980 OPTICS LETTERS / Vol. 16, No. 13 / July 1, 1991

Picosecond photoinduced absorption in photorefractive BaTiO3

Peixian Ye, Alain Blouin, Christian Demers, and Marguerite-Marie Denariez RobergeCentre d'Optique, Photonique et Lasers, Universit6 Laval, Sainte-Foy, Qu6bec G1V 1H8, Canada

Xing Wu

Institute of Physics, Chinese Academy of Sciences, Beijing, China

Received December 10, 1990

Picosecond photoinduced absorption in BaTiO3 has been observed for the first time to our knowledge using pico-second laser pulses and a time delay pump-probe technique. The pump intensity dependence as well as thedelay time dependence of the photoinduced absorption coefficient have been measured. The results can be ex-plained qualitatively by a secondary-center model.ondary centers is not larger than 60 ps.

Barium titanate (BaTiO3) has been recognized as atypical and excellent photorefractive material. Aseries of typical photorefractive effects, such asbeam fanning,' two-wave coupling,2 self-pumpedphase conjugation,3 and mutually pumped phase con-jugation,4 7̀ were found in this crystal either for thefirst time or with the highest efficiency. Also, manyapplications have been developed. However, littlehad been known about the species responsible forthe photorefractive effect in BaTiO3 until now.This situation prevents further improvement ofphotorefractive materials. Motes and Kim8 foundan intensity-dependent absorption in BaTiO3. Thephotoabsorption induced by the 514.5-nm line of acw laser was then investigated more extensively witha power of as much as 100 W/cm2, and a model hasbeen proposed to explain the experiments. 9 Thismodel supposes the existence of secondary centersin addition to the usual photorefractive primarycenters.

It is then evident that the population kinetics ofthese centers would be important to know in con-nection with the photorefractive effect.

Also, from the model of Ref. 9, it can be seen thatthe recombination rate of holes with ions in thesesecondary centers will be proportional to the holenumber Nh. We then expect a fast recombinationrate when a large number of holes are created sud-denly by absorption of a picosecond high-power pulse.The population kinetics of these secondary traps canthen be followed by time-delayed probe absorption.

In this Letter our preliminary studies of photo-induced absorption in BaTiO3 in the picosecond timescale by using a time-delayed pump-probe techniqueare presented. We demonstrate for the first timeto our knowledge that photoinduced absorption inBaTiO3 could also be generated by the pump pulse ofa picosecond laser. We have measured the pumpfluence dependence as well as the pump-probe delaytime dependence of the photoinduced absorption co-efficient and show that the secondary-center modelcould be applied to explain the results qualitatively.

From our results, the hole recombination time with sec-

Also, the hole recombination time with secondarycenters is estimated to be smaller than 60 ps forpulses of 10-mJ/cm2 fluence, and the hole populationin the secondary centers is shown to be still impor-tant after a few nanoseconds.

The basic geometry used in our experiment isshown in Fig. 1. With a beam splitter, two 30-ps(FWHM) laser pulses were derived from a single-pulse frequency-doubled mode-locked Nd:YAG laserto generate the pump beam and the time-delayedsignal beam. They were nearly counterpropagatingand overlapped inside a 6-mm-thick BaTiO3 crystalgrown at the Institute of Physics, Chinese Academyof Sciences. The beams were s polarized. The caxis of the crystal was orthogonal to the optical elec-tric field and to the pump beam direction. Withsuch a configuration, beam coupling is not ex-pected.10 Neutral filters were used to produce a sig-nal beam that was 1000 times weaker than the pumpbeam. The transmitted energy of the signalbeam was measured by a large-area fast photodiode(PD1). The relative delay of the two beams wasvaried. To avoid cumulative effects, the experi-ment was performed with an interval of the order of1 min between shots.

First, we observed that the transmitted signal Iswas reduced in the presence of the pump pulse for alarge range of delay times. These measurementsshowed that photoinduced absorption could occureven within a picosecond time scale. Then, thephotoinduced component of the absorption coeffi-cient, a,, could be defined and measured with therelation

Is (with pump pulse)Is (without pump pulse)

= exp(-ajd), (1)

where d is the interaction length. The main resultsof these measurements are summarized in Figs. 2and 3.

In Fig. 2, the variation of the induced absorp-tion (aid) with pump fluence (I,) is shown for typ-

0146-9592/91/130980-03$5.00/0 © 1991 Optical Society of America

July 1, 1991 / Vol. 16, No. 13 / OPTICS LETTERS 981

of 2.5 cm-' that appears for a beam fluence of5 mJ/cm2. And for longer delays, the induced ab-sorption is lowered.

tM1 Variations of aid with time have also beensketched for different pump beam fluences inFig. 3. We observed that the induced absorptionreaches its maximum within the first 60 ps andthen decays slowly.

All these observations may be well described bythe secondary-center model. In this description9 thepopulation kinetics of primary and secondary trapsare given by

M3

Fig. 1. Experimental setup, DL, delay line; PD's, photo-diodes; M's, mirrors; ILs lenses; F, filter; Ip, pump beam;Is, transmitted probe beam.

dN,= Y,(N1T - N0)Nh - PfN, - SiNPN1,dt

dN - 7 2(N 2T - N2)Nh - f32N2 - S2NpN 2 ,dt

9V

- V~~~~~V `

_ ~ ~~~~ vV v

_ v

V 0V0

_VOV OD

I 0

I I I I I I I IA.J A 14ti t

V v

V

%b o0

V

00O 0 0

a AI. .. .I.... I... I...

6 7 8 9 10

PUMP FLUENCE (mJ/cm2

Fig. 2. ald variations as a function of I, for differenttime delays T: T = -120 ps (A), X = 60 ps (V), T1440 ps (0).

1.8

1.5 wajd DVVV

1.2 _Vo3 VD

0.9 Z 03

_ Vo 0 0

0.6 0J0000 00 0 00 ~~~~~~0

0.3 0 0

0.0 l i s h A Il-300 0 300 600 900 1200 1500 1800 2100

DELAY (picoseconds)

Fig. 3. Variations of ald as a function of time delay X

for different values of I : I = 0.75 mJ/cm2 (A), Ip =3 mJ/cm2 (0), Ip = 5.3 mJ/cm (J), Ip = 10.5 mJ/cm 2 (V).

ical delays: before pump excitation, at delay formaximum absorption, and for a delay long afterexcitation.

From these curves we can see that there is nooriginal induced absorption before pump excita-tion (7 = -120 ps). Also, for a delay (T = 60 ps)there is a saturation in the induced a, of the order

where yl and Y2 are the recombination coefficients,NIT and N2T are the total density of the primary andsecondary traps, respectively, and N, and N2 are theoccupation levels of those traps.thermal ionization rates; Si andtion cross sections at 532 nm. Jtensity in number of photonssquared per second. N, andlated by the continuity equation.of pump fluence is incident upon

/31 and /32 are theS2 are the absorp-Np is the pump in-per centimetersN2 are then re-When 10 mJ/cm 2

the crystal, for anabsorbed energy of the order of 35% and with aquantum efficiency of 1 for the hole creation as-sumed, the hole density would reach a value of theorder of 7.6 x 10i5 cm-3 . With such Nh values, wecan see that the occupation of secondary traps is ini-tially favored when N2(0) is essentially zero. ThenN2 is increased until saturation is obtained. Thissaturation presumably appears for N2 of the orderof N2T.

The shape of the curve of the photoinduced ab-sorption coefficient versus the pump fluence (Fig. 2)may be explained by this kinetics: to observephotoinduced absorption, the pump fluence shouldbe high enough to produce a high occupation levelN2, and the saturation regime is related to the occu-pation rate of the secondary-center sites.

From the measured 60-ps time inside whichthe secondary centers become occupied and fromknowing Nh, we obtained from Eq. (1) a coefficientrate Y2 larger than 10-6 s- cmC3 , which is largerthan the value assumed in Ref. 9. Liberation ofholes from N2 is then regulated by thermal ioniza-tion. The discrepancy is not too surprising whenwe consider that both the secondary traps and theprimary traps may be entirely different in the twosamples.

The saturated absorption coefficient a, =2.5 cm-' permits an estimation of either the numberof secondary traps or their cross sections. Assum-ing a maximum population of 1016 cm-3, from thenumber of holes liberated, we obtained an absorp-tion cross section of 2.5 x 10-16 cm2 of the secondarycenters at that wavelength.

In our analysis, we have neglected other mecha-nisms such as two-photon and free-carrier absorp-

PD1

Is

DL

L1

1.E

1.5aid

1.2

0.9

0.6

0.3

(2)

(3)

iU. 0-7 ---- i-4 5- - I

982 OPTICS LETTERS / Vol. 16, No. 13 / July 1, 1991

tion. Both effects may change our numerical re-sults, but we cannot evaluate the importance ofthose effects from our data.

In conclusion, photoinduced absorption was ob-served for what is to our knowledge the first time inBaTiO3 by using a picosecond pulse laser and a time-delay pump-probe technique.

The pump fluence dependence and the delaytime dependence of the photoinduced absorptioncoefficient were measured. The results were ex-plained qualitatively by applying the secondary-center model. The hole recombination time withsecondary centers and the decay time of the holepopulation in secondary centers were estimated.This study may also be helpful for the investigationof the picosecond photorefractive effect in BaTiO3.

The authors are grateful to Nathalie McCarthy forfruitful discussions. This research has been partlysupported by the National Natural Foundationof China.

Peixian Ye is on leave from the Institute ofPhysics, Chinese Academy of Sciences, Beijing,China.

Note added in proof: Since the submission of thisLetter, a paper has been published by Boggess et al. "that also describes experimental measurements of

photoinduced absorption in BaTiO3 on a picosecondtime scale, but the pulse width and the pump powerdensity are quite different. In addition, their nu-merical estimation of the two-photon and free-carrier cross sections justifies our omission of thesecross sections within our experimental conditions.

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