A two-stage two-dimensional (2D) gas-dynamic model of laser ablation in an ambient gas atmosphere is proposed. The initial one-dimensional stage of the process is related to the ablation plume formation under the action of a laser pulse (duration of the order of 10 ns; fluence about several J/cm2; laser spot diameter about 1 mm) and describes heating, melting, and evaporation of the target, the target–vapor interaction in the Knudsen layer, and the vapor dynamics. The final 2D stage is responsible for the formation of the energy and angular distributions of the ablated material. Considerable compression of the ambient gas around the expanding plume of the laser-evaporated material and a shock front propagating through the undisturbed ambient gas are found. The pressure of the compressed ambient gas behind the shock may be much higher than the ambient one. However, at the investigated ambient pressures below 100 Pa, it remains still much lower than the vapor pressure during laser evaporation. Therefore, th...

Gas dynamics of laser ablation: Influence of ambient atmosphere

Toward High-Quality Epitaxial LiNbO3 and LiTaO3 Thin Films for Acoustic and Optical Applications

We performed a parametric study to suppress secondary phases in lithium niobate thin films by using pulsed laser deposition (PLD). By reducing the growth rate and changing the ambient gas pressure respectively, we found that the main parameter suppressing secondary phases is controlling the plume strength. For investigating the relation between film phases and the plume strength, extensive parametric studies were performed by changing the oxygen ambient pressure, the target to substrate distance, and also the Li content in the target. The surface morphologies of single phase films obtained with different deposition parameters are compared and deposition parameters to achieve lower loss single-phase films are discussed.

Pulsed laser deposition of single phase LiNbO3 thin film waveguides

Two-dimensional gas-dynamic model of laser ablation in an ambient gas

LiNbO 3 thin films were grown on (0001) sapphire substrates by a chemical route, using the polymeric precursor method. The overall process consists of preparing a coating solution from the Pechini process, based on metallic citrate polymerization. The precursor films, deposited by dip coating, are then heat treated to eliminate the organic material and to synthesize the phase. In this work, we studied the influence of the heat treatment on the structural and optical properties of single-layered films. Two routes were also investigated to increase the film thickness: increasing the viscosity of the coating solution and/or increasing the number of successively deposited layers. The x-ray diffraction u-2u scans revealed the c-axis orientation of the single- and multilayered films and showed that efficient crystallization can be obtained at temperatures as low as 400 °C. The phi-scan diffraction evidenced the epitaxial growth with two in-plane variants. A microstructural study revealed that the films were crack free, homogeneous, and relatively dense. Finally, the investigation of the optical properties (optical transmittance and refractive index) confirmed the good quality of the films. These results indicate that the polymeric precursor method is a promising process to develop lithium niobate waveguides.

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Epitaxially grown LiNbO3 thin films by polymeric precursor method

The origin of Li deficiency in films grown by laser ablation of single-crystal LiNbO3 targets in a buffer gas has been investigated by analyzing the stoichiometry of the deposited films as a function of the following parameters: the distance target-substrate, the nature of the buffer gas (He, O2, and Ar) and the deposition configuration. The results show that significant Li losses are related to scattering processes during the expansion regime which are higher the higher the mass of the gas species. The results show that the Li content of the films can be enhanced by setting the substrate either at distances larger than the plume length or in a configuration in which the substrate is not facing the target.

/pdf/li-deficiencies-in-linbo3-films-prepared-by-pulsed-laser-557fr8lqpw.pdf

Li deficiencies in LiNbO3 films prepared by pulsed laser deposition in a buffer gas

The correlation between the optical properties and composition of lithium niobate films is investigated in films deposited by laser ablation of single‐crystal LiNbO3 and Li enriched LiNbO3 sintered targets under different oxygen pressures and laser fluences. The results show that the variation of the film refractive index is in many cases related to the deposition process itself, rather than to a variation of the film composition. The experimental conditions to produce stoichiometric films with optical properties similar to those of the bulk material are discussed.

/pdf/correlation-between-optical-properties-composition-and-2o139vqx27.pdf

C. Ortega

Papers

Li deficiencies in LiNbO3 films prepared by pulsed laser deposition in a buffer gas

Correlation between optical properties, composition, and deposition parameters in pulsed laser deposited LiNbO3 films