| A series of Hf-doped LiNbO3 crystals(Hf:LiNbO3?Hf:Fe:LiNbO3?Hf:Fe:LiNbO3 with various Li/Nb ratio and Hf:Fe:Cu:LiNbO3),Zr-doped LiNbO3 crystals(Zr:Fe:LiNbO3 and Zr:Mn:Fe:LiNbO3 with various Li/Nb ratio),and Mn/Fe codoped LiNbO3 crystals(Sc:Mn:Fe:LiNbO3 and Mg:Mn:Fe:LiNbO3)were grown by Czochralski method.The grown crystals were annealed,polarized,oxided and reduced with the optimal technological conditions.All the crystals were transparent and free of macroscopic defect.The effective distribution coefficient of Mg:Mn:Fe:LiNbO3 crystals were measured by means of inductively coupled plasma atomic emission spectroscopy(ICP-AES).With the increase of Mg2+ concentration,the distribution coefficient of Mg decreased,the distribution coefficient of Mn increased firstly then decreased,and the distribution coefficient of Fe increased.All the distribution coefficient were around one.The distribution coefficient of Zr in Zr:Mn:Fe:LiNbO3 crystals was about 0.9.The crystal quality of grown crystals co-doped with Mg?Mn?Fe and Zr was good.The powder X-ray diffraction patterns of the doped LiNbO3 crystals were measured and the lattice parameters and unit cell volume were calculated.The patterns showed that there was no new absorption peak appeared.It was demonstrated that the doped elements were introduced into the crystal lattice.The intensity change of the peak was attributed to difference between the diameters of doping and substrate elements.The infrared transmittance spectra and ultraviolet-visible absorption spectra measured.It was found that the OH-absorption peaks and absorption edges of doped LiNbO3 crystals shifted.In doped LiNb03 crystals,the threshold concentration of the doping ions could be determined via the infrared transmittance spectra.The absorption peak of Mg2+ was located at 3535cm-1.The absorption peak of Zn2+ was located at 3529cm-1.The absorption peak of Sc3+ was located at 3508cm-1.The absorption peak oF Hf4+ was located at 3488cm-1.The absorption peak of Zr4+ was located at 3479cm-1.The OH-absorption peaks and the shifting mechanism were discussed.The shift of absorption edge was explained via the polarized ability of ions.The photorefractive properties of the crystals were measured and investigated by using Kr+laser(476nm,blue light),Ar+ laser(488nm,blue light)and He-Ne laser(633nm,red light).The photorefractive parameters including diffraction efficiency(?),response time(?r),dynamic range(M/#)and photorefractive sensitivity(S)were measured under blue laser irradiation.The results showed that the diffraction efficiency was as high as 75.4%,dynamic range as large as 20.24,sensitivity as high as 3.19cm/J.The exponential gain coefficient and effective carrier concentration were was 34.2cm-1 and 3.1 × 1015cm-3,respectively.The diffraction efficiency was decreased with increasing concentration of Mg2+,Hf4+ and Zr4+ when red laser was used as recording light.While the diffraction efficiency was increased when blue laser was employed.The blue photorefractive properties were better that red photorefractive properties.The enhancing mechanism of blue photorefraction was discussed.Taking Kr+ laser(476nm wavelength blue light)or Ar+ laser(488 nm wavelength blue light)as the writing beams,He-Ne laser(633nm wavelength red light)as reading light is called two-wavelength technique.While a shorter wavelength of light(Kr+ laser,Ar+ laser and ultraviolet laser)is used for sensitizing and a longer wavelength light(He-Ne)for writing is called two-color technique.In two-wavelength nonvolatile holographic measurement,Hf(4mol%):Fe:Cu:LiNbO3 crystal was used as recording medium.Kr+ laser and He-Ne laser were used as recording and reading light,respectively.The fixed diffraction efficiency was as high as 49%,response time was as fast as 180s,and nonvolatile sensitivity was 0.252cm/J.The results obtained from two-color experiment showed that the fixed diffraction efficiency,response time and nonvolatile sensitivity were 29%,650s and 0.015cm/J,respectively.When ultraviolet laser was used as sensitizing light,the fixed diffraction efficiency,response time and nonvolatile sensitivity were 30%,1680s and 0.0078cm/J,respectively.The results above showed that two-wavelength technology was more suitable than two-color technology.Generally,optical damage existed in LiNbO3 crystals(also called photorefraction or light-induced scattering),which would decrease the signal/noise ratio.Optical damage resistance could be enhanced by doping Mg,Mn,Zn,In,Sc,Hf and Zr.For Mg:Mn:Fe:LiNbO3,the optical damage resistance was enhanced by two orders of magnitude compared with Mn:Fe:LiNbO3 when Mg concentration exceeded 5mol%(threshold concentration).For Sc:Mn:Fe:LiNbO3,the optical damage resistance was enhanced by two orders of magnitude compared with Mn:Fe:LiNbO3 when Sc concentration exceeded 3mol%(threshold concentration).The threshold concentration of Hf and Zr were 4mol%and 2mol%,respectively.The optical damage resistance was measured by means of transmitted beam spot distortion method. |
PDF Full Text Request