Meiping Zhu

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.

Study on high-reflective coatings of different designs at 532 nm

Conventional HfO2/SiO2 and Al2O3/HfO2/SiO2 double stack high reflective (HR) coatings at 532 nm are deposited by electron beam evaporation onto BK7 substrates. The laser-induced damage threshold (LIDT) of two kinds of HR coatings is tested, showing that the laser damage resistance of the double stack HR coatings (16 J/cm2) is better than that of the conventional HR coatings (12.8 J/cm2). Besides, the optical properties, surface conditions, and damage morphologies of each group samples are characterized. The results show that laser damage resistance of conventional HR coatings is determined by absorptive defect, while nodular defect is responsible for the LIDT of double stack HR coatings.

Investigations on the catastrophic damage in multilayer dielectric films

HfO2/SiO2 coatings are always fluence-limited by a class of rare catastrophic failures induced by a nanosecond laser with a wavelength of 1053 nm. The catastrophic damage in HfO2/SiO2 coatings behaves as the damage growth with repeated laser irradiation, and thus eventually limits the mirror performance. Understanding the damage processes and mechanisms associated with the catastrophic damage are important for reducing the occurrence of the catastrophic failure and allowing the HfO2/SiO2 coatings to survive at the high fluence required by high laser systems. The rough damage behavior of the catastrophic failure at the proper critical fluence is present. The pit and delamination in the catastrophic failure are investigated to find the possible reasons leading to the catastrophic failure. The experimental results indicate that nodular defect originated from the substrate easily incurs the catastrophic damage. The electric field enhancements of the pit and the substrate impurities may contribute to this phenomenon. The delamination is always present on the left of the pit when laser irradiates from left to right at oblique incidence, which may be related to the plasma plume toward the laser incidence.

Preparation of high performance thin-film polarizers

The optical performance of thin film polarizers is highly sensitive to the layer thicknesses of thin film. The thicknesses of the sensitive layers are optimized in order to gain broader polarizing zone in such case when the total layer thickness does not increase. An automatic layer thickness control system is established, and errors caused by different monitoring methods are analyzed. With this thickness control system, thin-film polarizers with Tp higher than 98% and Tp/Ts higher than 200:1 (Tp and Ts are transmissions for p- and s-polarizations, respectively) with the bandwidth of 11 nm are prepared. Using the system allows for optimum repeatability of three successive runs.

Wide angle and broadband perfect absorber with compact multilayer structures

We design and numerically investigate a wide angle and broadband perfect absorber with compact multilayer structures. The proposed structure is composed of three stacks of semiconductor films and one metal layer. The absorber presents a broadband absorption between 450 nm and 700 nm with an average absorption above 99.2%. For oblique incidence, the absorber shows an average absorption about 90% for a wide range of incident angles from 0∘ to 60∘ with p polarization and s polarization. The electrical field intensity distributions are also studied to disclose the broadband absorption mechanism. This designed broadband absorber appears to have very promising applications in solar thermal energy harvesting, thermal emitters and detection.

Laser-resistance sensitivity to substrate pit size of multilayer coatings

Nanosecond laser-resistance to dielectric multilayer coatings on substrate pits was examined with respect to the electric-field (E-field) enhancement and mechanical properties. The laser-induced damage sensitivity to the shape of the substrate pits has not been directly investigated through experiments, thus preventing clear understanding of the damage mechanism of substrate pits. We performed a systematic and comparative study to reveal the effects of the E-field distributions and localized stress concentration on the damage behaviour of coatings on substrates with pits. To obtain reliable results, substrate pits with different geometries were fabricated using a 520-nm femtosecond laser-processing platform. By using the finite element method, the E-field distribution and localized stress of the pitted region were well simulated. The 1064-nm damage morphologies of the coated pit were directly compared with simulated E-field intensity profiles and stress distributions. To enable further understanding, a simplified geometrical model was established, and the damage mechanism was introduced.

Suppression of nano-absorbing precursors and damage mechanism in optical coatings for 3ω mirrors.

Damage precursors in the 3ω (351 nm) mirror for a high-power laser system are investigated as well as the relevant damage mechanisms. The precursors are classified into two ensembles according to the different laser resistance and damage features. The former is nano-absorbing precursors, which are sensitive to the standing wave electric field and vulnerable to the laser irradiation. The latter is submicrometer nodular defects, which have higher laser resistance and are sensitive to the adhesion strength between the fluoride coatings and oxide coatings. The damage due to nano-absorbing precursors is efficiently suppressed with the double stack design that screens the electric field in the oxides. Currently, the nodular seed is major originating from the Al2O3/SiO2 stack. Even for the same defect type and mirror, the final damage features are dependent on the local mechanical properties at the irradiation location. The investigations of the damage mechanisms provide a direction to further improve the laser-induced damage threshold of the 3ω mirror.

Effect of pulse duration on laser induced damage threshold of multilayer dielectric gratings

Multilayer dielectric gratings (MDGs) are more and more used to compress pulse in the next generation of chirped-pulse amplification (CPA) system for high-energy petawatt (HEPW)-class lasers due to their high efficiency and high damage threshold for picosecond pulses. The damage tests for MDGs were carried out with long pulse (12ns) in air and short pulse (0.66~9.7ps) in vacuum at 1053nm, respectively. The experiment methodologies and results were discussed. For both long and short pulse, the initial damage locates at the grating ridges opposite to the incoming wave, which is consistent with the maximum normalized electric field intensity (NEFI). For long pulse, the damage is characterized by melting and boiling. And for short pulse, the damage is ascribed to multiphoton-induced avalanche ionization because of the electric field enhancement in the grating groove structure. And Measurement results of the dependence of damage threshold on the pulse width are presented. And the damage threshold of MDG in beam normal is 4.4J/cm2 at 70° incidence angle for 9.7ps pulse.

Multilayer deformation planarization by substrate pit suturing

In the pursuit of 1064 nm high-power laser resistance dielectric coatings in the nanosecond region, a group of HfO2/SiO2 high reflectors with and without suture layers were prepared on prearranged fused silica substrates with femtosecond laser pits. Surface morphology, global coating stress, and high-resolution cross sections were characterized to determine the effects of substrate pit suturing. Laser-induced damage resistance was investigated for samples with and without suture layers. Our results indicate considerable stability in terms of the nanosecond 1064 nm laser-induced damage threshold for samples having a suture layer, due to decreased electronic field (e-field) deformation with simultaneous elimination of internal cracks. In addition, a suture layer formed by plasma ion-assisted deposition could effectively improve global mechanical stress of the coatings. By effectively reducing the multilayer deformation using a suture layer, electron-beam high-reflective coatings, whose laser-induced damage resistance was not influenced by the substrate pit, can be prepared.

Laser induced defect decrement in DKDP crystals varied with photon energy

In this paper, laser induced reactions of bulk defects in DKDP crystals were real-time detected by ultra-microscopy while high power laser irradiation with different photon energy, and the defect elimination processes were observed. It’s found that there were two kinds of bulk defects that can be eliminated: submicron-scale defect and nanoscale defect clusters. The decrement of submicron-scale defects was related to the laser parameters, such as laser fluence and photon energy. The defect decrement could achieve its maximum value at appropriate laser fluence, while the photon energy was fixed. It indicated that up to ~47% of defects could be eliminated by laser irradiation at 3.50eV (355nm). While the laser fluence was fixed, the amount of defects reduced by laser irradiation at 3.50eV was larger than that at 1.17eV (1064nm). The nanoscale defect clusters were hard to be eliminated by laser irradiation at 1.17eV, while most of them could be reduced by laser irradiation at 3.50eV.