Photovoltaic splitting of water microdroplets on a y-cut LiNbO3:Fe crystal coated with oil-infused hydrophobic insulating layers.

We demonstrate the successful photovoltaic splitting of water microdroplets on a $y$y-cut ${{\rm LiNbO}_3}:{\rm Fe}$LiNbO3:Fe substrate coated with an oil-infused hydrophobic layer. The temporal evolution of the microdroplet contact angle upon a central illumination and the distinct behaviors of two sub-droplets during a following boundary illumination reveal that both electrowetting and electroosmotic effects induced by the dipolar photovoltaic potential on the substrate contribute to the water microdroplet splitting. The reciprocal relationship between the splitting time and the illumination intensity verifies the inherent photovoltaic nature of the water microdroplet splitting. The splitting time is found to be linearly dependent on the initial microdroplet size. These points are quite important to the practicalization of lithium niobate (LN)-based microfluidic chips in the biological field.

All-optical splitting of dielectric microdroplets by using a y-cut-LN-based anti-symmetrical sandwich structure.

We demonstrate an all-optical active mode of dielectric microdroplet splitting in a sandwich structure consisting of two anti-symmetrical y-cut LN:Fe substrates. The dynamic process of the microdroplet splitting and the simulation of the electrostatic interaction inside the sandwich gap show that the combination of two anti-symmetrical substrates are capable to provide a sufficient dielectrophoretic force and to reduce the unbalance of the drag forces for a stable and efficient splitting of the microdroplet. The dependences of the splitting time on the illumination intensity and the initial microdroplet size are also studied, and the results show that the microdroplet splitting process is fully governed by the establishment of the superposed photovoltaic field inside the sandwich gap. A key ratio Er/E0, representing the microdroplet splitting difficulty for a given sandwich structure, is found linearly dependent on the initial microdroplet size. These points are quite important to the integration of splitting functionality on the LN-based microfluidic chip.

Visible-light-assisted condensation of ultrasonically atomized water vapor on LiNbO3:Fe crystals.

Optically massive trapping of the moisture in the air into an adjacent surface is a potential technique in the fields of bacterial adhesion and microfluidic generation, which is quite important to the development of LN-based biological lab-on-chips. Here we demonstrate on a LiNbO3:Fe substrate the visible-light-assisted condensation of the water vapor in a flowing stream created by an ultrasonic atomizer. Through analyzing the dynamic processes of the visible-light-assisted water condensation at different illumination intensities, it is found that the extent of the water condensation, the bending angle of water vapor trails and the interaction range of the condensation effect are highly dependent on the illumination intensity. According to these findings and the simulated trajectories of the water vapor stream at different illumination intensities, we propose that this visible-light-assisted water condensation is an aggregation process of tiny water droplets driven by the dielectrophoretic interaction of inhomogeneous photovoltaic field and also an electrostatic screening course of photovoltaic charges through the charged evaporation of condensed water. The prolonged condensation of water vapor after a high-intensity illumination and that of oil vapor at a super-low evaporation rate are also studied, and the agreement between the simulation and experimental results reinforces the above mechanism. The reported technique, employing the inexpensive, safe-for-cell visible laser beam, is quite convenient for the controllable generation of various biological microdroplets, and thus it is promising for the microfluidic functionality integration of LN-based biological lab-on-chips.

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.