Hongjian Chen

Investigations of the light-induced scattering varied with HfO2 codoping in LiNbO3:Fe crystals

Light-induced scattering in LiNbO3:Fe crystals codoped with different concentrations of HfO2 has been investigated. Unusual behaviors with varied HfO2 codoping are observed and explained. Li vacancies are suggested to be the main contributor to the dark conductivity of the crystal and subsequently to induce the threshold effect of the pump light intensity for the light-induced scattering. High photovoltaic field produced in the crystals is thought to be the cause of the serious light-induced scattering at high pump intensity. As a result, LiNbO3:Fe, Hf crystals exhibit an enhanced ability to suppress the light-induced scattering at moderate pump intensity.

Investigations on the UV photorefractivity of LiNbO 3 :Hf

The UV photorefractivity of LiNbO(3):Hf was investigated by beam distortion and two-wave coupling experiments. A sharp decrease of beam distortion was found between Hf concentrations of 2 and 2.5 mol.%, indicating a significant increase of photoconductivity there. The energy-transfer direction of the two-wave coupling and the effect of chemical reduction on the UV photorefractivity reveal that electrons are the carriers responsible for the UV photorefraction. Moreover, dissimilar with LiNbO(3) highly doped with Mg, Zn, and In, the grating recorded in LiNbO(3):Hf was found to be nearly stable in the dark but sensitive to red light, which can be attributed to the special location of Fe ions in the crystal lattice.

Tunable photocatalytic activity of photochromic Fe-Mn-codoped LiNbO_3 nanocrystals

The tunable photocatalytic activity of LN nanoparticles is a useful integratable function of LN-based lab-on-chip platform for unraveling the biological complexity in population. In this paper, Fe-Mn-codoped LiNbO3 nanoparticles were prepared by a soft-chemistry route. The nanoparticle absorption in UV-Vis range was modified through photochromic effect, and their influence on the photocatalytic activity of the nanoparticls was evaluated by the degradation of Rhodamine B under xenon lamp illumination. It was found that Fe-Mn-codoped LiNbO3 nanocrystals with less Vis absorption exhibited higher photocatalytic efficiency for photocatalytic dye solution. The mechanism of tuning the photocatalytic activity through photochromic effect was proposed for Fe-Mn-codoped LiNbO3 nanocrystals and the holes released through charge transfer from Mn3+ are emphasized for the explanation of the photocatalytic behavior.

Investigations of the OH− absorption bands in congruent and near-stoichiometric LiNbO3:Hf crystals

OH− absorption bands of both congruent and near-stoichiometric LiNbO3:Hf crystals were investigated. The abrupt blueshift of OH− absorption bands was found between the doping levels of 2 and 2.5 mol% in the congruent crystals, which confirms the exact optical-damage threshold for Hf doping. The absorption difference of OH− bands between the congruent and near-stoichiometric crystals shows that after Li incorporation H+ ions no longer prefer the sites of 3500 cm−1. Moreover, with the increase of Hf doping in the near-stoichiometric crystals, H+ ions were found to gradually transfer to the sites of 3489 cm−1. These results are explained by the breakdown of the association of HfNb− and H+ ions and the subsequent redistribution of H+ ions around HfLi3+ ions in VLi−-deficient lattice. Additionally, the link between the intrinsic defects and how they influence the optical-damage threshold upon adding Hf was discussed in detail.

Dielectrophoretic behaviours of microdroplet sandwiched between LN substrates

We demonstrate a sandwich configuration for microfluidic manipulation in LiNbO3 platform based on photovoltaic effect, and the behaviours of dielectric microdroplet under this sandwich configuration are investigated. It is found that the microdroplet can generate in the form of liquid bridge inside the LiNbO3-based sandwich structure under the governing dielectrophoretic force, and the dynamic process of microdroplet generation highly depends on the substrate combinations. Dynamic features found for different combinations are explained by the different electrostatic field distribution basing on the finite-element simulation results. Moreover, the electrostatic field required by the microdroplet generation is estimated through meniscus evolution and it is found in good agreement with the simulated electrostatic field inside the sandwich gap. Several kinds of microdroplet manipulations are attempted in this work. We suggest that the local dielectrophoretic force acting on the microdroplet depends on the distribution of the accumulated irradiation dosage. Without using any additional pumping or jetting actuator, the microdroplet can be step-moved, deformed or patterned by the inconsecutive dot-irradiation scheme, as well as elastically stretched out and back or smoothly guided in a designed pass by the consecutive line-irradiation scheme.

Polyol process synthesis of metal selenide nanomaterials and their photovoltaic application

A mixed solvent of monoethanolamine and thioglycolic acid is successfully used to dissolve Se powder to prepare a highly reactive and soluble Se solution at room temperature in air. The as-prepared Se solution is very stable for more than half a year without any observed precipitation even in air. We used the Se precursor solution to synthesize various metal selenide nanomaterials such as Sb2Se3 nanorods, CuSe nanoplates and CuInSe2 nanocrystals by using a facile and environmentally-friendly triethylene glycol process. The structure, morphology, chemical composition and optical properties of the synthesized metal selenide nanomaterials are characterized. Moreover, the CuInSe2 nanocrystals are used to prepare ethanol-based colloidal ink and then deposited thin films by a dip-coating process. The synthesized CuInSe2 nanocrystal thin films are annealed at 550 °C in a selenizing atmosphere. A power conversion efficiency of 2.65% has been achieved by using the CuInSe2 thin films after selenization as a light-absorbing layer under AM 1.5 illumination.

Photoinduced Ag-nanoparticle deposition on Fe-doped lithium niobate crystals

We study the photoinduced deposition of Ag nanoparticles on Fe-doped LiNbO3 crystals by using an in situ probe. SEM and XPS analysis show that Ag metallic nanoparticles are close-packedly deposited on the crystal surface. Both 405 and 532nm laser are found effective for the photo-induced Ag deposition. The Ag deposition on the y-surface shows obvious anisotropy as compared to that on the + z-surface. Moreover, the Ag deposition on the + z-surface is found to start easily surrounding the focal point of the laser rather than at its center. Both photogalvanic and diffusion effects of photo-excited electrons are suggested to account for these features.

Photochromic effect in LiNbO(3): Fe :Co.

In this paper, Co(2)O(3) was codoped in LiNbO(3): Fe crystals. It was found that Co codoping can give rise to the strong photochromic effect in LiNbO(3): Fe. Based on the UV-VIS-NIR absorption spectra, photorefractive sensitivities, and EPR results for both virgin and sensitized states of the crystal, the photochromic mechanism in this material was suggested as follows. During sensitization, electrons transfer from O(2-) to Fe(3+) directly while holes are thermally excited from O- into the valence band and then partly trapped by Co(2+), which leads to the darkening of the crystal. In bleaching process, the electrons are excited from Fe(2+) to the conduction band by green light and recombine with the holes on the Co(2+) level. Sensitizing/bleaching experiments were also carried out in LiNbO(3): Fe: Co crystals. Fast sensitization found for this material is beneficial to two-color recording.

Photo-assisted proton exchange and chemical etching on Fe-doped lithium niobate crystals

We report the photo-assisted proton exchange and chemical etching on Fe-doped LiNbO(3) crystals. Selective proton exchange and chemical etching are realized through the 455nm-laser irradiation on the crystal surface in pyrophosphoric acid. Optical microscopy and Micro-IR spectroscopy analysis show that the hydrogen incorporation is confined spatially by the laser irradiation. Moreover, under the laser irradiation, + z surface is found to be more easily etched than -z surface. This unexpected etching anisotropy is attributed to the photogalvanic effect of the crystal.

Photo-assisted splitting of dielectric microdroplets in a LN-based sandwich structure

We demonstrate an active and controllable photo-assisted splitting of dielectric microdroplets inside a LiNbO3-based sandwich structure by utilizing the pyroelectric and photovoltaic effects of LiNbO3 crystals. Basing on electrostatic simulation results, the mechanism of the photo-assisted splitting is explained; meanwhile, the microdoplet pre-polarizing and the inclined electrostatic force induced by the photovoltaic charges inside the sandwich structure are emphasized. We also study the dependence of the splitting part size on the substrate temperature variation, the irradiation intensity, and the duration. Featured dependence is found to follow the theoretical curve predicted by the compensation between the photo-excited and bound surface charges.