Microscopic mechanism of suppressing photorefraction in LiNbO3:Mg,Fe crystals

Solid State Communications, Vol. 98, No. 6, pp. 523-526, 1996 Copyright 0 1996 Published by Elsevier Science Ltd Printed in Great Britain. All rights reserved

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MICROSCOPIC MECHANISM OF SUPPRESSING PHOTOREFRACTION IN LiNbOs:Mg,Fe CRYSTALS Jianjnn Lin, WanIin Zhang, Cuangyin Department

Zhang

of Physics, Nankai University, Tianjin, 300071, P. R. China

(Received 5 January 1996; accepted 31 January 1996 by Z. Can)

The infrared absorption spectra of OX in LiN’bO,:Mg,Fe crystals have been investigated.

It

is shown that near the Mg concentration threshold the OH absorption bands successively shift from 3484 cm” to 2504 cm-’ and 3535 cm-‘. The intensity of the 3504 cm-’ band firstly increases to a maximum value, then decreases as the Mg content increases.

This result

contributed to the substitution of Fe ions into N’b sites due to Mg-doping in crystal. The site alteration of Fe ions from the Li sites to Nb sites is the origin of increasing the resistance against optical damage.

Keywords: D. optical properties, E. light absorption and reflection.

1. Introduction

tra in LiNbO,:Mg, tion threshold.

LiNbO,

crystal

cal material,

is a well-known

but its application

photorefractive

nonlinear opti-

is limited

by the

effect (e.g. optical damage).

Zhong

et a.111 reported that the resistance

of LI’NQO, to

optical damage was greatly imprwed

if more than

4.6mol% MgO was added to the congruent melt. Bryan et aL[2,3] confirmed the obserration existence

of the threshold

and found

effect with regard to the

Mg-doping level. The photoconductivity LiNbO,

crystal

is drasticalIy

hanced photoconductivity

of Mg-doped

increased.

reduced trapping cross-section and the smaller trapping

of Fe*+ for electrons,

cross section may be be-

cause of a changed substitutional

site for Fe’+[4].

The site of Fe seems to be a important the susceptibility

of LiNKA

fractive index inhomogeneities. significant

The en-

was attributed to a greatly

re-

The microscopic

origin of suppressing

pho-

torefraction is analysed.

2. Experimental The LiN60,

crystal was grown using the Czoch-

ralski method along the c-axis. 5mol% MgO and 0.05 mol% Fe,08

were added to the congruent melt. The

as-grown crystal was cut parallel to the c- axis and polished.

The OH-

absorption

sured with a Nicolet

spectra were mea-

710 FT-IR

spectrometer.

A

rectangular slit was used before the crystal in order to record the OH-

absorption

spectra in different

part of the crystal along the c-tis.

factor for

to the lasepinduced

3.

Results and Discussion

Thus, it will be very

to 6nd out the correlation

changes in defect structure

cussed.

Fe crystal near the Mg concentra-

The sites occupied by Fe ions are dis-

of crystal

between

the

and optical

damage center.

Fig.1 shows the OHc-axis.

Because

the effective

of Mg is greater than In this paper we study the OH- absorption spec-

absorption

ferent part of the LiNbO&fg,Fe 1 (k=

spectra in dif-

crystal along the

distribution

coefficient

1.2)(5], the Mg con-

tent in the crystal is found to change continuously

524

SUPPRESSING PHOTOREFRACTION IN LiNbO,:Mg,Fe CRYSTALS

dong the pulling axis.

per part of the crystal is above the threshold, in the lower part below the threshold part near the threshold.

and in the central

From the bottom

to the

top of Fig. 1, it is shown a successive transition havior of OH3504cm-i

and 3535cm-‘.

comes weak and a peak at 3504 cm-i dition to a peak at 3535cm-‘.

as Mg content increases the intensity

pearing in different valence states. Of special importance are Fe ions. Fe ions occur as Fe’+ and Fe’+. In both charge states Fe occupies the Li lattice sites(9].

be-

appears in ad-

It is noticeable

by impurity ap

Fe’* is a electron donor and Fe’+ is a electron accep

after that three peaks

The peak at 3484cm-’

determined

to

At first, only one strong

peak appears at 3484cm-‘;

refractive index changes in

LiNbOs are completely

be-

absorption bands from 3484cm-’

emerge simultaneously.

The photo-induced

The Mg content in the up-

Vol. 98, No. 6

that

of 3504cm-’

tor. Fe2+ is characterized by optical absorption near and near 2.6eV, they correspond to the ‘T,-‘E

l.leV

and Fe’+-Nb’+

d-d transition respectively[lO].

intervalence

Under illumination

transfer,

of visible light

the Fe’+-Nb6+

transition

conduct band.

The drift and diifusion of photoex-

produces free electrons in

band firstly increases to a maximum value! then de-

cited free electrons rest& in the generation of space

creases while that of the 3535cm-’

charge fields which modulates

from first to last. 3484em-’

band increases

band is the band slways

via the electro-optic

effect.

present in nominally pure or weakly doped LiNbOt

tion of Fe’*

crystals.

effect in visible region. The O’--Fe’+

The sppesrance

of 3535cm-’

indicates that

is responsible

the refractive index

The Fe’+-Nb6+

the Mg content in the crystal is above the threshold.

Fe’+

3535cm-’

the corresponding

band has been attributed

brations in M&-OH--Mg$

to hydroxys vi-

complex[b]. 3504cm-’

band emerges only in doubly doped LiNbOt with Mg and Fe in which Mg content exceeds the threshold. is interpreted

as an OH-

Fe&> complex(7,8].

vibration in MgtT-OH--

Our experimental

show that the 3504cm-’

It

results clearly

band begins to appear only

near the Mg concentration

threshold.

trani-

for the photorefractive transitions

ions produce the free holes in valence

superimposed

absorption

of

band,

band starts at 3.leV

on the fundamental

absorption

edge

3.8eV. In congruent LiNbO, crystals there are a lot of antisite

defects N6si due to Li deficiency[lO].

sults of a chemicd

analysis

advanced model calculation(ll] incorporated

and

indicate that Mg is

on Li sites replacing the anti&e

Nb,_i.

removal of Nb‘iy Mg ions simul-

After a complete taneously

Re-

of LiNbOs:Mg(5],

enter the Li sites and the Nb sites.

In

doubly doped LiNbO, with Mg and Fe, how do Mg ions influence the sites of Fe ions? Our experimental results reveal that the Fe ions are pushed to the Nb sites from the Li sites by the Mg ions when the Mg content exceeds the threshold. appears near the Mg threshold

3504cm-i

band

concentration

indi-

cates that the Fe ions are pushed to the Nb sites by Mg ions after a complete

removal of NIL;.

As the

Mg content increases there are more Fe ions entering the Nb sites to form the M&-OH--Fe&: 3504em-’

band becomes

ions are replaced, 3430

3480

3530

Wavelength

3580

(cm-=)

ing Mgir-iUg$

the LiNbOs:Mg,Fe

crystal along the c-axis

After slI FeLi

Because MgtT-M&

pairs.

complex are pulled to M&-M&

ing Me:,‘-OH--Mg$ absorption spectra in different part of

stronger.

ions enter Nb site formhas a

stronger force to attract H+, the H+ in the MgtT OH--Fe:>

Fig.1. OH-

Mg’+

complex,

of 3504em-’

complex.

form-

Thus, the intensity

band decreases while that of 3535em-’

band increases.

Though above the Mg threshold con-

(from the bottom to the top Mg content

centration

increases)

weaker, the Fe ions still occupy the Nb sites.

the 3504cm-’

band become weaker and

SUPPRESSING PHOTOREFRACTION IN LiNbO,:Mg,Fe CRYSTALS

Vol. 98, No. 6

The removal of Fe from Li site to Nb sites transto

forms the function of Fe ions from donor(Fe’+) the scceptor(Fe’+)

since Fe ions prefer to occur as

Li sites are not affected. At this situation, if the amount of the Fe’+ ions in LiNbOs:Mg,Fe crystal is increased, the photorefraction

can still reaches a

quite large value in spite of the increase of the photo-

Fe’+ rather than Fe’+ in Nb sites. The reduction of Fe’+ ions lead to the disappearance

525

conductivity

of the 2.6eV ab-

due to Mg-doping[lS].

Only when the

sorption band and a abrupt decrease of photovoltaic

Mg concentration

current(l2j.

ions are pushed to the Nb sites. The role played by

If there are not other donor centers in

the crystal it is impossible

to produce the photoex-

cited free electrons illuminated

is eliminated.

should be pointed out that the photorefraction relevance to the energy of the incident light. crystal is illuminated than 3.leV,

the Fe ions in the photorefraction

by visible light. Thus

the basic factor of photorefraction

all the Fe

is changed,

the

basic reason of optical damage is removed.

It

4. Conclusion

has If the

by a light which energy is higher

Our results show that near the Mg concentration

the Fe’+ ions at Nb sites can be excited

threshold the OR absorption band in LiNbO,:Mg,Fe crystal

The space

gradually evolves from 3484 cm-’ band to 3504 cm” band

to produce free holes in valence band.

charge fields resulted from the drift and diffusion of photoexcited

exceeds the threshold

free holes can also modulate the index

of refraction.

and 3535 cm-’ band with Mg content rising.

This fact

reveals that after complete Mg9’ substitution for Nb,, at Li sites, the Fe ions at Li sites begin to be pushed to the Nb sites. The change of Fe sites make the Fe” ions lose their

Bryan et a1.(2,3] found that as the Mg content increases the photoconductivity

drastically increases

donor properties leading to the disappearance of the 2.6 eV absorption band. The microscopic origin of suppressing the

and thought that the increase of the photoconducti-

photorefraction in LiNbO,:Mg,Fe crystals is the alteration

vity is primariIy responsible for the increase of the re-

of the Fe sites in the lattice by Mg-doping.

sistance against the optical damage. It is reasonable because

the photorefraction

ratio of photovoltaic vity.

is proportional

The increase of the photoconductivity

decreases

to the

current to the photoconducti-

the photorefraction

indeed

but it can not tho-

roughly eradicate the optical damages. content is below the threshold NbL; are gradually substituted,

When Mg

the antisite

Acknowledgment-

The work was supported by

a grant for Key Research Project in Climbing Program from the State Science and Technology mission of China, and by the National

defects

ence Foundation

but the Fe ions at

ence Foundation

Com-

Natural Sci-

of China, China Postdoctoral

Sci-

References

[l] Gi-Guo Zhong, JinJian, Zhong-Kang International

Quantum Electronics

Wu, 1 lth

Conference!

IEEE Cat. No.80, CH 61-0, P631, June,1980.

Appl. Phys. Lett., 44, 847(1984).

R. R. Rice,

R. Gerson? H. E. Tomaschke.

J.

79, 682(1986).

F. Otto!

Silva. M. Hage-Ali! J. A. Sans-Garcia,

J. C. soares, M. F. da

J. P. Stoquert,

P. Siffert.

G. Corradi, Zs. Szaller, K.

Polgar, J. Phys. Condens, 5, 781(1993). H. E. Halliburton,

D.A. Bryan, J. Appl. Phys., 60, 3553(1986). [S] B. C. Grabmaier,

Condens.

Matter, 5, 2423(1993). [8] L. Kovaes, L. Rebouta,

D. A. Bryan,

Appl. Phys., 57, 1036( 1985). [4] R. Gerson, J. E. Firchhoff,

Z. W. Yin, Sci. China A, 33, 108(1990). [7] Xi-Q; Feng, Tong B Tang, J. Phys.

(21 D. A. Bryan, Robert Gerson, H. E. Tomaschke,

[3] K. L. Sweeney, L. E. Halliburton,

[S] X. Q. Feng, Q. R. Zhong, J. F. Ying, J. C. Liu,

J. Crystal Growth,

[9] C. Prieto, C. Zaldo, Solid State Commun..

83,

819(1992). [lo] 0. F. Schirmer, 0. Thiemann,

M. Wohlecke. J.

Phys. Chem. Solids, 52, 185( 1991).

SUPPRESSING PHOTOREFRACTION IN LiNbO,:Mg,Fe CRYSTALS

526 Ill]

H. Donnerberg,

S. M. Tomlinson,

Catlow,

Sehirmer,

0.

F.

Phys.

C. R. A. Rev.B.

44.

4877(1991). [lZ] H. X. Feng, J. K. Weu, H. Wang, H. F. Wang,

Vol. 98, No. 6

Appl. Phys. A, 51, 394(1990). [13] Guangyin

Zhang, Jingjnn Xn, Simin Lin, Qian

Sun, Guoquan

Zhaug, Qiyin Fang, Chaoli Ma.

SPIE, 2529,14( 1995).