Recently, integrated photonics has attracted considerable interest owing to its wide application in optical communication and quantum technologies. Among the numerous photonic materials, lithium niobate film on insulator (LNOI) has become a promising photonic platform owing to its electro-optic and nonlinear optical properties along with ultralow-loss and high-confinement nanophotonic lithium niobate waveguides fabricated by the complementary metal–oxide–semiconductor (CMOS)-compatible microstructure engineering of LNOI. Furthermore, ferroelectric domain engineering in combination with nanophotonic waveguides on LNOI is gradually accelerating the development of integrated nonlinear photonics, which will play an important role in quantum technologies because of its ability to be integrated with the generation, processing, and auxiliary detection of the quantum states of light. Herein, we review the recent progress in CMOS-compatible microstructure engineering and domain engineering of LNOI for integrated lithium niobate photonics involving photonic modulation and nonlinear photonics. We believe that the great progress in integrated photonics on LNOI will lead to a new generation of techniques. Thus, there remains an urgent need for efficient methods for the preparation of LNOI that are suitable for large-scale and low-cost manufacturing of integrated photonic devices and systems.
By using ionic implantation and wafer bonding technologies, we peeled off a single-crystal LiNbO3 thin film in the atomic-layer smoothness from the surface of a bulk Z-cut LiNbO3 single crystal. X-ray diffraction patterns showed only (00l) orientation of the film. From positive-up-negative–down pulse characterization, we measured domain switching current transients under various short-pulse voltages, where we observed domain switching currents to occur separately at time after initial capacitor charging currents. This is similar to early observations in bulk ferroelectric single crystals, where apparent positive/negative coercive fields of domain switching determined from polarization-electric field hysteresis loops always equal maximum/minimum applied fields. However, after pulse stressing of the film for more than 1000 cycles, the domain switching speed is accelerated, where domain switching current overlaps with the initial capacitor charging current with a well-defined coercive field independent of the applied-field strength. Finally, we simulated the whole domain switching current transients with the assumption of the resistance degradation across interfacial passive layers between the film and electrodes.
Ferroelectric domain inversion and its effect on the stability of lithium niobate thin films on insulator (LNOI) are experimentally characterized. Two sets of specimens with different thicknesses varying from submicron to microns are selected. For micron thick samples (~28 m), domain structures are achieved by pulsed electric field poling with electrodes patterned via photolithography. No domain structure deterioration has been observed for a month as inspected using polarizing optical microscopy and etching. As for submicron (540 nm) films, large-area domain inversion is realized by scanning a biased conductive tip in a piezoelectric force microscope. A graphic processing method is taken to evaluate the domain retention. A domain life time of 25.0 h is obtained and possible mechanisms are discussed. Our study gives a direct reference for domain structure-related applications of LNOI, including guiding wave nonlinear frequency conversion, nonlinear wavefront tailoring, electro-optic modulation, and piezoelectric devices.
Cavitation and flashing formed inside and outside of an injector, respectively, have significant effect on liquid phase LPG (Liquefied Petroleum Gas) injection. Numerical simulations of internal flow of the liquid phase LPG inside different injector nozzles were performed using the FIRE CFD code. The results showed that the cavitation always occurred at the inlet corner of the nozzle with negative pressure and higher velocity regardless the nozzle geometries. The relationships between vapor void fraction at the exit of the nozzle and injection pressure were also investigated for different nozzle geometries. The spray of the liquid phase LPG was further simulated by using the results of the internal flow as initial conditions. During the simulation of the spray, the effect of superheat degree on evaporation was considered and a modified evaporation equation was employed. The comparison of the simulation with experimental results showed that, with the injection pressure increasing, spray tip penetration increased but SMD (Sauter mean diameter) decreased.
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