Estimation of the average residual reflectance of broadband antireflection coatings

Enhanced Transmission from Visible to Terahertz in ZnTe Crystals with Scalable Subwavelength Structures

The zinc blend nonlinear crystal of zinc telluride (ZnTe) is currently one of the most commonly used electro-optical material for terahertz (THz) probe and imaging. We report herein how to engineer the surface behavior of a ZnTe single crystal to design subwavelength structures (SWSs) for enhancing ultrabroadband transmission. Polystyrene (PS) nanoparticle monolayers with a maximum coverage of 85.2% were produced on the ZnTe crystal by an eccentric spin-coating technique combined with surface wettability engineering. Subsequently, the well-defined conical SWS arrays were fabricated on the ZnTe crystal by reactive ion etching over the PS monolayer template, with the size of the SWS arrays customized by optimizing the etching process. Finally, we demonstrated ultrabroadband antireflection on the surface structured ZnTe crystals in the visible-near-infrared, infrared, and terahertz regions with transmittance increase of 11.6%, 10.0%, and 24.8%, which are attributed to the decrease of surface Fresnel reflection by SWS. Notably, in 0.2-1.0 THz, the transmittance reached over 70%. Our work provides a new strategy to enhance the THz generation efficiency and detection sensitivity based on ZnTe crystals by surface engineering.

Broadband multilayer antireflection coating for quantum cascade laser facets

We demonstrate a broadband multilayer anti-reflection (AR) coating applied to a quantum cascade laser (QCL) facet. The bandwidth of the AR coating was optimized for the range of 8.0 to 12.0 μm to cover the entire emission spectrum of a broad-gain QCL. The laser facets are high-reflectance and AR-coated for use in an external cavity QCL. The AR coating is composed of layers of yttrium fluoride, zinc sulfide, and germanium, all deposited using electron-beam evaporation, and the modal reflectance of the laser facet across the wavelength range was reduced to 0.75%. The external cavity laser performance in a Littrow-type configuration is also described.

Design and production of antireflection coating for the 8-10 µm spectral region

A special design procedure allowing to trap layer thicknesses inside specified limits is applied for designing of antireflection coating (AR) for the infrared spectral band of 8-10 µm. The obtained AR design has no too thick layers that may cause delaminating of the deposited AR coating. A special monitoring procedure taking into account wavelength positions of monitoring signal extrema is applied for coating deposition. The manufactured coating features excellent AR properties in the requested spectral region and possesses high mechanical stability.

Stress compensation with antireflection coatings for ultrafast laser applications: from theory to practice

Each complicated coating, in particular, a dispersive mirror consists of dozens of layers. Thin films layers have mechanical stresses. After summing up stresses from all layers, the resulting stress is high enough to bend even a relatively thick substrate. To avoid this effect we suggest depositing an antireflection coating (AR) at the back-side of the substrate which together with suppression of unwanted reflections from the back side will also compensate this stress. We demonstrate unique, extremely thick and sophisticated AR coating consisting of 71 layers with the total physical thickness of 7.5 µm. This AR coating completely compensates stress from the dispersive mirror coated on the front side and minimizes unwanted reflections.

Antireflection efficiency comparison of single- and double-layered structures for photovoltaic glass covers

For single-layer antireflection (AR) on glass, a low refractive index (n) AR layer is required to achieve high AR efficiency, which limits the selection of materials. The double-layered AR structure has a lower requirement on materials' n but is typically used for narrow waveband AR, and photovoltaic glass covers require broadband AR to increase the whole-spectrum solar energy transmittance. With the help of a multilayered optical simulation, we optimized the n and thickness of the single and double layered AR structure and found that, for broadband AR, double-layered structure only showed AR efficiency advantages in very high or low top layers' n compared to single AR layer structure. For a n=1.45 top layer of the double layer structure, the optimized reflectance is 2.57% (single side), while the optimized reflectance of a single AR layer with n=1.45 is 2.87%, which is a negligible AR efficiency advantage (0.30%) when considering production costs. Moreover, in our experiment, using SiO2 and SiO2 and TiO2 composite layers, the absorption of short wavelengths by TiO2 ostensibly cancelled this advantage out (92.87% compare to the single layer's 92.98% for single side AR).

J. A. Dobrowolski (1931-2013): in memoriam

J. A. Dobrowolski, or George, as he was known to his friends and colleagues, passed away on February 12, 2013 in Ottawa, Ontario. George was a leading pioneer in the field of optical thin films. His work stretched over 60 years beginning with his graduate studies in 1953, and he impacted all areas of research in this field. This in memoriam outlines both his professional career and personal life; as befitting George, there is a comprehensive list of his numerous publications in books, papers, and patents. An in memoriam talk on George's life and career was presented at the 2013 Optical Interference Coatings Conference held in Whistler, B.C., Canada on June 16-21, 2013.

Tunable antireflection from conformal Al-doped ZnO films on nanofaceted Si templates

Photon harvesting by reducing reflection loss is the basis of photovoltaic devices. Here, we show the efficacy of Al-doped ZnO (AZO) overlayer on ion beam-synthesized nanofaceted silicon for suppressing reflection loss. In particular, we demonstrate thickness-dependent tunable antireflection (AR) from conformally grown AZO layer, showing a systematic shift in the reflection minima from ultraviolet to visible to near-infrared ranges with increasing thickness. Tunable AR property is understood in light of depth-dependent refractive index of nanofaceted silicon and AZO overlayer. This improved AR property significantly increases the fill factor of such textured heterostructures, which reaches its maximum for 60-nm AZO compared to the ones based on planar silicon. This thickness matches with the one that shows the maximum reduction in surface reflectance.

PACS

81.07.-b; 42.79.Wc; 81.16.Rf; 81.15.Cd.

Broadband and wide-angle antireflection realized by multireflection effect in a micro-∧-shape array

Antireflective surfaces are often realized by minimizing the refractive index contrast between the air and substrate using subwavelength microstructures. In this paper, we introduce another kind of geometry-induced antireflective surface using multireflection to suppress reflection to a very low extent. This surface is composed of micro-∧-shape array with the wedge size much larger than the wavelength of the incident light. Simulation and experimental results show that the micro-∧-shape array can effectively suppress the reflection within wide incident angles and large wavelength ranges. The enhanced light adsorption caused by the greatly increased light path length within the micro-∧-shape array is responsible for the antireflection phenomena. Such antireflective surface may find various applications, in solar cells, for example.

Optimal design of antireflection coatings with different metrics

We consider the problem of designing antireflection coatings by numerical optimization methods with merit functions in different metrics. The dependence of the optimal solution on the metrics is investigated through numerical experiments on several types of coatings, using the average and maximal values of the reflected power in the regions of interest. The results confirm existing statements and provide a few new findings, e.g., some specific metrics can yield particularly better solutions than others.

Modern design tools and a new paradigm in optical coating design

Several modern optical coating designs tools are discussed in the frame of a new design paradigm proposing the search not for a formally optimal solution with the lowest possible merit function value but for the most practical solution that takes into account additional feasibility demands. Considered design tools include a stochastic optimization procedure that takes into account upper and lower constraints for layer optical thicknesses. This procedure allows one to obtain multiple solutions to a design problem, which presents additional opportunities for choosing a practically optimal design. Two special design techniques involving integer optimization also take into account additional demands. The first one is aimed at designing multicavity narrow bandpass filters with quarter wave or multiple quarter wave layer optical thicknesses. It enables obtaining bandpass filters with extremely steep transmittance slopes, bandwidths of several tens of nanometers, and very small ripples in transmission zones. The second technique is aimed at covering design problems that have been traditionally solved using the theory of equivalent layers. One more technique considered in this paper is aimed at reducing the influence of noncorrelated thickness errors on design spectral characteristics.

Design, production, and reverse engineering of two-octave antireflection coatings

We deal with design and production of optimal two-component antireflection (AR) coatings for an ultra broadband spectral range from 450 nm to 1800 nm. We demonstrate the whole design-production chain including design selection, choosing monitoring technique, coating production, and reverse engineering of the deposited coatings. At each step of this chain we provide thorough analysis on the basis of theoretical results and adequate computational manufacturing experiments. In order to produce the designed AR coatings we use magnetron sputtering deposition technique and accurate time monitoring.

Ultrabroadband efficient intracavity XUV output coupler

We report an efficient intracavity XUV output coupler based on an anti-reflection-coated grazing incidence plate (GIP). Conceptually, GIP is an extension of a Brewster plate, affording low loss of the circulating fundamental light and serving as a highly efficient, extremely broadband output coupler for XUV. Due to the grazing incidence geometry, the short wavelength reflectivity can be extended to the keV range. The first GIP realized shows parameters close to the design. We discuss both the limitations of the GIP in comparison with other XUV output couplers and the applicability of the GIP extension at longer wavelengths, down to the MIR.

Estimations of production yields for selection of a practical optimal optical coating design

Modern design approaches enable one to construct a series of theoretical designs with excellent spectral properties for almost all optical coating design problems. Selection of a practical optimal design among a variety of possible theoretical designs becomes a key issue. We demonstrate how preproduction estimations of expected production yields can be used for selection of a practical optimal design. The question of reliability of such estimations is also addressed.

Combined in situ and ex situ optical data analysis of magnesium fluoride coatings deposited by plasma ion assisted deposition

The combination of in situ spectrophotometry during film deposition and ex situ spectrophotometry allows insight into the depth distribution of optical losses in plasma ion assisted deposition coatings. An adapted optical characterization strategy for absorbing coatings using only in situ transmittance data has been developed and is exemplified in application to magnesium fluoride coatings. Measurements and simulation results strongly indicate an increased absorption caused by local understoichiometry of the fluoride material close to the fused silica substrate.

Further guidance for broadband antireflection coating design

Recent investigations have added to and refined the understanding of the behavior of broadband antireflection coating designs and provided further guidance for achieving more nearly optimal designs. The ability to optimize designs wherein the overall optical thickness of the design is constrained to a specific value has allowed this investigation. A broader bandwidth than previously reported has been studied and statistically fit more precisely by a polynomial equation, and also two linear equations for routine approximations have been derived. It has also been found that the optimal number of layers in the design can be predicted as a function of the bandwidth.

Recent development and new ideas in the field of dispersive multilayer optics

A dispersive-mirror-based laser permits a dramatic simplification of high-power femtosecond and attosecond systems and affords promise for their further development toward shorter pulse durations, higher peak powers, and higher average powers with user-friendly systems. The result of the continuous development of dispersive mirrors permits pulse compression down to almost single cycle pulses of 3 fs duration. These design approaches together with the existing modern deposition technology pave the way for the manufacture of dielectric multilayer coatings able to compress pulses of tens of picoseconds duration down to a few femtoseconds.

All-oxide broadband antireflection coatings by plasma ion assisted deposition: design, simulation, manufacturing and re-optimization

A new all-oxide design for broadband antireflection coatings with significantly reduced impact of deposition errors to the final reflectance is presented. Computational manufacturing including re-optimization during deposition has been used in the design work to account for maximum insensibility of the design with respect to deposition errors typical for plasma ion assisted deposition PIAD. Repeated deposition runs with the deducted monitoring and re-optimization strategy verify the validity of the simulations and the stability of the derived design solution.

Monitoring error compensation in general optical coatings

Error compensation in optical monitoring was demonstrated many years ago for narrow bandpass and edge filters. It is shown here that the compensation effects can apply to a broader range of more general coatings. This is illustrated with a very broadband antireflection coating design of 20 layers and a ramp design of 13 layers. The choice of monitoring wavelength and monitoring strategy are important, and suggestions are made concerning those choices. The results are derived from and demonstrated by computer simulations of the monitoring processes. The importance of monitoring directly on only one part and with one wavelength throughout the process in order to obtain the benefits of error compensation is emphasized.

Wideband antireflection coatings by combining interference multilayers with structured top layers

The residual reflectance obtained for a broad wavelength range depends mainly on the refractive index of the last layer. Using interference layer stacks composed of naturally available low- and high-index materials, the residual reflection for a broad range cannot be adjusted below a certain limit. However, nanostructured (gradient) and porous layers are effective media with a refractive index lower than that of natural materials. Results demonstrate that an interference layer stack combined with a structured layer as the last layer yields better antireflection properties owing to the low effective index of the structure.