored.The main results are as follows:(Ⅰ)Crystal growth and optimization of growth parameters of LiNa5Mo9O30Large-sized crystals with high-quality have been grown by the top-seeded solution method with MoO3 as flux for the first time.The high-quality crystals in[100]-,[010]-and[001]-direction were successfully obtained with a narrow constant temperature zone,suitable cooling speed and crystallization rate through optimization.The theoretical morphology of the crystal was calculated and analyzed,and crystals in the direction of[101]and perpendicular to the[101]direction were grown in high-quality.What’s important,angle between the[101]-direction and 1064 nm-XOY-plane-type-Ⅰ noncritical-phase-matching direction were only 4.5° away.(Ⅱ)Crystal structure and basic properties of LiNasMo9O30The crystal quality was characterized by Laue diffraction and high-resolution X-ray diffraction.The Laue diffraction spots are clear and symmetrical;the peak shapes of the(100),(010)and(001)plane curves are sharp,and the FWHM are 17",24" and 57",respectively.The results indicate that the crystal is in high quality,satisfying the requirements for physical properties testing and devices fabrication.The crystal structure was analyzed,it belongs to orthorhombic space group Fdd2,in which Mo and O atoms link to be MoO6 octahedron,and adjacent MoO6 octahedrons are mainly connected along b-axis,and other a and c axes,and finally Li+and Na+interspersed to form a three-dimensional structure.The density at room temperature was determined to be 4.00 g/cm3,in agreement with the theoretical results,and the crystal density decreased as the temperature increasing.The hardness of(100),(010)and(001)planes of the crystal were 4.0,4.3,4.1,respectively,which inhabits the obvious anisotropic,and hardness of the crystal is moderate and easy to be processed.The crystal thermal properties were systematically tested.The specific heat varies continuously from 0.748 to 0.854 J/(g·K)in the temperature range from room temperature to 300℃,consistent with the theoretical result of 0.766 J/(g·K).At the range of 30～350 ℃,the crystal thermal expansion along crystallographic axes are listed as below:α11=16.57×10-6 K-1,α22=5.58×10-6 K-1,α33=27.75×10-6 K-1.Both thermal diffusion and thermal conductivity decrease with the temperature increasing,and the thermal conductivity at 48℃was determined to be:κa=2.69 W/(m·K),κb=3.61 W/(m·K),κc=3.22 W/(m·K).The crystal exhibits thermal anisotropy,attributing to the binding force along different axes.The Mo-O bond strength along b-axis is larger than that in a-and c-axis,corresponding to the lower probability of phonon collision and larger molecular free path,therefore the crystal along b-axis presents a smaller thermal expansion coefficient and better thermal conductivity.The optical properties of the crystal were systematically studied.The band gap of the crystal is 2.8 eV,and the transmission range is 0.31～5.35 μm.The Raman spectrum was tested,and the strongest Raman frequency shift peak was 946.6 cm-1,which attributing to the vibration of the strongest Mo-O bonds.The crystal refractive index was measured by the minimum deviation technique from 0.435 to 2.3.25 μm,and the refractive index dispersion curves were fitted The relationship between the optical principal and crystallographic axis is:Z∥b,Y∥c,X∥a.The electrical properties of LiNa5Mo9O30 crystal were systematically characterized.The resistivity of the crystal in all three axes are in the order of 1010 Ω·m at room temperature.The relative dielectric constants was determined to be:ε11=16.11,ε22=10.52,ε33=16.79;the piezoelectric constants are calculated as:d31=0.17 pC/N,d32=0.20 pC/N,d33=1.20 pC/N,d15=0.14 pC/N,d24=2.32 pC/N,which are comparable to α-SiO2.The spatial transformation of the piezoelectric constants of the crystal are also studied.According to the crystal structure,all the five MoO6 octahedra are strong distortion,but the dipole moment magnitude of octahedra are in an antiparallel manner,which resulted that the net dipole moment of the crystal unit cell is only-29.92×10-22 esu·cm/?3,consisting with the result of piezoelectric constants.(Ⅲ)The laser modulation performance of LiNasMo9O30 crystal.The polarized prism was designed and manufactured.When the light incident along the Y-axis,birefringence in the range of 0.435～5 μm is larger than 0.1408.The prism would be applied in the range from visible to mid-infrared band with the wedge angle of 31.94°～32.12°,and the extinction ratio is greater than 15000:1,meeting the acquirements for high-power lasers applications.In addition,the relationship between the maximum field angle,the structure angle of the prism and wavelength was analyzed for the first time.The second-order nonlinear optical frequency conversion of crystal was studied.The XOY-plane-phase-matching curves,the effective nonlinear coefficient of the noncritical phase-matching angle and related parameters were calculated,and the effective nonlinear coefficient of type-I non-critical phase-matching direction was 3.852 pm/V.With the means of one-way pass light,the relationship between crystal symmetry,length and frequency doubling efficiency was studied.The highest efficiency was determined to be 28.5%with the sample length of 10 mm.By optimizing the optical path,the conversion efficiency would be significantly improved.The stimulated Raman scattering laser output property of the crystal was studied.The spontaneous Raman spectra of different configurations along the crystal principal axes were measured,and the Raman gain coefficient of X(ZZ)X mode was 0.68×gYVO,the FWHM of maximum Raman frequency shift peak was 6.34 cm-1.The first-order Stokes Raman laser of 1184 nm was realized by the external cavity pumping,and the threshold energy was 49 MW/cm2,with the highest optical-to-optical conversion efficiency of 21%and the slope efficiency of 36.2%.With the frequency-doubling crystal of KTP put in,the yellow-orange light of 592 nm was obtained.And the "eye-safe" laser of 1524 nm was output by the 1064 nm laser pumping through third-order Stokes Raman effect.To date,the conversion efficiencies of the three Raman laser experiments have not reached the saturation state,and it is necessary to magnify the pumped laser energy and optimize the optical path to obtain high energy laser output.The self-frequency-doubled Raman laser output of the crystal was studied.The phase-matching curves in XOY-plane at 1184 nm was calculated,and the effective nonlinear optical coefficient of the type-I noncritical-phase-matching was 3.979 pm/V.The laser output characteristic was tested by external cavity pumping.The yellow-orange light of 592 nm was obtained with a threshold of 54 MW/cm2,and the maximum optical-to-optical conversion efficiency was determined to be 3.35%.Furthermore,the optical-to-optical conversion efficiency don’t reach to a saturated state,and it is beneficial to obtain high power laser output with crystal coating and optical path optimization.The acousto-optic device of the crystal was studied and fabricated.A 100 MHz free-space acousto-optical device was designed and manufactured with the light along a-direction and sound wave along b-direction.The corresponding diffraction efficiencies were 59.40%,68.30%and 67.90%,corresponding to the electrode lengths of 8,10 and 14 mm,respectively.Importantly,acousto-optic-frequency-doubling and acousto-optic-frequencydoubling-Raman devices were designed and fabricated for the first time,which indicated the feasibility of the crystal as multi-functional devices.It is expected that high diffraction efficiency through the optimization of electrode size and acousto-optic crystal length will be realized. |
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