The time development of a photorefractive grating created by intersecting 30 ps (532 nm) beams in a well-characterized Bi12SiO20 crystal (in a static field around 1 kV/cm) unambiguously reveals the mobility of photoexcited electrons to be 0.24±0.07 cm2/V s through what is essentially a ‘‘time-of-flight’’ measurement.
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.
In the only study to our knowledge of the photorefractive effect in n-type cubic Bi12SiO20 (n-BSO) at low temperatures (300 to 80 K), optical four-wave mixing via the photorefractive effect was found to decline over one order-of-magnitude as the temperature was lowered from ~ 300 K to below ~ 200 K.1 The mixing returned to normal upon warming. To better understand this unexpected result, we have undertaken both pulsed and dc photoconductivity studies at various temperatures on the same n-BSO crystal, called "CT1" (Crystal Technology, Inc., 1984), as well as on other n-BSO crystals. The photocurrent excited by a nanosecond or picosecond optical pulse is known to decay non-exponentially in time, with characteristic times in the nanosecond and microsecond regimes.2 In CT1 we see further long decay tails (~ seconds) at room temperature. However, in another n-BSO crystal called "SU1" (Sumitomo Corp., 1986), this long tail is essentially absent.
The mobility of photoexcited charge carriers in photorefractive insulators can be measured with a holographic time-of-flight technique.1 By illuminating the crystal with two interfering 30 ps laser pulses at the wavelength of 532 nm, we create an instantanuous sinusoidal pattern of photoexcited charge carriers. A strong electric field E0 is applied across the crystal causing the sinusoidal pattern of charge carriers to drift with a velocity μE0, where μ is the mobility. With a proper choice of the interference fringe spacing Λ, the superposition of this drifting charge pattern on the complementary pattern of photo-ionized traps creates an observable oscillating space charge field. We probe this oscillation by diffracting a weak cw He-Ne beam from the refractive index grating that is created via the electro-optic effect. The period Pt of the observed oscillation is the time required for photoexcited charge carriers to drift over one spatial period Λ.
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