S. X. Dou

Studies of shallow levels in undoped and rhodium-doped barium titanate

Spectral and temporal characteristics of light-induced absorption for an undoped and a Rh-doped as-grown BaTiO3 crystal were investigated and compared. Two dark-decay time constants corresponding to two shallow levels were observed: 40 ms and 1.2 s for undoped BaTiO3 and 39 ms and 3.4 s for Rh-doped BaTiO3. The light-induced absorption spectra were resolved into two components according to the dark-decay time constants. The resolved spectra reveal that one shallow level in Rh-doped BaTiO3 and one in undoped BaTiO3 are caused by a same impurity defect. The thermal activation energies of these levels were determined to be 0.53 and 0.7 eV for undoped BaTiO3 and 0.57 and 0.8 eV for Rh-doped BaTiO3. The 0.8-eV shallow level in BaTiO3:Rh is caused by Rh4+/5+; the other level, 0.57 eV, is the one that has the same defect origin as the 0.53 eV level in undoped BaTiO3. Other parameters for the shallow levels are also given.

Mechanism transformation with wavelength of self-pumped phase conjugation in BaTiO 3 :Ce

We report what is to our knowledge the first observation of mechanism transformation with wavelength of self-pumped phase conjugation (SPPC) in BaTiO(3):Ce crystals in the near infrared. It was found that the SPPC mechanism changed from a backscattering and four-wave-mixing mechanism to a cat mechanism when the pump wavelength was varied from 706 to 770 nm. In addition, SPPC was obtained when the pump beam entered the crystal by the +c face, and a variation with wavelength of optical beam paths in the crystal was also observed.

Studies on formation mechanisms of self-pumped phase conjugation in BaTiO 3 :Ce crystals at wavelengths from 570 to 680 nm

Formation mechanisms of self-pumped phase conjugation (SPPC) in two BaTiO3:Ce crystals with different doping concentrations have been experimentally studied. The pump beam was from dye lasers, with an overall tunable wavelength region lying between 570 and 680 nm. Variations of the optical beam path patterns as well as reflectivity of the phase conjugators with respect to the incident angle, the incident position, and the wavelength of the pump were investigated in two configurations. One configuration corresponds to that in which the pump enters an a face of the crystal; the other to that in which the pump enters the +c face of the crystal. It was found that SPPC is sometimes established in a way similar to that of a cat phase conjugator, but usually without the formation of a clear beam path loop. Sometimes SPPC is established by a stimulated photorefractive backscattering and four-wave mixing mechanism. In these cases, however, the crystal corners as well as the scattering centers on the crystal faces usually play an important role by seeding the stimulated photorefractive backscattering processes. In other cases SPPC is established by both the cat and the stimulated photorefractive backscattering and four-wave mixing mechanisms.

Wavelength dependence of the effective trap density in Rh-doped BaTiO3 : a comparison between theory and experiment

Abstract The wavelength dependence of the effective trap density N eff of a BaTiO 3 :Rh crystal has been studied theoretically and experimentally. The three-charge-state model and the two-beam coupling (TBC) were used for the theoretical and experimental studies, respectively. By comparing the theoretical and experimental results, we found that it is not enough to model the photorefractive effect in our BaTiO 3 :Rh crystal with the three-charge-state model at long wavelengths.

Plate-formed mutually pumped phase conjugator

We have demonstrated experimentally a new scheme for mutually pumped phase conjugation with two incoherent beams, using a 0.93-mm-thick plate-formed BaTiO3:Ce crystal. The highest phase was reflectivities obtained in our experiments are 30.5% (in the case of equal incident powers) and 71.6% (in the case of unequal incident powers). The variations of the phase-conjugate reflectivity with incident angles and is intensity ratio have been determined, as has the power-dependent response time.

Theoretical study on effects of stimulated photorefractive backscattering in self-pumped phase conjugators

Effects of stimulated photorefractive backscattering (SPB) interactions in both stimulated photorefractive backscattering and four-wave mixing (SPB-FWM) and total internal reflection (TIR) self-pumped phase conjugators have been analyzed theoretically. SPB interactions in the four-wave mixing regions are neglected. Calculations show that the operational threshold and the phase conjugation reflectivity of both phase conjugators are very sensitive to the backscattering two-wave mixing gain coefficient of the crystals. From these results the mechanism transition phenomenon of a self-pumped phase conjugator between SPB-FWM and TIR can be explained if the backscattering gain coefficient of the crystal varies with doping concentration or with wavelength.

Wavelength dependence of two‐beam coupling gain coefficients of BaTiO3:Ce crystals

Two‐beam coupling gain coefficients of transmission and reflection gratings in BaTiO3:Ce crystals versus wavelength between 570 and 680 nm have been measured. The former decreases more quickly than predicted by theory. The latter was found to vary significantly with wavelength, while theory shows that it should vary little. In order to explain the experimental results we assumed that the effective carrier number density Neff depends on wavelength. By calculating Neff from the measured results we found that Neff is approximately a linearly decreasing function of wavelength. The slopes and intercepts for the linear functions are given for the two crystals studied.

Theory of a “stimulated photorefractive backscattering and four-wave mixing” self-pumped phase conjugator

Abstract We give a theoretical analysis of a “stimulated photorefractive backscattering and four-wave mixing” (SPB-FWM) self-pumped phase conjugator (SPPC) in which the generation of the phase conjugate wave relies on both stimulated photorefractive backscattering (SPB) and four-wave mixing (FWM) processes. The two pumping beams in the FWM process are, respectively, the forward-propagating beam of which the generation is seeded by the fanning effect, and the backward-propagating beam that is generated by the SPB process. Assuming that the SPB interaction is seeded by the scattering centers in the crystal, we have calculated the threshold four-wave mixing coupling strength (γ 1 l 1 ) th as a function of the SPB two-wave mixing coupling strength (γ 2 l 2 ) corresponding to the 2k grating and of the reflectivity R ′ of the scattering centers. Above threshold, we have also calculated the phase-conjugate reflectivity with a numerical method. Our calculations show that the threshold as well as the reflectivity of a SPB-FWM SPPC strongly depend on γ 2 l 2 and R ′. It is also shown that a high phase-conjugate reflectivity can be easily realised in doped barium titanate crystals which are shown to have much larger γ 1 and γ 2 than an undoped one.

Self-pumped phase-conjugation properties of cerium-doped BaTiO 3 crystals in the near infrared

Studies of a new Ce-doped BaTiO(3) crystal as an efficient self-pumped phase conjugator in the near infrared are presented. The internal beam production during the establishment of self-pumped phase conjugation (SPPC) is closely observed, and the corresponding mechanisms of SPPC are identified accordingly. When the incident beam is in an a-face-incidence geometric arrangement, it is observed for the first time that SPPC mechanism has a transition from stimulated photorefractive backscattering and four-wave mixing to total internal reflection when λ is varied from 706 to 733 nm. Variations of the SPPC reflectivity with the λ, incident angle, and position of the input beam have been investigated, and a high reflectivity of approximately 80% has been obtained. In addition, SPPC reflectivity of approximately 40% is obtained when the incident beam enters the crystal by the +c face, and a variation with λ of optical beam patterns in the crystal has also been observed. Qualitative explanations of the phenomenon of SPP mechanism transition with λ that will be very useful in practical applications are given.

Origin of the wavelength-dependence of effective trap density in photorefractive BaTiO3:Ce

The wavelength dependence of the total effective trap density, Neff, in a photorefractive BaTiO3:Ce crystal is experimentally determined by two-beam coupling. The result shows that Neff decreases with increasing wavelength. The deep-shallow-trap model for charge transport well fits to the experimental data, indicating that the model can quantitatively explain the variation of Neff with wavelength in BaTiO3:Ce. It is concluded that the remarkable variation of the ratio of the photo ionization cross section of the deep center to that of the shallow trap with wavelength, and the higher density of the shallow traps than that of the uncompensated deep centers are significant to the variation of Neff with wavelength.