Direct laser writing combined with a phase-delay probe.

We develop on lithium niobate crystals a photorefractive direct-laser-writing approach, in which we combine in one beam both direct writing and phase-delay probing functionalities to extract the in situ information of the refractive index or the electrostatic field. The phase-delay signal, predicted well by the photorefractive theory, is used as feedback for tuning the exposure time or scanning speed of the focused laser in order to control the refractive index change (Δn) at single points and scanning lines. Different features found in creating Δn at the points and lines are explained by the different photorefractive responses in the two cases.

Impact of the crystal orientation of Fe-doped lithium niobate on photo-assisted proton exchange and chemical etching.

Photo-assisted proton-exchange (PAPE) is carried out on the +c- and y-surfaces of Fe-doped LiNbO3 crystals and the impact of the crystal orientation on the PAPE and the subsequent photo-assisted chemical etching (PACE) is investigated. The proton distributions and the morphologies of the proton-exchanged surfaces are studied by using Micro-FT-IR, Micro-Raman, optical and scanning electron microscopy. Through the PAPE process the proton-exchange can be confined in a specific region by an incident laser beam with fixed intensity profile. It is found that the y-surface is much more fragile than the +c-surface and that micro-cracks are easily generated on the y-surface during the PAPE process. Moreover, the range and number of these micro-cracks can be controlled by the experimental parameters of the PAPE process. The etching morphology of the y-surface shows apparent directional features along the c-axis of LiNbO3 crystal and the proton spatial distribution is found elongated along the c-axis. Both effects are attributed to the accumulation of photovoltaic charges at the two sides of the illumination area along the c-axis.

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.

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.

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.