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- Front Matter: Volume 7504
- Thin Films
- Fundamental Mechanisms
- Mini-Symposium: Femtosecond Laser Damage
- Surfaces, Mirrors, and Contamination
- Materials and Measurements
Front Matter: Volume 7504
Front Matter: Volume 7504
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This PDF file contains the front matter associated with SPIE Proceedings Volume 7504, including the Title Page, Copyright information, Table of Contents, the International Program Committee, Symposium Welcome, and the Summary of Meeting listing.
Thin Films
The effect of nitrogen doping on the multiple-pulse subpicosecond dielectric breakdown of hafnia films
Show abstract
A film of hafnium oxide, doped with 5 atomic % nitrogen, was prepared by dual-ion-beam-assisted deposition. The
properties were compared to a pure hafnium oxide film. The damage threshold is lower for the nitrogen-doped film.
However, the multiple-pulse damage threshold for a 1 kHz train of 800 fs pulses shows no drop relative to the singlepulse
value. These results are discussed within the context of a
multiple-damage model, based on midgap trapping states.
Studies of femtosecond laser induced damage of HfO2 thin film in atmospheric and vacuum environments
Show abstract
The single pulse femtosecond laser induced damage threshold (LIDT) of hafnia and silica films is not affected by the
ambient gas pressure. In vacuum, the multiple pulse LIDT drops to ~10% (~10%) of its atmospheric value for hafnia
(silica). The water vapor content of the ambient gas was found to control the change in the LIDT. The LIDT of bulk
fused silica surfaces did now show any dependence on the ambient gas pressure. Hydrocarbons (toluene) did not change
the multiple pulse LIDT for Hafnia films
Surface damage of thin AlN films with increased oxygen content by nanosecond and femtosecond laser pulses
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AlN films deposited on sapphire substrates were damaged by single UV nanosecond (at 248 nm) and IR femtosecond
(at 775 nm) laser pulses in air at normal pressure. The films had high (27-35 atomic %) concentration of oxygen
introduced into thin surface layer (5-10 nm thickness). We measured damage threshold and studied morphology of the
damage sites with atomic force and Nomarski optical microscopes with the objective to determine a correlation between
damage processes and oxygen content. The damage produced by nanosecond pulses was accompanied by significant
thermal effects with evident signatures of melting, chemical modification of the film surface, and specific redistribution
of micro-defect rings around the damage spots. The nanosecond-damage threshold exhibited pronounced increase with
increase of the oxygen content. In contrast to that, the femtosecond pulses produced damage without any signs of
thermal, thermo-mechanical or chemical effects. No correlation between femtosecond-damage threshold and oxygen
content as well as presence of defects within the laser-damage spot was found. We discuss the influence of the oxygen
contamination on film properties and related mechanisms responsible for the specific damage effects and morphology of
the damage sites observed in the experiments.
Laser damage resistant pits in dielectric coatings created by femtosecond laser machining
Show abstract
Replacing growing damage sites with benign, laser damage resistant features in multilayer dielectric films may enable
large mirrors to be operated at significantly higher fluences. Laser damage resistant features have been created in high
reflecting coatings on glass substrates using femtosecond laser machining. These prototype features have been damage
tested to over 40 J/cm2 (1064nm, 3ns pulselength) and have been shown not to damage upon repeated irradiation at
40J/cm2. Further work to optimize feature shape and laser machining parameters is ongoing.
Thermal robustness of ion beam sputtered TiO2/SiO2, TiO2/Al2O3, and Al2O3/SiO2 IR anti-reflective coatings on YAG and sapphire substrates
Show abstract
As optical coatings are deployed in more extreme environments and applications, mechanical and environmental
robustness must be taken into account when designing the film(s). Even minor degradation of the film structure from
these outside factors can affect final fluence handling capabilities in operation, and limit the life of the coating. We
present the results of a study of maximum thermal handling capabilities of Broad-Band IR Anti-Reflective coatings in
the mid-IR (3 to 5 micron) regime. We prepared a family of coated optics utilizing different coating material sets on
different substrate materials, and exposed them to a range of increasing temperatures. We examined the damage
morphologies under dark field, bright field, and Nomarski microscopy.
Optimization of scandia thin films for high power laser coating applications
Show abstract
We discuss the physical and optical properties of Sc2O3 single layers deposited by the dual ion beam sputtering technique
at oxygen partial pressures ranging from 1.7×10-5 to 5.1×10-5 Torr. The films are amorphous with crystallite size ~10
nm and have surface roughness RMS values of 1.2±0.3 nm. The refractive index at 1 μm is 1.95. Absorption loss is shown
to be sensitive to the oxygen partial pressure during growth. Multiple-pulse damage experiments suggest that the scandia
film deposited at the higher oxygen partial pressure accumulates laser-induced trap defects more slowly than the scandia
film deposited in a lower oxygen partial pressure atmosphere.
Laser conditioning of high reflectivity mirrors used in OPOs by 266 and 355 nm nanosecond pulses
Show abstract
Experimental investigation of the laser conditioning efficiency by nanosecond pulses at 266 and 355 nm in high
reflectivity mirrors used in optical parametric oscillators (OPOs) is present in this report. The high reflection coatings
were deposited on the fused silica substrates. The materials used for e-beam coating deposition were ZrO2 and SiO2.
Laser conditioning was investigated as function of number of pulses, wavelength and conditioning protocol. Ramped-fluence
pre-exposure was used as a method to explore optimal improvement to the damage performance at 266 and
355 nm. No conditioning effect was observed using nanosecond pulses at 266 nm, but the mirror conditioning with
355 nm pulses increased the laser-induced damage threshold (LIDT) by 2.5-3 times. The experimental results support the
concept that the laser conditioning effect observed in high quality optical thin films is associated with intrinsic electronic
defects in the films.
193 nm laser induced spectral shift in HR coated mirrors
Show abstract
High reflectance mirrors, using fluoride coating materials, have been irradiated for extended time periods by a 193 nm
kilohertz repetitive laser source. This irradiation promoted a spectral shift in the reflectance band towards shorter
wavelength. In efforts to determine the mechanism for the observed spectral shifts, various models were investigated by
employing such techniques as: spectrophotometry, surface profile interferometry, coating design simulation and x-ray
diffraction. The result of the investigation indicates that layers near the top surface of the coating structure underwent
densification which resulted in the observed spectral shift.
Design consideration for high damage threshold UV-Vis-IR mirrors
Show abstract
Millijoule-energy femtosecond laser pulses at high repetition rates constitute major workhorses for nonlinear optics and
ultrafast science. The evolution of dielectric multilayer mirror technology within last 15 years allows one to demonstrate
low-loss and dispersion-controlled optics. Unfortunately, dispersive optics has lower laser damage threshold in
comparison to standard quarter-wave stacks. Nevertheless only multilayer optics is able to support high energy laser
pulses together with capability to impart almost any spectral or phase feature on the reflected light. There exist only a
limited number of materials suitable for deposition of UV-Vis-IR mirrors. The highest damage threshold was
demonstrated with alternating HfO2/SiO2 layers deposited by electron beam without ion-assistance. Utilizing
performance of this materials pair we present a novel design approach that allows us to increase the damage threshold of
dispersive UV-Vis-IR optics. It is based on the reduction of the electric field inside the multilayer stack.
Investigation in oxide mixture coatings with adapted gradient index profiles
Show abstract
The stability of thin film coatings for applications especially in the UV spectral range is oftentimes a limiting factor in
the further development of radiation sources and beam delivery systems. Particularly, functional coatings on laser and
conversions crystals as well as resonator mirrors show an insufficient lifetime due to laser-induced degradation. Previous
investigations in the power handling capability of UV coatings mostly concentrate on the properties of pure oxide
materials and particle mitigation.
Recent innovations in ion beam sputtering technology enabled efficient deposition of mixture coatings of different oxide
materials. In combination with an advanced thickness monitoring equipment, the described IBS deposition systems are
capable of employing designs with sub-layers of a few nm thickness. In the present investigation, the stability of classical
designs using pure oxide materials is compared with gradient index design concepts based on mixture materials.
Reflecting and transmitting thin film coatings employing classical and gradient index approaches manufactured under
comparable conditions are characterized in respect to their power handling capability. The results are analyzed before the
background of theoretical expectations regarding contributions from field enhancement and absorptance effects.
Meeting thin film design and production challenges for laser damage resistant optical coatings at the Sandia Large Optics Coating Operation
Show abstract
Sandia's Large Optics Coating Operation provides laser damage resistant optical coatings on meter-class optics required
for the ZBacklighter Terawatt and Petawatt lasers. Deposition is by electron beam evaporation in a 2.3 m × 2.3 m × 1.8
m temperature controlled vacuum chamber. Ion assisted deposition (IAD) is optional. Coating types range from antireflection
(AR) to high reflection (HR) at S and P polarizations for angle of incidence (AOI) from 0° to 47°.
This paper reports progress in meeting challenges in design and deposition of these high laser induced damage threshold
(LIDT) coatings. Numerous LIDT tests (NIF-MEL protocol, 3.5 ns laser pulses at 1064 nm and 532 nm) on the coatings
confirm that they are robust against laser damage. Typical LIDTs are: at 1064 nm, 45° AOI, Ppol, 79 J/cm2 (IAD 32
layer HR coating) and 73 J/cm2 (non-IAD 32 layer HR coating); at 1064 nm, 32° AOI, 82 J/cm2 (Ppol) and 55 J/cm2
(Spol ) (non-IAD 32 layer HR coating); and at 532 nm, Ppol, 16 J/cm2 (25° AOI) and 19 J/cm2 (45° AOI) (IAD 50 layer
HR coating). The demands of meeting challenging spectral, AOI and LIDT performances are highlighted by an HR
coating required to provide R > 99.6% reflectivity in Ppol and Spol over AOIs from 24° to 47° within ~ 1% bandwidth at
both 527 nm and 1054 nm.
Another issue is coating surface roughness. For IAD of HR coatings, elevating the chamber temperature to ~ 120 °C and
turning the ion beam off during the pause in deposition between layers reduce the coating surface roughness compared to
runs at lower temperatures with the ion beam on continuously. Atomic force microscopy and optical profilometry
confirm the reduced surface roughness for these IAD coatings, and tests show that their LIDTs remain high.
Influence of subsurface defects on 355 nm laser damage resistance of monolayer and multilayer coatings
Show abstract
In this study, fused silica substrates were etched to different depths, ranging from about 200 nm to 600 nm. And then
single-layer, anti-reflection (AR) and high reflection (HR) coatings were deposited to the etched substrates using E-beam
evaporation. It was found that 355nm laser induced damage thresholds (LIDTs) of single-layer and AR coatings on the
etched substrates were much higher than that on un-etched substrates, and the existing of the top 200 nm of the substrate
was the most important factor influencing the damage behaviors of single-layer and AR coatings. Step Profiler was
employed in measuring the depth of damage sites in the coatings deposited on the un-etched substrates. It showed that
laser damage was initiated from the coating-substrate interface for single-layer and AR coatings, while from the inside of
coating stack for HR samples. On the basis of above results, we concluded that subsurface defects in the redeposition
layer of substrate could be a serious factor that lowered the 355nm LIDTs of single-layer and AR coatings.
Anti-reflection coating damage threshold dependence on substrate material
Ove Lyngnes,
Aiko Ode,
Dale C. Ness
Show abstract
This paper presents a study of the laser damage threshold (LDT) of ion beam sputtered (IBS) Ta2O5/SiO2 AR coated and
uncoated samples ranging from low grade plate glass through neutral density glass to laser grade fused silica substrates.
We find that the damage threshold varies by just over a factor of 3 from 9 to 33 J/cm2 for 1064nm pulsed laser light with
20ns pulse length and 20 Hz repetition rate. The damage threshold of the uncoated samples ranges from 80 J/cm2 to
greater than 200 J/cm2. We have estimated the single pulse temperature increase in the uncoated neutral density glasses
to be as much as 400-500°C at the damage fluence. This is unexpected as multiple pulse exposure would rapidly bring
the glass up to melting temperatures. A possible explanation is saturation of the absorption at high fluence levels
resulting in lower temperature than estimated. We conclude that the absorption level in the IBS coating is too low to
give a significant contribution to the LDT of the AR coated samples.
Optical coatings with ultralow refractive index SiO2 films
Show abstract
Advantages of utilizing ultralow refractive index layers in optical coatings include broadband and wide angle
spectral performance. Ultralow refractive index films can be fabricated by chemical method and physical
vapor deposition. In the chemical method, ultralow refractive indices are controlled by sol-gel derived
nanostructures. In the physical method, ultralow refractive indices are obtained by self-shadowing nature in
oblique angle deposition. Techniques for ultralow refractive index silica formation and characterizations are
reviewed. Chemical, mechanical and optical properties of the ultralow refractive index silica are presented.
Optical coatings consisting of the ultralow refractive index silica layers are discussed in a spectral regime
from IR down to DUV.
Fundamental Mechanisms
Ultrafast laser-induced modifications of energy bands of non-metal crystals
Show abstract
Ultrafast laser-induced variations of electron energy bands of transparent solids significantly influence ionization and
conduction-band electron absorption driving the initial stage of laser-induced damage (LID). The mechanisms of the
variations are attributed to changing electron functions from bonding to anti-bonding configuration via laser-induced
ionization; laser-driven electron oscillations in quasi-momentum space; and direct distortion of the inter-atomic
potential by electric field of laser radiation. The ionization results in the band-structure modification via accumulation
of broken chemical bonds between atoms and provides significant contribution to the overall modification only when
enough excited electrons are accumulated in the conduction band. The oscillations are associated with modification of
electron energy by pondermotive potential of the oscillations. The direct action of radiation's electric field leads to
specific high-frequency Franz-Keldysh effect (FKE) spreading the allowed electron states into the bands of forbidden
energy. Those processes determine the effective band gap that is a laser-driven energy gap between the modified
electron energy bands. Among those mechanisms, the latter two provide reversible band-structure modification that
takes place from the beginning of the ionization and are, therefore, of special interest due to their strong influence on the
initial stage of the ionization. The pondermotive potential results either in monotonous increase or oscillatory variations
of the effective band gap that has been taken into account in some ionization models. The classical FKE provides
decrease of the band gap. We analyzing the competition between those two opposite trends of the effective-band-gap
variations and discuss applications of those effects for considerations of the laser-induced damage and its threshold in
transparent solids.
Laser-induced damage and nonlinear absorption of ultrashort laser pulses in the bulk of fused silica
Show abstract
In the last decade, investigations in fs-laser damage mechanisms have proven that the interaction of ultra-short laser
pulses with dielectric matter is driven by electronic processes. This fact leads to a deterministic damage behavior and a
well defined dependency of the laser-induced damage threshold (LIDT) on the absorption gap of the dielectric optical
material.
These facts have been considered in detail for dielectric thin films. Additionally, numerous investigations in
filamentation, supercontinuum generation (SCG) and optical breakdown in bulk material have been published. The
phenomena are also traced back to the interaction of ultra-short laser pulses with dielectric material. In contrast to thin
films, these effects are based on a large interaction length of the beam with the dielectria. Consequently, additional
nonlinear effects can contribute to the damage process of bulk materials.
In the presented work, nonlinear absorption and the LIDT of fused silica are investigated. The influence of the
propagation distance of the laser beam in the solid was in the focus of interest for an experiment on a series of fused
silica samples of identical material and different thickness. The results of this study show that nonlinear absorption and
laser-induced damage strongly vary in dependence of the sample thickness. The variations in the damage threshold
range over more than one order of magnitude.
Influence of Na-related defects on DUV nonlinear absorption in CaF2: nanosecond versus femtosecond laser pulses
Show abstract
The influence of Na stabilized F and M centers on the DUV absorption behavior of CaF2 is comparatively studied for
nanosecond and femtosecond laser pulses by in-situ transmission and laser induced fluorescence measurements.
For 193 nm nanosecond pulses the steady state transmission of ArF laser pulses through CaF2 is measured in dependence
on the incident fluence H ≤ 10 mJ cm-2 pulse-1. The related absorption coefficients αst(H) are proportional to H and
rationalized by effective 1- and 2-photon absorption coefficients αeff and βeff, respectively. The αeff and βeff values
increase with the Na content of the CaF2 samples as identified by the fluorescence of Na related MNa centers at 740 nm.
This relation is simulated by a complex rate equation model describing the ArF laser induced MNa generation and
annealing. MNa generation starts with intrinsic 2-photon absorption in CaF2 yielding self-trapped excitons (STE). These
pairs of F and H centers can separate upon thermal activation and the F centers combine with FNa to form MNa centers.
MNa annealing occurs by its photo dissociation into a pair of F and FNa centers.
Comparative transmission measurements with DUV femtosecond pulses are done using the fourth harmonic of a Ti:Safs-
laser at 197 nm. The resulting βeff values virtually show no dependence on the MNa center concentration. Furthermore,
the absolute βeff values are lower by a factor of three compared to those obtained for nanosecond pulses. This is
explained by additional two-step absorption for nanosecond pulses after formation of self-trapped excitons (STE).
A microscopic model for long-term laser damage in calcium fluoride
Show abstract
Single crystal calcium fluoride (CaF2) is an important lens material in deep-ultraviolet optics, where it is exposed
to high radiation densities. The known rapid damage process in CaF2 upon ArF laser irradiation cannot account
for irreversible damage after long irradiation times. We use density functional methods to calculate the properties
of laser-induced point defects and to investigate defect stabilization mechanisms on a microscopic level. The
mobility of the point defects plays a major role in the defect stabilization mechanisms. Besides stabilization by
impurities, we find that the agglomeration of F-centers plays a significant role in long-term laser damage of CaF2.
We present calculations on the stability of defect structures and the diffusion properties of the point defects.
Biaxial flexural strength of optical window materials
Show abstract
The design of high-energy laser windows critically depends on the availability of appropriate numbers for the allowable tensile stress. Relying on a "modulus of rupture" in conjunction with a "safety factor" usually results in overestimating the required thickness, which degrades the optical performance. The primary purpose of this paper is to clarify issues relating to Weibull's theory of brittle fracture and make use of the theory to assess the results of equibiaxial flexure testing that was carried out on laser-window material candidates. Specifically, we describe the
failure-probability distribution in terms of the characteristic strength σC--i.e., the effective strength of a uniformly stressed 1-cm2 area---and the shape parameter m, which reflects the dispersion of surface-flaw sizes. A statistical analysis of flexural strength data thus amounts to obtaining the parameters σC and m, which is best done by directly fitting estimated cumulative failure probabilities to the appropriate expression derived from Weibull's theory. In this light, we demonstrate that (a) at the 1% failure-probability level, fusion-cast CaF2 and OxyFluoride Glass perform poorly compared to CVD-ZnSe; (b) available data for fused SiO2 and sapphire confirm the area-scaling principle, thus validating Weibull's theory; and (c) compressive coatings enhance the characteristic strength but degrade the shape parameter, which mitigates their benefit. In Appendix, it is shown that four-point bending data for fusion-cast CaF2 do not obey a simple two-parameter model but are indicative of a bimodal surface-flaw population.
Modeling wet chemical etching of surface flaws on fused silica
Show abstract
Fluoride-based wet chemical etching of fused silica optical components is useful to open up surface fractures for
diagnostic purposes, to create surface topology, and as a possible mitigation technique to remove damaged material. To
optimize the usefulness of etching, it is important to understand how the morphology of etched features changes as a
function of the amount of material removed. In this study, we present two geometric etch models that describe the
surface topology evolution as a function of the amount etched. The first model, referred to as the finite-difference etch
model, represents the surface as an array of points in space where at each time-step the points move normal to the local
surface. The second model, referred to as the surface area-volume model, more globally describes the surface evolution
relating the volume of material removed to the exposed surface area. These etch models predict growth and coalescence
of surface fractures such as those observed on scratches and ground surfaces. For typical surface fractures, simulations
show that the transverse growth of the cracks at long etch times scales with the square root of etch time or the net
material removed in agreement with experiment. The finite-difference etch model has also been applied to more complex
structures such as the etching of a CO2 laser-mitigated laser damage site. The results indicate that etching has little
effect on the initial morphology of this site implying little change in downstream scatter and modulation characteristics
upon exposure to subsequent high fluence laser light. In the second part of the study, the geometric etch model is
expanded to include fluid dynamics and mass transport. This later model serves as a foundation for understanding
related processes such as the possibility of redeposition of etch reaction products during the etching, rinsing or drying
processes.
Modeling of light intensification by conical pits within multilayer high reflector coatings
Show abstract
Removal of laser-induced damage sites provides a possible mitigation pathway to improve damage resistance of coated
multilayer dielectric mirrors. In an effort to determine the optimal mitigation geometry which will not generate
secondary damage precursors, the electric field distribution within the coating layers for a variety of mitigation shapes
under different irradiation angles has been estimated using the finite difference time domain (FDTD) method. The
coating consists of twenty-four alternating layers of hafnia and silica with a quarter-wave reflector design. A conical
geometrical shape with different cone angles is investigated in the present study. Beam incident angles range from 0° to
60° at 5° increments. We find that light intensification (square of electric field, |E|2) within the multilayers depends
strongly on the beam incident direction and the cone angle. By comparing the field intensification for each cone angle
under all angles of incidence, we find that a 30° conical pit generates the least field intensification within the multilayer
film. Our results suggest that conical pits with shallow cone angles (≤ 30°) can be used as potential optimal mitigation
structures.
Mini-Symposium: Femtosecond Laser Damage
Analysis in wavelength dependence of electronic damage
Show abstract
Many investigations in laser induced damage thresholds (LIDT) have shown a very deterministic behavior for the ultra
short pulse regime. From the current understanding, femtosecond laser damage is driven by electronic photon-matter
interaction. This process can be theoretically described by photo-, and avalanche-ionization and the respective relaxation
processes. On the basis of a wavelength dependency of the laser-induced damage threshold of titania, a dominant
influence of multi-photon absorption (MPI) on the damage behavior could be demonstrated. This particular
characteristic could be observed in a stepwise increase of the LIDT in the transition ranges of the orders of multi-photon
absorption.
This paper presents an analysis of the wavelength dependence of femtosecond LIDT with different theoretic models and
a comparison of simulated data with the measured wavelength dependence of titania. Both, the wavelength position and
the quantitative change of the laser power resistance in the range of the n to the n+1 photon absorption are calculated in
a theoretical analysis. Additionally, the influence of different microstructures of titania on the quantized MPIcharacteristic
is investigated.
Time-resolved digital holography: a versatile tool for femtosecond laser-induced damage studies
Show abstract
Digital holography (DH) technique allows obtaining quantitative amplitude- and phase-contrast images. However,
there are only several attempts to use advantages of digital holography in time resolved manner. Herewith
we demonstrate that combination of conventional off-axis DH and ultrashort probing laser pulses results in
versatile tool suitable for ultra-fast phenomena studies. We demonstrate its applications in spatial-temporal
characterization of ionizing pre-damage propagation of femtosecond pulses in transparent material as well
as characterization of post-damage ablation processes in metals. Phase-and amplitude-contrast images are
obtained in single-shot mode with the best to our knowledge temporal resolution of ≈ 30 fs.
Fundamental processes controlling the single and multiple femtosecond pulse damage behavior of dielectric oxide films
Show abstract
In this contribution we will summarize the fundamental mechanisms that lead to subpicosecond laser
damage in dielectric films, discuss the resulting scaling laws of single pulse (1-on-1) damage with respect
to pulse duration and bandgap, of the multiple pulse (S-on-1) damage threshold as a function of pulse
number, and compare these findings to recent experimental results.
Subpicosecond pulse laser damage behavior of dielectric thin films prepared by different techniques
Show abstract
A new instrument dedicated to laser damage measurement in subpicosecond scale has been developed at the Fresnel
Institute (3 ps to 100 fs, 1030 nm). The objective of this work is to realize a comparative study of the behavior of hafnia
thin films prepared by different techniques (Reactive Low Voltage Ion Plating, Electron Beam Deposition, Dual Ion
Beam Sputtering) under subpicosecond pulse irradiation in the near infra red. Laser-induced damage thresholds are
measured for one-on-one procedures. Laser damage setup and first results at 1 ps are presented and initiation
mechanisms are studied thanks to damage morphologies and optical properties characterization. Results show a
dependence of damage threshold with deposition techniques and so with microstructure of the film.
Mixed metal dielectric gratings for pulse compression applications
Show abstract
A Petawatt facility called PETAL (PETawatt Aquitaine Laser) is under development near the LIL (Ligne d'Integration
Laser) at CEA Cesta, France. PETAL facility uses chirped pulse amplification (CPA) technique [1]. This system needs
large pulse compression gratings exhibiting damage threshold of more than 4 J/cm2 beam normal at 1.053μm and for
500fs pulses. In this paper, we study an alternative design to the classic Multidielectric (MLD) gratings [2] called "mixed
metal-multidielectric grating" (MMLD). In this design, the dielectric mirror stack of the MLD grating is replaced by a
gold reflective layer covered with a few pairs of HfO2/SiO2 [3]. The number of pairs must be high enough to ensure a
sufficient reflection coefficient in order to prevent damage of the gold layer. On the top of the stack, a silica layer is
coated to receive the grating. After some considerations on the grating design and optimization, a comparison between
MLD and MMLD mirrors is also carried out. We finally detail the measured diffraction efficiencies obtained on MMLD
gratings.
Thin film femtosecond laser damage competition
Show abstract
In order to determine the current status of thin film laser resistance within the private, academic, and government sectors,
a damage competition was started at the 2008 Boulder Damage Symposium. This damage competition allows a direct
comparison of the current state of the art of high laser resistance coatings since they are tested using the same damage
test setup and the same protocol. In 2009 a high reflector coating was selected at a wavelength of 786 nm at normal
incidence at a pulse length of 180 femtoseconds. A double blind test assured sample and submitter anonymity so only a
summary of the results are presented here. In addition to the laser resistance results, details of deposition processes,
coating materials and layer count, and spectral results will also be shared.
Surfaces, Mirrors, and Contamination
Aspects of laser optics qualification for space applications
Show abstract
As a consequence of the ongoing interest for deployment of laser systems into space, suitable optical components have to be
developed and must be extensively space qualified to ensure reliable, continuous, and autonomous operation. The exposure
to space environment can adversely affect the longevity of optics, mainly coatings, and lead to system degradation. An
increased operational risk is due to the air-vacuum effect, which can strongly reduce the laser damage resistance of optical
coatings. For this purpose, a vacuum laser damage test bench has been developed and is operated at DLR. In extensive test
campaigns, all damage-prone optics of the ALADIN laser system (being the laser source of the upcoming ESA ADM
Aeolus mission) were tested under operative conditions at the fundamental and at the harmonic wavelengths of Nd:YAG.
Further operational risks are due directly to operation under high vacuum. In the past, several space-based laser missions
have suffered from anomalous performance loss or even failure after short operation times. This degradation is due to
selective contamination of laser-exposed optical surfaces fed by outgassing constituents. These volatile components are
omnipresent in vacuum vessels. Various organic and inorganic species were tested at our facilities for their criticality on
deposit built-up. Finally, active optical components like Q-switch crystals or frequency converter crystals can also suffer
from bulk absorption induced by high-energy radiation (gray tracking) and dehydration. To analyze these effects, an ultrahigh
vacuum phase matching unit was set up to test various combinations of SHG and THG frequency converters.
The effects on glass surfaces of shrapnel and debris emissions from Petawatt laser driven solid targets
Show abstract
Chirped pulse amplification [CPA] has been implemented on laser facilities to produce high irradiance conditions in a
vacuum chamber for the study of novel plasma physics processes. When such focussed laser beams interact with solid
targets, the material is disrupted and leads to ejecta as solids, liquids, vapours and radiation. These target by-products can
degrade and damage optical or diagnostic surfaces in the interaction chamber. This paper describes the effects of such
emissions on surfaces by the use of metrology, microscopy, image processing, fragment capture in foams and autoradiography.
Target plume divergence is discussed so that precautions for future experiments can be evaluated.
Impact of outgassing organic contamination on laser induced damage of optics
Show abstract
The impact of molecular contamination on the lifetime of fused silica UV optics used in high power laser facility is
studied. In our particular case corresponding to Laser MégaJoule (LMJ) beams, the irradiation conditions are a fluence
higher than 10 J/cm2 at a wavelength of 351 nm (3 ω) for 3 ns pulse duration and a single shot/day frequency. A confine
environment, a long period of exposition and proximity of the optical components with outgassing materials are critical
parameters for the optics contamination. Consequently, experiments were performed in the UV section of the Ligne
d'Intégration Laser (LIL), actual prototype of the LMJ. Moreover, the optics storage conditions were studied. Indeed, to
ensure an efficient replacement of the optics on the laser bundle, many optical components are fabricated long before.
They are stored during months in polypropylene frames put in containers. Then, we intentionally contaminated silica
samples with one sort of polypropylene. We evidenced an important increase of laser induced damage density on
samples contaminated by both bundle and storage environments. Surface analyses have been used to identify the
potential causes of this effect. Various hypotheses of damage mechanisms are proposed.
Chemical characterizations of optical materials: a tool for high quality components
Show abstract
Recent works have shown that for low contaminants level, damage density is independent of the amount of
contaminants. In these conditions, sub-surface defects (cracks), generated along the optical process, is considered as the
main source of damage. Hence, efforts have been made to improve SSD measurement in order to improve its suppression
during industrial process. We have developed a method to measure SSD depth which is detailed in this presentation. This
method is based on successive acid etching steps. The principle is to establish contamination level (ICP-AES
measurement) as a function of etched thickness of SiO2. The experimental setup has been specially designed to minimize
contaminations, reduce hydrofluoric acid quantities and to ease the etch rate determination. SSD depth is given by the
asymptotic impurities. This method has been applied to a grinded fused silica intentionally doped in barium tracer.
Results have been successfully compared to other characterization techniques such as MRF dimpling or empirical law
correlating SSD and surface roughness.
Laser damage precursors in fused silica
Show abstract
There is a longstanding, and largely unexplained, correlation between the laser damage susceptibility
of optical components and both the surface quality of the optics, and the presence of near surface
fractures in an optic. In the present work, a combination of acid leaching, acid etching, and confocal
time resolved photoluminescence (CTP) microscopy has been used to study laser damage initiation
at indentation sites. The combination of localized polishing and variations in indentation loads
allows one to isolate and characterize the laser damage susceptibility of densified, plastically flowed
and fractured fused silica. The present results suggest that: 1) laser damage initiation and growth are
strongly correlated with fracture surfaces, while densified and plastically flowed material is
relatively benign, and 2) fracture events result in the formation of an electronically defect rich
surface layer which promotes energy transfer from the optical beam to the glass matrix.
Diagnostics tools for subsurface damage characterization of ground silica parts
Show abstract
Subsurface cracks in fused silica optics are known to be damage initiators under laser irradiation. Each step of optic
production, from sawing to polishing, creates its own type of cracks. An efficient optic manufacturing process requires
that each production step removes cracks from the previous step. The extent of cracks has to be measured for each
production step. We review and compare different subsurface damage (SSD) characterization techniques applied to
ground and fine ground fused silica samples.
Birefringence and residual stress induced by CO2 laser mitigation of damage growth in fused silica
Show abstract
We investigate the residual stress field created near mitigated sites and its influence on the efficiency on the CO2 laser
mitigation of damage growth process. A numerical model of CO2 laser interaction with fused silica is developed that
take into account laser energy absorption, heat transfer, thermally-induced stress and birefringence. Specific photoelastic
methods are developed to characterize the residual stress near mitigated sites in fused silica samples. The stress
distribution and quantitative values of stress levels are obtained for sites treated with the CO2 laser in various conditions
of energy deposition (beam size, pulse duration, incident power). The results obtained also show that the presence of
birefringence/residual stress around the mitigated sites has a critical effect on their laser damage resistance.
Residual stress and damage-induced critical fracture on CO2 laser treated fused silica
Show abstract
Localized damage repair and polishing of silica-based optics using mid- and far-IR CO2 lasers has been shown to be
an effective method for increasing optical damage threshold in the UV. However, it is known that CO2 laser heating
of silicate surfaces can lead to a level of residual stress capable of causing critical fracture either during or after laser
treatment. Sufficient control of the surface temperature as a function of time and position is therefore required to
limit this residual stress to an acceptable level to avoid critical fracture. In this work we present the results of 351
nm, 3ns Gaussian damage growth experiments within regions of varying residual stress caused by prior CO2 laser
exposures. Thermally stressed regions were non-destructively characterized using polarimetry and confocal Raman
microscopy to measure the stress induced birefringence and fictive temperature respectively. For 1~40s square
pulse CO2 laser exposures created over 0.5-1.25kW/cm2 with a 1-3mm 1/e2 diameter beam (Tmax~1500-3000K), the
critical damage site size leading to fracture increases weakly with peak temperature, but shows a stronger
dependence on cooling rate, as predicted by finite element hydrodynamics simulations. Confocal micro-Raman was
used to probe structural changes to the glass over different thermal histories and indicated a maximum fictive
temperature of 1900K for Tmax≥2000K. The effect of cooling rate on fictive temperature caused by CO2 laser
heating are consistent with finite element calculations based on a Tool-Narayanaswamy relaxation model.
Study of CO2 laser smoothing of surface roughness in fused silica
Show abstract
Small micrometer-sized roughness on optical surfaces, caused by laser damage and/or redeposition of laser ablated
material, can cause local electric field intensification which may lead to damage initiation both on the optics and/or
downstream. We examined the smoothing of etched periodic surface structures on SiO2 substrate with 10.6μm CO2
laser using atomic force microscopy. The characteristic surface tension driven mass flow of the glass under
different laser parameters were simulated using computational fluid dynamics and correlated with experimental
results. We found that during CO2 laser polishing the estimate viscosity of the silica glass appears to be higher than
typical literature values measured at a temperature similar to the laser heating conditions. This discrepancy can be
explained by the observation that at high temperature, a significant portion of the hydroxyl content in the layer of
heated silica glass can diffuse out resulting in a much stiffer glass.
The effect of pulse duration on the growth rate of laser-induced damage sites at 351 nm on fused silica surfaces
Show abstract
Past work in the area of laser-induced damage growth has shown growth rates to be primarily dependent on the laser
fluence and wavelength. More recent studies suggest that growth rate, similar to the damage initiation process, is affected
by a number of additional parameters including pulse duration, pulse shape, site size, and internal structure. In this study,
we focus on the effect of pulse duration on the growth rate of laser damage sites located on the exit surface of fused
silica optics. Our results demonstrate, for the first time, a significant dependence of growth rate at 351 nm on pulse
duration from 1 ns to 15 ns as τ0.3 for sites in the 50-100 μm size range.
Magnetorheological finishing (MRF) of potassium dihydrogen phosphate (KDP) crystals: nonaqueous fluids development, optical finish, and laser damage performance at 1064 nm and 532 nm
Show abstract
Over the past year we have been working on specialized MR fluids for polishing KDP crystals. KDP is an extremely
difficult material to conventionally polish due to its water solubility, low hardness, and temperature sensitivity. Today,
KDP crystals are finished using single-point diamond turning (SPDT) tools and nonaqueous lubricants/coolants. KDP
optics fabricated using SPDT, however, are limited to surface corrections due to tool/method characteristics with surface
quality driven by microroughness from machine pitch, speed, force, and diamond tool character. MRF polishing offers a
means to circumvent many of these issues since it is deterministic which makes the technique practical for surface and
transmitted wavefront correction, is low force, and is temperature independent. What is lacking is a usable nonaqueous
MR fluid that is chemically and physically compatible with KDP which can be used for polishing and subsequently
cleaned from the optical surface. In this study, we will present the fluid parameters important in the design and
development of nonaqueous MR fluid formulations capable of polishing KDP and how these parameters affect MRF
polishing. We will also discuss requirements peculiar to successful KDP polishing and how they affect optical
figure/finish and laser damage performance at 1064 nm and 532 nm.
Materials and Measurements
Nonlinear spectroscopy: absorption and refraction
Show abstract
We have been developing tools for nonlinear spectroscopy aimed toward the ultimate goal of building a nonlinear
spectrophotometer analogous to the ubiquitous linear spectrophotometer where a sample is placed in the instrument, a
button is pushed, and the absorption spectrum is obtained sometime later. This paper describes our progress toward this
goal, describing many difficulties and complications as well as opportunities. We also show spectroscopic data and
analysis of a variety of materials that we have taken with preliminary nonlinear spectroscopic instrumentation we have
already developed. One of the more interesting observations obtained along this research path is the realization that
linear dispersion theory can also be applied to nonlinear systems when formulated properly such that Kramers-Kronig
relations can be used to connect the dispersion of nonlinear refraction to the spectrum of nonlinear absorption. In some
circumstances this can be more easily applied to nonlinear systems than to linear systems since the nonlinear absorption
spectrum can be limited in wavelength. In addition, we have developed tools that can simultaneously give the spectrum
of nonlinear absorption as well as the dispersion of the nonlinear refraction over an octave spectral range from 400nnm
to 800 nm, the so called White-Light-Continuum Z-scan. Much of the research on nonlinear optical materials has been a
collaborative effort requiring the skills and expertise of organic chemists and materials manufacturers. The goals of this
part of the research are to determine predictive structure-property relation capabilities. The database needed for this
research makes the nonlinear spectrophotometer a necessity.
Ultrafast photoluminescence as a diagnostic for laser damage initiation
Show abstract
Using high-sensitivity confocal time-resolved photoluminescence (CTP) techniques, we report an ultra-fast
photoluminescence (40ps-5ns) from impurity-free surface flaws on fused silica, including polished, indented or
fractured surfaces of fused silica, and from laser-heated evaporation pits. This fast photoluminescence (PL) is not
associated with slower point defect PL in silica which has characteristic decay times longer than 5ns. Fast PL is
excited by the single photon absorption of sub-band gap light, and is especially bright in fractures. Regions which
exhibit fast PL are strongly absorptive well below the band gap, as evidenced by a propensity to damage with 3.5eV
ns-scale laser pulses, making CTP a powerful non-destructive diagnostic for laser damage in silica. The use of CTP
to provide insights into the nature of damage precursors and to help develop and evaluate new damage mitigation
strategies will be presented.
Photoluminescence and photothermal deflection measurements in KDP crystals for high power applications
Show abstract
KDP is a non linear optical crystal used for frequency conversion in high power applications. In this paper we present
results of optical non-destructive measurements on different fast growth KDP crystals in order to characterize the
precursors responsible for laser-induced damage. We performed two types of analysis: the first one is temporally
resolved photoluminescence excited by a pulsed Nd:YAG laser at 4.66 eV and 3.49 eV (266 nm and 355 nm). The
second is spatially resolved photothermal deflection pumped by a CW Argon laser at 3.53 eV (351 nm).
With these two complementary techniques we highlight the presence of large scale inhomogeneities and we discriminate
nano-scale defects statistically distributed in the bulk of the crystals.
Imaging system to measure kinetics of material cluster ejection during exit-surface damage initiation and growth in fused silica
Show abstract
Laser-induced damage on the surface of optical components typically is manifested by the formation of microscopic
craters that can ultimately degrade the optics performance characteristics. It is believed that the damage process is the
result of the material exposure to high temperatures and pressures within a volume on the order of several cubic microns
located just below the surface. The response of the material following initial localized energy deposition by the laser
pulse, including the timeline of events and the individual processes involved during this timeline, is still largely
unknown. In this work we introduce a time-resolved microscope system designed to enable a detailed investigation of
the sequence of dynamic events involved during surface damage. To best capture individual aspects of the damage
timeline, this system is employed in multiple imaging configurations (such as multi-view image acquisition at a single
time point and multi-image acquisition at different time points of the same event) and offers sensitivity to phenomena at
very early delay times. The capabilities of this system are demonstrated with preliminary results from the study of exitsurface
damage in fused silica. The time-resolved images provide information on the material response immediately
following laser energy deposition, the processes later involved during crater formation or growth, the material ejecta
kinetics, and overall material motion and transformation. Such results offer insight into the mechanisms governing
damage initiation and growth in the optical components of ICF class laser systems.
High temperature thermographic measurements of laser heated silica
Show abstract
In situ spatial and temporal surface temperature profiles of CO2 laser-heated silica were obtained using a
long wave infrared (LWIR) HgCdTe camera. Solutions to the linear diffusion equation with volumetric and surface
heating are shown to describe the temperature evolution for a range of beam powers, over which the peak surface
temperature scales linearly with power. These solutions were used with on-axis steady state and transient
experimental temperatures to extract thermal diffusivity and conductivity for a variety of materials, including silica,
spinel, sapphire, and lithium fluoride. Experimentally-derived thermal properties agreed well with reported values
and, for silica, thermal conductivity and diffusivity are shown to be approximately independent of temperature
between 300 and 2800K. While for silica our analysis based on a temperature independent thermal conductivity is
shown to be accurate, for other materials studied this treatment yields effective thermal properties that represent
reasonable approximations for laser heating. Implementation of a single-wavelength radiation measurement in the
semi-transparent regime is generally discussed, and estimates of the apparent temperature deviation from the actual
outer surface temperature are also presented. The experimental approach and the simple analysis presented yield
surface temperature measurements that can be used to validate more complex physical models, help discriminate
dominant heat transport mechanisms, and to predict temperature distribution and evolution during laser-based
material processing.
Early thermal damage in optical coatings identified by infrared spectral signatures
Sangho S. Kim,
Nicholas T. Gabriel,
Wing S. Chan,
et al.
Show abstract
Laser induced damage can take several forms. Perhaps the most common is thermal breakdown, caused by heating
of the coating to a catastrophic failure induced by local melting, delamination, evaporation, or some other change.
We demonstrate that micromachined dielectric membranes show strong differences in their hydroxyl signatures
as measured by Fourier transform infrared spectroscopy. The changes correspond to regions of high fluence
(3200 J/cm2) from a Nd:YAG laser. It is found that the absorption peaks associated with O-H stretching mode
decrease after laser treatment, indicating a reduction in the number of film hydroxyl groups.
Process for rapid detection of fratricidal defects on optics using linescan phase-differential imaging
Show abstract
Phase-defects on optics used in high-power lasers can cause light intensification leading to laser-induced damage of
downstream optics. We introduce Linescan Phase Differential Imaging (LPDI), a large-area dark-field imaging
technique able to identify phase-defects in the bulk or surface of large-aperture optics with a 67 second scan-time.
Potential phase-defects in the LPDI images are indentified by an image analysis code and measured with a Phase
Shifting Diffraction Interferometer (PSDI). The PSDI data is used to calculate the defects potential for downstream
damage using an empirical laser-damage model that incorporates a laser propagation code. A ray tracing model of LPDI
was developed to enhance our understanding of its phase-defect detection mechanism and reveal limitations.
Laser-induced surface damage density measurements of fused silica optics: a parametric study
Show abstract
The rasterscan test procedure implemented in order to determine low laser damage density of large aperture UV
fused silica optics was improved in terms of accuracy and repeatability. Tests have been carried on several facilities
using several pulse durations and spatial distributions. We describe the equipment, test procedure and data analysis to
perform this damage test with small beams (Gaussian beams, about 1mm @ 1/e, and top hat beams). Then, beam overlap
and beam shape are the two key parameters which are taken into account in order to determine damage density. After
data analysis and treatment, a repeatable metrology has been obtained. Moreover, the consideration of error bars on
defects distributions permits to compare data between these installations. This allows us to look for reproducibility, a
necessary condition in order to share results and to make reliable predictions of laser damage resistance. For that, a
careful attention has been paid to beam analysis.
Numerical analysis of laser-induced damage threshold search algorithms and their uncertainty
Show abstract
Determination of absolute Laser-Induced Damage Threshold (LIDT) value from experimentally obtained statistical data
is still very important metrological problem in terms of accuracy and repeatability. In fact experimentally estimated
LIDTs are always affected by many factors such as (temporal, energy, pointing and beam shape) stability of laser pulses
used for damage testing as well as properties (homogeneity and limited size) of the sample to be tested. These problems
are especially important in case of small aperture limited samples when testing with nanosecond pulses where the
mechanism of damage is usually driven by defects. Several known experimental techniques or its modifications (for
example raster scan) are typically applied for LIDT estimation in 1-on-1 mode namely Damage Frequency Method
(DFM) and General Binary Search Technique (GBST). Almost all methods lead to the same value of LIDT under ideal
experimental conditions and sufficiently large number of interrogated test sites. The goal of this numerical study is to
analyze the performance (accuracy and repeatability) of above mentioned algorithms with respect to damage limiting
surface defect density under non-ideal experimental conditions: energetic instabilities of laser radiation and aperture
limited to max. 300 sites. Herewith we also introduce "moving average" measurement concept. The conclusions are
drawn about the precision of all above mentioned methods.
Spot-size dependence of the LIDT from the NIR to the UV
Show abstract
It has been well known that laser-induced damage in optical thin films depends on the applied test laser beam
diameter, if the damage initiation is dominated by inclusions and defects. This correlation has been subject
of numerous discussions and has also been implemented in the corresponding ISO standard (ISO 11254-2).
There, a minimum beam diameter is advised which further reduction affects the result significantly. Previous
work indicates, depending on sample properties, a constant damage threshold towards large beam diameters.
However, experimental data for this behavior is limited. This paper presents a detailed investigation within the
ns-timescale on a series of four comparable sample sets of HfO2 /SiO2 high reflectors. The respective samples
are designed and manufactured for ISO damage testing at 1ω, 2ω, 3ω, and 4ω irradiation of the Nd:YAG laser.
Adaptive laser-induced damage detection
Show abstract
Laser-Induced Damage Threshold (LIDT) measurements are typically performed in order to characterize the optical
resistance of laser components. However the sensitivity of online damage detection techniques is often limiting factor for
the accuracy and reproducibility of so called S-on-1 and 1-on-1 measurements. In fact the sensitivity of damage detection
has the biggest impact to precision of these experiments. In this paper we describe the idea of making improvements on
scattered light registration based damage detection. It was learned, that scattered light intensity is linearly proportional to
incident energy of laser pulses while material is not damaged. However in case of induced damage the linear
proportionality becomes nonlinear: this feature is used in order to detect optically induced surface and bulk changes in
material. According to the base of those theoretical considerations the adaptive scattering detector was proposed and
made up. It was put into practice by projecting it on discrete element schematics. The improved sensitivity of laserinduced
damage detection was reached. This technique helps to avoid degradation of damaged site and pollution of
surrounding area due to laser ablation during the S-on-1 tests since it allows blocking the repetitive irradiation
immediately when damage appears. Numerous tests were made, that shows, that adaptive scattering detector can
precisely detect damages in their initiation state, independently from material ability of light scattering. Calibration of
this detector can be automated, therefore the influence of human factor is minimized. This fact opens up the possibility to
run whole damage threshold measurement procedure automatically.
Optical damage testing at the Z-Backlighter facility at Sandia National Laboratories
Show abstract
To enable laser-based radiography of high energy density physics events on the Z-Accelerator[4,5] at Sandia
National Laboratories, a facility known as the Z-Backlighter has been developed. Two Nd:Phosphate glass lasers are
used to create x-rays and/or proton beams capable of this radiographic diagnosis: Z-Beamlet (a multi-kilojoule laser
operating at 527nm in a few nanoseconds) and Z-Petawatt (a several hundred joule laser operating at 1054nm in the subpicosecond
regime) [1,2]. At the energy densities used in these systems, it is necessary to use high damage threshold
optical materials, some of which are poorly characterized (especially for the sub-picosecond pulse). For example,
Sandia has developed a meter-class dielectric coating capability for system optics. Damage testing can be performed by
external facilities for nanosecond 532nm pulses, measuring high reflector coating damage thresholds >80J/cm2 and antireflection
coating damage thresholds >20J/cm2 [3]. However, available external testing capabilities do not use
femtosecond/picosecond scale laser pulses. To this end, we have constructed a sub-picoseond-laser-based optical
damage test system. The damage tester system also allows for testing in a vacuum vessel, which is relevant since many
optics in the Z-Backlighter system are used in vacuum. This paper will present the results of laser induced damage
testing performed in both atmosphere and in vacuum, with 1054nm sub-picosecond laser pulses. Optical
materials/coatings discussed are: bare fused silica and protected gold used for benchmarking; BK7; Zerodur; protected
silver; and dielectric optical coatings (halfnia/silica layer pairs) produced by Sandia's in-house meter-class coating
capability.
Angle resolved scattering: a method for in situ investigations of laser-induced damage?
Show abstract
Light scattering from interface imperfections or defects is a topic of persistent interest. On the one hand, light scattering
can be a critical issue that limits throughput and imaging quality of optical components even if high-end polishing and
coating techniques are employed. This in particular holds for multilayer coatings and for short wavelengths of
application. On the other hand, scattered light also carries valuable information about its origins, such as the structural
properties of multilayer coatings.
A large variety of instruments for scatter measurements in the visible, ultraviolet, infrared spectral ranges has been
developed at the Fraunhofer IOF in Jena during the past years. In addition, software tools have been developed based on
existing scattering theories which enable an advanced analysis of the observed scattering properties.
For the rather complicated scattering of thin film coatings, a simplified model was developed recently which introduces
two simple parameters to describe the roughness evolution from interface to interface and optical thickness deviations.
An iterative solution of the inverse scattering problem yields information about alterations of the structural and optical
properties inside multilayer coatings based on angle resolved scatter measurements. Experimental results will be
presented for highly reflective metal fluoride coatings for 193 nm as well as for a Rugate filter after laser damage tests.
It will be discussed how the procedure could be implemented in a laser damage test environment to enable the damage
behavior to be characterized in-situ during irradiation. This can provide valuable information about the fundamental
damage processes even prior to the ultimate damage events.
Life testing for laser optics: a first look
Show abstract
This short paper lays out the goals for a new series of investigations into the proper means of estimating the lifetime of
an optic exposed to many, S, laser pulses. The problem of properly identifying a means to estimate component life as
important, interesting and being needed is discussed. The fundamental issue in estimation of life is the extrapolation
from small to large values of S, frequently this extrapolation is many orders of magnitude. The need for a universal
fitting function to enable this extrapolation is the seminal point of these investigations. A key question is the universality
of the form of the fitting function. This paper makes the case that such a fitting function requires experimental
confirmation. The paper concludes with a call for colleagues to provide such life test data to help in the search of the
universal fitting function or functions.
Damage and degradation of optics and sensors under intense EUV radiation from a table-top laser produced plasma source
Show abstract
In recent years, technological developments in the area of extreme ultraviolet lithography (EUVL) have
experienced great improvements. Currently, the application of EUV radiation apart from microlithography
comes more and more into focus. Main goal of our research is to utilize the unique interaction between soft
x-ray radiation and matter for probing, modifying, and structuring solid surfaces.
In this contribution we present a setup capable of generating and focusing EUV radiation. It consists of a
table-top laser-produced plasma source. In order to obtain a small focal spot resulting in high EUV fluence,
a modified Schwarzschild objective consisting of two spherical mirrors with Mo/Si multilayer coatings is
adapted to this source, simultaneously blocking unwanted out-of-band radiation. By demagnified (10x)
imaging of the plasma an EUV spot of 5 μm diameter with a maximum energy density of ~7.4 J/cm² is
generated (pulse length 8.8 ns).
We present first applications of this integrated source and optics system, demonstrating its potential for
high-resolution modification and structuring of solid surfaces. As an example, etch rates for PMMA, PC
and PTFE depending on EUV fluences were determined, indicating a linear etch behavior for lower energy
densities.
In order to investigate damage tests on EUV sensors and optics,
1-on-1 damage tests were performed on
grazing incidence gold mirrors, Mo/Si multilayer mirrors and mirror substrates. To our knowledge, this is
the first time that such experiments using nanosecond EUV-pulses were carried out.
Modeling laser conditioning of KDP crystals
Show abstract
When potassium dihydrogen phosphate crystals (KH2PO4 or KDP) are illuminated by multi-gigawatt nanosecond
pulses, damages may appear in the crystal bulk. One can increase damage resistance through a conditioning that
consists in carrying out a laser pre-exposure of the crystal. The present paper addresses the modeling of laserinduced
conditioning of KDP crystals. The method is based on heating a distribution of defects, the cooperation
of which may lead to a dramatic temperature rise. The conditioning is assumed to be due to a decrease in
the defect absorption efficiency. Two scenarios associated with various defect natures are proposed and these
account for certain of the observed experimental facts. For instance, in order to improve the crystal resistance
to damage, one needs to use a conditioning pulse duration shorter than the testing pulse. Also, a conditioning
scenario based on the migration of point (atomic-size) defects allows the reproduction of a logarithmic-like
evolution of the conditioning gain with respect to the number of laser pre-exposures. Moreover, this study
aims at refining the knowledge regarding the precursor defects responsible for the laser-induced damage in KDP
crystals. Within the presented modeling, the best candidate permitting the reproduction of major experimental
facts is comprised of a collection of one-hundred-nanometer structural defects associated with point defects as
for instance cracks and couples of oxygen interstitials and vacancies.
Laser conditioning process combining N/1 and S/1 programs to improve the damage resistance of KDP crystals
Show abstract
Multi-parametric experiments were carried out to investigate laser conditioning efficiency for KDP crystals as a function
of fluence step, laser fluence, and S/1 pulse sequence by using a tripled Nd:YAG laser (355nm) with a pulsewidth of
about 6 ns. It was disclosed that the laser conditioning enhancement was mainly depended on the maximum fluence
which was reached in conditioning process. Compared with increasing steps, higher fluence can more efficiently improve
the laser damage resistance capability. Moreover, S/1 pulse sequence was better for stabilizing defects and could enhance
the damage threshold further. Based on the results, an optimized laser conditioning process was put forward. Firstly, an
N/1 conditioning program can be used to achieve the maximum fluence without causing damage. Secondly, an S/1
program shall be adopted to further enhance the damage threshold with the maximum fluence which is achieved in the
foregoing N/1 program. After the laser conditioning process, damage tests showed that laser damage threshold was
almost doubled. The model of laser conditioning was discussed on the basis of the size reduction of absorber by the local
decomposition of the surrounding material. And then the dependence of conditioning efficiency on the process
parameters was analyzed.
Nanosecond-laser induced damage at 1064 nm, 532 nm, and 355 nm in LiB3O5
Show abstract
LiB3O5, short LBO, is an important nonlinear optical material for frequency conversion. As efficient frequency
conversion requires high intensities nonlinear optical crystals are often subject to laser induced damage even in
commercial laser systems. In this work we studied nanosecond laser induced damage in LBO at the three harmonic
wavelengths of the Nd:YAG laser: 1064nm, 532nm and 355nm. Similarly to KTP and RTP a polarization dependent
anisotropy of the laser induced damage threshold has been found, being strongest at 1064nm wavelength. The weakest
point of this material regarding the three tested wavelengths is the bulk damage threshold at 355nm. The fatigue effect
was found to be negligible at the UV wavelength and most important for the IR light. Despite the fact that the green light
has been generated by external frequency doubling, it caused an intermediate fatigue effect. Our measurements also
confirmed the high bulk laser damage threshold for the IR wavelength being approximately a factor 1.5 higher than the
one of synthetic fused silica.
Laser induced damage of sapphire and titanium doped sapphire crystals under femtosecond to nanosecond laser irradiation
Show abstract
The use of large Ti:Sapphire crystals in ultra fast high peak power laser amplifiers makes crucial the problem of
crystal laser induced damage. These works aim to quantify the laser induced damage threshold (LIDT) of Sapphire and
Ti:Sapphire crystals under femtosecond, picosecond and nanosecond laser pulse irradiations, which are typically
encountered in such laser chains. Furthermore, a study of the influence of cryogenic conditions on the LIDT of
Ti:Sapphire crystals and of their anti-reflection coating has been performed. The results are important to understand the
mechanisms leading to the damage, and to reveal the key parameters which will have to be optimized in future high peak
power laser chains.
Deterministic single shot and multiple shots bulk damage thresholds for doped and undoped crystalline and ceramic YAG
Show abstract
We measured the single-shot and multiple-shot damage thresholds of pure and Nddoped ceramic Yttrium Aluminum Garnet (YAG), and of pure, Nd-doped, Cr-doped, and Yb-doped crystalline YAG. We used 9.9 ns, single-longitudinal-mode, TEM00 pulses
tightly focused inside the ceramic and crystalline YAG. The 8 microns radius of the focal
spot was measured using surface third harmonic generation. With this tight focus the
damage threshold powers for both the ceramic and crystalline YAG were below the SBS
threshold, and the effect of self focusing was small.
We found the single-shot and multiple-shots damage thresholds to be deterministic.
The single-shot damage of YAG occurs on the trailing edge of the laser pulse, in contrast
to fused silica in which damage always occurs at the peak of the laser pulse. However,
the multiple-shot damage threshold of YAG occurs at the peak of the nth laser pulse.
We find the damage thresholds of doped and undoped, ceramic and crystalline YAG
range from 1.1 to 2.2 kJ/cm2. We also report some damage morphologies in YAG.
Study of haze in 193nm high dose irradiated CaF2 crystals
Show abstract
Crystalline calcium fluoride is one of the key materials for 193 nm lithography and is used for laser optics, beam
delivery system optics and stepper/scanner optics. Laser damage occurs, when light is absorbed, creating defects in the
crystal. Haze is known as a characteristic optical defect after high dose irradiation of CaF2 - an agglomeration of small
scattering and absorbing centers. In order to prevent unnecessary damage of optical components, it is necessary to
understand the mechanism of laser damage, the origin of haze and the factors that serve to prevent it. Stabilized M
centers were described as reversible absorbing defects in CaF2, which can be annealed by lamp or laser irradiation. In
this study the irreversible defects created by 193 nm laser irradiation were investigated.
Development of large size MgF2 single crystal grown by the CZ method
Show abstract
In the semiconductor lithography technology, the polarized illumination system is applied to make the resolution more
microscopic, therefore the polarizer material with excellent durability against the high power ArF laser has been
required. Magnesium fluoride (MgF2) is one of a suitable material because of its laser durability and high transparency
in VUV region. Previously we reported MgF2 single crystal with diameter of 100mm by using the Czochralski (CZ)
method. By optimizing the crystal growth conditions, MgF2 single crystals with over 150mm in diameter have been
stably grown. Also these crystals show good optical properties and crystallinity.
Laser-induced damage thresholds in silica glasses at different temperature
Show abstract
The laser-induced damage thresholds in silica glasses at different temperature conditions (123 K - 473 K) by Nd:YAG
laser fundamental (wavelength 1064 nm) and third harmonic (wavelength 355 nm) 4 ns of pulses were measured. In the
results, the damage thresholds increased at low temperature. At 1064 nm, the temperature dependence became strong by
the concentration of impurities. However, at 355 nm, the temperature dependences of almost sample were almost the
same for different concentration of impurities.