Multilayer Dielectric Coatings Research Articles

Global optimization of multilayer dielectric coatings for precision measurements.

We describe the design of optimized multilayer dielectric coatings for precision laser interferometry. By setting up an appropriate cost function and then using a global optimizer to find a minimum in the parameter space, we were able to realize coating designs that meet the design requirements for spectral reflectivity, thermal noise, absorption, and tolerances to coating fabrication errors. We also present application of a Markov-Chain Monte Carlo (MCMC) based parameter estimation algorithm that can infer thicknesses of dielectric layers in a coating, given a measurement of the spectral reflectivity. This technique can be a powerful diagnostic tool for both commercial coating manufacturers, and the community using dielectric mirrors for precision metrology experiments.

Striated composite layers of silica and hafnia offering advantageous properties for short-pulse optical coatings.

Monolayers containing subnanometer striations of silica and hafnia to form composite materials at varying ratios are explored as a method to develop high-index dielectric layers with increased laser-induced-damage thresholds (LIDTs). These layers can then be used in multilayer dielectric coatings for short-pulse, high-peak-power laser applications, particularly in regions of the highest electric-field intensity. Fabrication is achieved by means of exposure to two different evaporant vapor plumes, where local exposure to each plume is controlled via shielding to prevent simultaneous exposure. The LIDT of the resulting layers has been evaluated at 1053 nm with 600-fs pulses. The results indicate that such hafnia/silica layers exhibit LIDTs similar to silica for a refractive index of ≤1.65. These results suggest that the use of these layers in locations subjected to high electric-field intensity within multilayer dielectric coatings may significantly improve the LIDT, with this deposition process providing particular benefit for scaling to large-aperture, high-fluence components.

Ultrafast Laser Material Damage Simulation-A New Look at an Old Problem.

The chirped pulse amplification technique has enabled the generation of pulses of a few femtosecond duration with peak powers multi-Tera and Peta–Watt in the near infrared. Its implementation to realize even shorter pulse duration, higher energy, and higher repetition rate laser systems relies on overcoming the limitations imposed by laser damage of critical components. In particular, the laser damage of coatings in the amplifiers and in post-compression optics have become a bottleneck. The robustness of optical coatings is typically evaluated numerically through steady-state simulations of electric field enhancement in multilayer stacks. However, this approach cannot capture crucial characteristics of femtosecond laser induced damage (LID), as it only considers the geometry of the multilayer stack and the optical properties of the materials composing the stack. This approach neglects that in the interaction of an ultrashort pulse and the materials there is plasma generation and associated material modifications. Here, we present a numerical approach to estimate the LID threshold of dielectric multilayer coatings based on strong field electronic dynamics. In this dynamic scheme, the electric field propagation, photoionization, impact ionization, and electron heating are incorporated through a finite-difference time-domain algorithm. We applied our method to simulate the LID threshold of bulk fused silica, and of multilayer dielectric mirrors and gratings. The results are then compared with experimental measurements. The salient aspects of our model, such as the implementation of the Keldysh photoionization model, the impact ionization model, the electron collision model for ‘low’-temperature, dense plasma, and the LID threshold criterion for few-cycle pulses are discussed.

Design of Metal–Dielectric Multilayer Coatings for Energy‐Efficient Building Glazing

A systematic approach to the design of metal–dielectric multilayer coatings is proposed for energy‐efficient building glazing application. Thirty‐six coatings were modeled with alternating metal (Ag, Au, Cu) and dielectric (TiO2, Ta2O5, ITO, ZrO2, SiO2, Al2O3) thin films in three‐layer (3L) and five‐layer (5L) configurations. It is found that the optical performance is less affected by the bottom dielectric layer compared with the metal and middle/top dielectric layers. The 5L coatings offer a competitive modulation of optical properties, ensuring an optimal balance between luminous transmittance and infrared reflectance. 5L configuration ensures a higher visible transmittance in the whole visible region with peak values close to those of unfunctionalized glass (≈85%), i.e., 3–4% higher compared with the 3L configuration. The optical properties of the optimized multilayers are then used as inputs to building energy models of typical office buildings located in extreme cold and hot weather conditions. The 5L–Ag configurations achieve significant reductions in cooling loads under hot climate conditions (around 15%) without compromising access to daylight (important driver of occupants’ comfort and wellbeing). However, under cold climate conditions, cooling energy savings are outweighed by unintended increases in heating loads, confirming the need for more climate‐driven glazing research efforts.

Preferential sputtering of metal oxide mixture thin films

Metal oxide mixture materials enable the production of dielectric multilayer coatings for highest power laser applications. During thin film deposition, when using sputtering techniques in combination with composite target materials, preferential sputtering occurs on the target surface. The quantitative analysis of the mixture thin film composition, usually performed by ion beam based depth profiling methods, is also affected by preferential sputtering. To gain a deeper understanding, the atomic composition variation of sputtered mixture material surfaces is calculated applying the Monte Carlo simulation program tridyn. The simulation results are compared to the atomic composition gradient measured via depth profiling x-ray photoelectron spectroscopy for mixture thin films composed of HfO2, Sc2O3, Al2O3, and SiO2. The deviations between the experimental and simulated data are discussed with respect to the different mixture material combinations.

Tuning of Laser Damage Threshold by Adjusting Field Zones in Multilayer Dielectric Coatings

Tuning of Laser Damage Threshold by Adjusting Field Zones in Multilayer Dielectric Coatings

Evaluating the Accuracy of Reconstruction of the Electrical and Geometric Parameters of Multilayer Dielectric Coatings by a Multifrequency Radio-Wave Method for Slow Surface Electromagnetic Waves

One of the most important problems in the diagnostics of multilayer dielectric materials and coatings is the development of methods for quantitative interpretation of the results of monitoring the electrical and geometric parameters of these materials. Results are presented from a study of the potential informativeness of a multifrequency radio-wave method for slow surface electromagnetic waves for reconstruction of the electrical and geometric parameters of multilayer dielectric coatings. A simulation model is shown for evaluating the accuracy of reconstructing the electrical and geometric parameters of multilayer dielectric coatings. The model takes into account the electrical and geometric parameters of the coating, the level of noise in the measurement data, and the measurement bandwidth. Simulation modeling and experimental test data for the reconstruction of the relative dielectric constants and thicknesses of one- and two-layer equal-thickness dielectric coatings based on polymethyl methacrylate, ftoroplast (teflon) F-4D, and RO3010 for different values of the mean square deviation of the noise level in the measured attenuation coefficients for the field of a slow surface electromagnetic wave. The accuracy of the reconstruction of the geometric and electrical parameters of the layers is found to decrease as the number of estimated parameters and the noise level increase, as well as when the dielectric constant and thickness of the layers are reduced. Experimental data confirm the adequacy of the simulation model developed here. This model can be used for a specific measurement complex employing a multifrequency radio-wave technique for slow surface electromagnetic waves to estimate quantitatively the potential possible accuracy of reconstructing the geometric and electrical parameters of multilayer dielectric materials and coatings. A simulation model and experimental study of a multilayer dielectric coating show that for a measurement bandwidth of 1 GHz the errors in estimating the dielectric constants and thicknesses of the layers do not exceed 10% with a confidence coefficient of 0.95 for a mean square deviation of 0.003–0.004 in the noise level.

Integral field unit for the existing imaging and spectroscopy instrument, FOCAS

Abstract The Faint Object Camera and Spectrograph (FOCAS) is an optical imaging and spectroscopy instrument for the Subaru Telescope. It has been a workhorse instrument since the first-light phase of the telescope. We describe an integral field unit (IFU) that has recently been installed in FOCAS. The IFU utilizes an image slicer that divides a ${13{^{\prime \prime }_{.}}5}$ × ${10{^{\prime \prime }_{.}}0}$ field of view into 23 stripes, with a width of ${0{^{\prime \prime }_{.}}435}$. A sky spectrum separated from an object by approximately ${5{^{\prime }_{.}}2}$ can be obtained at the same time as an object spectrum. Test observations confirmed that the image quality of the IFU does not degrade the ${0{^{\prime \prime }_{.}}435}$ sampling, and that slice width and length are consistent with the design. Highly reflective multilayer dielectric coatings were coated on all the mirrors in the IFU, thereby offering a high mean IFU throughput of ∼85% over the field. However, the outer part of the field showed throughput degradation, which was mainly caused by vignetting as a result of misalignment. The flat-fielding accuracy was degraded by the vignetting, with the variation depending on the direction of the telescope.

Mechanisms of picosecond laser-induced damage in common multilayer dielectric gratings.

The modifications of multilayer dielectric (MLD) gratings arising from laser-induced damage using 0.6-ps and 10-ps laser pulses at 1053 nm are investigated to better understand the damage-initiation mechanisms. Upon damage initiation, sections of the affected grating pillars are removed, thereby erasing the signature of the underlying mechanisms of laser damage. To address this issue, we performed paired studies using macroscopic grating-like features that are 5 mm in width to reveal the laser-damage morphology of the different grating sections: pillar side wall, sole, and pillar top. The results suggest that, similarly to MLD coatings, there are two damage-initiation mechanisms corresponding to the different pulse durations.

Femtosecond Laser-Induced Damage Characterization of Multilayer Dielectric Coatings

The laser-induced damage threshold (LIDT) of optical components is one of the major constraints in developing high-power ultrafast laser systems. Multi-layer dielectric (MLD) coatings-based optical components are key parts of high-power laser systems because of their high damage resistance. Therefore, understanding and characterizing the laser-induced damage of MLD coatings are of paramount importance for developing ultrahigh-intensity laser systems. In this article, we overview the possible femtosecond laser damage mechanisms through damage morphologies in various MLD optical coatings tested in our facility. To evaluate the major contributions to the coating failure, different LIDT test methods (R-on-1, ISO S-on-1 and Raster Scan) were carried out for a high reflective hybrid Ta2O5/HfO2/SiO2 MLD mirror coating at a pulse duration of 37 fs. Different LIDT test methods were compared due to the fact that each test method exposes the different underlying damage mechanisms. For instance, the ISO S-on-1 test at a higher number of laser pulses can bring out the fatigue effects, whereas the Raster Scan method can reveal the non-uniform defect clusters in the optical coating. The measured LIDT values on the sample surface for the tested coating in three test methods are 1.1 J/cm2 (R-on-1), 0.9 J/cm2 (100k-on-1) and 0.6 J/cm2 (Raster Scan) at an angle of incidence of 45 deg. The presented results reveal that the performance of the tested sample is limited by coating defects rather than fatigue effects. Hence, the Raster Scan method is found to be most accurate for the tested coating in evaluating the damage threshold for practical applications. Importantly, this study demonstrates that the testing of different LIDT test protocols is necessary in femtosecond regime to assess the key mechanisms to the coating failure.

Control of defects in the multilayer dielectric materials and coatings in the microwave range

The development of modern engineering is inextricably linked with the development of the new types of multilayer dielectric materials. Existing radio wave methods for monitoring interlayer defects in such materials exhibit low accuracy in reconstructing the geometric parameters of defects. The results of studying extended interlayer defects in the three-layer coating consisting of polymethyl methacrylate, F-4D PTFE, and semi-hard rubber by the method of surface electromagnetic waves are presented. The method is based on the solution of inverse problems in the reconstruction of the geometric parameters of extended interlayer defects of special multilayer materials and coatings from the frequency dependence of the attenuation coefficient of the field of a slow surface electromagnetic wave. Unlike the methods that make use from the complex reflection coefficient we proposed to increase the accuracy of the reconstruction of the geometric parameters of extended interlayer defects taking into account the linear frequency dependence of the attenuation coefficient as well as reducing the number of fixed measurement frequencies. Moreover, the determination procedure is rather simple, since only the field strength of the surface electromagnetic wave is measured, and there is no need for phase measurements. Experimental data obtained on a multifrequency measuring complex in the range of 10 – 11 GHz were used for reconstruction of the interlayer defects in the coating under study. The developed method provided a relative error of thickness estimation below 10%. The proposed method approach can be used in studying multilayer dielectric coatings on the metal for detection of delamination in the lack of glue or poor adhesion between the layers. The developed method is also suitable for control of the defects in semiconductors, ferrite and composite materials.

Evaluation of the accuracy of reconstruction of the electrophysical and geometric parameters of multilayer dielectric coatings by the multi-frequency radio wave method of a slow surface electromagnetic waves

One of the most important diagnostic problems multilayer dielectric materials and coatings is the development of methods for quantitative interpretation of the checkout results their electrophysical and geometric parameters. The results of a study of the potential informativeness of the multi-frequency radio wave method of surface electromagnetic waves during reconstruction of the electrophysical and geometric parameters of multilayer dielectric coatings are presented. The simulation model is presented that makes it possible to evaluate of the accuracy of reconstruction of the electrophysical and geometric parameters of multilayer dielectric coatings. The model takes into account the values of the electrophysical and geometric parameters of the coating, the noise level in the measurement data and the measurement bandwidth. The results of simulation and experimental investigations of reconstruction of the structure of relative permittivitties and thicknesses of single-layer and double-layer dielectric coatings with different thicknesses, with different values of the standard deviation (RMS) of the noise level in the measured attenuation coefficients of the surface slow electromagnetic wave are presented. Coatings based on the following materials were investigated: polymethyl methacrylate, F-4D PTFE, RO3010. The accuracy of reconstruction of the electrophysical parameters of the layers decreases with an increase in the number of evaluated parameters and an increase in the noise level. The accuracy of the estimates of the electrophysical parameters of the layers also decreases with a decrease in their relative permittivity and thickness. The results of experimental studies confirm the adequacy of the developed simulation model. The presented model allows for a specific measuring complex that implements the multi-frequency radio wave method of surface electromagnetic waves, to quantify the potential possibilities for the accuracy of reconstruction of the electrophysical and geometric parameters of multilayer dielectric materials and coatings. Experimental investigations and simulation results of a multilayer dielectric coating demonstrated the theoretical capabilities gained relative error permittivity and thickness of the individual layers with relative error not greater than 10 %, with a measurement bandwidth of 1 GHz and RMS of noise level 0,003–0,004.

Reconstruction of The Structure of the Electrophysical Parameters of Multilayer Dielectric Materials and Coatings from The Frequency Dependence of the Attenuation Coefficient of the Field of a Surface Electromagnetic Wave

A new radio-wave method of reconstruction of the relative permittivities and thicknesses of multilayer materials and coatings is proposed. The method is based on the solution of inverse problems in the reconstructionof the structure of the electrophysical parameters of special multilayer materials and coatings from the frequency dependence of the attenuation coefficient of the field of a slow surface electromagnetic wave. The principal differences of the new method from well-known radio wave methods of determining the electrophysical parameters of multilayer materials are demonstrated. In contrast to methods that involve an informative parameter – the complex reflection coefficient – the proposed method yields an increase in the precision and reliability of the reconstruction of the structure of the relative permittivitties and thicknesses. Improved precision and reliability is achieved by taking into account the linear dependence of the attenuation coefficient on frequency as well as reducing the number of fixed measurement frequencies. The results of experimental studies that confirm the linear dependence of the attenuation coefficient of the field of a surface electromagnetic wave on frequency as well as a decrease in the number of fixed measurement frequencies are presented. The method implements a simple measurement procedure, since only the values of the field strength of the surface electromagnetic wave are measured and there is no need for phase measurements in the method. Reconstruction of the permittivity of a three-layer coating consisting of polymethyl methacrylate, F-4D PTFE, and semi-hard rubber with precision acceptable for practical applications is realized with the use of experimental results of measurements in a multifrequency measurement complex in the range 9–10 GHz by means of the new method. Experimental investigations of a multilayer dielectric coating demonstrated the theoretical capabilities gained with measurement of the relative permittivity and thickness of the individual layers with relative error not greater than 8 and 6%, respectively.

Dynamics of secondary contamination from the interaction of high-power laser pulses with metal particles attached on the input surface of optical components.

We investigate the interaction of 355-nm and 1064-nm nanosecond laser pulses with nominally spherical metallic particles dispersed on the input surface of transparent substrates or high-reflectivity (HR) multilayer dielectric coatings, respectively. The objective is to elucidate the interaction mechanisms associated with contaminant-induced degradation and damage of transparent and reflective optical elements for high-power laser systems. The experiments involve time-resolved imaging capturing the dynamics of the interaction pathway, which includes plasma formation, particle ejection, and secondary contamination by droplets originating from the liquefied layer of the particle. The results suggest that HR coatings are more susceptible to secondary contamination by liquid droplets produced by the particles because of the different geometry of excitation and the location of plasma initiation. Modeling results focus on better understanding the melting of the particle surface, leading to ejections of liquid droplets and the pressure applied to the substrate, leading to mechanical damage.

Mechanisms of picosecond laser-induced damage in common multilayer dielectric coatings

The physical mechanisms and ensuing material modification associated with laser-induced damage in multilayer dielectric high reflectors is investigated for pulses between 0.6 and 100 ps. We explore low-loss multilayer dielectric SiO2/HfO2 mirrors which are commonly employed in petawatt-class laser systems. The spatial features of damage sites are precisely characterized, enabling the direct correlation of the observed damage morphology to the location of energy deposition and the corresponding standing-wave electric-field intensities within the layer structure. The results suggest that there are three discrete damage-initiation morphologies arising from distinctly different mechanisms: the first prevailing at laser pulse lengths shorter than about 2.3 ps, while the other two are observed for longer pulses. Modeling of the thermomechanical response of the material to localized laser-energy deposition was performed for each type of damage morphology to better understand the underlying mechanisms of energy deposition and subsequent material response.

Reconstruction of the structure electrophysical parameters of multilayer dielectric materials and coatings according to the frequency dependence of the attenuation coefficient of the field of surface electromagnetic wave

Reconstruction of the structure electrophysical parameters of multilayer dielectric materials and coatings according to the frequency dependence of the attenuation coefficient of the field of surface electromagnetic wave

Laser-induced damage of nodular defects in dielectric multilayer coatings

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.

Theoretical study of multilayer coatingreflection taking into account third-order optical nonlinearities.

A theoretical approach is presented that allows calculating the reflectance of dielectric multilayer coatings taking third-order optical nonlinearities into account. The description is based on third-order optical susceptibility so that nonlinear refraction and two-photon absorption processes are automatically considered in terms of the real and imaginary parts of the susceptibility. Two model systems are calculated in order to demonstrate the physical validity of the approach: a mirror that is optimized for maximum reflectance at a given wavelength and a given incident intensity and a mirror that provides a rather flat dependence of the reflected intensity on the incident one. Model calculations are performed for laser wavelength values of 1064nm, 800nm, and 532nm.

Effect of elevated substrate temperature deposition on the mechanical losses in tantala thin film coatings

Brownian thermal noise in dielectric multilayer coatings limits the sensitivity of current and future interferometric gravitational wave detectors. In this work we explore the possibility of improving the mechanical losses of tantala, often used as the high refractive index material, by depositing it on a substrate held at elevated temperature. Promising results have been previously obtained with this technique when applied to amorphous silicon. We show that depositing tantala on a hot substrate reduced the mechanical losses of the as-deposited coating, but subsequent thermal treatments had a larger impact, as they reduced the losses to levels previously reported in the literature. We also show that the reduction in mechanical loss correlates with increased medium range order in the atomic structure of the coatings using x-ray diffraction and Raman spectroscopy. Finally, a discussion is included on our results, which shows that the elevated temperature deposition of pure tantala coatings does not appear to reduce mechanical loss in a similar way to that reported in the literature for amorphous silicon; and we suggest possible future research directions.

Experimental and numerical study of the nonlinear response of optical multilayers.

Dielectric multilayer coatings exhibiting steep reflectance in an extremely narrow transition zone, highly sensitive to any variations of layer refractive indices and therefore suitable for studying the nonlinear properties are produced and characterized. Increase of reflectance at growing intensity reveals the presence of the optical Kerr effect. A new method calculating intensity dependent spectral characteristics of multilayer optical coatings in the case of nonlinear interaction with high intensity laser pulses is developed. The method is based on the numerical solution of a boundary-value problem derived from the system of Maxwell equations describing the propagation of light through a multilayer system. The method opens a way to synthesis of optical coatings with predictable nonlinear properties. Comparison of our numerical modelling with experimental data enabled us to accurately determine the Kerr coefficients n2 of the widely-used thin-film materials Ta2O5 and Nb2O5.