Papers by Author: Yan Shen Wang

Abstract: By focusing femtosecond laser pulses in the bulk of a quartz wafer, sub-surface waveguides were microexploded. The material around the femtosecond laser induced microexplosive zone was densified, which altered local refractive index. Changes in material density can take corresponding variations in nanomechanical properties, which were proved by the depth dependent modulus and hardness variations tested by nanoindentation in the area around the microexplosive zone. Changes in refractive index were correlated with residual strains along radial direction of the waveguide cross section. An equation of residual strain in such area that based on nanoindentation data was set up, so as to discover density and refractive index variations in the adjacent areas of femtosecond laser microexplosive zone indirectly.

Abstract: Femtosecond laser pulses are irradiated on a single-side polished single-crystalline silicon wafer. Metamorphic zone appears around the ablated zone and the morphology changes gradually in the metamorphic zone. Typical phenomena of thermal ablation such as melt-resolidification and subtransparent glassy materials occur in the marginal area of the ablated zone. No apparent changes are found among microscopic morphologies of the ablated, metamorphic and unirradiated zones. There are flaws and spallations on the smooth back surface of the wafer, which are caused by the stress inside the sample. Nanomechanical properties of the sample surface hardly change in the small scope of the backside around the ablated zone. While regular changes occur in large scope, which is the conjunct result of such stresses caused by many different collateral damages.

Abstract: Ripples in the area of femtosecond laser irradiated discrete points and continuous lines were studied. The characteristics of interference-induced ripples (LSFL) in the area irradiated by single shot were investigated by AFM. During single point irradiation, morphology of the irradiated area changed with energy deposition. Morphologies of the irradiated continuous lines with and without ablated groove inside were both investigated. The intensity of interfered light waves varied in different positions of each energy stripes. Thus the characteristics of ripples in the irradiated area varied with different positions. Ripples much larger than LSFL were found perpendicular to laser polarization.

Abstract: High-energy femtosecond laser pulses were utilized to ablate single-crystalline silicon wafer. Collateral damage areas around the ablation zone can be observed in microscope. The morphology in such areas changes gradually. The microscopic morphology and nanomechanical properties of the pre-polished back surface were measured by AFM and Hysitron TriboIndenter respectively. The topography and roughness in the ablated, metamorphic and unaffected zone are almost equal. Yet the elastic ratio and hardness on the back surface vary gradually with indent positions, which coincide with the gradual morphological changes in the metamorphic zone on the front surface. Such regular changes in nanomechanical properties, to some extent, reflect the distribution of collateral damages near the ablated zone on the back surface. And they also testify the occurrence of the ill effects that go against micromachining during high-energy femtosecond laser irradiation.