Searching for optimal mitigation geometries for laser-resistant multilayer high-reflector coatings

Growing laser damage sites on multilayer high-reflector coatings can limit mirror performance. One of the strategies to improve laser damage resistance is to replace the growing damage sites with predesigned benign mitigation structures. By mitigating the weakest site on the optic, the large-aperture mirror will have a laser resistance comparable to the intrinsic value of the multilayer coating. To determine the optimal mitigation geometry, the finite-difference time-domain method was used to quantify the electric-field intensification within the multilayer, at the presence of different conical pits. We find that the field intensification induced by the mitigation pit is strongly dependent on the polarization and the angle of incidence (AOI) of the incoming wave. Therefore, the optimal mitigation conical pit geometry is application specific. Furthermore, our simulation also illustrates an alternative means to achieve an optimal mitigation structure by matching the cone angle of the structure with the AOI of the incoming wave, except for the p-polarized wave at a range of incident angles between 30° and 45°.     

Camera for coherent diffractive imaging and holography with a soft-x-ray free-electron laser

We describe a camera to record coherent scattering patterns with a soft-x-ray free-electron laser (FEL). The camera consists of a laterally graded multilayer mirror, which reflects the diffraction pattern onto a CCD detector. The mirror acts as a bandpass filter for both the wavelength and the angle, which isolates the desired scattering pattern from nonsample scattering or incoherent emission from the sample. The mirror also solves the particular problem of the extreme intensity of the FEL pulses, which are focused to greater than 10(14) W/cm2. The strong undiffracted pulse passes through a hole in the mirror and propagates onto a beam dump at a distance behind the instrument rather than interacting with a beam stop placed near the CCD. The camera concept is extendable for the full range of the fundamental wavelength of the free electron laser in Hamburg (FLASH) FEL (i.e., between 6 and 60 nm) and into the water window. We have fabricated and tested various multilayer mirrors for wavelengths of 32, 16, 13.5, and 4.5 nm. At the shorter wavelengths mirror roughness must be minimized to reduce scattering from the mirror. We have recorded over 30,000 diffraction patterns at the FLASH FEL with no observable mirror damage or degradation of performance.     

Correlation between He-Ne scatter and 2.7-microm pulsed laser damage at coating defects

A reported correlation between defect-initiated pulsed laser damage and local predamage scatter in multilayer infrared mirror coatings has been analyzed in detail. Examination of a much larger data base confirms the previous result on dielectric-enhanced reflectors with polished substrates over a wide range of energy densities above the damage onset. Scatter signals from individual undamaged defects were detected using a He-Ne scatter probe with a focal spot that nearly coincides with the 150-microm-diam (D1/e(2)) focal spot of the damage-probe beam. Subsequent damage frequency measurements (1-on-1) were made near normal or at 45 degrees incidence with 100-ns pulses at 2.7-microm wavelength. The correlation is characterized by an increase in damage frequency with increasing predamage scatter signal and by equivalence of the defect densities indicated by the two probes. Characteristics of the correlation are compared with a simple model based on focal spot intensity profiles. Conditions that limit correlation are discussed, including variable scatter from defects and background scatter from diamond-turned substrates. Results have implication for nondestructive defect detection and coating quality control.     

Damage threshold prediction of hafnia-silica multilayer coatings by nondestructive evaluation of fluence-limiting defects

A variety of microscopic techniques were employed to characterize fluence-limiting defects in hafnia-silica multilayer coatings manufactured for the National Ignition Facility, a fusion laser with a wavelength of 1.053 mum and a pulse width of 3 ns. Photothermal microscopy, with the surface thermal lens effect, was used to map the absorption and thermal characteristics of 3 mm x 3 mm areas of the coatings. High-resolution subaperture scans, with a 1-mum step size and a 3-mum pump-beam diameter, were conducted on the defects to characterize their photothermal properties. Optical and atomic force microscopy were used to identify defects and characterize their topography. The defects were then irradiated by a damage testing laser (1.06 mum and 3 ns) in single-shot mode until damage occurred. The results were analyzed to determine the role of nodular and nonnodular defects in limiting the damage thresholds of the multilayer coatings. It was found that, although different types of defect were present in these coatings, the fluence-limiting ones had the highest photothermal signals (up to 126x over the host coating). The implication of this study is that coating process improvements for hafnia-silica multilayer coatings should have a broader focus than just elimination of source ejection, since high photothermal signals frequently occur at nodule-free regions. The study also demonstrates that, for optics subject to absorption-induced thermal damage, photothermal microscopy is an appropriate tool for nondestructive identification of fluence-limiting defects.     

Design, fabrication, and analysis of chirped multilayer mirrors for reflection of extreme-ultraviolet attosecond pulses

Chirped Mo/Si multilayer coatings have been designed, fabricated, and characterized for use in extreme-ultraviolet attosecond experiments. By numerically simulating the reflection of the attosecond pulse from a multilayer mirror during the optimization procedure based on a genetic algorithm, we obtain optimized layer designs. We show that normal incidence chirped multilayer mirrors capable of reflecting pulses of approximately 100 attoseconds (as) duration can be designed by enhancing the reflectivity bandwidth and optimizing the phase-shift behavior. The chirped multilayer coatings have been fabricated by electron-beam evaporation in an ultrahigh vacuum in combination with ion-beam polishing of the interfaces and in situ reflectivity measurement for layer thickness control. To analyze the aperiodic layer structure by hard-x-ray reflectometry, we have developed an automatic fitting procedure that allows us to determine the individual layer thicknesses with an error of less than 0.05 nm. The fabricated chirped mirror may be used for production of 150-160 as pulses.     

Investigation of processes leading to damage growth in optical materials for large-aperture lasers

Damage growth in optical materials used in large-aperture laser systems is an issue of great importance to determine component lifetime and therefore cost of operation. Small size damage sites tend to grow when exposed to subsequent high-power laser irradiation at 355 nm. An understanding of the photophysical processes associated with damage growth is important to devise mitigation techniques. We examine the role of laser-modified material and cracks formed in the crater of damage pits in the damage growth process using fused-silica and deuterated KDP samples. Experimental results indicate that both of the above-mentioned features can initiate plasma formation at fluences as low as 2 J/cm2. The intensity of the recorded plasma emission remains low for fluences up to approximately 5 J/cm2 but rapidly increases thereafter, accompanied by an increase of the size of the damage crater.     

Temporal dynamics of a ring dye laser with a stimulated Brillouin scattering mirror

We present a numerical study of the temporal dynamics of a stimulated Brillouin scattering ring resonator. A coaxial flash-lamp-pumped Rh6G dye laser is assumed. The influence of the most important parameters on the temporal evolution of the resonator is analyzed, namely, the acoustic decay time of the nonlinear material, the features of the external injection pulse - its pulse width, energy, and spatial quality - and the coupling mirror reflectivity. We found the conditions to initiate and maintain laser oscillation in the ring resonator as long as the duration of the pumping system pulse persisted.     

Femtosecond laser-induced damage of HfO₂/SiO₂ mirror with different stack structure

Laser-induced damage of the "standard" (λ/4 stack structure) and "modified" (reduced standing-wave field) HfO₂/SiO₂ mirrors were investigated by a commercial 800?nm Ti:sapphire laser system. Three kinds of pulse duration of 50?fs, 105?fs, and 135?fs were chosen. The results show that the single-shot damage threshold of the "modified" mirror was about 14%-23% higher compared to that of the "standard" mirror. A model based on the rate equation for free electron generation was adopted to explain the threshold results. It took in account the transient changes in the dielectric function of material during the laser pulse. The simulated threshold agreed with the experimental very well. Besides, for two kinds of mirror, typical breakdown craters for both the single-shots and multi-shots damage tests reveal striking distinct characteristics. Interestingly, the multi-shots damage crater with zigzag-like edge was observed only on the "standard" mirror. These phenomena were illustrated reasonably by the distribution features of the electric field intensity within the mirrors.     

Uniformity of the Soft-X-Ray Emissions from Gold Foils Irradiated by OMEGA Laser Beams Determined by a Two-Mirror Normal-Incidence Microscope with Multilayer Coatings

A two-mirror normal-incidence microscope with multilayer coatings was used to image the soft-x-ray emissions from planar foils irradiated by OMEGA laser beams. The bandpass of the multilayer coatings was centered at a wavelength of 48.3 ? (257-eV energy) and was 0.5 ? wide. Five overlapping OMEGA beams, without beam smoothing, were typically incident on the gold foils. The total energy was 1500 J, and the focused intensity was 6 x 10(13) W cm(-2). The 5.8x magnified images were recorded by a gated framing camera at various times during the 3-ns laser pulse. A pinhole camera imaged the x-ray emission in the energy range of >2 keV. On a spatial scale of 10 mum, it was found that the soft-x-ray images at 257 eV were quite uniform and featureless. In contrast, the hard-x-ray images in the energy range of >2 keV were highly nonuniform with numerous features of size 150 mum.     

Broadband and high efficiency metal–multilayer dielectric grating based on non-quarter wave coatings as reflective mirror for 800 nm

A broadband and high efficiency metal–multilayer dielectric grating (MMDG), which was used to compress and stretch an ultra-short laser pulse in a chirped-pulse amplification (CPA) system, was designed. The diffraction characteristics of the MMDG were analyzed using the method of rigorous coupled wave analysis. The reflective mirror used for the broadband and high diffraction grating is made up of non-quarter wave metal–multi-layer dielectric coatings. Taking the diffraction efficiency of the ?1 order as merit function, the parameters such as groove depth, residual thickness and reflective mirror were optimized to obtain broadband and high diffraction efficiency. The optimized MMDG shows an ultra-broadband working spectrum with an average efficiency exceeding 97% over 120?nm wavelength centered at 800?nm and TE polarization. The optimized MMDG should have potential application in CPA systems.     

Atomistic simulations of spallation dynamics in multilayer thin-film interface excited by femtosecond laser

This paper presents an atomic-scale analysis of delaminating dynamics for characterizing microscopic mechanisms of interfacial spallation at multilayer thin-film interface excited by femtosecond pulse laser. For the first time via a molecular dynamics (MD) approach to investigate the interfacial spallation induced by pulse laser, the standard form of 12–6 Lennard-Jones (L-J) model and a solid-state argon interface are introduced. To allow MD modeling of interfacial spallation being conducted effectively, various laser incident energy densities and pulse durations are employed to characterize the dynamic behaviors and evolutions of interfacial spallation at multilayer thin-film interface. Based on the results of simulation, three different progressive stages, including void nucleation, coalescence leading to crack, and interfacial spallation, are classified via the transient temperature, pressure and density trajectories. The extraordinary expansive dynamics and tension stress induced by relaxation of thermal and pressure wave are major factors leading to detrimental defects growth and enlargement. The same conclusion can be further verified from the viewpoint of energy trajectories. Moreover, the ultra-high strain rate of the order 10⁹ s⁻¹ is estimated. The result is analogous to that of the experimental result of metal-film spallation excited by pulse laser. Finally, a critical strain-rate is evaluated and the dominant mechanism of the interfacial fracture is also presented.     

Wolter type I x-ray focusing mirror using multilayer coatings

A multilayer coating is a useful addition to a mirror in the x-ray region and has been applied to normal incidence mirrors used with soft x rays. When a multilayer coating is used on grazing incidence optics, higher performance can be achieved than without it. Cr/Sc multilayers coated on a Wolter type I mirror substrate for a soft x-ray microscope are considered. The reflectivity and effective solid angle are calculated for Wolter type I mirrors with uniform and laterally graded multilayer coatings. The laterally graded multilayer mirror showed superior x-ray performance, and the multilayer tolerances were relaxed. This multilayer mirror could be especially useful in the soft x-ray microscope intended for biological applications.     

Development of a pumping laser system for x-ray laser research

A two-beam chirped-pulse-amplification Nd:glass laser system dedicated to x-ray laser research is described. Each beam provides an output energy of 20 J with a typical pulse duration of 1.3 ps. A prepulse of variable duration is generated by use of a novel, to our knowledge, optical system. A reflection optical system, comprised of an off-axis parabolic mirror and a spherical mirror, produces a line focus with 6-mm length and 165-microm width without chromatic aberration. By use of this pumping laser system, the nickel-like silver x-ray laser at a wavelength of 13.9 nm has been demonstrated.     

Phase effects owing to multilayer coatings in a two-mirror extreme-ultraviolet schwarzschild objective

The aberrations of a multilayer-coated reflective Schwarzschild objective, which are influenced both by mirror surface profiles and by multilayer coatings, are evaluated with a phase-shifting point diffraction interferometer operating in the extreme ultraviolet. Using wave-front measurements at multiple wavelengths near 13.4 nm, we observed chromatic aberrations and wavelength-dependent transmission changes that were due to molybdenum-silicon multilayer coatings. The effects of chromatic vignetting due to limited multilayer reflection passbands on the imaging performance of the Schwarzschild optic are considered. The coating characteristics extracted from the interferometry data on the two-mirror optical system are compared with previously reported coating properties measured on individual mirror substrates.     

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.     

Coupling effect of multi-wavelength lasers in damage performance of beam splitters at 355 nm and 1064 nm

The coupling effect between a 355 nm laser and a 1064 nm laser in damage initiation and morphology formation was investigated on beam splitters. When extra 1064 nm pulse energy was low, 355 nm laser-induced damage thresholds (LIDTs) increased because of laser conditioning, and when 1064 nm pulse energy was high enough, 355 nm LIDTs decreased. Damage morphologies were also studied to explore the damage mechanism at respective wavelengths. For the entirely different electric field intensity distributions, 355 nm laser-induced damages were mainly from nanometer-sized absorbers at upper interfaces, while initiators for the 1064 nm laser were located at substrate-coating interface or substrate subsurface. Under simultaneous illumination, the sensitive defects were still the precursors, and damages also showed the representative damage characteristics induced by a single laser, namely, 355 nm laser-induced small pits and 1064 nm laser-induced large delamination. Further studies also showed that, although the 1064 nm laser fluence was kept unchanged, delamination area grew with the increase of pits, which were induced by the 355 nm laser. A possible mechanism was proposed to interpret the delamination area growth phenomenon.     

High-performance Mo-Si multilayer coatings for extreme-ultraviolet lithography by ion-beam deposition

An ion-beam deposition system has been used to fabricate Mo-Si multilayer coatings for masks and imaging optics to be used for extreme-ultraviolet lithography. In addition to high reflectivity and excellent profile control, ion-beam deposition has the capability to smooth rough substrates. For example, we achieved reflectivity of 66.8% on a substrate with 0.39-nm roughness. Smoothing can be further enhanced with a second ion source directed at the multilayer coating. The smoothing capabilities relax the requirement on the finish of the mirror and the mask substrates and could dramatically reduce the cost of these components. Thickness profile control is in the +/-0.01% range, and the figure error added to the mirror substrate by errors in the multilayer thickness is less than 0.1 nm. Peak reflectivities obtained on smooth substrates are 67.5-68.6%.     

Multilayer silicon cavity mirrors for the far-infrared p-Ge laser

Multilayer mirrors capable of > 99.9% reflectivity in the far infrared (70-200 microm wavelengths) were constructed using thin silicon etalons separated by empty gaps. Calculations indicate that only three periods are required to produce 99.9% reflectivity because of the large difference between the index of refraction of silicon (3.384) and the vacuum (1). The mirror was assembled from high-purity silicon wafers, with resistivity over 4000 omega cm to reduce free-carrier absorption. Wafers were double-side polished with faces parallel within 10 arc sec. The multilayer mirror was demonstrated as a cavity mirror for the far-infrared p-Ge laser. Dependence of reflectivity on design accuracy was considered.     

多层介质膜脉冲压缩光栅激光损伤特性研究进展

本文结合国内外研究情况,概括性介绍了用于啁啾脉冲放大系统中的多层介质膜脉冲压缩光栅的激光损伤特性研究进展,包括多层介质膜的损伤、表面浮雕结构的损伤,以及介质膜光栅损伤的影响因素。在关于介质膜光栅激光损伤的影响因素中又分别介绍了槽型结构、制备工艺、激光参数、脉冲数量、热应力和杂质缺陷对其抗激光损伤阈值的影响。最后,从结构设计、制备工艺以及后期处理等方面,介绍了提高多层介质膜光栅抗激光损伤阈值的常用方法。     

Operation of a Ti:sapphire laser pumped by a 499-nm green laser

We report, for the first time, to our knowledge, the operation of a tunable Ti:sapphire laser pumped by a third-order Raman XeCl-H(2) laser system at 499 nm with a 60-ns pulse duration. The slope efficiency is 59% for this laser, producing pulses of 20-ns duration. The highest conversion-energy efficiency obtained is 41%, with an output energy of 1.2 mJ. The tuning range for a single set of cavity mirrors is 680-834 nm and is limited mainly by the mirror reflectivity. This study shows that a combined laser system based on a XeCl excimer laser can offer wavelength diversity.