Dadi Wang

Effects of partial substitution of transition metals for cobalt on the high-temperature thermoelectric properties of Ca3Co4O9+δ

Polycrystalline samples of Ca3Co4−xMxO9+δ (M=Ni, Fe, Mn, Cu; x=0–0.6) have been prepared using sol-gel procedure followed by spark plasma sintering. Their thermoelectric properties have been carefully examined from room temperature to 1000 K. Substitution of Ni and Fe for Co caused the significant increase of electrical resistivity, but the thermopower changed only slightly. The magnitude of the increases in electrical resistivity and thermopower for the Mn-substituted samples was the most pronounced as compared with Ni- and Fe-substituted samples in the same substitution content, which could be explained by the strong localization of charge caused by Mn substitution. Temperature dependence of electrical conductivity showed that the hole hopping conduction mechanism was dominant for all samples. The activation energy increased with increasing substitution content. On the other hand, the Cu substitution resulted in decreases of both electrical resistivity and thermopower, and the activation energy remained...

Observations on the coupling channel of two mutually incoherent beams without internal reflection in BaTiO3

Abstract A new scheme of mutually-pumped phase conjugator that works not relying on the internal reflection has been demonstrated experimentally in both one BaTiO 3 crystal and two separated BaTiO 3 crystals. These new phase conjugators differ from the well known double phase-conjugate mirror in that a channel coupling two incident beams has been observed and shared photorefractive holograms with two or more interaction regions in the crystal have to be formed. Thus, it is also demonstrated that coupling channels of two incident beams can be formed not only with internal reflections inside the crystal but also without internal reflection.

PHOTOREFRACTIVE PROPERTIES AND SELF-PUMPED PHASE CONJUGATION OF TETRAGONAL FE-DOPED KTA1-XNBXO3 CRYSTAL

This letter reports the two‐wave mixing experimental results of tetragonal Fe‐doped KTa1−xNbxO3. A 7.3 cm−1 gain coefficient and 42% diffraction efficiency have been obtained. For the first time internally self‐pumped phase conjugation is being observed in this crystal. At room temperature, the conjugate reflectivity measured is 21%.

Studies of third-order optical nonlinearities inC 60 –toluene and C 70 –toluenesolutions

Third-order optical nonlinearities in C 60 - and C 70 -toluene solutions were studied by using a time-delayed degenerate four-wave-mixing technique with 30-ps pulses at 0.532 μm in both parallel- and cross-polarization configurations. The concentration dependences of χ (3) 1212 is were measured for C 60 - and C 70 -toluene solutions

INSTABILITIES IN A MUTUALLY PUMPED PHASE CONJUGATOR OF BATIO3

Various instabilities, such as regular and irregular pulsation, periodic oscillation, and optical chaos, have been observed by selecting geometric parameters in a mutually pumped phase conjugator (PC) of BaTiO3. The behavior of the PC output in the periodic oscillation state was studied in detail. Some explanations, for instance, competition among the coupling channels and interference between two PC beams coming from different channels, are proposed.

Photorefractive diffraction dynamic during writing in paraelectric KTN crystals

The photorefractive diffraction dynamic during writing in paraelectric KTN crystals is studied for the first time to our knowledge. A decay process of the diffraction efficiency during writing after a maximum was reached is observed at a high enough writing intensity. The variation of the time evolution of the diffraction efficiency with the writing intensity and with the external dc electric field is measured. An electron–hole transport model and a nonequilibrium screening model are applied to explain qualitatively the major observations.

Mutually coherent beam induced self‐pumped phase conjugate reflection in BaTiO3

A self‐pumped phase conjugate reflection induced by another mutually coherent incident beam has been observed and explained successfully for the first time in a photorefractive crystal BaTiO3. Other relevant phenomena, such as the destruction of the coupling channel when two incident beams are mutually coherent, have also been studied. Possible applications are pointed out.

Studies of third-order optical nonlinearities in C_60–toluene and C_70–toluene solutions

Third-order optical nonlinearities in C 60 - and C 70 -toluene solutions were studied by using a time-delayed degenerate four-wave-mixing technique with 30-ps pulses at 0.532 μm in both parallel- and cross-polarization configurations. The concentration dependences of χ (3) 1212 is were measured for C 60 - and C 70 -toluene solutions

Dynamics of photorefractive diffraction in KTa/sub 1-x/Nb/sub x/O/sub 3/ crystals

Although the photorefractive properties of KTaNbO/sub 3/ (KTN) crystal, one of the first identified photorefractive crystals, have been investigated in more detail in recent years, its photorefractive diffraction dynamics have been rarely studied. However, these studies might be important for understanding the mechanism of the photorefractivity including the need for a complicated photorefractive model and the influences of the internal dc electric field induced during writing with an applied field. In this presentation, our experimental investigations of photorefractive diffraction dynamics during writing, reading and erasing in a cubic KTN crystal will be reported systematically. Here, we studied not only the diffraction from the first-order grating but also the diffraction from the second-order grating.<<ETX>>

Studies of photorefractive diffraction dynamic in paraelectric KTa1−xNbxO3 crystals

The dynamics of photorefractive diffraction in paraelectric KTa1−xNbxO3 crystals during writing, reading, and erasing have been studied systematically. Experimental observations related not only to the diffraction from the first‐order photorefractive grating but also to that from the second‐order photorefractive grating, and where a writing beam with spots smaller than, as well as larger than, the entrance surface of the crystal was used. All the complicated behaviors observed have been explained qualitatively. An electron‐hole transport model and a nonequilibrium screening model have been applied, and the existence of an internal dc electric field induced during writing simultaneously by an optical field and an externally applied field has been suggested and demonstrated. The significant influence of this induced field is indicated.