Cryst. Res. Technol. 36 2001 6 527533 © WILEY-VCH Verlag Berlin GmbH, 13086 Berlin, 2001 0232-1300/01/0607-0527 $ 17.50+.50/0 Acoustically induced optical second harmonic generation (SHG) and two-photon absorption (TPA) in ferroelectric Pb 4.7 Ba 0.3 Ge 3 O 11 crystals have been found. We have found that with increasing acoustical power, the SHG for YAG:Nd laser light (OPm) increases and achieves its maximum value at acoustical power density about 1.75 W/cm 2 . The evaluated SHG values were 23 % less comparing with F 222 tensor of the KDP single crystals. With decreasing temperature, the acoustically induced SHG signal strongly increases below 29 K. The maximal acoustically induced SHG has been observed at acoustical frequencies lying within the ranges 12-17 kHz, 22-23 kHz and above 26 kHz. This behavior reflects nonlinear superposition of the nonlinear optical susceptibilities stimulated by externally- induced electron-phonon anharmonicity. We have observed substantial increase of the TPA (for the acoustical power W= 1.8 W/cm 2 ) at high hydrostatic pressures (about 16 GPa) and low temperatures (below 16 K). This one confirms complicated influence of the electron-phonon interactions in the ferroelectrics on the observed nonlinear optical dependences. Keywords: nonlinear optical crystals, lead germanate (Received May 7, 2001; Accepted June 6, 2001) 1. Caption The effect of the acoustically-induced optical second harmonic generation (AIOSHG) was discovered by (NELSON and LAX (1971)]. Recently we have found AIOSHG in several composites containing nanocrystallites (MAJCHROWSKI et al. (2000)). One of the main physical origin of occurrence the acoustically-induced non-centrosymmetry consists in an asymmetry of photo-carrier excitation under applied external acoustical power (BIELINCZER et al. (1980)). Particularly, it was shown that this asymmetry is caused by interference of electron-hole and electron-phonon interaction. Especial interest may present the ferroelectric crystals possessing spontaneous polarization. From general theory of ferroelectrics it is well known (BLINC et al. (1972)) that the occurrence of ferroelectricity is caused by condensation of low-frequency phonon modes. These modes are called soft modes and are directly connected with anharmonic electron-phonon interactions (BUSSMANN-HOLDER et al. (1992); DALAL et al. (1998)). One of the most sensitive method for detection of such kind of interactions are time dependent nonlinear optical methods (SAHRAOUI et al. (1999); KITYK et al.(1999)). In the present works it was shown that the picosecond pump-probe nonlinear optical technique is sensitive to the such kinds of anharmonicity and allows to use the mentioned effect in the studying of observed phenomena. Ferroelectric crystals possessing substantial electron-phonon anharmonicity are directly connected with the macroscopic spontaneous polarization and may be of a separate interest. Among the ferroelectric crystals the better nonlinear light-induced properties possess (Pb 1-x Ba x )Ge 3 O 11 single crystals (YU et al. (1998)). J. ZMIJA, H. KADDOURI**, A. MAJCHROWSKI, Z. MIERCZYK, I. V. KITYK* Institute of Applied Physics, MUT, Warsaw, Poland *Institute of Physics Czestochowa, Poland **LP2A, Universite of Perpignan, France Pb 4.7 Ba 0.3 Ge 3 O 11 Crystals as New Acoustically Induced Nonlinear Optical Materials