Presentation on theme: "Tetravalent Ions Doped Lithium Niobate Crystals Yongfa Kong, Shiguo Liu, Shaolin Chen, and Jingjun Xu School of Physics and TEDA Applied Physics School."— Presentation transcript:
Tetravalent Ions Doped Lithium Niobate Crystals Yongfa Kong, Shiguo Liu, Shaolin Chen, and Jingjun Xu School of Physics and TEDA Applied Physics School
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? 1. Introduction
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. Materials Update: Material of the month November 2002: Lithium niobate Indeed, because of its availability, widespread use and versatility, it has been dubbed by many as the “silicon of nonlinear optics”.
Silicon of photonics Lithium niobate (LiNbO 3 ), also called the ‘silicon of photonics’, is indispensable in advanced photonics and nonlinear optics. M. Kösters1, et al., Nature Photonics 3, 510 (2009)
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. Lithium niobate (LiNbO 3, LN)
“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."
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).
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; ………. What have been done on Lithium niobate crystal?
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 ………. Good enough?
Optical damage resistance Photorefraction Domain engineering Crystal growth Micro-mechanism of some effects and structural design What can tetravalent dopants do?
Optical damage Light-induced optical damage, now also named as photorefraction, was discovered in LiNbO 3 and LiTaO 3 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. A. Ashkin, et al., Appl. Phys. Lett. 9, 72(1966) 2. Optical damage resistance
A solution: doping 1980, Mg 2+ ions, LN:Mg; “Star of China” 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). It promotes the practical applications of LN in nonlinear optics at high light intensities. 1990, Zn 2+ ions, LN:Zn; 1992, Sc 3+ ions, LN:Sc; 1995, In 3+ ions, LN:In.
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).
HfO 2 doped LiNbO 3 (LN:Hf) E.P. Kokanyan et al., J. Appl. Phys (2002); Appl. Phys. Lett. 48, 1980 (2004).
Optical damage resistance of LN:Hf LN:Hf4 is able to withstand a light density of 5×10 5 W/cm 2 without noticeable beam smearing, which is comparable to that observed in 6.5mol% MgO doped LN (LN:Mg6.5) crystal. (a) 2 mol% Hf ； (b) 4 mol% Hf ； (c) 6 mol% Hf ； (d) 6.5 mol% Mg The light intensity for (a) is 10 4 W/cm 2 and 5×10 5 W/cm 2 for (b), (c), and (d). S. Li et al., J. Phys.: Condens. Matter. 18, 3527 (2006).
As the doping concentration reaches 2.0 mol% ZrO 2, LN:Zr crystals can withstand a light intensity as high as 2.0 10 7 W/cm 2. At the same experimental conditions, the light intensity that 6.5 mol% Mg doped LN (LN:Mg 6.5 ) can withstand is about 5.0 10 5 W/cm 2. (a), (b) and (c) LN:Zr 1.7 ; (d) LN: Zr 2. The light intensity for (a) is 1.3 10 3 W/cm 2, (b) 1.3 10 4 W/cm 2, (c) and (d) 2.0 10 7 W/cm 2. (a)(a)(c)(c)(b)(b) (d) (d) Y. Kong et al., Appl. Phys. Lett. 91, (2007). ZrO 2 doped LiNbO 3 (LN:Zr)
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. Light-induced change of the refractive index in saturation as a function of dopants
The distribution coefficient of Zr The maximum value is 1.04 and the minimum value is Therefore, the distribution coefficient of Zr is much closer to one than that of Mg.
SnO 2 doped LiNbO 3 (LN:Sn) L. Wang et al., Opt. Lett. 35, 883 (2010). 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) 2.5×10 2 W/cm 2, (b) 4.7×10 3 W/cm 2, (c) 4.8×10 5 W/cm 2, and (d) 5.4×10 5 W/cm 2.
The distribution coefficient of LN:Sn Dependence of the distribution coefficient of Sn 4+ ions in Sn:LN crystals on the doping levels of SnO 2.
Ultraviolet photorefraction (UVPR) Enhancement of UVPR in LN:Mg J. Xu, et al., Opt. Lett. 25, 129(2000)
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.
The UVPR of LN:Zn and LN:In H. Qiao, et al., Phys. Rev. B 70, (2004).
The resistance of LN:Zr to UVPR Fig.2 Beam distortion of the transmitted UV light passing through LiNbO 3 crystals (wavelength 351 nm, intensity 1.6×10 5 W/cm 2 ). (a) PLN; (b) LN:Zr1; (c) LN:Zr2; (d) LN:Zr5. 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. F. Liu, et al., Opt. Lett. 35, 10 (2010)
The UVPR of LN:Hf Fig.1 Distortion of transmitted UV beam spots after irradiation of 5 min (wavelength 351nm, intensity 18.5 kW/cm 2 ); 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)
By now, Fe 2 O 3 doped LiNbO 3 (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. 3. Photorefraction Fe 2 O 3 doped LiNbO 3 (LN:Fe) The problems: low response speed, strong light-induced scattering, volatility.
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, Fe 3+ ions and part of Fe 2+ ions on Li sites will be repelled to Nb sites, improves the response speed. apparently decreases the diffraction efficiency. G. Zhang, Proc. SPIE 2529, 14 (1995).
HfO 2 and Fe 2 O 3 co-doped LiNbO 3 (LN:Fe,Hf) S. Li, et al., Appl. Phys. Lett. 89, (2006) Samples Doping concentrations Photorefractive properties Fe (wt.%) Mg (mol%) Hf (mol%) η sat (%) τ r (s) S (cm/J) LN:Fe LN:Fe:Mg LN:Fe:Mg LN:Fe:Hf LN:Fe:Hf LN:Fe:Hf
ZrO 2 and Fe 2 O 3 co-doped LiNbO 3 (LN:Fe,Zr) Samples Doping concentrations Photorefractive properties Fe (wt.%) Mg (mol%) Zr (mol%) η sat (%) τ r (s) S (cm/J) LN:Fe LN:Fe,Zr LN:Fe,Zr LN:Fe,Zr LN:Fe,Zr LN:Fe,Zr Y. Kong et al., Appl. Phys. Lett. 92, (2008)
The OH - absorption spectra of LN:Fe,Zr 3507 cm -1 : Fe 3+ in Nb-site LN:Fe:Mg LN:Fe,Zr: from top to bottom are for 1, 2, 3, 4, and 5 mol% Zr, respectively; 0.03 wt% Fe
The UV-Visible spectra of LN:Fe,Zr and LN:Fe,Hf Fe 2+/3+ ions still remain at Li sites when the doping concentration of ZrO 2 or HfO 2 goes above its threshold value! 400~700 nm ： Fe 2+ Nb 5+ intervalence transfer 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.
Comparison of LN:Fe, LN:Fe,Mg, LN:Fe,Hf and LN:Fe,Zr Samples Doping concentrations Photorefractive properties Fe (wt.%) Mg (mol%) Hf (mol%) Zr (mol%) η sat (%) τ r (s) S (cm/J) LN:Fe LN:Fe:Mg LN:Fe:Hf LN:Fe:Zr S. Li, et al., Appl. Phys. Lett. 89, (2006) Y. Kong et al., Appl. Phys. Lett. 92, (2008)
Nonvolatile holographic storage LiNbO 3 :Fe,Mn K. Buse, et al., Nature 393, 665 (1998) one-center two-center
Energy level diagram of LiNbO 3 The co-doping of Zr eliminates undesired intrinsic electron traps, which greatly enhances the charge transition speed for nonvolatile holographic storage Nb Li 4+/5+ Nb Nb 4+/5+ Nb Li 4+/5+ Conduction band 2.8 eV 2.6 eV 2.5 eV 1.6 eV Mn 2+/3+ Fe 2+/3+ E Fermi Conduction band 2.8 eV 2.6 eV Mn 2+/3+ Fe 2+/3+ E Fermi CLN:Mn,FeLN:Zr,Fe,Mn
LiNbO 3 :Zr,Fe,Mn Oxidation time I rec /I sen (mW/cm 2 ) s (%) f (%) S’ (cm/J) r (s) 24h800/ h600/ h400/ h400/ h400/ Y. Kong et al., Opt. Lett 34, 3896 (2009)
Comparison of LN:Zr,Fe,Mn, LN:Mg,Fe,Mn, and LN:In,Fe,Mn
LiNbO 3 :Zr,Cu,Ce Oxidation Time I sen /I rec (mW/cm 2 ) η sat (%) η non (%) S (cm/J) S’ (cm/J) 13h40/ h40/ h40/ h40/ F. Liu et al., Opt. Express 18, 6333 (2010) The light intensity dependence of the measured light-induced scattering in the samples of triply doped LiNbO 3 crystals. The lines are guides to the eyes.
The sensitivity of LiNbO 3 co-doped with different ions for nonvolatile holographic storage Crystal componentS(cm/J)S’(cm/J)Reference LiNbO 3 :Fe,Mn─0.07K. Buse, et al., Nature 393, 665 (1998) sLN (Li/Nb=49.65/50.35) 0.03L. Hesselink, et al., Science 282, 1089 (1998) LiNbO 3 :Cu,Ce0.022─Y. Liu, et al., Opt. Lett. 25, 908 (2000). LiNbO 3 :Fe,Cu0.035─D. Liu, et al., Appl. Opt. 41, 6809 (2002). LiNbO 3 :Ce,Mn0.0025─Q. Dong, et al., Appl. Opt. 43, 5016 (2004). sLiNbO 3 :Cu,Ce (Li/Nb=49.57/50.43) ─0.107X. Li, et al, Appl. Opt. 46, 7620 (2007). LiNbO 3 :Mg,Fe,Mn0.047─W. Zheng, et al., Cryst. Res. Tech. 43, 526 (2008). LiNbO 3 :Zr,Fe,Mn Y. Kong et al., Opt. Lett 34, 3896 (2009) LiNbO 3 :Zr,Cu,Ce F. Liu et al., Opt. Express 18, 6333 (2010)
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 question remains: Why LN:Zr has such outstanding properties as compared with LN:Hf, LN:Sn, and LN:Mg? 4. Concluding remarks
Silicon single crystal Fig. 1. Range of electrical resistivities of pure and donor-doped silicon single crystals shown in comparison with metals and insulators. 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. H. Queisser, et al., Science 281, 945 (1998)
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.
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!