A new spectroscopic method to get the optical constants of an absorbing material: The case of SrTiO3

To illustrate a previous paper (1) we compute the complex refractive index = n -jk of a SrTiO₃ single crystal plate cut persendicular to the axis, at =270 cm^–1 with a new spectroscopic method where the SrTiO₃ plate is covered with a thin Ge film. Irregular spaced interferences fringes lead to a direct determination of n and k at the minima and maxima. In the case of SrTiO₃ the results are in good agreement with Kramers-Kronid determinations made by Barker and Tinkham (2).     

Cost-effective double-layer antireflection coatings in mid-wavelength infrared band

In this letter, design and preparation of cost-effective double-layer antireflection (AR) coatings in mid-wavelength infrared band is investigated. The method of vector graphics which is simple and visualized for coatings designs with only a few layers is applied to design narrow-band double-layer AR coatings with high performance and low cost for laser applications. The experiment result verifies the design very well with peak transmittance very close to the theoretical limit.     

Applications of antireflection coatings in sonic crystal-based acoustic devices

The unwanted reflection seriously baffles the practical applications of sonic crystals, such as for various acoustic lenses designed by utilizing the in-band properties of sonic crystals. Herein we introduce the concept of the antireflection coating into the sonic crystal-based devices. The efficiency of such accessorial structures is demonstrated well by an originally high reflection system. Promising perspectives can be anticipated in extending the antireflection coating layers into more general acoustic applications through a flexible design process.     

Broad band antireflection coating on zinc sulphide simultaneously effective in SWIR, MWIR and LWIR regions

In recent years multi-spectral imagery is steadily growing popularity. Multi-channel imaging which includes short-wave infrared (SWIR), mid-wave infrared (MWIR) and long-wave infrared (LWIR) systems are useful for threat detection, tracking, thermal signature detection and terrain analysis. In this paper, a broad band antireflection coating on ZnS substrate, simultaneously effective in SWIR, MWIR and LWIR is reported. The coating design approach was evolved using gradient index concept, where refractive index varies gradually from incident media to the ZnS (n = 2.2) substrate. The gradient index profile depicted by 4th degree polynomial n(t) = −0.45t⁴ + 1.9t³ − 2.7t² + 1.9t + 1,where n(t) is the refractive index at the distance t from ambient, and t is the thickness in micron. The profile is best approximated by eight discrete step index layers, whose first layer is thorium fluoride (n = 1.42; lowest index stable material available). Other seven layers are replaced by two equivalent layer system of real materials thorium fluoride and zinc sulphide. Final 15 layers design is deposited by e-beam evaporation. The maximum layer thickness was restricted around 0.7 μm to overcome the stress problem in the film. This 15 layers coating has shown average transmission 95% in 0.9–10.5 μm spectral band having peak 99% at 9 μm.     

利用Rugate膜系实现宽角度减反射膜的设计

阐述了利用Rugate膜系理论设计宽角度减反射薄膜的方法,从理论上分析了在宽角度的情况下,偏振产生透过率不同的原因,模拟设计了入射角从0°到80°宽角度减反射膜系,采取随机与共轭梯度相结合的方法进行优化,得到了较理想的结果这一研究方法将能充分地提高光能的利用率,在能源利用方面和光通信等领域具有重要的应用价值.     

Measurements and analysis of antireflection coatings reflectivity related to external cavity lasers

Facets of semiconductor optical amplifiers (SOA) designed for external cavity lasers must be coated with an antireflection (AR) film of high quality and extremely low reflectance. Therefore measurements of facet reflectance play a crucial role in the fabrication of such AR coating. The reflectance can be estimated by studying the optical power reflected from the Fabry-Perot cavity formed for that purpose by the examined SOA facet and the end of a single-mode fibre. We have made analysis of practical suitability of such measurement method. Theoretical calculations show that, for the low reflectance coatings, losses due to light coupling into optical fibre cannot be omitted in the analysis of the experimental results. To verify this conclusion a theoretical model was tested for a low reflectance surface and we have found that the relative error of the measurements supported by the theoretical model is on the order of 8%.     

Wafer-level fabrication and optical characterization of nanoscale patterned sapphire substrates

The wafer-level aperiodic nanostructures were fabricated atop the sapphire substrates in order to increase the transmittance over broadband spectra. The fabrication was presented along with characterization of their optical properties. The nanostructures were patterned using natural lithography with nickel silicide as a hard mask, and the subsequent etching was performed using inductively coupled plasma dry-etching method. The sapphire substrates with nanostructures compared to conventional sapphire substrates, which exhibit antireflective characteristics over broadband spectra at a wide range of incident angles. The nanostructures reduce the reflection down to 5% in the visible spectrum for normal incidence. The transmittance of visible to near-IR spectra was found to be 94% at normal incidence and over 90% at an incident angle of 45°. In the mid-IR spectrum, the transmittance exceeds 88% until the reflection is no longer suppressed by nanostructures. The polarization properties have also been investigated. The nanostructures can enhance the reflectivity ratio 90% for wavelengths shorter than 400 nm. As the amplitude ratio, enhanced from 50% to 80% over the whole visible spectrum.     

A study of the optical properties and adhesion of zinc sulfide anti-reflection thin film coated on a germanium substrate

To conduct this study, zinc sulfide (ZnS) thin films deposited on germanium (Ge) substrates were prepared by an evaporation method. The effects of deposition rate and annealing on the optical properties and adhesion of the ZnS thin films were investigated. The transmission intensity and the X-ray diffraction (XRD) pattern of the samples showed that the transmittance of the samples decreases by increasing the evaporation rates. However, with the increase of the annealing temperature, crystallinity of the thin films improves which, in turn, results in the enhancement of the transmission intensity in a far infrared region. The maximum grain size was obtained at the annealing temperature of 225 °C. Our experimental results also show that evaporation rate and annealing influences the adhesion of ZnS thin films to Ge substrates.     

Broadband antireflection of silicon nanorod arrays prepared by plasma enhanced chemical vapor deposition

Hydrogenated nanocrystalline Si (nc-Si:H) nanorod arrays were cost-effectively prepared on electrodeposited nickel nanocones substrates by very high frequency plasma enhanced vapor deposition. The antireflection properties of the obtained Si nanorod arrays were investigated carefully for the possible application in solar cells. It was found that the structures of nc-Si:H nanorod arrays can be tuned to obtain a very low reflectance especially in the near infrared region. The obtained Si nanostructure with well-separated nanorods, each of which had an average diameter of 200 nm and height of 700 nm, showed a reflectance value of <5% at normal incident over a wide wavelength of 400-1100 nm.     

Formation of antireflection nanostructure for silicon solar cells using catalysis of single nano-sized silver particle

Antireflection nanostructure was formed by simple wet chemical etching using catalysis of silver (Ag) nanoparticle. Single nano-sized Ag particle dispersion solution was coated onto Si(1 0 0) substrate with polished surface. Then, the samples were soaked in an aqueous etching solution of hydrofluoric acid and hydrogen peroxide. The surface of 9-min-etched Si substrate appeared black, and the reflectivity was reduced to below 5% throughout the entire spectrum from 200 to 1000 nm owing to the formed nanostructure. The absorption was significantly increased after the formation of antireflection structure with 9 min etching, and the conversion efficiency of solar cell with antireflection structure increased from 8.25 to 10.0% owing to the increase of short-circuit current.