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Green-induced infrared absorption in MgO doped LiNbO₃

Y. Furukawa

a)

OXIDE Corporation, 9633 Kobuchisawa, Kitakoma, Yamanashi 408-0044, Japan

K. Kitamura

National Institute for Research in Inorganic Materials, 1-1 Namiki, Tsukuba 305-0044, Japan

A. Alexandrovski, R. K. Route, and M. M. Fejer

E.L. Ginzton Laboratory, Stanford University, Stanford, California 94305

G. Foulon

Crystal Technology Inc., 1040 East Meadow CIRcle, Palo Alto, California 94303

Received 25 September 2000; accepted for publication 2 February 2001

Green-induced infrared absorption GRIIRA was investigated by a photothermal technique for

undoped and Mg-doped LiNbO₃

crystals that have different Li/Nb ratios. Threshold effect on

GRIIRA was found, threshold MgO concentrations being the same for GRIIRA and photorefraction.

We suggest that GRIIRA is associated with the formation of the small polaron that is located on Nb

antisite defect. The remarkable decrease of GRIIRA in Mg:LiNbO₃

can then be attributed to the

elimination of this intrinsic defect, Nb in Li, following the incorporation of Mg on Li sites. For

nonlinear optical applications, LiNbO₃

doped with MgO at concentrations over threshold has a

combined advantage of having almost no GRIIRA and photorefraction. © 2001 American Institute

of Physics. DOI: 10.1063/1.1359137

In recent years there has been increasing interest in the

use of quasi-phase-matched QPM nonlinear crystals

1

for a

variety of frequency conversion applications. Periodically

poled lithium niobate PPLN has been demonstrated to give

efficient second-harmonic generation

2

and optical parametric

oscillation

3

in both the cw and the Q-switched regimes.

However, the performance of high power operating PPLN-

SHG devices has been limited by material issues such as

photorefractive beam distortion and green-induced infrared

absorption GRIIRA .

4

It has been demonstrated that the

former problem can be solved by either high temperature

device operation or MgO doping to the LiNbO₃

crystal, how-

ever, the latter problem still remains to be investigated. De-

fect models of the phenomena of green-induced infrared ab-

sorption have not been developed. Recently we reported that

photorefraction in LiNbO₃

was completely eliminated by the

doping of small amounts of MgO in crystals with near-

stoichiometric composition.

5

These notable changes in mate-

rial properties are strongly related to the elimination of Nb

antisite defects (Nb

Li

) by the substitution of Mg on Li sites.

In this letter, we investigate the influence of MgO dop-

ing on the GRIIRA properties in LiNbO₃

, and demonstrate

that LiNbO₃

crystals doped with MgO show remarkably

lower GRIIRA than undoped LiNbO₃

crystals.

LiNbO₃

single crystals grown by two different methods

were used in this study. They are denoted CLN when theIR

stoichiometry, expressed as the ratio c

Li

=[Li] /( [Li]+

[Nb] ), has a congruent-composition value of 48.4% and

SLN when it deviates from this value in the direction of the

stoichiometric composition of 50%. crystals denoted as SLN

and Mg:SLN were grown as described in earlier reports

6

by

use of a top-seeded solution growth technique from a sto-

ichiometric melt with the addition of 11 mol % of K

2

O as

flux and of MgO as dopant. crystals denoted as CLN and

Mg:CLN were grown by the conventional Czochralski

method from congruent melt compositions. Chemical com-

position, Curie temperature, OH absorption wave number,

and photorefractive damage threshold of the LiNbO₃

single

crystals used in this study are summarized in Table I.

7

The

corresponding chemical formulas were characterized by

chemical analysis as follows: The crushed samples were dis-

solved with HNO

3

–HF solution in a closed Teflon vessel at

150 °C for one night. After being filtered for the separation

of precipitated MgF

2

, the yielded solution was passed

through as SA-a anion exchanger for further separation of Li

and Nb ions. The Li and Mg content were analyzed by an

inductively coupled plasma atomic emission spectrometry

ICP-AES . The Nb ions eliminated from the resin by

HCl–HF solution were precipitated by cuperon, and the pre-

cipitate was dried and incinerated. The weight of the yielded

Nb

2

O

5

gave the Nb content. The accuracy for Li / Nb cat-

ion ratios in Mg:SLN and Mg:CLN was 0.6% while that for

MgO content amounted to 0.8%. As shown in Table I, it is

clear that the SLN crystals contain lower intrinsic defect den-

sities than conventional CLN crystals, but they still contain

Nb

Li

at levels of thousands of ppm. On the other hand, ex-

trinsic defects such as Fe were determined by ICP analysis to

be in the ppm levels. In Table I, optical damage threshold

refers to the onset of optical damage. The high MgO concen-

tration samples, according to the data presented, displayed

no optical damage, therefore the optical damage threshold

was never reached. The high MgO concentration samples

showed a shift in the OH absorption band from 3466 to 3485

to 3552 cm

-1

which suggests that at those doping levels, the

excess Nb

5+

on Li sites has been completely replaced by

Mg

2+

.

5

a

APPLIED PHYSICS LETTERS

VOLUME 78, NUMBER 14

2 APRIL 2001

1970

© 2001 American Institute of Physics

---

Page 2

The technique for absorption measurements used in this

work was photothermal common-path interferometry

8

which

has the sensitivity of 0.1 ppm/cm. We measured absorption

of an infrared pump beam wavelength 1064 nm with the

power density of 21 kW/cm

2

, which was focused into 0.5

mm thick crystals. Both green and infrared light were propa-

gated along the z axis in order to separate refractive index

change caused by thermal effect from photorefractive effect.

A green beam, with a wavelength of 532 nm and with the

power density in the range 0.03–3.6 kW/cm

2

, was opened

and closed manually. Green and IR beams were aligned to

overlap inside the crystal so that the change of IR absorption

in the presence of green light could be monitored. Values for

photorefraction in the presence of green light were estimated

under the same conditions as for the GRIIRA measurements.

These values are estimated from the basic feature of the pho-

tothermal system, which detects any phase distortion of the

probe beam caused by temperature change or photorefrac-

tion. Note that for the specific configuration used light

propagating along the optic axis, thin crystal the observed

index change is approximately two orders of magnitude

lower than for the conventional geometry with extraordinary

polarized light propagating in the dIRection normal to the

optic axis.

Figure 1 shows an example of green-induced infrared

absorption and photorefraction as a function of time for an

undoped, nearly stoichiometric LiNbO₃

crystal ( Li / Nb

0.988). As seen in this figure, undoped SLN shows small

initial IR absorption (0.0017 cm

-1

) and a factor of five in-

crease in IR absorption in the presence of green light. The

green induced IR absorption observed here recovered to its

original value when the green pump beam was turned off.

Figure 2 shows the effect of Li / Nb stoichiometry and

Mg doping on the GRIIRA in LiNbO₃

. For low doping lev-

els GRIIRA increased with the increase in Mg concentration,

however, doping with Mg concentrations that exceeded cer-

tain threshold levels showed remarkable decreases of

GRIIRA in both SLN and CLN cases. Mg:SLN requIRes a

smaller Mg concentration than Mg:CLN for low GRIIRA

state to be reached. The data plotted by open circles and

open triangles in Fig. 2 show samples that exhibit no mea-

surable photorefractive damage at 532 nm to intensities of as

much as 8 MW/cm

2

in Table I.

Figure 3 shows photorefraction as a function of Li /

FIG. 1. Example of the change of infrared absorption and photorefraction by

green light irradiation in an undoped nearly stoichiometric LiNbO₃

crystal

listed in Table I.

TABLE I. Chemical formula, Curie temperature, wave number of OH absorption, and photorefractive damage

threshold of samples.

Sample

Chemical formula

Curie temperature

°C

Position of

OH absorption

band (cm

1

)

Optical damage

threshold

a

(kW/cm

2

)

Nondoped CLN

Li

0.950 0.04

Nb

1.01

O

3

1145.0

3485

1

Mg30 CLN

Li

0.925

Mg

0.030 0.042

Nb

1.003

O

3

1188.5

3485

10

Mg45 CLN

Li

0.901

Mg

0.045 0.052

Nb

1.0018

O

3

1209.0

3485

75

Mg60 CLN

Li

0.886

Mg

0.060 0.055

Nb

0.9987

O

3

1220.0

3532

8000

b

Nondoped SLN

Li

0.990 0.008

Nb

1.002

O

3

1192.0

3466

0.1

Mg06 SLN

Li

0.9856

Mg

0.0072 0.0066

Nb

1.0006

O

3

1205.5

3466

10

Mg18 SLN

Li

0.972

Mg

0.019 0.011

Nb

0.9980

O

3

1211.0

3532

8000

b

Mg36 SLN

Li

0.957

Mg

0.034 0.14

Nb

0.9950

O

3

1210.5

3532

8000

b

a

Photorefractive damage threshold is defined as the cw green-light intensity where the transmitted laser beam is

distorted as a result of photorefraction after 10 min of IRradiation see Ref. 5 .

b

No photorefraction was observed at this intensity level.

FIG. 2. Green induced infrared absorption GRIIRA vs MgO concentration

in SLN and CLN crystals. Chemical formula and other material properties

of these samples are listed in Table I.

1971

Appl. Phys. Lett., Vol. 78, No. 14, 2 April 2001

Furukawa

et al.

---

Page 3

Nb and MgO concentrations. The refractive index changes

were measured along with the GRIIRA measurementss shown

in Fig. 2. The photorefraction in both SLN and CLN crystals

decreased monotonically with increase of MgO concentra-

tion. SLN crystals with high Li / Nb ratios show faster de-

creases of photorefraction than congruent crystals. The fea-

ture similar to the GRIIRA behavior is the threshold doping

level over which photorefraction disappears along with the

disappearance of GRIIRA.

The reasons for the threshold behavior of both photore-

fraction and GRIIRA are probably related. Moreover, in

Table I, the optical damage disappearance and the shift in the

OH absorption band from 3466 to 3485 to 3532 cm

-1

both

correspond to the same threshold MgO concentrations. The

comparison with the chemical formulas strongly suggests

that at these doping levels the excess Nb

5+

on Li sites has

been completely replaced by Mg

2+

.

5

We think that GRIIRA is associated with small polaron

absorption band that is centered in the near IR, much like it

occurs in reduced crystals though the absorption is several

orders of magnitude less for crystals measured in this work.

The small polaron is believed to be formed by electron

trapped by antisite Nb.

9,10

The results of an early work

11

suggest that at room temperature that this trap is unstable, the

decay time being in the range of milliseconds. This explains

the rapid response of induced absorption on the green light

power observed in Fig. 1. The significant decrease in GRI-

IRA over a threshold concentration can be attributed to the

complete elimination of intrinsic defects of Nb

Li

following

the incorporation of Mg on Li sites.

The increase of GRIIRA with doping in the region under

threshold needs some comments. Both the rate of excitation

of electrons to the conduction band and small polaron life-

time will contribute to the IR absorption along with Nb

Li

concentration. As to the concentration, it is clear that un-

doped SLN crystals contain lower intrinsic defect densities

than conventional CLN crystals, but they still contain Nb

Li

at

a level of thousands of ppm. Since SLN crystals show stron-

ger GRIIRA than CLN crystals the other factors should play

a very important role.

As for the excitation rate, it should be proportional to the

concentration of the corresponding defects responsible for

the green absorption. Fe impurity is known to contribute to

excitation of electrons to the conduction band. The concen-

tration of Fe was determined by chemical ICP analysis to

be in the ppm levels both in SLN and CLN. This number is

well below the concentration of Nb

Li

shallow traps, even in

SLN, so that it is likely that most excited electrons will be

trapped by Nb

Li

. This is why the dominant factor in

GRIIRA, for the same excitation rate, is probably small po-

laron lifetime. In this way it is possible that small polaron

lifetime increases dramatically near the threshold. This as-

sumption has to be checked experimentally.

In summary, we have investigated green-induced infra-

red absorption and photorefraction by means of the photo-

thermal common-path interferometry technique. We ob-

served that MgO doping had similar results for reducing

GRIIRA in LiNbO₃

, namely that less MgO was necessary to

eliminate GRIIRA in stoichiometric LiNbO₃

than in congru-

ent LiNbO₃

. LiNbO₃

crystals doped with MgO at levels ex-

ceeding threshold concentrations of 1 mol. % for SLN and 5

mol. % for CLN exhibit no measurable GRIIRA with an IR-

radiation up to 3.5 kW/cm

2

of green light. This remarkable

decrease of GRIIRA in Mg:LiNbO₃

is attributed to the elimi-

nation of antisite intrinsic defects of Nb

Li

.

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FIG. 3. Photorefractive index change vs MgO concentration in SLN and

CLN crystals. Chemical formula and other material properties of these

samples are listed in Table I.

1972

Appl. Phys. Lett., Vol. 78, No. 14, 2 April 2001

Furukawa

et al.