Transient beam fanning effects in fiber-like BTO-crystals, that occur if an external electric field is applied, have been studied.

Beam fanning means an asymmetrically amplified scattering. By this process an important part of the energy of the incident signal beam can be transferred to the fanning. This effect normally occurs in photorefractive materials that have high values of the electrooptical coefficients and therefore provide a high coupling gain (e.g. BaTiO₃). In sillenite type crystals (BTO) this effect only occurs if a very high external field is applied. Therefore we used fiber-like [1,2,3] BTO-crystals (Fig.1).

Figure 1: Fiber-like BTO-crystals

Because of their little width (1...4 mm) high electric fields can be applied using moderate voltages, and because of their length (8...20 mm) the interaction area is very large.

Figure 2: Experimental arrangement

These properties lead to new effects that can not be observed in bulk materials.

Fig.2 shows the experimental set-up that was used for the investigations. A square-wave voltage (U=5kV, E=50kV/cm) with low frequency (maximally some Hz) was applied. So every switching of the voltage could be considered as a single event. It turned out that the effects occur in the same way if the voltage changes its polarity of if the voltage is switched on or off [4,5]. The effect only depends on changing the electric field.

Fig.3 shows what could be seen on the screen.

The intensity pattern shows the transmitted signal beam in the central point and the fanning in direction of the x-axis. The fanning has the shape of a club and is not very closely concentrated near the x-axis. The reason is that the electrooptical coefficients of BTO are equal (unlike BaTiO₃) and so the coupling strength does not very strong depend on the direction.

Figure 3: Image on the screen

Because of the small aperture of the crystal at the output, there was a reflected part of the fanning at the opposite side of the central point. It could be observed that a really strong fanning only occurs in the moment when the applied voltage and with that the electric field was switched.

Figure 4: Course in time of the intensity of the fanning in a certain point.

The course in time of the intensity on a point shape detector in the fanning was recorded (Fig.4).

Figure 5: Build-up time of the fanning at different positions

A peak of the intensity occurs rather fast (approximately 20 ms) after switching the voltage (at t=0). In the other time period when the electric field is stationary the fanning keeps on a much lower intensity level.

We measured the rise time of the peak (time from switching the field till to reaching the maximum of the peak) for different distances of the detector from the central point along the x-axis (Fig.5).

Figure 6: Temporal series of pictures of the fanning (drawing)

The rise time increases with growing distance to the central point (according to a growing angle of the fanning with respect to the transmitted signal beam).

This means that the fanning is a chain process in this case. The energy is transferred from the signal beam to the first appearing fanning with a small angle to the signal beam, then from this part of the fanning to the next part (larger angle) and so on.

Fig.6 shows a drawn temporal series of intensity patterns of the fanning.

This way, something like a "fanning wave" can be seen moving over the screen.

The maximum intensity of the peaks (Fig.4) depends on the position along the x-axis (Fig.7).

Figure 7: Peak amplitude (intensity) as a function of the position on the x-axis

The highest intensities are obtained near the central point (according to small angles between signal beam and fanning). The intensity decreases with increasing distance.

However, a transient fanning effect has been studied experimentally. This effect occurs in fiber-like BTO crystals when the applied high voltage changes.

Acknowledgment

This research has been partially supported by the Deutsche Forschungsgemeinschaft (DFG) within the Innovationskolleg "Optische Informationstechnik" (INK 1/A1) at the Friedrich-Schiller-Universität Jena.

References

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[4] E. Raita, A.A. Kamshilin, V.V. Prokofiev, and T. Jaaskelainen, Appl. Phys. Lett. 70, 1641 (1997)

[5] S.I. Stepanov and M.P. Petrov, Opt. Commun. 53, 292 (1985)