Materials for Nonlinear Optics - American Chemical Society

Chapter 25

Defect Chemistry of Nonlinear Optical Oxide Crystals Patricia A. Morris

Downloaded by SUNY STONY BROOK on October 16, 2014 | http://pubs.acs.org Publication Date: March 11, 1991 | doi: 10.1021/bk-1991-0455.ch025

Central Research and Development Department, Ε. I. du Pont de Nemours and Company, Wilmington, DE 19880-0306

The defect chemistry of a specific crystal is determined by both its structural characteristics and the growth, or processing, of the material. Structurally, the nonlinear optical oxides contain anionic oxide groups (i.e. TiO , NbO , PO , AsO , B O , B O ) which are the basic structural units responsible for the second order nonlinear optical susceptibility. The relatively large contribution of covalent bonding in the anionic groups to the total lattice energy appears to allow the structures to accommodate nonstoichiometric defects on the other, more "ionic", cation sublattice or sublattices with a relatively small cost of energy. This enhances the incorporation of many isovalent and aliovalent impurities into the crystals. The nonlinear optical oxide crystals recently developed are grown by flux or solution techniques to prevent decomposition or to obtain a low temperature phase. The intrinsic nonstoichiometry and the impurity contents of the as-grown crystals are determined by the solutions and temperatures used for growth. Recent work on the defects present in KTP, K T A , B B O and L B O crystals shows that the intrinsic defect concentrations in these materials are relatively low, compared to the more traditional nonlinear optical oxides having the perovskite, perovskite-like and tungsten bronze type structures. As a result, their defect structures can be dominated by impurities present at relatively small concentrations. The defect chemistry of nonlinear optical oxide crystals can affect many of the materials' properties required for device applications and several examples are described. 6

6

4

4

3

6

3

7

The defect chemistry of nonlinear optical oxide crystals can affect many of the materials' properties required for device applications. Applications of these crystals, 2

having high second order nonlinear optical susceptibilities (λ( )), include frequency convenors for laser systems, electro-optic modulators and switches, and holographic and phase conjugate optics.(1-5) The materials' requirements for device applications 2

include: 1) large %( ), 2) optical transparency in the wavelength range of interest, 3) low ionic and electrical conductivity for photorefractive, electro-optic and waveguide devices, 4) high optical damage threshold in frequency generation and Q-switching applications, 5) high sensitivity and fast response time of the photorefractive effect for

0097-6156/91/0455-0380$06.00/0 © 1991 American Chemical Society In Materials for Nonlinear Optics; Marder, S., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1991.

Nonlinear Optical Onde Crystals

25. MORRIS

381

holographic and phase conjugate optics and 6) homogeneity with respect to the optical properties and conductivity. Both intrinsic (i.e. nonstoichiometry) and extrinsic (i.e. impurities) defects may be present in nonlinear optical oxide crystals which affect the materials' properties of interest. The defect chemistry of a specific crystal is determined by both its structural characteristics and the growth, or processing, of the material. The purpose of this paper is to summarize the current understanding of the defect chemistry of nonlinear optical oxide crystals and specifically the relationship of the defects present to 1) the structure and growth, or processing, of the material and 2) the properties of interest for device applications. The defects in traditional nonlinear optical oxide crystals (i.e. BaTi03, L i N b 0 3 , S r i - B a N b 2 0 6 , B a 2 N a N b 5 0 i 5 , K 3 L i 2 N b 5 0 i 5 ) are reviewed. Our recent work on the defect chemistry of new x

x


nonlinear optical oxide crystals (i.e. K T 1 O P O 4 , K T 1 O A S O 4 , P-BaB204, then discussed.

ΠΒ3Ο5)

is

Induced Polarization and Origin of the Second Order Nonlinear Susceptibility A polarization is induced in a material when subjected to laser radiation or dc electric fields. The following expression (1-3), Pi (ω) = ε

0

[λ

(ω) Ej (ω) + 2 χ

ijk

(2) (ω = α>ι + ω ) Ej (ωΟ E 2

k

(ω )] 2

for the induced polarization (Pi) includes the first two terms in the series, where Ε is the electric field strength associated with the incident radiation or dc electric field, and ω is the frequency. The first term describes the linear optical effects: absorption, refraction, emission and reflection. The second term is responsible for the second order nonlinear polarization processes, such as second harmonic generation, parametric sum or difference mixing and the linear electro-optic or Pockels effect. Second harmonic generation and parametric generation are typically used to extend the frequency range of solid state lasers. The Pockels electro-optic effect is used in applications involving Q-switches for laser systems, optical modulators and switches, and the photorefractive effect for real-time holography and phase conjugation. The basic structural units responsible for the second order nonlinear optical susceptibility in most oxide crystals are the acentric anionic groups. (4,6) The macroscopic X@) is determined by the microscopic nonlinear susceptibility of the bonds in the acentric oxide group, the number and orientation of equivalent groups in a unit cell, and the number of unit cells per unit volume. The following are the acentric oxide 2

groups contributing to 3C3

Ba(i. )TiO(3. ):x = 0.01

V W . Vo"

LiNb0

Li i. )NbO :x = 0.036

Nbn"", V b

x


3

(

Defects** 7

x

x

3

Sr i. )Ba Nb 06 : χ = 0.39

Ba2NaNb50i

Ba2Nao.72Nb50is

K Li Nb50i5

K .786Lil.989Nb50i

KT1OPO4

K^TiPOcs.^rxHO.OOOS "

x

x

6

5

3

2

β-Β Β0 3 2

3

2

x

x

4

+ +

+ + 4

+ + 5

(7-9)

(10,11)

(12,13)

2

VtfJ

VK

5

4

KTiOAs0

LiB 0

(

7 / / / /

N

Sr i. )Ba Nb20 (

References

7

(14,15)

,Vl/

V K ' , Vo"

VR , VO", 7

(16,17)

(30,31)

{ASTiO

77

{VBa } , {Vo"}

{ V u O . (Vo"}

* ** + ++

Representative of the range of intrinsic nonstoichiometry in as-grown crystals. The defects are presented as being fully ionized. Represents typical flux grown crystals. Defect structure in crystals presently grown are thought to be extrinsically controlled. { } Presence of this defect is suspected; insufficient data exists for confirmation. Example: V K / is a vacant potassium site with an effective negative one charge. Example: NbLi

is a niobium on a lithium site with an effective positive four charge.

In Materials for Nonlinear Optics; Marder, S., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1991.


384

MATERIALS FOR NONLINEAR OPTICS: CHEMICAL PERSPECTIVES

presently used to grow KTP because the crystal decomposes upon melting. (32) Much work has been done to understand the defects in KTP crystals in the past two years and the results will be summarized here. For further discussion see references.(30,31,33) The intrinsic defects present in KTP crystals are vacant potassium ( V K ) and vacant oxygen (Vo) sites. This results in a very limited range of nonstoichiometry, relative to that observed in traditional nonlinear optical oxides (Table I). This mechanism of intrinsic defect formation in K T P has been confirmed by mass spectroscopic analysis of the gases evolved from typical KTP crystals. The intrinsic defect concentrations are dominant in typical KTP crystals grown by the flux technique and are very temperature dependent over the range of temperatures practical for flux growth. The calculated defect formation energy, using the bulk ionic conductivities of crystals grown by the flux technique over a range of temperatures (Figure 1) is approximately 5 eV per defect. Protons are the dominant defects in hydrothermally grown K T P and are most likely the primary defect contributing to the formation of V K sites in these crystals. Protons are present in K T P grown by both the flux and hydrothermal techniques and are considered to be present in the form of OH". The relative amounts of OH" found in KTP crystals as a function of the growth technique are high temperature hydrothermal > low temperature hydrothermal > flux, but all concentrations are within the same order of magnitude, estimated to be in the range of a hundred ppm.(33,34) Protons present as O H " in K T P can be distributed in multiple sites on each of the eight inequivalent oxygen sites in the unit cell. The distribution of OH" sites in KTP is a function of the technique and conditions used for growth (i.e. activity of H 2 O and the K/P ratio, or effective pH, in the solution or flux, growth temperature, etc.). Protons in KTP can be charge compensated by the formation of V K ^ or TiTi^ sites in the crystal. The compensating defect formed is believed to depend on the location of the oxygen site where the OH" is present. Several isovalent ions form solid solutions with K T P (Table II), showing that this structure is relatively tolerant, with respect to isovalent impurities, as are the traditional nonlinear optical oxide crystal structures. But due to the relatively limited range of nonstoichiometry in KTP, aliovalent impurities, such as divalent Ba, Sr and Ca introduced through ion exchange in nitrate melts, which substitute on the Κ site, are incorporated at concentrations less than one mole percent.(36) Typical impurity concentrations present in flux and hydrothermally grown KTP are shown in Table III. Control of the ionic conductivity of KTP is important in both the processing of optical waveguides and to electro-optic waveguide device stability. (27) A moderate ionic conductivity is necessary to form waveguides in the material, but if excessive, the mode distribution in the waveguides can be altered during device processing. As discussed above, the intrinsic V K defect concentration is primarily responsible for the potassium ion conductivity of typical flux grown K T P crystals. Protons present in spécifie sites in hydrothermal KTP are compensated by V K sites, increasing the ionic conductivity above that expected due to the intrinsic V K defect concentration formed at the growth temperatures involved. This is shown in Figure 1. The line drawn through the data for K T P crystals grown over different temperature ranges by the flux technique represents the ionic conductivity in KTP dominated by the intrinsic V K defect concentrations. It is clear from the data for the high and low temperature hydrothermally grown K T P that some other defect mechanism is contributing to the ionic conductivity of these materials. The variations in ionic conductivities observed cannot be explained as due to conventional impurities. The concentrations of cation and anion impurities in flux and hydrothermal crystals of KTP, shown in Table III, are indistinguishable within the precision of techniques used. (The impurity concentrations in low temperature hydrothermally grown KTP are comparable to those grown using


25. MORRIS

Nonlinear Optical Oxide Cryslah

385

Table Π. Ions Forming Solid Solutions in Several Nonlinear Optical Oxide Crystals Crystal References Solid Solution Forming Ions or Ion Pairs BaTi03

Sr , Pb , (Na +Nb ), ( La +In ), 2+

2+

1+

5+

3+

Ce +, C a , S i , Zr *, Ge , L a 3

LiNb03

2+

4+

4

4+

3 +

T a , (Mg +Ti ) 5+

2+

(21)

4+

M g , C o , Z n , C r , Sc , S n 2+


(21)

3+

2+

2+

3+

3+

(22)

4+

(21)

L a , T i , W6+

BaiNaNbsOis

3+

4 +

Li*+, Kl+, Sr +, C a , Ta^+

(23)

R b , Ή1+, N F U , As5+,

(32)

Na , Agl+, C s

(35)

2

1+

KT1OPO4

1+

1+

0.0008

2+

0.0010

1 +

0.0012

0.0014

1/T (Growth) (1/K) Figure 1. The room temperature bulk ionic conductivity, along z, of KTP crystals as a function of the reciprocal of their midpoint growth temperatures, φ , Philips flux; • , DuPont flux; Ο» Airtron high temperature hydrothermal; • , Airtron low temperature hydrothermal.


386

MATERIALS FOR NONLINEAR OPTICS: CHEMICAL PERSPECTIVES

Table ΙΠ. Typical Impurity Content of K T 1 O P O 4 and K T 1 O A S O 4 Crystals Grown by the Flux and Hydrothermal Techniques (Parts Per Million by Weight)


Element

Β F Na Mg Al Si Ρ S Cl Ca V Cr Μη Fe Co Ni Cu As Sr Y Zr Nb Sn Sb Ba Pt Total

Flux KTP 0.05 0.5 (8.3) (

Materials for Nonlinear Optics - American Chemical Society

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