Measurement of the complex polarizability of electron traps in Bi(12)SiO(20) by a moving-grating technique

We measure the optical two-beam coupling gain in three nominally undoped cubic n-type Bi(12)SiO(20) crystals as a function of the beam frequency difference Omega and the beam intensity. The crystal geometry is chosen so that the beam coupling through the ordinary photorefractive effect is absent. From the dependence of the gain on Omega the full complex polarizability difference alpha(fe) between a full and an empty deep trap at the wavelength of 515 am is deduced to be (-1.8 - 4.6i +/- 0.7 +/- 1.1i) x 10(-39) F m(2) in SI units or (-1.6 - 4.1i +/- 0.6 +/- 1.0i) x 10(-23) cm(3) in Gaussian units. This suggests that the hole photoexcitation cross section sigma(h) is larger than that for an electron, sigma(e). Our data are consistent with the electron and hole parameters deduced from extensive previous measurements (in one of the crystals) analyzed with the standard electron-hole competition equations. This consistency requires that the average density of full traps be at least 20 times larger than the average density N(A) of empty traps and that sigma(h) be (2.4 +/- 0.8) x 10(-17) cm(2), while N(A) is (1.4 +/- 0.4) x 10(16) cm(-3) and sigma(e) is less than ~6 x 10(-8) cm(2). This is to our knowledge the first determination of these parameters in a sillenite crystal.