Tetravalent Ions Doped Tetravalent Ions Doped Lithium Niobate CrystalsLithium Niobate Crystals

Yongfa Kong, Shiguo Liu, Shaolin Chen, and Jingjun Xu School of Physics and TEDA Applied Physics School

OutlineOutline

1. Introduction1. Introduction 2. Optical damage resistance2. Optical damage resistance 3. Photorefraction3. Photorefraction 4. Concluding remarks4. Concluding remarks

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. Introduction1. 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 monthNovember 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 (LiNbO3), 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, piezo

electric, pyroelectric, ferroelectric, nonlinear optic, etc.

Multi-applications： Waveguides, modulators, isolators, frequency t

ransformers, optical parametric oscillators, filters, sensors, holographic storage, etc.

Property controllability： Good solubility to many dopants, Properties change with different dopants and d

oping concentrations.

Lithium niobate (LiNbO3, 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, 043903 (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 LiNbO3 and LiTaO3 crystals.

Photorefraction (PR) Can be used in

holographic storage, information processing, light control of light. low response speed,

volatility.

Optical damageHinders the applications:

frequency doublers, optical parametric

oscillators, Q-switches, optical waveguides.

A. Ashkin, et al., Appl. Phys. Lett. 9, 72(1966)

2. Optical damage 2. Optical damage resistanceresistance

A solution: doping1980, Mg2+ 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, Zn2+ ions, LN:Zn; 1992, Sc3+ ions, LN:Sc; 1995, In3+ 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).

HfO2 doped LiNbO3 (LN:Hf)

E.P. Kokanyan et al., J. Appl. Phys. 92 1544 (2002); Appl. Phys. Lett. 48, 1980 (2004).

Optical damage resistance of LN:Hf

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.

(a)    2 mol% Hf； (b) 4 mol% Hf； (c) 6 mol% Hf； (d) 6.5 mol% MgThe light intensity for (a) is 104 W/cm2 and 5×105 W/cm2 for (b), (c), and (d).

S. Li et al., J. Phys.: Condens. Matter. 18, 3527 (2006).

As the doping concentration reaches 2.0 mol% ZrO2, LN:Zr crystals can withstand a light intensity as high as 2.0107 W/cm2.

At the same experimental conditions, the light intensity that 6.5 mol% Mg doped LN (LN:Mg6.5) can withstand is about 5.0105 W/cm2.

(a), (b) and (c) LN:Zr1.7; (d) LN: Zr2. The light intensity for (a) is 1.3103 W/cm2, (b) 1.3104 W/cm2, (c) and (d) 2.0107 W/cm2.

(a) (c)(b) (d)

Y. Kong et al., Appl. Phys. Lett. 91, 081908 (2007).

ZrO2 doped LiNbO3 (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.

0 .0 1 .0 2 .0 3 .0 4 .0 5 .0 6 .0 7 .0D o p in g L e v e l (m o l% )

1 .0

1 0 .0

1 0 0 .0

Cha

nge

of re

frac

tive

inde

x (1

0 )-6 :M g O

:H fO:Z rO

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 0.97.

Therefore, the distribution coefficient of Zr is much closer to one than that of Mg.

0 .0 1 .0 2 .0 3 .0 4 .0 5 .0 6 .0D o p in g L ev e l (m o l% )

0 .8

0 .9

1 .0

1 .1

1 .2

Dis

tribu

tion

Coe

ffic

ient

SnO2 doped LiNbO3 (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) 2.5×102 W/cm2, (b) 4.7×103 W/cm2, (c) 4.8×105 W/cm2, and (d) 5.4×105 W/cm2.

The distribution coefficient of LN:Sn

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

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, 094101(2004).

The resistance of LN:Zr to UVPR

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.

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/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)

CrystalsProperties LN:Mg LN:Hf LN:Zr LN:SnOptical damage resistance (W/cm2, 514.5nm)

5105 * 5105 >2107 4.8105

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______

*6.5 mol% MgO; **5 mol% MgO in melt.

Comparison of LN:Mg, LN:Hf, LN:Zr and LN:Sn

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.

3. Photorefraction3. PhotorefractionFe2O3 doped LiNbO3 (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, 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.

G. Zhang, Proc. SPIE 2529, 14 (1995).

HfO2 and Fe2O3 co-doped LiNbO3

(LN:Fe,Hf)

S. Li, et al., Appl. Phys. Lett. 89, 101126 (2006)

Samples

Doping concentrations

Photorefractive properties

Fe (wt.%)

Mg(mol%)

Hf (mol%)

ηsat

(%)

τr

(s)

S (cm/J)

LN:Fe 0.01 70 160LN:Fe:Mg2 0.01 2 70 60LN:Fe:Mg6 0.01 6 15 15LN:Fe:Hf2 0.03 2 68.0 17.2 3.99LN:Fe:Hf4 0.03 4 47.6 12.6 4.36LN:Fe:Hf5 0.03 5 55.4 10.7 5.23

ZrO2 and Fe2O3 co-doped LiNbO3

(LN:Fe,Zr)

SamplesDoping concentrations Photorefractive properties

Fe (wt.%)

Mg(mol%)

Zr (mol%)

ηsat (%)

τr (s)

S (cm/J)

LN:Fe 0.01 70 160LN:Fe,Zr1 0.03 1 25.5 2.2 13.46LN:Fe,Zr2 0.03 2 32.0 1.8 12.87LN:Fe,Zr3 0.03 3 32.7 1.8 13.48LN:Fe,Zr4 0.03 4 32.5 1.8 13.40LN:Fe,Zr5 0.03 5 42.2 2.2 12.61

Y. Kong et al., Appl. Phys. Lett. 92, 251107 (2008)

The OH- absorption spectra of LN:Fe,Zr

3507 cm-1:Fe3+ in Nb-site

LN:Fe:MgLN: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

Fe2+/3+ ions still remain at Li sites when the doping concentration of ZrO2 or HfO2 goes above its threshold value!

400~700 nm ： Fe2+Nb5+ 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

SamplesDoping concentrations Photorefractive properties

Fe (wt.%)

Mg(mol%)

Hf (mol%)

Zr (mol%)

ηsat

(%)

τr

(s)

S (cm/J)

LN:Fe 0.01 70 160LN:Fe:Mg6 0.01 6 15 15LN:Fe:Hf5 0.03 5 55.4 10.7 5.23LN:Fe:Zr2 0.03 2 32.0 1.8 12.87

S. Li, et al., Appl. Phys. Lett. 89, 101126 (2006)Y. Kong et al., Appl. Phys. Lett. 92, 251107 (2008)

Nonvolatile holographic storage

LiNbO3:Fe,Mn

K. Buse, et al., Nature 393, 665 (1998)one-center two-center

Energy level diagram of LiNbO3

The co-doping of Zr eliminates undesired intrinsic electron traps, which greatly enhances the charge transition speed for nonvolatile holographic storage

NbLi4+/5+ NbNb

4+/5+

NbLi4+/5+

Conduction band

2.8 eV2.6 eV

2.5 eV

1.6 eV

Mn2+/3+

Fe2+/3+EFermi

Conduction band

2.8 eV2.6 eV

Mn2+/3+

Fe2+/3+EFermi

CLN:Mn,Fe LN:Zr,Fe,Mn

LiNbO3:Zr,Fe,Mn

Oxidation time

Irec/Isen (mW/cm2)

s

(%)f

(%)S’

(cm/J)r

(s)

24h 800/40 54.3 14.9 0.65 2.4

24h 600/40 52.1 14.5 0.88 2.2

24h 400/40 62.0 13.6 1.78 1.2

48h 400/40 57.0 7.8 1.13 2.0

20h 400/40 62.5 14.0 2.10 0.88

Y. Kong et al., Opt. Lett 34, 3896 (2009)

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

LiNbO3:Zr,Cu,Ce

OxidationTime

Isen/Irec

(mW/cm2)ηsat

(%)ηnon

(%)S

(cm/J)S’

(cm/J)

13h 40/400 62.4 6.3 0.312 0.099

24h 40/400 72.6 6.6 0.079 0.024

24h 40/600 74.2 4.3 0.033 0.008

24h 40/800 72.7 3.0 0.025 0.005 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 LiNbO3 crystals. The lines are guides to the eyes.

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)

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 remarks4. 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 tha

t 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!

Thank you for your Thank you for your attention!attention!