MEASUREMENT OF THE COMPLEX POLARIZABILITY OF ELECTRON TRAPS IN BI12SIO20 BY A MOVING-GRATING TECHNIQUE

We measure the optical two-beam coupling gain in three nominally undop ed cubic n-type Bi12SiO20 crystals as a function of the beam frequency difference Omega and the beam intensity. The crystal geometry is chos en is that the beam coupling through the ordinary photorefractive effe ct is absent. From the dependence of the pain on Omega, the full compl ex polarizability difference alpha(fe) between a full and an empty dee p trap at the wavelength of 515 nm ia 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 p hotoexcitation cross section sigma(h) is larger than that for an elect ron, sigma(e). Our data are consistent with the electron and hole para meters deduced from extensive previous measurements (in one of the cry stals) analyzed with the standard electron-hole competition equations. This consistency requires that the average density of full traps be a t least 20 times larger than the average density N-A of empty traps an d that ah 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 similar to 6 x 10(-18) cm( 2). This is to our knowledge the first determination of these paramete rs in a sillenite crystal.