Xiaonong Shen

Recording of second-harmonic index gratings in photorefractive (K(0.5) Na(0.5))(0.2) (Sr(0.75) Ba(0.25))(0.9) Nb(2)O(6) crystals.

We propose and demonstrate a novel method for recording second-harmonic volume index gratings in photorefractive crystals. The fundamental component of the index grating is eliminated by means of a pi -phase shifted double-exposure process that uses the Mach-Zehnder interferometric technique. Experimental results for (K(0.5) Na(0.5)) (0.2)(Sr> (0.75)Ba (0.25))(0.9)Nb (2)O(6) crystals are presented and discussed.

Electrical domain fixing of photorefractive index gratings in (K0.5Na0.5)0.2(Sr0.75Ba0.25)0.9Nb2O6 crystals

Electrical domain fixing in Cu- and Mn-doped (K0.5Na0.5)0.2(Sr0.75Ba0.25)0.9Nb2O6 (KNSBN) crystals is presented. The relation between the applied field and the revealed grating in single and multidomain crystals is investigated. In multidomain Mn-doped KNSBN, a revealing efficiency of over 233% is obtained. The mechanism of domain fixing in single and multidomain crystal is discussed.

PHOTOREFRACTIVE PROPERTIES OF CU-DOPED (K0.5NA0.5)0.2(SR0.75BA0.25)0.9NB2O6 CRYSTALS WITH DIFFERENT DOPING LEVELS AND DIFFERENT DIMENSIONS

In this article, we studied the photorefractive properties of Cu-doped (K0.5Na0.5)0.2(Sr0.75Ba0.25)0.9Nb2O6 (Cu:KNSBN) crystals with different doping levels. The properties include two-wave mixing and self-pumped phase conjugation, absorption properties, and the growth conditions as well. We also studied the effect of the dimensions on the photorefractive properties. The mechanism of Cu-doped KNSBN and the influence of the different sites occupied by Cu+ on photorefractive properties are also discussed. All the experimental results showed that doping levels have much effect on the photorefractive properties. The crystal with a higher doping level has a higher effective charge carrier density, a larger coupling coefficient, and a faster response rate, which is suitable for fast optical information processing. Dimension has much influence on the photorefractive properties. The thinner crystal with the same doping level has a larger coupling gain coefficient, and a shorter response time.

Systematic studies on photorefractive crystal growth and properties of Cu- and Mn-doped (K0.5Na0.5)0.2(Sr0.75Ba0.25)0.9Nb2O6

In this paper, photorefractive (PR) crystals of Cu and Mn- doped (K0.5Na0.5)0.2(Sr0.75Ba0.25)0.9Nb2O6 were symmetrically studied. We reported the growth conditions, the absorption properties and the photorefractive properties including two wave mixing (TWM) and self-pumped phase conjugation (SPPC). The mechanism of Cu-doped KNSBN was referred briefly, and the influence of the different sites occupied by doping ions on PR properties was also discussed. All these results reveal that: (1) In Cu-doped KNSBN crystals, the photorefractive properties become better with the increase of the doping level, such as higher effective charge carrier density, larger coupling coefficient, faster response rate, higher SPPC reflectivity. (2) The thinner sample shows larger coupling gain coefficient, which is due to the light creep effect and beam fanning effect. The response time becomes shorter with the decrease of the thickness. (3) In Cu:KNSBN crystals, the coupling gain coefficient of TWM was obtained up to 35 cm-1. (4) In Cu:KNSBN and Mn:KNSBN samples, the SPPC reflectivity up to 73% have been obtained, respectively. (5) The C-site in the tungsten-bronze structure of KNSBN has an effect on the PR properties of the crystal. All these results show that doping in KNSBN is an effective method to improve the PR properties. The Cu-doped KNSBN crystal is a very promising photorefractive material.

Photorefractive properties of Cu-doped KNSBN crystal with fluorine replacing oxygen.

We report the photorefractive properties of a new kind of Cu-doped KNSBN crystal in which a small fraction of oxygen (O) atoms are replaced with fluorine (F) atoms. The replacement of oxygen with fluorine in Cu:KNSBN crystals leads to a much shorter response time of 30 ms at an incident intensity of 5 W/cm(2). We also discuss the effect of electron-hole competition on the coupling constant.

Domain fixing process in Cu- and Mn-doped KNSBN crystals

In this paper, we reported the electrical fixing results of photorefractive index gratings in Cu- and Mn-doped KNSBN crystals at room temperature. Due to the higher Curie temperature of KNSBN, the fixed grating may have a longer lifetime than SBN crystals, and the fixing results are different from the observation in SBN crystals. The relation between the applied field and the revealed grating is studied by analyzing the domain fixing process. We also reported the comparison of the fixing results in the single domain crystals and the multi-domain crystals. In the multi-domain crystals, the diffraction efficiency of the revealed grating is as about three times as that of the recorded grating. The probable reason is also discussed.

Properties of harmonic gratings in Mn:KNSBN in the process of electrical domain fixing

Photorefractive crystals have many applications in holographic storage, optical communications, and nonlinear optical information processing. One problem is that the photorefractive holographic index gratings can be erased in the readout process. Several non-destructive approaches have been proposed and demonstrated in different photorefractive materials. Based on the initial research of domain fixing on KNSBN crystals, we studied the high order harmonic gratings during electrical domain fixing process in KNSBN crystals. In this paper, we mainly focus on the properties of the first order and second order harmonic index gratings of Mn-doped KNSBN crystal during electrical domain fixing process at room temperature. Domain fixing in Rh:SBN was also investigated, and the results in Mn:KNSBN and Rh:SBN were compared. In Mn:KNSBN crystals, the second order harmonic grating becomes stronger after fixing. This may be the main reason of the high noise in the revealed grating, which affects the image quality and fidelity. In addition, the effect of the accumulated charges in the revealing process was also discussed.

Four-wave mixing with partially coherent waves in photorefractive media. I. Transmission grating approximation

We investigate the effect of beam temporal coherence on four-wave mixing via transmission gratings in photorefractive media. We take into account the propagation of mutual coherence and self-coherence in space and the evolution of mutual coherence and self-coherence with time. Analytical solutions for nondepleted-pump cases and numerical results for general cases are obtained. We find that the mutual coherence of the signal beam and the pump beam can be enhanced or reduced, depending on the coupling constant and the beam ratio of the signal beam and the pump beam. A decrease of the initial mutual coherence can lead to a lower phase-conjugate reflectivity in the nondepleted-pump case. Even with low initial mutual coherence, high phase-conjugate reflectivity can still be obtained in the case with pump depletion. As a result of the shift of the grating profile, partial coherence can lead to an enhancement of the phase-conjugate reflectivity when the signal–pump-beam ratio is small. We also found that when the initial value of the phase-conjugate beam is zero, the phase-conjugate beam and the pump beam are in phase during propagation. Results for beam intensity and mutual coherence are presented and discussed to provide an insight into four-wave mixing with partially temporally coherent waves.

Photorefractive properties of Mn-doped KNSBN crystals

The photorefractive properties of Mn-doped KNSBN crystals, especially the crystal with a higher doping level (0.09 wt.%), was studied. Experiment showed that the properties of Mn:KNSBN crystals with 0.09 wt.% are different from others. This maybe provides a good material to study the electron- hole competition, which exists widely in PR crystals. The high order harmonic properties were also investigated. Five orders of harmonics were observed. It was found that the maximum of the second order index grating shifts towards a small incident angle with respect to that of the fundamental grating. Based on the strong harmonics in this kind of crystals, the recording of harmonic index gratings was achieved by using a novel method in experiment.

Selective erasure of holographic index gratings in photorefractive crystals

We proposed a novel method to realize selective erasure of holographic index gratings in the photorefractive crystals. This scheme is based on the self-enhancement of the index gratings in the photorefractive materials. It is a save-erase- restore procedure, namely, save wanted information-erase unwanted information-restore wanted information. Simple experimental results are also presented. The requirement in this scheme is that the material has the effect of self- enhancement, i.e. the index grating is transmission grating, the product of the coupling constant and the thickness is above the threshold of the self-enhancement. But it eliminates the critical requirement of preventing vibration in the previous (pi) -phase-shift scheme.