Simple transient solutions for photoconduction and the space-charge field in a photorefractive material with shallow traps

Abstract

We have found simple analytical solutions for the coupled differential equations that are commonly used to describe unipolar photoconduction in a homogeneous material with shallow traps. Our solutions cover transient photoconduction in the dark after an initial low-intensity pulse of light creates some spatial pattern of photoexcited carriers. We assume that there is no conduction in equilibrium in the dark.Our solutions introduce two parameters to describe the shallow traps: a mean time between trapping events in the conduction band and a mean time spent in a shallow trap. When used to analyze extensive experimental data on a particular sensitive crystal of n-type cubic Bi»Si02O, our solutions predict an observed, but unexpected, dependence of carrier grating decays (in the dark) on applied electric field. The solutions also correlate transient photoconductivity results with grating-decay behavior both in the dark and under uniform illumination, giving a mean time between shallow traps in the conduction band of 26+3 ns, and a mean time of 800+300 ns spent in a shallow trap. A value p, =5+3 cm /(V s) is inferred for the electron mobility in the conduction band between trapping events. Also, the electron diffusion length inferred from our solutions matches that observed previously by light-induced grating decays.

<title>Charge trapping in photorefractive polymers</title>

“…The samples were made by sandwitching 200 jim thick pellets between two ITO covered glass plates by heating above the glass transition temperature. The phase shift ofthe photorefractive gratings in these samples was the same as in the casted ones, indicating that no strange artifacts are In transient photorefractivity experiments, an interference pattern from two pico or nanosecond laser pulses creates a sinusoidal distribution of mobile carriers, which drifts under the influence of a strong dc electric field [10,11]. As charge separation advances, a space charge field builds up that can be probed with a cw laser beam through the electrooptic effect.…”

“…Further drift causes a decrease of the space charge field until coincidence is reached again and so on. The diffraction efficiency versus time shows > oscillatory behaviour and from the time tm that corresponds to the first maximum the drift mobility 3dr Cfl be extracted [11]:…”

Light-induced properties of ruthenium-doped Bi4Ge3O12 crystals

“…Figure 4 shows that the simple exponential law can fit the experimental data very well. 17 In the BGO crystal structure, each Ge 4+ ion is coordinated by four oxygen ions, forming a tetrahedron, and each Bi 3+ ion is coordinated by six oxygen ions. 16 The observed diffraction efficiency temporal behavior resembles that reported for undoped and Cr-doped BGO crystals, and is slightly different from those of Fe-and Mndoped crystals, where the writing dynamics is more complicated, starting with fast growth decreasing and finally approaching the steady-state value.…”

<title>Space-charge field formation in poly(N-vinylcarbazole)-based photorefractive composites</title>

“…As charge separation advances, the space charge field builds up and it can be probed with a cw laser beam through the electrooptic effect. One measures the difftaction efficiency 1(t) [31]:…”