Solid StateCommunicationsVol. 10, pp.1229—l23l, 1972. PergamonPress. Printed in GreatBritain

THE POCKELS EFFECT IN ADP ABOVE ROOM TEMPERATURE

L B. Harris and G.J. Vella

School of Physics,University of New SouthWales, Kensington,N.S.W. 2033, Australia.

(Received14 March 1972 by T. Nagamiya)

Longitudinal electro-opticstudies in paraelectricADP haverevealedan anomalousvariation of the Pockelscoefficient r

6~with temperatureabove 90CC. This behavior is shownto be part of a more generalvariation of r~3with surfacecondition of the specimen.

AN INVESTIGATION hasbeenmadeof the linear 3.5 Eelectro-optic effect above room temperatureincrystals of ammoniumdihvdrogenphosphate(ADP).

3O~~Early observationsshowedthat continuousappli-cationof sufficiently large electric fields pro-

duced conductioncurrentsthat overheatedthe — 25specimens.Heating effects wereabsentfrom A

subsequentexperiments(see later), in which 2 0 ~.valuesof the Pockelscoefficient r~ were deter-

mined from measurementsof the changein lightintensity transmittedby specimensto which ‘—‘a 15voltagepulses 2 to 3kV in magnitudeand20—25 I

20 60 60 80 100 120msec in durationwere applied across vacuum-evaporatedsilver electrodes. The Pockelsco -TEMPERATURE (°C)efficient r63 , defined as the componentof thelinear electro-optic tensorapplicable to ADP FIG. 1 The temperaturevariation of thewhen the electric field and light beamare both Pockelscoefficient r63 , showingtheparallel to the optic axis, was measuredby means increaseBC obtainedat constanttempera-of the equation ture above 90°C.Closed circles obtained

during heatingrun, open circles duringcooling run.r63 =

Vr2 ~

where F is the retardationin units of wavelength It was found that r63 appearedto increaseA the wavelengthof the light beam, 1 the applied with increasingtemperatureabove90°C.Thisvoltage, and n~the ordinary refractive index. The effect was tracedto a time-dependentinstabilityrisetime of the voltagepulses was approximately duringwhich readingsof r63 very slowly but pro-1 msec,which is longer than the risetimeexpected gressively increasedin magnitudeuntil a final

to producepiezoelectricresonances,and hence equilibrium value was reached. Equilibriumthe measuredcoefficient r63 was the sum of the valuesof r~3as a function of temperaturefor adirect Pockelscoefficient r~ andthe elasto- typical ADP specimenare given in Fig. 1. Foroptic contribution. The electrodeshad a ring any one specimenthe temperaturevariation belowconfiguration.

2 90°C(section AB in Fig. 1) was reproducible

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1230 THE POCKELS EFFECT IN ADP ABOVE ROOM TEMPERATURE Vol. 10, No. 12

and showedtheCurie Law dependenceof r~3being mechanically-polishedlight-transmittingsurfaces

inverselyproportional to absolutetemperature. of crystals, either ADP or KDP, the value ofThe vertical line BC representsan increaseof r~3was increasedby up to 20%. Sincethe total

r63 with time, over a period of approximately60 increaseproduced in ADP by thehigh-temperaturehours for the specimenof Fig. 1, whilst the treatmentwas larger than this, theremust be somesectionDE representsthe variation of r63 with additional mechanismto account for thedifference.tempersturesubsequentto the high-temperature A clue to the nature of this mechanismis givenheat-treatment.The sectionDE wasalso repro- by a time-decayobservedin the electro-opticducible for any onespecimenand also approxi- response.mately followed the Curie Law. The increaseBC, which correspondsto a rise in the magnitude

of r~3of between40—80 per cent for differentspecimens,could be obtainedby holding the

specimen at any temperaturebetween90 and Max Mm140°C,the latter beingthe temperatureat which

~-:

specimensdeterioratedtoo rapidly for measure- ~9.merits to be consideredreliable.

zA parallel study over the temperaturerange

20—180°Cshowedno correspondingincreasein —~ 5 ~— TIMEr63 for crystals of potassiumdihydrogenphosphate ~ 4 msec

(KDP). However, a randomscatterwas observedin the magnitudeof r6~ from specimento specimenfor both ADP and KDP. This scatterwas attributedto variations in surfacecondition arising from themechanicalpolishing requiredto give an optical ~i)

finish to thespecimens,and further investigation 4

of ADP showedthat deliberatevariation of sur-face condition producedsignificant changesin

FIG. 2. Oscillogram tracesof electro-opticr63. For a typical ADP specimen, for example, responseof ADP during applicationofr63 at room temperature(in iO~esu)was 2.23 high-voltage pulse, risetime 1 msec,fall-

for roughly-polishedsurfaces,2.53 for finely- time approximately7msec. (a) at 40°Cinpolished surfacesand2.72 for a water-polished region AB of Fig. 1, (b) samespecimenat

44°Cin region DE.finish. Theseand other measurementssuggestthat r~3 increaseswith decreasingsurfacedamage.A particular instanceof this trend was provided Figure 2 illustrates the fact that whenmeasure-by specimensgiven a high-temperaturetreatment mentswere made in the region AB of Fig. 1, thethat left them in the electro-opticcondition corres- value of r~ decreasedduring the 20 to 25msecponding to point E in Fig. 1: mechanicalre- durationof the appliedhigh-voltage pulse,whilstpolishing of the surfacesof thesespecimenstended no suchdecaywas observedfor measurementsinto reducetheir room-temperatureelectro-optic the regionDE. Such a decay, besidesconfirmingresponseto a lower value muchcloser to point A. the absenceof any spurious heatingof the speci-

men, implies the presenceof spacecharge,pro-In terms of the above behavior the increase bably associatedwith chargedsurfacedislocations.

in r63 above90°Ccan be partly attributedto the It is suggestedthat there is, in addition toremovalof mechanically-introducedsurfacedamage evolution of ammoniaat the surface,a pipe-by the etchingactionof thephosphoricacid left diffusion of ammoniamoleculesalongdislocationsbehind on the surfaceof ADP when ammoniais adjacentto the surface,so that thesurfacetendsevolved from thesecrystals.

3 Separateexperiments to decomposewithin a surfacelayer of finitecarried out at room temperaturedemonstratedthat thickness. Such a processcould explain not onlywhenphosphoricacid was smearedon the an increasein r~ greaterthan that produced by

Vol. 10, No. 12 THE POCKELS EFFECT IN ADP ABOVE ROOM TEMPERATURE 1231

direct surfaceetching, but also the disappearance large increasein r6~for ADP above 90°Candofof a time-decaydue to spacechargenear disloca- the randomvariation in Te3 betweendifferenttions. Note that, for consistency, the valuesof specimensof ADP and KDP, andshows that user63 alongregion AB of Fig. 1 were alwaysdeter- of our measurementsto suggestthereis anmined from the initial maximum of the transmitted anomalyin the volume susceptibility of ADP ~light intensity, as shown in Fig. 2(a), though the is misinterpretation. The most intriguing aspect

trend of behavior would have remainedunaltered of this work is the fact that surfacepropertiesif the final minimum intensity had beenused. havebeenshownto affect theelectro-optic

effect which, till now, hasalwaysbeenconsideredAll experimentalevidencepoints directly in termsof bulk propertiesof the crystal.

to a surfacemechanismas thecauseboth of the

REFERENCES

1. BILLINGS B., .1. Opt. Soc. Amer. 39, 797 (1949).

2. BILLINGS B., Optics in Metrology p.119 PergamonPress,New York (1960).

3. HARRIS LB. and VELLA G.J., J. Chem.Phvs. (in press).

4. RICHMOND P., Phys.Leti. 32A, 52 (1970).

Longitudinale elektro-optischeUntersuchungenan paraelektrischenAmmoniumdihvrodogenphosphat(NH4H2PO4)habeneineanomaleAnderungdesPockelsKoeffizienten r~ mit der Temperaturoberhalb90°Cergeben. ~s wird gezeigt, dass diesesVerhalten mit einerailgemeinerenAnderungdes r63 mit dem Zustandder OberflächedesKristalls zusammenhängt.