Effect of the interface on femtosecond laser damage of a metal-dielectric low dispersion mirror.

Metal-dielectric low dispersion mirrors (MLDM) have a promising application prospect in petawatt (PW) laser systems. We studied the damage characteristics of MLDM and found that the damage source of MLDM (Ag + Al2O3+SiO2) is located at the metal-dielectric interface. We present the effect of the interface on the femtosecond laser damage of MLDM. Finite element analysis shows that thermal stress is distributed at the interface, causing stress damage which is consistent with the damage morphology. After enhancing the interface adhesion and reducing the residual stress, the damage source transfers from the interface to a surface SiO2 layer, and the damage threshold can be increased from 0.60 J/cm2 to 0.73 J/cm2. This work contributes to the search for new techniques to improve the damage threshold of MLDM used in PW laser systems.

High-precision measurement of optical constants of ultra-thin coating using surface plasmon resonance spectroscopic ellipsometry in Otto-Bliokh configuration.

In this paper, a surface plasmon resonance (SPR) spectroscopic ellipsometry, based on Otto-Bliokh configuration, is developed for the measurement of thickness and optical constants of ultra-thin coatings. This technique combines sensitivity of surface plasmon with accessibility of optical constants and other advantages of ellipsometry. Surface plasmons (SP) are generated in the sample under test in total reflectance mode and SP geometric distribution over the sample surface is influenced by the coating thickness and optical properties on one hand, and by the air gap thickness on the other hand. Nanoscale control of the thickness of the air gap between a convex surface and the sample was assured using a micron-size beam spot irradiating the contact zone. The amplitude and phase change induced by SPR in the visible and near-infrared spectral range were obtained to determine the dispersion of optical constants and the thickness of the ultra-thin layer. The extracted optical constants were found to be in excellent agreement with the results obtained using TEM and XRR techniques. Both theoretical analysis and experimental results demonstrated high sensitivity and precision of the proposed technique for the analysis of coatings of both metals and dielectrics on metals.

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.

Defect analysis of UV high-reflective coatings used in the high power laser system.

By considering the rapid change of standing-wave electric-field and assuming the interface defect distribution, an improved model is developed to analyze the defect density distribution and assess the damage performance of high-reflective coatings. Two kinds of high-reflective coatings deposited by e-beam evaporation (EBE) and ion beam sputtering (IBS) techniques are analyzed with this method. The lower overall damage threshold is the major feature for the coatings deposited by IBS method according to the defect parameters extracted from the model. Typical damage morphologies of coatings are also measured and analyzed. The assumption of interface defects is supported by the damage behavior. The damage mechanisms of two high-reflective coatings are attributed to the formation of molten pool and mechanical ejection. The influence of the incident angle on the damage probability is also considered and numerically calculated. The defect analysis model improved here is suitable for high-reflective coatings.

Scattering analysis for random antireflective structures on fused silica in the ultraviolet.

Random antireflective structures are fabricated on fused silica by the thermal dewetting process and reactive ion etching, which shows a broadband antireflective effect over the whole visible wavelength. However, the transmittance in the ultraviolet is limited by the scattering from the etched structures. A graded refractive index model ignoring the scattering in the visible range is applied to extract the etched profile. Then the Lubachevsky-Stillinger algorithm is used to reconstruct the random antireflective structures with the extracted profile. Bidirectional scattering distribution for the reconstructed structures is simulated with the finite-difference time-domain method, which indicates the importance of transmissive scattering the scattering directivity. The scattering directivity is explained well with an effective grating model. The period of the effective grating can guide the prepared technique in the ultraviolet.

Intrafilm separation of solgel film under nanosecond irradiation.

We have observed large-scale intrafilm separation after the irradiation of solgel film with a single Nd:YAG pulse (1064 nm, 12 ns) in air. The irradiated but undamaged surface or the surface after intrafilm separation is densified. These damage features are distinctly different from the scalding surface of the electron beam evaporation coating or the ripple structures on the rear surface of fused silica, which indicates the extreme pressure gradients at the free surface-film interface. The submicrometer size melted cavity in the center of damage site is related with the nanoscale absorber. A phenomenological description that combines the defect-induced incubation phase and laser-supported surface breakdown wave is used to explain the damage process.

High-Performance and Stable Perovskite X-ray Detection and Imaging Based on a Ti Cathode.

High-energy radiation detectors with a good imaging resolution, fast response, and high sensitivity are desired to operate at a high electric field. However, strong ion migration triggered by electrochemical reactions at the interface between a high-potential electrode and an organic-inorganic hybrid perovskite limits the stability of radiation detectors under a high electric field. Herein, we demonstrate that such ion migration could be effectively suppressed in devices with a Ti cathode, even at a high electric field of 50 V mm-1, through time-of-flight secondary-ion mass spectrometry. X-ray photoelectron spectroscopy illustrates that Ti-N bonds formed at the interface of MAPbBr3 perovskite single crystals/Ti electrode effectively inhibit the electrochemical reaction in organic-inorganic hybrid perovskite devices and ultimately improve the operating stability under a high electric field. The device with a Ti electrode reaches a high sensitivity of 96 ± 1 mC Gyair-1 cm-2 and a low detection limit of 2.8 ± 0.3 nGy s-1 under hard X-ray energy.

Application of Flow Field Analysis in Ion Beam Figuring for Ultra-Smooth Machining of Monocrystalline Silicon Mirror.

X-ray free-electron lasers are large modern scientific devices that play an important role in fields such as frontier physics and biomedicine. In this study, a light source is connected to an experimental station through beam lines, which requires numerous ultra-smooth and high-precision X-ray mirrors. Monocrystalline silicon is an ideal substrate material where ion-beam figuring is required. However, the ultra-smooth surface is damaged after the ion-beam figuring. Through an analysis of the machined surface, it is found that in the process of vacuum pumping, the impurities in the cavity adhere to the machined surface and increase the roughness after processing. Therefore, an optimized vacuum-pumping scheme is proposed. The experiment demonstrates that the original value of the processed surface roughness remains unchanged.

Wide Band BAW Filter Based on Single-Crystalline LiNbO3 Thin Film With Insulating Bragg Reflector.

Bulk acoustic wave (BAW) filter with large bandwidth is an urgent need in fifth-generation (5G) communication systems. In the present work, 43° Y-cut lithium niobate (LN) single-crystal film is prepared on multilayer oxide film, and bulk acoustic filter with oxide Bragg reflector (BR) is successfully achieved. The design method of the filter and the fabrication process are presented. Atomic force microscope (AFM) and scanning electron microscope (SEM) are used to characterize the quality of thin films. The results demonstrate the feasibility of transferring single-crystal film onto multilayer oxide, which is efficient for the confinement of acoustic energy. The resonator has effective electromechanical coupling coefficient of 14.6% and figure of merit (FOM) of 32.94. The filter with a compact size of 600 [Formula: see text] has a relative bandwidth of 10.3% at a center frequency of 3.128 GHz, which is promising for the application of 5G systems.

Interface Study on the Effect of Carbon and Boron Carbide Diffusion Barriers in Sc/Si Multilayer System.

Sc/Si multilayers are one of the promising material combinations commonly used in the spectral range of 35-50 nm. However, diffusion and silicidation at the interfaces of Sc/Si multilayers limit widespread applications of this material combination. To improve the properties of Sc/Si multilayers, the scheme of barrier layers is utilized. In this work, a series of Sc/Si multilayers with boron carbide and carbon barrier layers were designed and fabricated to compare the properties including interface quality and thermal stability. The effect on the multilayer structure and quality before and after annealing were investigated by using grazing-incidence X-ray reflection, X-ray diffraction, rocking-curve X-ray diffuse scattering, transmission electron microscopy, and selected area electron diffraction. The results indicate that severe interdiffusion and crystallization occur in the multilayer with a carbon barrier after annealing. However, a boron carbide barrier layer improves thermal stability up to 550 °C since the interfaces remain abrupt and clear after annealing. The multilayer quality is confirmed to be improved significantly.