Jiasen Zhang

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.

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.

Fine structure of the plasmon resonance absorption peak of Ag nanoparticles embedded in partially oxidized Si matrix

Ag/Si nanocomposite films were prepared by the radio-frequency magnetron cosputtering method. The fine structure of the plasmon resonance absorption peak was found in film samples. X-ray photoelectron spectroscopy analysis indicated that the samples were composed of a two-layer structure, which accounted for the structure of the optical absorption spectra. The peak located near 445 nm is the plasmon resonance absorption peak of Ag nanoparticles embedded in a partially oxidized Si matrix. Its intensity decreases with decreasing film thickness and disappears in a very thin sample. The peak located near 380 nm originates from the plasmon resonance absorption of the thoroughly oxidized surface layer of the sample. Its intensity does not change with increasing thickness, but it cannot be observed in the very thick sample

Silicon membrane resonant-cavity-enhanced photodetector

A Si resonant-cavity-enhanced (RCE) photodiode was fabricated on a silicon membrane. The Si membrane was formed by etching from the back side of the silicon-on-insulator substrate with the buried SiO2 layer as etch-stop layer. A gold layer was deposited serving as an electrode layer and bottom mirror of the RCE photodiode. The photodiode had an external quantum efficiency of 33.8% at the resonant wavelength of 848 nm and a full width at half maximum (FWHM) of 17 nm. The responsivity was 4.6 times that of a conventional Si p-i-n photodiode with the same absorption layer thickness. (c) 2005 American Institute of Physics.

LIGHT-INDUCED INFRARED ABSORPTION SPECTRA AND PROPERTIES OF IMPURITY LEVELS IN DOPED PHOTOREFRACTIVE BATIO3

Light-induced absorption spectroscopy, the decay process of light-induced absorption, and its temperature dependence were investigated to study the properties of the photorefractive impurity levels in doped BaTiO3. Light-induced absorption spectra in one Rh-doped and two Ce-doped BaTiO3 crystals from 0.7 to 3.0 μm were measured and it was found that the light-induced absorption of all these crystals approached zero above ∼1.3u2009μm. A two-shallow-level model is proposed to explain the biexponential behavior of the decay of the induced absorption in BaTiO3:Ce. The activation energies are deduced to be 0.72 eV for the single-shallow level in BaTiO3:Rh, 0.50 and 0.66 eV for the two shallow levels in the 8 ppm Ce-doped BaTiO3, and 0.50 and 0.74 eV for that in the 30 ppm Ce-doped BaTiO3.

Reduction of fanning influence in two‐wave mixing coefficient measurements in thick crystals

We propose and demonstrate a new method to measure the two‐wave mixing coefficient with slight influence of beam fanning in thick crystals with high coupling constants. The strong beam fanning was suppressed by introducing an additional erase beam. By measuring the effective two‐wave mixing coefficient with the additional erase beam, the coupling coefficient without the additional erase beam can be calculated.

Variation of mechanism transition wavelength of self-pumped phase conjugation with Ce-content in BaTiO3:Ce Crystals

The phenomena of SPPC (self-pumped phase conjugation) mechanism transition from stimulated photorefractive backscattering and four-wave mixing (SPB-FWM) to cat (or TIR) with wavelength have been investigated in BaTiO3:Ce crystals with different Ce-doping concentrations. In addition, it is observed that SPPC is generated with SPB-FWM mechanism in an undoped BaTiO3 crystal at wavelengths shorter than 488 nm while it is well known that cat mechanism is usually responsible for the SPPC generation in BaTiO3 crystals at longer wavelengths. A red shift of the mechanism transition wavelength with the doping concentration in BaTiO3 Crystals is clearly evident from our experiments. This is believed to be due to the increase of the stimulated photorefractive gain coefficient of the crystals with the doping concentration.