We propose to construct broadband thin-film polarizers (TFPs) by combining the polarizing regions of long-wave pass and short-wave pass stacks. The polarization bandwidth can be broader than traditional TFPs by roughly a factor of 2. We designed and fabricated two HfO2/SiO2 Brewster's angle TFPs with a high contrast ratio in the spectral range of 1064±25 nm. The laser-induced damage was investigated and illustrates that the two designs possess quite different laser damage properties due to the different electric field distributions. Nevertheless, broadband TFPs with high laser-induced damage threshold can be achieved with proper coating design.
Laser-induced damage (LID) of optical coatings has been extensively studied since the invention of the laser. It has been found that the defects, which are unavoidable in real-world optical coatings, are the main reason for triggering laser damage of optical components at low fluences. In particular, embedded nodules in dielectric multilayer coatings are the main limiting defects found in reflective optics operating in nanosecond regimes. During laser irradiation, thermomechanical damage occurs preferentially at nodules because of enhanced energy absorption due to electric-field intensity (EFI) enhancement and the degradation of mechanical stability due to discontinuous boundaries. This report reviews the recent studies of the LID due to nodular defects in dielectric multilayer coatings. First, we present statistical studies on the geometric model and laser damage mechanism of nodules in the real world and introduce the solutions to control the formation of nodules. However, the low density and diverse properties of real nodular defects make the systematic study of LID initiating from localized defects a time-consuming and challenging task. In this regard, experimental and theoretical studies of localized defect-driven LID using artificial defects with properties that can be controlled are highlighted. We also present recent research progress on the damage mechanism of artificial nodules interpreted from aspects of mechanical properties and electric-field enhancement. In addition, approaches for modifying the deposition process or multilayer design are examined to reduce the EFI enhancement in the nodules and to improve the laser damage resistance of near-infrared high-reflective coatings based on the deeper understanding of the underlying physics of the damage process.
Ultra-low loss optical thin films find broad applications in fields such as vertical-cavity surface-emitting lasers and optical atomic clocks. The main optical losses in AlGaAs/GaAs distributed Bragg reflectors (DBRs) prepared using metal-organic chemical vapor deposition (MOCVD) arise from absorption loss caused by free carriers within the layers and scattering loss caused by surface roughness. In this study, we fabricated AlGaAs and GaAs single-layer thin films with varying Al compositions on substrates of three crystal orientations and under different V/III ratios. The dependence of carrier concentration and surface morphology on different substrates and growth conditions was investigated. Thin films grown on substrates with three different crystal orientations exhibited three distinct growth modes (step-flow mode, SK mode, and FM mode). The impact of the V/III ratio on the growth mode was found to be complex. Higher V/III ratios resulted in poorer morphology for films grown on (100) substrates, while better morphology was observed on (211) B substrates. Furthermore, the surface morphology of films grown on (100) 15° off substrates showed less sensitivity to changes in the V/III ratio. With increasing Al composition, the carrier concentration of the films significantly increased. Elevating the V/III ratio proved effective in suppressing the incorporation of carbon, thereby reducing the carrier concentration of AlGaAs films. GaAs films exhibited a low carrier concentration at an appropriate V/III ratio. Additionally, the distinct abilities of different substrates to adsorb impurities exerted a significant impact on the carrier concentration of the films. This study demonstrates that, under optimal conditions, it is feasible to fabricate AlGaAs/GaAs Bragg mirrors with low carrier concentration and relatively small roughness on (100) 15° off substrates.
The influence of the incidence angle and polarization state on the damage site characteristics of fused silica under 355 nm laser irradiation was investigated. The initial damage morphologies and growth behaviors of the damage sites on the exit surface at incidence angles of 0° and 45° as well as in P and S states were compared to investigate the effects of various angles and polarization states. The relationships between the size of the initial damage sites and the laser fluence, as well as the growth threshold, were discussed. The damage morphologies of the craters and cracks at different incidence angles and polarization states were then investigated. Finally, the growth characteristics of the lateral size, crater depth, and crack depth were compared and analyzed.
A nanopillar assisted multilayer antireflection (AR) coating that combines the Ta2O5/SiO2 multilayer with SiO2 nanopillars was investigated to improve the light absorption of quadruple-bandgap photovoltaics. After clarifying that the performance of the traditional multilayer AR coating is restricted by the available refractive index of the top layer, periodic SiO2 rhombic nanopillars that work in the subwavelength regime were used to vary its effective index from 1 to 1.46. Then, the effective index and thickness of SiO2 nanopillars were optimized together with the Ta2O5/SiO2 stack using the global optimization algorithm to further reduce the reflection loss. When the SiO2 nanopillars have an effective index of 1.15 and a thickness of 108 nm, the best AR performance was achieved with a reflectance of 3.9% in the target spectral range of 300–1700 nm. Using laser interference lithography and ion assisted deposition technologies, the nanopillar assisted AR coating was realized with a reflectance of 4.5%. Compared to the traditional multilayer AR coating, this hybrid approach can not only achieve better AR performance but also reduce the disparities of the reflection loss among different bandgaps, which helps us to effectively improve the current matching and enhance the overall efficiency of quadruple-bandgap photovoltaics.
Perfect microwave absorbers, which absorb electromagnetic waves completely, play pivotal roles in electromagnetic shielding, and stealth technologies. Existing microwave absorber technologies rely on either electromagnetic properties of absorptive materials, the resonance behavior of meta-atoms, or a combination of both. So far, achieving simultaneous broadband absorption, high efficiency, and compact sizes remains a great challenge. Inspired by atomic doping techniques employed in conventional optical materials to broaden spectral bandwidths, a single-layer microfluidic metasurface microwave absorber is proposed with the assembly of two distinct types of water meta-atoms. By manipulating electromagnetic resonances of these water meta-atoms, the metasurface maintains impedance matching over a broad working range. A microwave absorber design with a thickness equivalent to 0.2 times the central wavelength is showcased, measuring over 93% absorption across both K and Ka bands (17.5-40.0 GHz). The results highlight unprecedented superiorities of microwave absorbers based on a 2D doped water meta-atom lattice when compared to previously reported metasurface absorbers utilizing identical meta-atoms. This absorber has advantages including small thickness, broad bandwidth, and cost-effectiveness, making it promising for applications in electromagnetic shielding, camouflage, and multi-spectral stealth.
Laser-induced damage of high reflection (HR) coatings, working at near ultraviolet (NUV) and near infrared (NIR) regions was investigated. For NIR HR coatings, the nodules still remain the most limiting defects. The E-field intensity (EFI) enhancement in nodules plays a central role for triggering laser-induced damage. We established a simple model for EFI enhancement in nodules using the focusing and light penetrating concept. With the help of finite-difference time domain (FDTD) simulations, we found that refractive indices and nodular geometries affected the focal length as well as the size of focal spots. Furthermore, the angular reflection bandwidth (ARB) of nodules determined the fraction of light that can penetrate to the focal region. For NUV HR coatings, we explored the increase of the laser-induced damage threshold (LIDT) by increasing the incident angle from 0 degrees to 65 degrees for S-polarization. The EFI in a 65 degree HR coating is more than 4 times lower compared to 0 degree HR coatings, which suggests that the LIDT of 65 degree HR coating is much higher compared to 0 degree HR coating. However, we found some contradictory results. For small testing laser beam size with a diameter of 20 μm, the LIDT of 65 degree HR coating is 3.5 times higher compared to a 0 degree HR coating. However, for a large sized testing laser beam with a diameter of 1000 μm, the LIDT of 65 degree HR coating is 2 times lower compared to a 0 degree HR coating. Possible reasons for the observed damage phenomena are discussed.
Half-wave-hole problem, mainly caused by refractive index inhomogeneity of hafnia, had greatly influenced the spectra and application of HfO2/SiO2 dichroic laser mirrors. Two approaches to eliminate the half-wave-hole effect had been proposed.
The paper investigates production of the multiband filter by broadband monitoring. A special 4-line filter was designed and the simulation demonstrated the layer thickness errors self-compensation effect that was quite significant.
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