Zhenkun Yu

Nanosecond laser pulse induced concentric surface structures on SiO 2 layer

We report the periodic concentric surface structures on SiO2 layer induced by a single shot nanosecond laser pulse at 1.06 μm. The fringe period of the structures ranges from 7.0 μm to 26.8 μm, depending on the laser fluence and the distance from central defect precursor. The size and depth of the damage sites increase almost linearly with the laser fluence from 19.6 J/cm(2) to 61 J/cm(2). Plasma flash was clearly observed during the damage process. We attribute the formation mechanism of the structures to the interference between the reflected laser radiations at the air/shock-front and the shock-front/film interfaces.

Damage threshold influenced by the high absorption defect at the film-substrate interface under ultraviolet laser irradiation

The laser-induced damage threshold (LIDT) of a single-layer coating at the nanosecond (ns) regime is obviously lower than an uncoated substrate or a high reflectivity coating coated by the same material. To elucidate this phenomenon, we demonstrate the LIDT of three types of samples at 355 nm with 8 ns. High absorption defects are found at the film-substrate interface by comparing their LIDTs and damage morphologies. These defects originate from the substrate and appear during the coating process. Simulation results show that these defects, coupled to the coating, are mainly responsible for decreasing the damage threshold.

Investigations on single and multiple pulse laser-induced damages in HfO 2 /SiO 2 multilayer dielectric films at 1064 nm

Nanosecond single and multiple pulse laser damage studies on HfO₂/SiO₂ high-reflective coatings were performed at 1064 nm. The evolution of LIDT and 100% damage probability threshold under multiple irradiations revealed that fatigue effects were affected by both laser fluence and shot numbers. And the damage probability curves exhibiting different behaviors confirmed experimentally that this fatigue effect of the dielectric coatings was due to material modification rather than statistical effects. By using a model assuming Gaussian distribution of defect threshold, the fitting results of LID probability curves indicated the turning point appeared in the damage probability curves under large shot number irradiations was just the representation of the existence of newly created defects. The thresholds of these newly created defects were exponential decrease with irradiated shot numbers. Besides, a new kind of damage morphologies under multiple shot irradiations were characterized to further expose the fatigue effect caused by cumulative laser-induced material modifications.

Coupling effect of subsurface defect and coating layer on the laser induced damage threshold of dielectric coating

The laser damage resistance of coatings for high power laser systems depends greatly on the surface quality of substrate. In this work, experimental approaches with theoretical simulation were employed to understand the coupling effect of subsurface defect and coating on the laser resistance of coating. 1064 nm anti-reflection coating was deposited by E-beam deposition on fused silica. Substrate with and without micro-scale pits were fabricated precisely by femtosecond laser processing. Experimental results indicate that impurities induced in the finishing process shifted to the substrate surface and aggregated during the heating process. Theoretical simulation result shows that the coupling effect of the aggregated impurities and coating are mainly responsible for the low LIDT of E-beam deposition coating.

Effect of laser conditioning on the LIDT of 532nm HfO2/SiO2 thin films reflectors

The laser-induced damage threshold (LIDT) of optical thin film is influenced by certain preconditioning processes. HfO2/SiO2 532nm high reflective multi-layers were prepared by electron beam evaporation and were preconditioned by 532nm laser. The 532nm LIDT, surface condition, and damage morphology of the sample were characterized and compared before and after laser conditioning process. Results are presented that the LIDT of e-beam deposited multilayer HfO2/SiO2 thin films can be increased after laser conditioning. Possible reasons for such enhancement have been analyzed.

Mechanism for 355 nm nanosecond-pulse-driven damage in HfO2/SiO2 high reflectivity coating

The effect of oxygen vacancy on the laser-induced damage behaviors of HfO2/SiO2 high reflective coating was investigated by single 355nm-8ns laser. The oxygen vacancy was tuned by controlling the oxygenation of the outmost HfO2 layer during the deposition procedure. The reflectivity of the coating with higher oxygen vacancy drops by 0.2% and the damage threshold drops by 60%, compared with the normal coating. Damage morphologies of samples were obtained by optical microscope, AFM, SEM and FIB technology. Typical morphologies of these coatings show little difference. Average oxygen vacancy of single HfO2 layer prepared on the BK7 substrate measured by XPS is about 43%. Theoretical analysis with a nonlinear thermodynamics model shows that the damage can be attributed to nonlinear thermal process. Moreover, the size of damage crater can be interpreted by a mechanics model.

Research on the laser damage performance of high reflection coatings at 355 nm

The development of 355 nm high reflection (HR) coatings with high laser-induced damage threshold (LIDT) is one of the continuing challenges for the high power laser field. To increase the LIDT, many efforts have been done concerning the coating material, coating design and deposition process. By optimization of the coating design and deposition parameters, the 355 nm HR coating with LIDT higher than 18J/cm2 has been prepared. The development of the measuring technique has promoted the investigation of laser damage precursors, enabling a better understanding of laser damage mechanism.

Multiple-defect coupling effect of nanosecond-pulsed laser-induced damage in fused silica

The laser induced damage threshold (LIDT) of fused silica samples was investigated with laser damage testing setup (355 nm, 6 ns). A new model, which improved the theory of defect-induced damage, was proposed to describe the multiple-defect coupling effect of the nanosecond-pulsed laser induced damage. The correlation between the damage probability and the damage threshold of this model was also reported.