Picosecond holography and four-wave mixing in BSO

PICOSECOND Jean-Louis

HOLOGRAPHY

FERRIER,

AND FOUR-WAVE

Jost GAZENGEL,

Fluid Optics Laboratory. U.A. CNRS 49045 Angers Cedex, France Received

1 July

OPTICS COMMUNICATIONS

Volume 58, number 5

1986

MIXING IN BSO

Xuan NGUYEN

PHU and Genevieve

RIVOIRE

780, Bouleoard Lauoisrer, 4, Angers Unruersit~v,

17 July 1985; revised manuscript

received

3 February

1986

Picosecond excitations have been used in order to investigate the efficiency, the rising and the decay times of the gratings induced in BSO, either in two-wave mixing or in degenerate four-wave mixing experiments. The high intensities of the exciting waves reveal the existence of new phenomena, particularly a nonlinear local effect.

1. Introduction The photorefractive effect, describing the photoinduced refractive index changes, has been observed in a variety of electrooptic materials [l] and particularly in B12Si0,,(BSO). The refractive index vari: tion, consequence of a photocurrent modulation by a nonuniform illumination, has been intensively investigated with cw laser excitation. This photocurrent is due to the space charge field created by the diffusion and (or) the drift of the photoinduced electrons. In very short pulse excitation - i.e. picosecond laser - the migration of the photoinduced carriers happens after the illumination [2]. Two experimental configurations can be used: 1. I. Four-wave mixing experiments

(FWM)

The crystal is illuminated with three intense single pulse beams. A fourth beam is created by nonlinear effects, particularly by the third order one, like in other media previously studied such as CS, [3]. 1.2. Two-wave mixing experiments

The observed beam either created by nonlinearities (sect. 1 ,I) or diffracted by the holographic grating (sect. 1.2) can be the consequence of different physical phenomena: their efficiencies, rise times and decay times can be very different. We present picosecond TWM and FWM experiments in BSO with and without an external field applied on the crystal. In order to obtain more temporal informations they will be completed with intermediate experiments between FWM and TWM: in the TWM set-up, an intense pulse beam is used as the reading beam instead of the low intensity cw beam and will be delayed relatively to the two intense interfering beams. The results will be compared to those obtained in cw [4] and nanosecond [5] excitations.

(TWM)

Two intense single pulse beams illuminate simultaneously the crystal where they interfere. The recorded induced grating is read by a cw laser of low intensity which can be present during the intense illumination. 0 030-4018/86/$03.50 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)

2. Experimental set-up The experimental set-up for the four-wave mixing and two-wave mixing experiments are respectively shown on figs. la and lb. The beams indicated by 1 or 3 are 25 ps single pulse provided by a frequency doubled Nd-YAG laser. The beam 2 is provided either by the same Nd-YAG laser (fig. la) or a cw HeNe laser (fig. lb). The notation of the waves index will be the same in the two configurations; the intensity of the ith wave will be noted Ii, 343

Volume 58. number

(a)

\

\

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5

1

I

H~.N~

532 nm

Fig. 1. (a) Schematic four wave mixing configuration: pumping beams, 3 probe beam, 4 observed generated V high voltage generator, 8 typically 2, Gl Glan prism. Schematic two wave mixing configuration: 1,3 pulsed _ 532 nm, 2 reading beam, 1 diffracted beam, V high generator (typically 2 kV), o typically 2.

1,2 beam, (b) beams voltage

The intensity and the polarization of the different beams may be controlled by Clan prisms and h/2 plates. These configurations are similar to those used in continuous and nanosecond excitations, nevertheless they present adjustment difficulties due to the shortness of the pulse. Optical adjustable delays have been inserted on each of the beams. In order to achieve a perfect coincidence of the beams 1 and 3, in picosecond, we use a Jamin interferometer as reported in previous work [6]. The dimensions of the BSO crystal are 3.3 X 5 X 2 mm. An external electrostatic field E, can be applied on the crystal.

close, while the intensity of the probe wave I3 is of the order of 3% of them. The maximum total energy E on the crystal is about 1 mJ in 25 ps, the corresponding intensity being 500 MW/cm2. The beams are linearly polarized, perpendicular to the incidence plane. When the waves 1,2 and 3 are simultaneously sent on the crystal, the wave 4 is observed, either with the external electric field E. applied on the crystal or without it. The efficiency p = I4/I3 is of the order of 10e4 and is independent of E,. A second reading a few seconds (one or more) later by the beam 2 alone does not give any signal 4. When several pulses (on the beams 1,2 and 3), separated by 7 ns, issued from the same laser pulse train, are sent on the crystal, several signals corresponding to each of these pulses can be observed on the wave 4. However the pulses of small intensity are inefficient in the creation of the fourth wave, suggesting the possibility of a threshold. The fig. 2 shows this similar temporal evolution for the beams 3 and 4 detected by fast photocells (risetime
aA,/az

a x’~‘A~A~A;

)

where the nonlinear susceptibility xC3) = x’ + j x” is the sum of a real part x’ (nonlinear refractive index

3. Four-wave mixing experiments (fig. la) The beams 1 and 2 are counterpropagating; the angle 0 between the beams 1 and 3 is typically 2”. The intensities of the pumping waves Z1 and I2 are 344

Fig. 2. Beam 3 and 4 detected by two fast photocells 4 has been 120 ns delayed relative to 3).

(signal

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1 July 1986

variation) and an imaginary part x” accounting for the two-photon absorption and the scatterings. The nonlinear refractive index changes can mostly be due to intraband phenomena (anharmonic response of bound electrons). In the same picosecond FWM experiments, the efficiencies in BSO and CS, [8] are of the same order involving close Ix 1values in the two media. In CS,, with the same laser, we measured 1x1 = 5 X lo-l3 esu [6].

4. Two-wave mixing experiments The interfering beams 1 and 3 of equal intensities are linearly polarized perpendicularly to the incidence plane. The fringe spacing is typically 15 pm. The diffracted beam (intensity 1,) of a low power HeNe laser (less than 0.1 mW on the crystal) in the first order of the holographic grating is observed with a photomultiplier associated with a storage oscilloscope. The rise and decay times of the whole are respectively 60 ns and 300 ~.ts.In order to reduce the background noise, a prism has been used to separate the diffracted and direct beams, the photomultiplier being protected by interferential and colored filters (OG 550). This experimental set-up has also been used with nanosecond pulses provided by the same laser: in this case, the maximal energy E is 10 mJ in 15 ns corresponding to an intensity of 1400 kW/cm2 (the waves cross sections are different in the nanosecond and picosecond range). However subnanosecond structures appear in the pulses and the coherence length measured by interferometry is of the order of some cm, only scarcely larger than the picosecond one. 4.1. Observations without

external electric field

The diffracted signal evolution is the same for the two excitation ranges nano and picosecond (fig. 3). The risetime defined as the time needed for the signal to reach its maximum (under HeNe illumination) is about 0.5 ms, while the decay time (defined as the time between the maximum value of the signal and this value divided by e) is about 10 ms. In order to observe a signal, an incident energy of at least 1 mJ is required, explaining why it was not reported in a previous publication [9] in which the ex-

Fig. 3. Temporal evolution of the diffracted beam without external electric field (ns excitation, incident energy 10 mJ 1400 kW/cm’).

had been performed with a less powerful periments laser. The diffraction efficiency q =Idmax/IHe/Ne incid., where Id max is the peak value of the diffracted beam, is about 10-5 when E = 1 mJ and can reach lop4 when E = 10 mJ in nanosecond excitation. It must be noted that the observed risetime of the diffracted beam is far larger than the incident pulse duration. These results may be compared with the nanosecond observations by Lesaux et al. [lo]. For incident energy density D < 1 mJ/cm2 these authors observed a very fast diffracted beam (risetime 1 mJ/cm2. In our experiments, where D > 1 mJ/cm2, and apparatus risetime 60 ns, we observe this slower effect which can conceal on the detector a faster effect if the total energy of this faster effect is not far larger than that of the slower one. 4.2. Observations with an external electric field The external electric field E, applied to the crystal is typically 6 kV/cm. The capacitor value C (some nF) is parallel on the crystal has no influence on the diffracted signal evolution when the high voltage provided by the generator is continuously applied to the crystal. The measured photocurrent is proportional to E, and to the energy of the light pulse. This photocurrent follows (within the apparatus response time) the light pulse (fig. 4). 345

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Fig. 4. Electrical response of the BSO crystal to two optical picosecond excitations separated by 8 ns, external electric field 6 kV/cm.

As in the previous case, without external field, the observations give close results under the two excitation ranges. Two phenomena appear; for conve-

Fig. 5. Temporal evolution of the diffracted 0.25 mJ, (b) 0.5 mJ, (c) 1.7 mJ, (d) 10 mJ.

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beam with external

nience, they will be denoted by P1 (the faster) and P2 (the slower). Fig. 5 shows the relative importance of the two phenomena for increasing exciting intensities. P, is the same as the one described without external electric field, the rise and decay times and the efficiencies are identical. The decay time is independent of the HeNe incident intensity, indicating that P, is a transient effect and does not lead to the registration of a permanent grating. P2 has a risetime, independent from the electric field E,, which increases with II from some tenths to some hundredths of ms. This signal is erased by the reading HeNe beam. Its decay time, typically of some seconds, is inversely proportional to the HeNe laser intensity and proportional to the external electric field E, [ 111. These observations must be related with the preservation of the registered grating. In the dark, Eu being applied, there is no measurable loss after 20

electric

field = 6 kV/cm

(ns excitation)

for incident

energy:

(a)

1 July 1986

OPTICS COMMUNICATIONS

Volume 58, number 5 31 6.&.

x

f 2 .lO”. X

e X

+ x

0

OF

0 1

5

10

50

0,kJ

100 150 1;nJ

500

1500 1O;nJ

> KWh?

E

Fig. 6. Efficiency versus incident energy (and intensity for nanosecond pulses): o without external field; + for “fast” part of I, * for overall phenomenon (cf. 5a) all with external field.

hours.

When EO is suppressed,

the grating

disappears

faster; two readings separated by two hours show a 50% loss of the registered information. A complete erasing of the grating under uniform pulsed illumination of about 1 mJ.

(beam

1 or 3) needs an energy

All these results indicate that P, registers a permanent grating. However no cumulative effect has been observed when several exciting pulses are successively sent. Fig. 6 gives the diffraction efficiency in terms of the incident energy E.The maximum efficiencies obtained in our experimental conditions either for PI or P,,or both phenomena when they are not temporarily separated, are slightly larger than 10e4. When the fringe spacing A is varied from 15 I.trn to 2.5 pm (with or without external field Eo), the rise and decay times of the two phenomena are not modified and the efficiencies are of the same order. All these TWM observations, performed with high intensities and short pulse durations, lead to the following conclusions : A transient grating, risetime 0.5 ms, decay time 10 ms exists in the medium, neither due to the diffusion nor to the drift, but perhaps mal effects [lo].

A

related

to ther-

permanent grating with a very long risetime

X

for “slow” part of I,

(from 10 ms to 500 ms) is written in the presence of an external electric field only. The increase of its risetime and the efficiency saturation with increasing values of the incident intensity I,, suggest that this grating is written by a space charge field created by the free carriers drift but with large recombination times due to a saturation of the trapping centers. In fact, in our experiments with high intensity exciting pulses, the number of created free carriers N x 3 X 1016/cm3 may be larger than that of the trapping centers number NA = 1016/cm3. Consequently , the mobility, the recombination times and the space charge can be strongly affected. It has to be noticed here that the theoretical model proposed by Valley [2] applied to BSO in our experimental conditions leads to results in large disagreement with our measurements; specially the calculated response times are far smaller than the measured ones, which confirms the presence of physical phenomena not taken into account in ref. [2]. In order to obtain more informations about the temporal evolution of the gratings written in BSO, using the DFWM configuration, we delayed the reading beam 2 relatively to the writing beams 1 and 3. For technical reasons, it has only been possible to vary continuously the delay between 0 and 300 ps, 347

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around 10 ns, between 100 ps and 400 ps, and beyond 1 s. Without electric field E, applied on the crystal a fourth beam is created except when the delay is above 1 s. These observations agree with those performed in the TWM experiments (fig. 3). They will be thoroughly carried on, with the measurement of the efficiency, in order to investigate continuously the change over from the transient grating written in DFWM to the slower transient grating and the permanent grating written in TWM. In conclusion, we can consider that the excitation of the BSO crystal by high intensity beams in the ps range leads to intricate phenomena ~ some of them possibly due to thermal effect - which disturb the evolution of the photorefractive effect. Without an external field applied on the crystal, a transient grating is observed, while in the presence of an external field a permanent grating appears after the transient one, but with a very long risetime probably due to saturation effects. However the excitation of BSO in DFWM by three intense picosecond beams reveals the presence of a very fast nonlinear effect.

Acknowledgements The authors wish to thank J.P. Herriau and J.P. Huignard for valuable discussions. The technical

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assistance of R. Chevalier and J.P. Lecocq has been greatly appreciated. This work was supported by A.T.P. C.N.R.S. no. 9.83.86.

References [l] P. Gunter, Physics Reports 93 (1982) 199. [2] G.C. Valley, IEEE J. Quant. Electron. QE19 (1983) 1637. [3] J.L. Eerrier, Z. Wu, X. Nguyen Phu and G. Rivoire, OpticsComm. 41 (1982) 207. [4] J.P. Huignard, J.P. Herriau, P. Aubourg and E. Spitz, Optics Lett. 4 (1979) 21. [S] J.P. Hermann, J.P. Herriau and J.P. Huignard, Appl. Optics 20 (1981) 2173. [6] N. Phu Xuan, J.L. Ferrier, J. Gazengel and G. Rivoire, Optics Comm. 51 (1984) 102. [7] R.K. Jam, Optical Engineering 21 (1982) 199. [S] G. Rivoire, J.L. Ferrier, J. Gazengel and N. Phu Xuan, J. de Physique, Colloque C2,44 (1983) 81. [9] J.L. Ferrier, J. Gazengel, G. Rivoire, J.P. Herriau and J.P. Huignard, Proc. ECOOSA 84, Amsterdam Bellingham, USA) p. 307. SPIE 492. [lo] G. Lesaux, G. Roosen and A. Brun, Optics Comm. 56 (1986) 374. [ 1 l] J.L. Ferrier, J. Gazengel, G. Rivoire, J.P. Herriau and J.P. Huignard, Proc. Opto 85, Paris (ES1 Publications, Paris) p. 135.