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Tetravalent Ions Doped Lithium Niobate Crystals

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1 Tetravalent Ions Doped Lithium Niobate Crystals
Yongfa Kong, Shiguo Liu, Shaolin Chen, and Jingjun Xu Good morning, Ladies and Gentlemen, It is my honor to take part in this conference. As we known, Institute of Physics is the leader of physics research in China. In our country, there is a proverb, that is “throw out a brick to attract a jade”, Now I throw out a kind of crystals: zirconium doped lithium niobate. I am Yongfa Kong, come from School of Physics and TEDA Applied Physics School. My main collaborators are Shiguo Liu, Shaolin Chen, and Prof. Jingjun Xu, School of Physics and TEDA Applied Physics School

2 Outline 1. Introduction 2. Optical damage resistance
3. Photorefraction 4. Concluding remarks The outline of my talk is: a brief introduction of the background; the optical damage resistance and photorefraction; then the concluding remarks.

3 1. Introduction The topic of this workshop is on Optics and New materials. Lithium niobate crystal is dull compared with the vast variability of today’s deliberately engineered materials. Is there any news?

4 Materials Update: Material of the month
November 2002: Lithium niobate In the field of nonlinear optics there have been many contenders for the title of all-star material of the world. But for day-to-day applications, the most successful of these nonlinear materials is lithium niobate. And was selected as the material of the month by Nature Materials Update, it has been dubbed as the silicon of nonlinear optics. Indeed, because of its availability, widespread use and versatility, it has been dubbed by many as the “silicon of nonlinear optics”.

5 Silicon of photonics Lithium niobate (LiNbO3), also called the ‘silicon of photonics’, is indispensable in advanced photonics and nonlinear optics. M. Kösters1, et al., Nature Photonics 3, 510 (2009)

6 Lithium niobate (LiNbO3, LN)
Multi-functions: electro-optic, acousto-optic, elasto-optic, piezoelectric, pyroelectric, ferroelectric, nonlinear optic, etc. Multi-applications: Waveguides, modulators, isolators, frequency transformers, optical parametric oscillators, filters, sensors, holographic storage, etc. Property controllability: Good solubility to many dopants, Properties change with different dopants and doping concentrations. As we have known, lithium niobate has multi-functions and multi-applications, and most importantly, its properties can be controlled by dopants.

7 “Optical silicon” New materials renew life for integrated optics Lawrence Gasman WDM Solutions, November, 2001 Material systems based on silica on silicon, gallium arsenide, lithium niobate, and indium phosphide are contenders for the role of "optical silicon." So lithium niobate has been considered as one of the main contenders of optical silicon.

8 Workshop on Optics and New Materials II
The topics include metamaterials, plasmonics, optical lattice, photonic crystals, and novel quantum effects of light-matter interaction. S. Zhu, et al., Quasi-phase-matched third-harmonic generation in a quasi- periodic optical superlattice. Science 278, 843–846 (1997). N. G. R. Broderick, et al., Hexagonally poled lithium niobate: a two- dimensional nonlinear photonic crystal. Phys. Rev. Lett. 84, 4345–4348 (2000). V. Ilchenko, et al., Nonlinear optics and crystalline whispering gallery mode cavities. Phys. Rev. Lett. 92, (2004). C. Canalias, et al., V. Mirrorless optical parametric oscillator. Nature Photon. 1, 459–462 (2007). A. Guarino, et al., Electro-optically tunable microring resonators in lithium niobate. Nature Photon. 1, 407–410 (2007). R. C. J. Hsu, et al., All-dielectric photonic-assisted radio front-end technology. Nature Photon. 1, 535–538 (2007). W. Yang, et al., Non-reciprocal ultrafast laser writing. Nature Photon. 2, 99– 104 (2008).

9 What have been done on Lithium niobate crystal?
In 1965, Ballman et al. firstly succeeded in growing lithium niobate single crystal: SAW Filter: 4~5 inch single crystals; Electro-optic modulator: 3~4 inch single crystals; Photorefraction: Fe, Cu, Mn, or Ce doped crystals; Optical damage resistance : Mg, Zn, In, or Sc doped crystals; Property enhancement : nearly stoichiometric crystals; Optical waveguide: H+, Ti; QPM: PPLN, PPMgLN; ………. Cerium; Scandium; Titanium

10 Good enough? Acoustic grade crystals: inhomogeneous stress, low electricity; Optical grade crystals: graining stripes; Photorefraction: long response time, low sensitivity; Optical damage resistance: poor optical quality, only in visible range; QPM:PPLN, low optical damage resistance, PPMgLN, hard to fabricate, poor thermal stability; NS crystals: very difficult to grow, very poor optical quality; Defect structures Energy levels Mechanism ……….

11 What can tetravalent dopants do?
Optical damage resistance Photorefraction Domain engineering Crystal growth Micro-mechanism of some effects and structural design

12 2. Optical damage resistance
Light-induced optical damage, now also named as photorefraction, was discovered in LiNbO3 and LiTaO3 crystals. Photorefraction (PR) Can be used in holographic storage, information processing, light control of light. low response speed, volatility. Optical damage Hinders the applications: frequency doublers, optical parametric oscillators, Q-switches, optical waveguides. Laser-induced optical damage was found in lithium niobate and lithium tantelate at Bell laboratory. This effect, later named as photorefraction, can be used in holographic storage, information processing and light control of light, on the other hand, hinders the applications of LN as frequency doublers, optical parametric oscillators, Q-switches and optical waveguides. So the control of optical damage is a very important task for LN and other electro-optical materials. A. Ashkin, et al., Appl. Phys. Lett. 9, 72(1966)

13 A solution: doping 1980, Mg2+ ions, LN:Mg; “Star of China”
It promotes the practical applications of LN in nonlinear optics at high light intensities. 1990, Zn2+ ions, LN:Zn; 1992, Sc3+ ions, LN:Sc; 1995, In3+ ions, LN:In. For LN, a solution is doping. In 1980, Zhong Jiguo et al. found the optical damage resistance can be improved by two orders of magnitude as doping magnesium [mæɡ΄ni:zjəm] above 4.6 mol%. This report promotes practical applications of LN in nonlinear optics at high light intensities. LN:Mg crystal has been considered as “Star of China”, and one of the most important contribution of China to World in the field of synthetic [sin΄θetic] crystal.   G. Zhong et al., J. Opt. Soc. Am. 70, 631 (1980).   T. R. Volk et al., Opt. Lett. 15, 996 (1990). J. K. Yamamoto et al., Appl. Phys. Lett. 61, 2156 (1992).    Y. Kong et al., Appl. Phys. Lett. 63, 280 (1995).

14 The problems of doped LN
It is difficult to grow high optical quality crystals. Large amounts of doping concentrations; (such as usually 5 mol% Mg for CLN) Distribution coefficient far from 1.0; (such as 1.2 for Mg) Some properties are still not satisfied: Resistance not high enough, Enhanced ultraviolet photorefraction (UVPR). However, there are still some problems in doped LN. It is difficult to grow high optical quality crystal, because the large amounts of doping concentrations and distribution coefficient far from 1.0. And some properties are still not satisfied: such as the resistance is not high enough and the enhanced ultraviolet photorefraction.

15 HfO2 doped LiNbO3 (LN:Hf)
Accidentally hafnia E.P. Kokanyan et al., J. Appl. Phys (2002); Appl. Phys. Lett. 48, 1980 (2004).

16 Optical damage resistance of LN:Hf
(a)    2 mol% Hf;(b) 4 mol% Hf;(c) 6 mol% Hf;(d) 6.5 mol% Mg The light intensity for (a) is 104 W/cm2 and 5×105 W/cm2 for (b), (c), and (d). LN:Hf4 is able to withstand a light density of 5×105 W/cm2 without noticeable beam smearing, which is comparable to that observed in 6.5mol% MgO doped LN (LN:Mg6.5) crystal. S. Li et al., J. Phys.: Condens. Matter. 18, 3527 (2006).

17 ZrO2 doped LiNbO3 (LN:Zr)
As the doping concentration reaches 2.0 mol% ZrO2, LN:Zr crystals can withstand a light intensity as high as 2.0107 W/cm2. At the same experimental conditions, the light intensity that 6.5 mol% Mg doped LN (LN:Mg6.5) can withstand is about 5.0105 W/cm2. The optical damage resistance can be directly examined by light spot distortion As the doping concentration of Zr reaches 2.0 mol%, LN:Zr crystals can withstand a light intensity as high as 2.0107 W/cm2 (the 7th power of 10, or ten to the 7th power, watt [wɔt] per centimeter square) At the same experimental conditions, the light intensity that 6.5 mol% Mg doped LN can withstand is about 5.0105 W/cm2. (a) (b) (c) (d) (a), (b) and (c) LN:Zr1.7; (d) LN: Zr2. The light intensity for (a) is 1.3103 W/cm2, (b) 1.3104 W/cm2, (c) and (d) 2.0107 W/cm2. Y. Kong et al., Appl. Phys. Lett. 91, (2007).

18 Light-induced changes of refractive indices
As the doping concentration of Zr above 2.0 mol%, the refractive index changes of LN:Zr crystals are one order of magnitude smaller than that of LN:Hf and LN:Mg. As the doping concentration above 2.0 mol%, the refractive index changes of LN:Zr crystals are one order of magnitude smaller than that of LN:Hf and LN:Mg. Light-induced change of the refractive index in saturation as a function of dopants

19 The distribution coefficient of Zr
The maximum value is 1.04 and the minimum value is 0.97. Therefore, the distribution coefficient of Zr is much closer to one than that of Mg. the distribution coefficient of Zr is much closer to one than that of Mg. fluorescence [fluə΄resns]

20 SnO2 doped LiNbO3 (LN:Sn)
Distortion of transmitted argon laser beam spots after 5 min of irradiation. (a)-(d) for Sn1:LN, Sn2:LN, Sn2.5:LN, and Sn5:LN, respectively. The light intensities are (a) 2.5×102 W/cm2, (b) 4.7×103 W/cm2, (c) 4.8×105 W/cm2, and (d) 5.4×105 W/cm2. L. Wang et al., Opt. Lett. 35, 883 (2010).

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22 The distribution coefficient of LN:Sn
Dependence of the distribution coefficient of Sn4+ ions in Sn:LN crystals on the doping levels of SnO2.

23 Ultraviolet photorefraction (UVPR)
Enhancement of UVPR in LN:Mg From 1980, LN:Mg had been considered as an optical damage resistance crystal, this concept was broken through in 2000 by Prof. Jingjun Xu. He observed that its photorefraction was enhanced in UV region. Then Haijun Qiao et al. observed UVPR enhancement in Zn, In and Na (sodium) doped LN. J. Xu, et al., Opt. Lett. 25, 129(2000)

24 Pulsed UV image amplification for programmable laser marking
A laser at 355 nm, with 5 mJ, 10 ns pulse duration, a repetition rate of 20 Hz.

25 The UVPR of LN:Zn and LN:In
H. Qiao, et al., Phys. Rev. B 70, (2004).

26 The resistance of LN:Zr to UVPR
But for LN:Zr, the UV photorefraction was apparently decreased. Fig.1 The dependence of UV photorefractive diffraction efficiency and saturated refractive index change of LN:Zr on the doping concentration of Zr. The open symbols show the data for LN:Mg5. Fig.2 Beam distortion of the transmitted UV light passing through LiNbO3 crystals (wavelength 351 nm, intensity 1.6×105 W/cm2). (a) PLN; (b) LN:Zr1; (c) LN:Zr2; (d) LN:Zr5. F. Liu, et al., Opt. Lett. 35, 10 (2010)

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29 The UVPR of LN:Hf Fig.1 Distortion of transmitted UV beam spots after irradiation of 5 min (wavelength 351nm, intensity 18.5 kW/cm2); a–e correspond to LN doped with 2, 2.5, 3, 4, and 6 mol.% Hf. W. Yan, et al., Opt. Lett. 35, 601 (2010)

30 Comparison of LN:Mg, LN:Hf, LN:Zr and LN:Sn
Crystals Properties LN:Mg LN:Hf LN:Zr LN:Sn Optical damage resistance (W/cm2, 514.5nm) 5105 * 5105 >2107 4.8105 Saturation refractive index change (514.5nm) 7.8 10-6 * 8.4 10-6 7.1 10-7 7.65 10-6 Doping threshold (mol% in melt) 4.6 ~2.5 2.0 2.5 Distribution coefficient 1.2 0.93 0.97 0.98 UV Photorefraction (351nm) 2.1 10-5 ** ____ 1.1 10-6 ______ Here is a comparison of Mg, Hf and Zr doped LN. We can see Zr doped LN has the highest optical damage resistance, smallest saturation refractive index change, the lowest doping threshold, and a distribution coefficient nearest 1, and low UVPR *6.5 mol% MgO; **5 mol% MgO in melt.

31 Fe2O3 doped LiNbO3 (LN:Fe)
3. Photorefraction Fe2O3 doped LiNbO3 (LN:Fe) By now, Fe2O3 doped LiNbO3 (LN:Fe) is one of the most excellent candidate materials for optical data storage due to its: high diffraction efficiency, high data storage density, long storage lifetime. iron [΄aiən]; manganese [͵mæŋge΄ni:z] As we known, Fe doped LiNbO3 is main candidate materials for optical data storage. due to its high diffraction efficiency, high data storage density, and long storage lifetime. But there are still some problems, such as low response speed, strong light-induced scattering, and volatility. The problems: low response speed, strong light-induced scattering, volatility.

32 A solution to increase the response speed
Co-doping with damage-resistant elements such as Mg, Zn, In and Sc, has been found to be a useful way to increase the response speed and resistance to scattering. When the doping concentrations are above the threshold, Fe3+ ions and part of Fe2+ ions on Li sites will be repelled to Nb sites, improves the response speed. apparently decreases the diffraction efficiency. A solution proposed by Zhang Guangyin is doping with damage-resistant elements such as Mg, Zn, In and Sc. But this solution is a double-edged sword. though the response speed remarkably improves. but the diffraction efficiency apparently decrease. G. Zhang, Proc. SPIE 2529, 14 (1995).

33 HfO2 and Fe2O3 co-doped LiNbO3 (LN:Fe,Hf)
Samples Doping concentrations Photorefractive properties Fe (wt.%) Mg (mol%) Hf ηsat (%) τr (s) S (cm/J) LN:Fe 0.01 70 160 LN:Fe:Mg2 2 60 LN:Fe:Mg6 6 15 LN:Fe:Hf2 0.03 68.0 17.2 3.99 LN:Fe:Hf4 4 47.6 12.6 4.36 LN:Fe:Hf5 5 55.4 10.7 5.23 S. Li, et al., Appl. Phys. Lett. 89, (2006)

34 ZrO2 and Fe2O3 co-doped LiNbO3 (LN:Fe,Zr)
Samples Doping concentrations Photorefractive properties Fe (wt.%) Mg (mol%) Zr ηsat (%) τr (s) S (cm/J) LN:Fe 0.01 70 160 LN:Fe,Zr1 0.03 1 25.5 2.2 13.46 LN:Fe,Zr2 2 32.0 1.8 12.87 LN:Fe,Zr3 3 32.7 13.48 LN:Fe,Zr4 4 32.5 13.40 LN:Fe,Zr5 5 42.2 12.61 when Zr and Fe co-doped into LN, though the saturation diffraction efficiency is decreased to about one half of that of LN:Fe, the recording time is shorten to only 2 second, so the photorefractive sensitivity is greatly increased. Y. Kong et al., Appl. Phys. Lett. 92, (2008)

35 The OH- absorption spectra of LN:Fe,Zr
3507 cm-1: Fe3+ in Nb-site To investigate what causes the difference of LN:Fe,Zr and LN:Fe,Mg, the OH absorption spectra was measured. As we known, when the doping concentration of Mg exceeding threshold, the OH absorption peak corresponding to Fe3+ occupying Nb-site will appear at about 3507 cm-1. (the reciprocal of cm) [ri΄siprəkəl], but in the OH absorption spectra of LN:Fe,Zr, there is no such absorption peak, so in LN:Fe,Zr, the Fe3+ are still occupying Li-sites. LN:Fe,Zr: from top to bottom are for 1, 2, 3, 4, and 5 mol% Zr, respectively; 0.03 wt% Fe LN:Fe:Mg

36 The UV-Visible spectra of LN:Fe,Zr and LN:Fe,Hf
400~700 nm: Fe2+Nb5+ intervalence transfer The ultraviolet-visible absorption spectra of LN:Zr are also measured. As we known, the wide absorption from 400 nm to 700 nm corresponds the inter-valence transfer from Fe2+ to Nb5+. From this figure, we can see there are no obvious difference when the doping concentration of Zr increases from 1 mol% to 5 mol%, which indicates that the site occupation of Fe2+ does not change while the doping concentration of Zr below and above the threshold. Therefore, the absorption spectra show Fe2+/3+ ions still remain at Li sites when the doping concentration of Zr goes above its threshold value! Fe2+/3+ ions still remain at Li sites when the doping concentration of ZrO2 or HfO2 goes above its threshold value! LN:Fe, Zr: A, B, C, D, and E are for 1, 2, 3, 4, and 5 mol% Zr, and X and Y are for 2 and 5 mol% Hf, respectively; 0.03% Fe.

37 Comparison of LN:Fe, LN:Fe,Mg, LN:Fe,Hf and LN:Fe,Zr
Samples Doping concentrations Photorefractive properties Fe (wt.%) Mg (mol%) Hf Zr ηsat (%) τr (s) S (cm/J) LN:Fe 0.01 70 160 LN:Fe:Mg6 6 15 LN:Fe:Hf5 0.03 5 55.4 10.7 5.23 LN:Fe:Zr2 2 32.0 1.8 12.87 Here is a comparison of Fe doped, Fe and Mg, Hf, or Zr co-doped LN. We can see Fe and Zr co-doped LN has the shortest response time and highest photorefractive sensitivity. S. Li, et al., Appl. Phys. Lett. 89, (2006) Y. Kong et al., Appl. Phys. Lett. 92, (2008)

38 Nonvolatile holographic storage
LiNbO3:Fe,Mn A big shortcoming of the common holographic storage is the volatility, that is the reading light will gradually erase the grating. In 1998, Karstern Buse accomplished non-volatile holographic storage in LN:Fe,Mn by two light. However, as we see the response time is too long, about 10 mins. one-center two-center K. Buse, et al., Nature 393, 665 (1998)

39 Energy level diagram of LiNbO3
Conduction band Conduction band 1.6 eV NbLi4+/5+ 2.5 eV 2.6 eV 2.6 eV 2.8 eV 2.8 eV Then we revised the energy level diagram proposed by Karstern Buse. The real energy level diagram of CLN should certainly include the intrinsic photorefractive centers. It is the co-doping of Zr the eliminates the un-desired intrinsic photorefractive centers. NbLi4+/5+ NbNb4+/5+ EFermi Fe2+/3+ EFermi Fe2+/3+ Mn2+/3+ Mn2+/3+ CLN:Mn,Fe LN:Zr,Fe,Mn The co-doping of Zr eliminates undesired intrinsic electron traps, which greatly enhances the charge transition speed for nonvolatile holographic storage

40 LiNbO3:Zr,Fe,Mn Y. Kong et al., Opt. Lett 34, 3896 (2009)
Oxidation time Irec/Isen (mW/cm2) s (%) f S’ (cm/J) r (s) 24h 800/40 54.3 14.9 0.65 2.4 600/40 52.1 14.5 0.88 2.2 400/40 62.0 13.6 1.78 1.2 48h 57.0 7.8 1.13 2.0 20h 62.5 14.0 2.10 As Zr is co-doped with LN:Fe,Mn, we can see the response time is shorten to only 2s. And the sensitivity is two order of magnitudes higher than the results of Hessilink. Y. Kong et al., Opt. Lett 34, 3896 (2009)

41 Comparison of LN:Zr,Fe,Mn, LN:Mg,Fe,Mn, and LN:In,Fe,Mn

42 LiNbO3:Zr,Cu,Ce Oxidation Time Isen/Irec (mW/cm2) ηsat (%) ηnon S (cm/J) S’ 13h 40/400 62.4 6.3 0.312 0.099 24h 72.6 6.6 0.079 0.024 40/600 74.2 4.3 0.033 0.008 40/800 72.7 3.0 0.025 0.005 The light intensity dependence of the measured light-induced scattering in the samples of triply doped LiNbO3 crystals. The lines are guides to the eyes. F. Liu et al., Opt. Express 18, 6333 (2010)

43 The sensitivity of LiNbO3 co-doped with different ions for nonvolatile holographic storage
Crystal component S(cm/J) S’(cm/J) Reference LiNbO3:Fe,Mn 0.07 K. Buse, et al., Nature 393, 665 (1998) sLN(Li/Nb=49.65/50.35) 0.03 L. Hesselink, et al., Science 282, 1089 (1998) LiNbO3:Cu,Ce 0.022 Y. Liu, et al., Opt. Lett. 25, 908 (2000). LiNbO3:Fe,Cu 0.035 D. Liu, et al., Appl. Opt. 41, 6809 (2002). LiNbO3:Ce,Mn 0.0025 Q. Dong, et al., Appl. Opt. 43, 5016 (2004). sLiNbO3:Cu,Ce (Li/Nb=49.57/50.43) 0.107 X. Li, et al, Appl. Opt. 46, 7620 (2007). LiNbO3:Mg,Fe,Mn 0.047 W. Zheng, et al., Cryst. Res. Tech. 43, 526 (2008). LiNbO3:Zr,Fe,Mn 3.47 1.31 Y. Kong et al., Opt. Lett 34, 3896 (2009) LiNbO3:Zr,Cu,Ce 0.312 0.099 F. Liu et al., Opt. Express 18, 6333 (2010)

44 4. Concluding remarks The above results indicate that tetravalent ions are excellent choice for the control of optical damage or photorefraction of LN. These results also open a door for us to understand the micro-mechanism of optical damage resistance. These results give us suitable choices for crystal design. The above results suggest that Zr is an excellent choices for the control of optical damage or photorefraction of LN. The remaining question is, why LN:Zr has such outstanding properties as compared with LN:Hf and LN:Mg? The question remains: Why LN:Zr has such outstanding properties as compared with LN:Hf, LN:Sn, and LN:Mg?

45 Silicon single crystal
Fig. 2. Cross-sectional view of the defect-free, near-surface region of a silicon wafer. The lower portion of the figure shows silicon dioxide precipitates used for impurity gettering. Fig. 1. Range of electrical resistivities of pure and donor-doped silicon single crystals shown in comparison with metals and insulators. H. Queisser, et al., Science 281, 945 (1998)

46 Optical fiber In 1966, Prof. Kao and Hockham proposed that when the loss of glass fiber was less than 20 dB/km it could be used as a conductor for optic communication, however at that time the loss of the best optical glass in the world was as large as 1000 dB/km. In 1970,Corning Incorporated made optical fibers with loss of 20dB/km. In 1974, the loss of optical fiber reduced to 2 dB/km as the purity of raw materials increased to 8N. In 1976, the loss of optical fiber reduced to 0.5 dB/km as the concentration of OH in raw materials controlled in the order of ppm. In 1980, the transport loss of optical fiber dropped to only 0.2 dB/km, which is closed to the theoretical value of 0.15dB/km. Kao Kuen, the Vice-Chancellor of CUHK

47 How about lithium niobate crystals?
Though lithium niobate has been dubbed as “optical silicon” or “photonic silicon”, compared with silicon single crystal and optical fiber, its research is rather preliminary. We do not exactly know: the defect structures, even the intrinsic defects, the function of every dopant, the relationship between defects and optical or photonic properties. We are far from what we expect: The control of defects; The growth of high quality single crystals. Our dream!

48 Thank you for your attention!
That is all, thank you for your attention! And Welcome to Chinese Physical Society 2010 Fall Conference


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