encyclopedia of modern optics || optical coatings | laser damage in thin film coatings

Microabrasives

Poly-C is also a very promising material for micro-abrasives. Micromachined poly-C MEMS structureshave recently been fabricated for possible use formicroabrasives. Figure 11 shows micromachinedMEMS structures which are expected to enhancethe life of a tool. Such abrasives by design areinherently cost-effective because they are batchfabricated.

See also

Light Emitting Diodes.

Further Reading

Asmussen J and Reinhard D (2002) Diamond Handbook.New York: Marcel Dekker.

Kim U and Aslam DM (2003) Field emission electrolumi-nescence on diamond and carbon nanotube films.Journal of Vaccum Scientific Technology B 21(4):1291–1296.

McMurry J (1996) Organic Chemistry. New York: Brooks/Cole.

Pan LS and Kania DR (1996) Diamond: ElectronicProperties and Applications. Boston, MA: Kluwer.

Spear KE and Dismukes JP (1994) Synthetic Diamond:Emerging CVD Science and Technology. New York:Wiley.

Laser Damage in Thin Film Coatings

D Ristau, Laser Zentrum Hannover e.V., Hannover,Germany

q 2005, Elsevier Ltd. All Rights Reserved.

Introduction

Since the very beginning of laser technology, Laser-induced Damage Thresholds (LIDT) of opticalcomponents were always an obstacle for the devel-opment of laser systems operating at high powerlevels. In such systems, the surface or the bulk of theoptical components can be damaged by laser radia-tion of sufficiently high power or energy densityresulting in an uneconomical standstill of the laserand its application. In the early days of high powerlaser development, mainly inclusions in laser rodmaterials were discussed as a major complication foran augmentation of the output power in solid statelaser systems. Nowadays, in the course of thedevelopment of optical materials with excellentquality and power handling capability, theproblem of laser-induced damage has shifted fromthe bulk to the surface of the optical component. Theoptical surface is subjected to various productionsteps and environmental influences, which modifyits structure and composition. Especially, the thinfilm coating, which is deposited on the optical surfaceto adapt its reflectance and transmittance to theapplication, contributes predominantly to thereduction of the LIDT values. As a consequence,the measurement and optimization of the powerhandling capability of thin films is considered as oneof the primary research areas in modern opticstechnology and is supported by an extensive scientificcommunity.

In the following, a brief review will be given onselected fundamental damage mechanisms in thinfilms, considering also the scaling of LIDT values fordifferent operation conditions. Also, current stan-dards for the measurement of LIDT will be described,and examples illustrating some practical aspects ofhigh power optical coatings will be presented.Finally, a summary of the present state of the artwill be given and recent trends in laser technology willbe discussed in respect to research in laser-induceddamage.

Fundamental Damage Mechanismsin Thin Films

During thirty years of intense research in laserdamage of thin films, a broad spectrum of differentdamage and degradation mechanisms had beendiscovered and studied for a variety of laser operationconditions and wavelengths. For prominent high-power laser systems often applied in industrialproduction fields or fundamental research, specificmodels for laser-induced breakdown have beendeveloped, which are also of practical relevance. Inmany experiments, damage sites with the mor-phology illustrated in Figure 1 are observed. Thecoating is delaminated from the surface of the opticalcomponent in the center of the laser beam area.Obviously, mechanical stress has built up in thecoating by heating, which is induced by the absorbedlaser power. At a certain stress level, exceedingthe adhesion strength of the coating to substratesurface, the thin film cracks and may even delaminatefrom the component. In other cases of this absorp-tion-induced damage effect, the film reaches itsmelting point prior to the mechanical threshold and

OPTICAL COATINGS / Laser Damage in Thin Film Coatings 339

evaporates or changes its crystalline structure. Themorphology of such damage sites exhibits a discolor-ing or an increased surface roughness in the center ofthe laser beam (see Figure 2). Absorption-induceddamage is dominated by the instantaneous heating ofthe coating material in the area of interaction with thelaser beam and can be described on the basis of theheat diffusion equation. In this approach, the sourceterm in the heat diffusion equation is given by the heat

coupled into the system by the absorbed laser power,and the boundary conditions are determined by thegeometry and the structure of the layer system as wellas the spatial laser beam profile. Apparently, absorp-tion-induced damage has to be considered only forcomponents with significant absorptance at theiroperation wavelength. Typical examples for coatingmaterials and wavelength regimes, where absorption-induced damage is frequently observed, are thewavelength 10.6 mm of the CO2-laser with ZnSe,ZnS, and fluorides as coating materials, or the DUV/VUV-wavelengths dominated by excimer lasers andfluoride coating materials. The effect of absorption-induced damage in optical materials is also ofpractical relevance for cw- and long-pulse operationconditions of the laser system corresponding to longthermal diffusion lengths. For cw-irradiation, thetemperature rise in the component can be calculatedby numerical methods involving finite elements ordifferences. For boundary conditions with circularsymmetry and a Gaussian beam profile (beamdiameter w; power P), an analytical expression canbe derived for the temperature rise DT in the center ofan irradiated circular component

DT ¼2bsP

p 3=2kwtan21

�16ktI

w2

�1=2

½1�

In this model the temperature rise is dependenton the thermal properties (k: thermal conductivity,k: thermal diffusivity), on the surface absorption ofthe component ðbSÞ; and on the beam diameter. Forirradiation times tI; long compared to the typical heatdiffusion time w2=k; eqn [1] reduces to the asymptoticdependence

DT !bsP

p1=2kwtI !1 ½2�

Obviously, the temperature rise DT scales with theP=w for these long-time irradiation conditions com-pared to the short pulse regime, where the scaling ofthe temperature rise is proportional to P=w2: For theapplication of optical components in high powercw- and long-pulse lasers, this P=w scaling law has tobe considered carefully, because the power handlingcapability depends critically on the size of theirradiated area. Also, the onset of damage may bedelayed in respect to the start of irradiation by timeconstants in the range of several 10 seconds.

In the course of the development of improvedcoating processes, optical absorptance could bereduced to very low levels in the near infrared andvisible spectral region. For example, high reflectingmirrors for the wavelength of the Nd:YAG-laser withtotal losses below 1 ppm can be produced with

Figure 1 SEM-picture of a damage site on an alternating

coating of SiO2/HfO2. The system was irradiated by a Nd:YAG-

laser with an energy density of 45 J/cm2. As a dominant damage

mechanism, a delamination of the coating by extreme mechanical

stress is suggested. Reproduced with permission from Ristau D,

Ebert J and Welling H (1989) Optische Beschichtungen fur

Hochleistungslaser. Laser und Optoelektronik 21(4): S.49.

Figure 2 Nomarski micrograph (magnification 250£ ) of a

damage site on an antireflecting coating of SiO2 / Ta2O5. The

system was irradiated with an energy density of 51 J/cm2 at a

wavelength of 1.064 mm. A delamination of the coating in the

center of the beam is surrounded by a recrystallization zone.

340 OPTICAL COATINGS / Laser Damage in Thin Film Coatings

advanced ion beam sputtering processes nowadays.Therefore, another principal damage mechanism,which is based on inclusions or defects in thecoatings, is often found in the near infrared spectralrange. A detail of a damage site representing inclusiondominated damage induced by a pulsed Nd:YAG-laser in a coating of HfO2/SiO2 is shown in Figure 3.In the laser beam area numerous craters are located,which extend from circular voids with differentdiameters in a range below 1 mm. At the highmagnification of the employed electron microscope,the multilayer structure of the coating system can beresolved indicating an origin of damage approxi-mately in the same depth of the layer structure at thesubstrate surface. The underlying damage mechanismis initiated by inclusions or other defects with anabsorption, which is significantly higher than for thesurrounding thin film material. As a consequence, theinclusion is rapidly heated during the interaction withthe laser beam and explodes removing the coveringlayer structure. This inclusion breakdown can bemodeled by calculating the temperature behavior TðrÞat the radial position r of a sphere (radius g; thermalconductivity kE; diffusivity kE) which is embeddedin a medium with defined properties (see eqn [3]).The interaction of the sphere, which has a size in therange of the wavelength l and a refractive index n0,with the laser irradiation, is described by the Mieabsorption coefficient Q: For the instant of damage, acritical temperature at the outer surface of the sphereðr ¼ gÞ is assumed, which is often related to themelting point of the layer material

TðrÞ ¼3QI

4pkEg

264 kE

3kþ

1

6

12

r2

g2

!

22gb

rp

ð1

0dyexp

2

y2tp

t1

!

£ðsiny2ycosyÞðsinry=gÞ

y2hðcsiny2ycosyÞ2þb2y2 sin2 y

i375

½3�

Q ¼ pg2

"1þ

2expð28pn0g=lÞ

8pn0g=lþ

expð28pn0g=lÞ21

ð8pn0g=lÞ2

#

t1 ¼g2

kE

b¼k

kE

ffiffiffiffikE

p ffiffik

p

!c ¼ 12

k

kE

Besides the thermal properties of the host material(thermal conductivity k; diffusivity k), the propertiesand the statistic distribution of the inclusions have to

be known for a quantitative description of inclusionbreakdown.

In contrast to the nearly deterministic behavior ofabsorption induced damage, defect dominateddamage mechanisms often exhibit a statistical nature.Since the inclusions are not homogeneously distri-buted in the layer structure, a variation of the thres-hold value is observed for different damage sites on asingle sample. The assessment of the inclusion para-meters is extremely difficult in practice. Therefore, theinclusion model is mainly employed for the predictionof tendencies for the LIDT as a function of the beamparameters and the properties of the layer materials.

Different aspects related to the substrate polishing,cleaning, and deposition process have to be con-sidered for an investigation in the various origins ofinclusions or defects in a layer structure. Besidescontamination of the substrate surface prior to thedeposition process, particles may be generated duringthe deposition process by mechanical abrasion ofmaterial from moving parts in the plant or bydelamination of material coated on the inner walls.Also, the deposition source may contribute to theformation of defects by sputtering of particles ordroplets from the melt or the target. Especially in lowenergetic thermal deposition processes, defects areoften the origin for the formation of nodules. Theseconical faults in the layer structure reduce the LIDTbecause they are less resistant against intensive laserradiation than the bulk of the coating system.

Besides models, which are based on the transform-ation of laser radiation into heat, direct electronicexcitation has to be considered as a breakdownmechanism for short-pulse lasers. In these intrinsicbreakdown models for dielectric materials, the

Figure 3 SEM-picture of damage site on a high reflecting

coating of SiO2/HfO2. This inclusion-dominated breakdown

mechanism was initiated at an energy density of approximately

130 J/cm2 with a Nd:YAG-laser.


generation of a high electron density in the conduc-tion band is assumed. This carrier generation can beexplained by high field ionization of electrons in thevalence band of the dielectric material. Manytheoretical concepts introduce the avalanche effectas an important mechanism contributing to thegeneration of free electrons. Electrons excited bystrong field ionization can interact with the electricfield and accumulate excitation energy. By collisionswith the lattice, this energy can be also transferred tothe electrons in the valence band. Thus, when theenergy of the free electrons exceed a critical value,other electrons can be excited to the conduction band,and a carrier generation like an avalanche arises.Another theoretical approach is the multiphotonmodel which is formulated on the basis of multi-photon absorption for the generation of free elec-trons. In most intrinsic models it is assumed, thatlaser-induced damage breakdown occurs when theelectron density in the conduction band reaches acritical value of approximately 1021 1/cm3. At thishigh electron density, a plasma state is reached andthe impinging radiation is coupled efficiently into thelayer structure resulting in its destruction.

Intrinsic damage is often characterized by adeterministic damage behavior with a well definedthreshold value which is also characteristic fordamage in bulk materials. As a prominent example,damage induced by ultra-short pulses with a durationbelow 1 ps can be often attributed to intrinsicmechanisms, because the diffusion length of thermaleffects can be neglected in comparison to the intensiveinteraction of the laser radiation with the electrons ofthe coating materials. In this case, the generation ratedn=dt of electrons in the conduction band can bedescribed on the basis of the impact ionization rateand multiphoton excitation

dn

dt¼ anðtÞqIðtÞ þ bm

hqIðtÞ

im½4�

In this model IðtÞ is the power density of the laserradiation, and q is a correction factor representinginterference effects in the coatings, which may resultin a local power exceeding the power density ofthe radiation impinging on to the layer system. Theelectron excitation mechanisms are described by theimpact ionization coefficient a and the m-photonabsorption coefficient bm: The relative contributionsof both excitation mechanisms to laser-inducedbreakdown are depending on the pulse duration andthe bandgap energy of the layer material.

In a rough approximation, the damage thresholdincreases with the bandgap energy of the materials asa consequence of the photon absorption coefficient

bm decreasing with its order m: For short pulsedurations below a few 10 fs, multiphoton processescontribute significantly in the range of some 10% tothe free-carrier generation. In most materials, ava-lanche ionization develops to the dominant gener-ation mechanism for pulse durations above 100 fs.These tendencies could be demonstrated for singlelayers of selected oxide material deposited by ionbeam sputtering on quartz substrates, which weretested in a pulse duration regime from 20 fs to 1 ps.According to the theoretical model, the expectedincrease of the damage also influencing values withthe pulse duration was observed in the damageexperiments.

In practice, the power handling capability ofoptical components is also often limited by imperfec-tions like scratches, digs, and areas with high rough-ness on the optical surface. In most depositiontechniques applied in optical technology, the layersystem tends to replicate or even enhance surfaceimperfections of the substrate. In the application,additional weak points may be introduced byimproper handling or contamination of the opticalsurfaces. If the contaminants are not removed by theimpinging high-power laser radiation, they increasethe surface absorption and act as initiation points forlaser-induced damage. Voids, grooves, pores, orscratches reduce the power capability of the opticalelement, because they act as concentrators for theelectric field.

Units and Scaling of Laser-InducedDamage Threshold

According to the theoretical models of laser-induceddamage, the appropriate units of measurement forLIDT values are mainly given by the dominantdamage mechanism and the irradiation time. Forpulsed laser radiation and dielectric breakdowneffects, the laser-induced damage threshold is usuallyreported in W/cm2. In the case of absorption-induceddamage or inclusion dominated breakdown and apulse duration short compared to the thermaldiffusion time in the layer structure, LIDT valuesare often scaled in J/cm2. The unit of linear powerdensity (W/cm) is indicated for the LIDT of long-pulse, cw-lasers or other sources, which induce atemperature increase in the optical component withrise times in the regime of seconds.

Laser-induced damage thresholds of optical thinfilms are critically dependent on the operationconditions of the applied laser system and on thedesign of the layer structure. For example, even thedamage mechanism can change if the wavelengthor the pulse duration is varied. Therefore, LIDT


values should be scaled with the irradiationparameters only for small intervals, in caseswhere the fundamental damage mechanism isclarified. In all other situations, the extrapolationof threshold values is extremely difficult and maylead to inaccurate results. Especially, an over-estimation of the LIDT value holds severe dangersin practical applications, where the replacement ofa damaged component and the repair of otherparts impaired by the damage event causes highexpenses. If the damaged components consist oftoxic materials (e.g., ZnSe, GaAs, CdTe, ThF4,chalcogenides, Be, Cr, etc.) severe health hazardsmay occur, and an expensive decontamination ofthe environment may be necessary. Therefore, thedetermination of the laser-induced damagethreshold using the actual laser parameters undercontrolled environmental conditions is alwaysrecommended for unclear cases.

In the present state of research in laser-induceddamage mechanisms, only few tendencies aregenerally accepted as confident for scaling of LIDTvalues. As a function of the pulse duration, laser-induced damage thresholds increase for longer pulses.For inclusion and absorption dominated breakdown,a tp

1/2-law is often used in the pulse duration regimebetween 10210 to 1028s. This dependency can beextended to other pulse regimes up to pulse lengths inthe ms-range, if the exponent is replaced by a valuebetween 0.5 to 2. In respect to the laser wavelength, adecrease of the LIDT value with decreasing wave-length is observed for most materials and operationalconditions. Investigations in the influence of the beamdiameter have been performed by many researchgroups, indicating a decrease of the LIDT values forincreasing beam diameters. Especially for inclusiondominated breakdown, the event of damage for acertain laser irradiated site will be dependent on thedistribution of inclusions at that position. If the beamsize is small, the probability for interrogating a defectvulnerable to damage is low. By increasing the spotdiameter, this probability will asymptotically reachunity, because at a certain beam diameter, at leastone defect will always be covered by the beam.

Therefore, the onset of laser-induced damage is notdependent on the beam diameter for inclusion-dominated breakdown, which is often encounteredin conventional optical coating systems. Anotherspecial case is the scaling of the cw-damage thresholdwith the beam diameter. Since the cw-LIDT isexpressed in linear power density, the power handlingcapability increases more slowly with the beamdiameter for the cw-lasers than expected from thenormal pulsed operation. For example, extrapolatingthe LIDT value of 100 W at a beam diameter of 1 mmfor a laser mirror to a beam diameter of 10 mm,results in a maximum power load for the componentof 1 kW according to the correct P/w-law. Thethreshold power would be extremely overestimatedto approximately 3.2 kW, if the general P/w2-depen-dence is applied.

Measurement of Laser-InducedDamage Thresholds

As a consequence of the complicated relationbetween the laser damage mechanism and a broadspectrum of thin film properties and laser para-meters, laser-induced damage threshold measure-ments have to be performed under well-definedconditions. To investigate the comparability ofLIDT measurements, an extended internationalround-robin experiment has been conducted oncoated optics for the wavelength of 1.064 mm atthe beginning of the 1980s. This experimentindicated the need for a clearly specified LIDT-measurement procedure, and conceptual work wasinitiated to develop a corresponding ISO-Standardseries. During recent years, an International Standard(ISO 11254) has been adopted covering testingconditions relevant for most typical laser appli-cations. In the first part of ISO 11254 1 on 1-testingof optical surfaces, in respect to laser damage, isdescribed. The fundamental approach of the stan-dard measurement procedure is illustrated inFigure 4. A laser source operating in transversaland longitudinal single mode is employed forthe irradiation of the sample surface. The beam

Figure 4 Fundamental setup for the measurement of laser-induced damage thresholds.


parameters of the laser are assessed by a beamdiagnostic system which monitors the spatial andtemporal profile as well as the energy of the laserradiation in the target plane. In order to achieve thehigh energy density levels necessary to destroy thesurface of the specimen, a well characterizedfocusing system is installed. For adjustment of thelaser energy in the target plane, an attenuatingsystem is employed. In the 1 on 1 measurementprotocol, each site on the sample surface is subjectedto a single laser pulse once only. In a test sequence,the test surface is examined by pulses of differentenergies, covering low values without damage andhigh values causing damage. After the test, thespecimen will be inspected with a Nomarski inter-ference contrast or a darkfield microscope at amagnification of 150£ or higher to identify thedamaged sites. For the evaluation of the damage test,the damage probability method is recommended. Inthis evaluation scheme, the ratio of the number ofdamaged sites to the total number of sites objected toa certain energy or power level is interpreted as thedamage probability. The plot of these damageprobability values as a function of energy orpower, which is called the survival curve of theoptical component, provides an insight into thedamage mechanisms involved. The damagethreshold is given by the highest quantity of laserradiation for which the extrapolated probability ofdamage is zero. A typical example for a 1 on 1damage test at the wavelength 1.064 mm and theextrapolated LIDT value is depicted in Figure 5.

In most catalogs of optics manufacturers, the 1 on 1LIDT values are used to illustrate the power handling

capability of their products, even though 1 on 1 dataare of limited importance for practical applications,where an optical component is always subjected morethan one time to a laser beam. This customarysituation is covered by Part 2 of the ISO-LIDT-Standard which describes a damage test procedure fora series of pulses (S on 1-tests). For an assessment ofthe reliability, the concept of the characteristicdamage curve is introduced by this standard. Thiscurve is deduced directly from the S on 1 test data byplotting the energy density for a selected damageprobability as a function of the number of pulses (seeFigure 6). By an extrapolation of the characteristicdamage curve to high pulse numbers in the order of109 to 1012 shots, the lifetime of the opticalcomponent can be roughly estimated. For a certifica-tion of optical components in respect to their powerhandling capability, a third part of ISO 11254 isunder development, which is concentrated on differ-ent testing protocols of a defined surface fraction atpower levels expected in the application. The funda-mental approach of these tests is a simulation ofconditions at the upper limits of operation parametersencountered in practice.

Optical Coatings for High PowerLasers

For the development and application of coatingsystems with high LIDT values, several major aspectshave to be considered. In the first approach, appro-priate materials and processes have to be selectedwhich deliver coatings with sufficient power resist-ance. In many studies, a correlation of the LIDTvalues of the constituent single layers to the power

Figure 5 Survival curve of an anti-reflective layer system (two

layer V-coating, SiO2/ Ta2O5) on fused silica substrate for the

Nd:YAG-laser wavelength. The measurement was performed at a

wavelength of 1.064 mm, a beam diameter of 420 mm, and a pulse

duration of approximately 15 ns, respectively.

Figure 6 Characteristic damage curve of a high reflecting mirror

of SiO2/TiO2 for fs-laser systems. The layer structure was tested

with a high repetition (1 kHz repetition rate) fs laser system

operating at a pulse duration of 150 fs.


handling capability of a layer system was observed.Therefore, single layers of potential depositionmaterials were investigated at the wavelengths ofprominent laser systems from the VUV- to the FIR-spectral region. For specific deposition processes,often a clear ranking of the coating materials inrespect to their power handling capability could bedemonstrated. An example for a Nd:YAG-laserwith a pulse duration of 15 ns is illustrated byTable 1. With the exception of the ZrO2-layer,which exhibited severe inhomogeneity and a highinclusion density, a correlation can be assumedbetween the LIDT value and the melting point of thebulk materials under these operation conditions. Thisdependency can be attributed to absorption-induceddamage as well as to the inclusion model, andtherefore, the damage morphology has to be studiedin each case to identify the dominating damagemechanism. In the fs-regime or the UV/VUV-spectralrange, often a dependence of the power handlingcapability on the bandgap of the materials is found.According to the relationships of the avalanche andother dielectric breakdown models, materials withhighest bandgaps are frequently encountered at thetop of an LIDT ranking.

For the influence of the coating process on the laser-induced breakdown of optical coatings, no cleartendencies can be detected. For example, Ion BeamSputtering (IBS), which is considered as the depo-sition process for coatings with extremely low lossesand contamination, cannot always surpass conven-tional thermal deposition processes in laser stability.In general, ion or plasma assisted deposition tech-niques produce coatings with lower LIDT values thanconventional coating processes for most operationconditions. Sol-gel processes, which were developedfor the deposition of removable coatings on largeoptics in laser fusion, can reach superior powerhandling capabilities. In general, as a consequence ofthe complicated relation between the production

parameters and laser-induced breakdown, pro-duction processes for optical coatings have to beoptimized separately for different wavelength regimesand irradiation conditions.

Besides the fundamental production parameters,the design of the coating system is of similarimportance for the achievement of high LIDT values,because damage is directly driven by the electric fieldstrength in the layer structure. A standing wave fieldpattern is depicted in Figure 7 for a high reflectingstack of Ta2O5 and SiO2. The design consisting oflayers with an optical thickness of 1 QWOT is typicalfor most standard laser mirrors. The power densityreaches extreme values always at the interfacesbetween the layers and even exceeds the incomingirradiation power density (100%). Interfaces betweenthe layers can be considered as weak points in acoating system, because additional contaminationmay occur during the switching of the material in theproduction process. Also, the adhesion between theadjacent layers may be reduced, and mechanicalstress may be built up by the different materials. Toimprove damage thresholds of laser mirrors, thethickness of the first few layer pairs can be adjusted toshift the points of maximum field strength into thebulk of the layer with higher damage resistivity (seeFigure 8). Also, a thick layer can be attached to thesystem in order to stabilize the outer layer pair inrespect to thermal or mechanical stress. Anothertechnique, which can be applied to enhance thestability of the interface is the codeposition ofmaterials, resulting in a gradual interface with amixing zone of materials between the layers. Thisregion of codeposited material exhibits a higherresistance against mechanical stress resulting in animprovement of LIDT values of up to 20%.

The effect of the internal electric field strengthdistribution on the power handling capability of alayer system should be most apparent for intrinsicdamage mechanisms, which are dominant in the ultrashort pulse regime below 1 ps. According to thefundamental model (see eqn [4]), a direct relationshipof the damage threshold to the maximum fieldstrength value within the layer system is expected.As an example, an investigation in the thresholdbehavior of ion beam sputtered coating systems withdifferent field strength values is illustrated in Figure 9.In this experiment, the maximum field strength in thelast low index layer of SiO2 has been adjusted tofactors between 0.4 and 1.6 of the impinging fieldstrength (see upper diagram in Figure 9) by depositingdifferent designs on a basic 1-QWOT layer stack ofTiO2/SiO2. In the lower part of Figure 9, the S on 1LIDT values, measured with a fs-laser, are depictedfor selected pulse numbers N. For all layer systems,

Table 1 Laser induced damage thresholds of selected single-

layer coatings in relation to the melting points of the corresponding

bulk materials. In the last column, LIDT values of anti-reflective

coatings (AR-Coating) composed of the high index material and

SiO2 are compiled to illustrate the correlation between single LIDT

values and the damage threshold of layer systems

Material Melting point

( 8C)

LIDT value

(J/cm2) single layer

LIDT value

(J/cm2) AR-coating

TiO2 1775 13 ^ 1 14 ^ 1

Ta2O5 1918 28 ^ 2 32 ^ 5

HfO2 2758 41 ^ 3 46 ^ 6

ZrO2 2700 34 ^ 4 28 ^ 1

Al2O3 2072 39 ^ 1

SiO2 1723 34 ^ 7


Figure 7 Distribution of the power density in a high reflecting layer stack of SiO2/Ta2O5. The first nine layers of the system and a final

thick stabilizing layer of SiO2 next to the air interface are depicted. The incoming energy density is calibrated to 100%. Reproduced with

permission from Ristan D, Ebert J and Welling H (1989) Optische Beschichtungen fur Hochleistungslaser. Laser und Optoelektronik

21(4): S.53.

Figure 8 Distribution of power density in a high reflecting layer stack of SiO2/Ta2O5. The first nine layers of the system, which is

designed for reduced power density at the interface in the first layer pair are depicted. Reproduced with permission from Ristan D,

Ebert J and Welling H (1989) Optische Beschichtungen fur Hochleistungslaser. Laser und Optoelektronik 21(4): S.53.


which were deposited in separate deposition runs, astrong correlation between the maximum fieldstrength and the laser threshold is observed. Thisexperiment clearly demonstrates the role of theelectric field strength distribution in high-powercoatings and the potential of advanced thin filmdesign strategies.

Besides the properties of the coatings, the quality ofthe substrate has to be considered in respect to surfaceimperfections and to the polishing procedure. Forexample, substrates for the production of high-quality optics with enhanced power handling capa-bility in the VIS/NIR-spectral range should bepolished to a surface roughness of less than 1 nmrms

with a surface imperfection value 5=1 £ 0; 010according to ISO 10110. For an illustration ofthe effect of the polishing grade on the LIDTvalues, selected results are compiled in Table 2 forsubstrates of BK7-glass and high reflecting mirrors.

Substrates with the polishing Type I and Type II wereprocessed with powder of different grain diameter(2 mm and 3 mm) in conventional pitch polishing. Forthe LIDT values of the bare substrate and thecoatings, a clear relation to the surface roughnesscan be observed. Especially for optical coatings withsignificant transmittance, subsurface damage in thesubstrate has to be taken into account. As aconsequence of the chemical and mechanical inter-action of the surface with the polishing compounds,impurities and dislocation are introduced in thesurface structure resulting in a reduction of thedamage threshold.

Summary

In the course of the rapid development of lasertechnology, a large background in high power opticalcoatings had been built up during the last threedecades. Nowadays, the corresponding experience inthe production of high power coatings is mainlylocated at industrial companies and a few researchinstitutes, which are also involved in the characteriz-ation of optical coatings. For an illustration of thepresent state in thin film technology, damagethreshold values of advanced optical componentsare summarized in Table 3 for laser systems andoperation conditions often applied in industrialproduction environments.

Besides the spectacular experiments in laser fusion,isotope separation, and fundamental physics, indus-trial applications of lasers in material processing,medicine, information technology, and semiconduc-tor lithography are considered as major pacemakersfor the progress of high power optics. Therefore,trends in these technology fields will dominantlygovern the future development of thin film techno-logy. For example, in semiconductor lithography at awavelength of 157 nm, which would open the way tofeature sizes well below 100 nm, optical coatingswith extended lifetimes are still on rank 6 of the list ofchallenges to achieve an effective lithographic pro-duction facility. In next generation lithography, optics

Figure 9 Damage thresholds (lower diagram) and internal field

strength relative to the field strength of the impinging wave (upper

diagram). The damage thresholds were measured with an ultra-

short pulse laser operating at a repetition rate of 1 kHz, a pulse

duration of 150 fs, and a beam diameter of 100 mm on the sample

surface. Damage thresholds for selected numbers between N ¼

30 to N ¼ 30 000 of pulses are indicated by columns. The plotted

maximum field strength in the last SiO2-layer is adjusted on the

basis of design variation.

Table 2 Laser-induced damage threshold values of samples

polished using compounds of different grain size. Besides the

LIDT values of the uncoated surfaces, also data for anti-reflective

coatings of selected materials on these surfaces are summarized

Coating type LIDT value

(J/cm2) polishing type I

LIDT value

(J/cm2) polishing type II

Bare surface 76 ^ 1 68 ^ 12

Ta2O5/SiO2 44 ^ 2 38 ^ 1

HfO2/SiO2 47 ^ 16 35 ^ 5

Nd2O3/MgF2 34 ^ 6 24 ^ 8


of even smaller wavelengths, around 13 nm, have tobe optimized to achieve the 60 nm node. Ultra-shortpulse lasers gain importance as innovative tools formaterial processing, laser medicine, and biology, aswell as the analysis and control of chemical reactions.For the development of fs lasers needed in theseapplications, special high-power broadband coatingsystems are required. In addition to high LIDT values,these coatings have to fulfil demands with respect totheir group delay dispersion for the compensation ofdispersion effects in the laser systems. Other chal-lenges have to be expected from applications, where acombination of the high-power handling capabilitywith additional properties has to be achieved. As atypical example, high-power laser coatings withimproved mechanical or chemical stability forapplications in laser medicine can be considered.

List of Units and Nomenclature

dn=dt generation rate of electrons in theconduction band

g radius of a spherical inclusion; (cm)IðtÞ power density of laser radiation,k thermal conductivity; (W/(cm 8C))kE thermal conductivity of a spherical

inclusion; (W/(cm 8C))LIDT laser induced damage threshold

(J/cm2; W/cm2; W/cm)n0 refractive index of a spherical

inclusion; (1)P output power of a laser; (W)q correction factor representing inter-

ference effects in coatingsQ Mie absorption coefficient of a

spherical inclusion;

QWOT quarter wave optical thickness: unitfor the thickness of the layer

tI irradiation time for a cw-laser; (s)TðrÞ temperature in a spherical inclusion

at the radial position r; (8C)w diameter of a laser beam with

Gaussian profile; (cm)a impact ionization coefficient; (1)bm m-photon absorption coefficient; (1)bS surface absorption of an optical

component; (1)DT temperature rise in the center of an

irradiated circular spot; (8C)k thermal diffusivity; (cm2/s)kE thermal diffusivity of a spherical

inclusion; (cm2/s)l wavelength of a laser; (cm)

See also

Optical Coatings: Thin-Film Optical Coatings.

Further Reading

Bloembergen N (1973) Role of cracks, pores andabsorbing inclusions on laser damage thresholdof transparent dielectrics. Applied Optics 12(4):661–664.

Genin FY and Stolz CJ (1996) Morphologies of laserinduced damage in hafnia-silica multilayer mirror andpolarizer coatings, Third International Workshop onLaser Beam and Optics Characterization. In: Morin Mand Giesen A (eds). Proc. SPIE 2870, 439–448.

Guenther AH, et al. Proceedings of the Symposium onLaser Induced Damage in Optical Materials, NBS(NIST) Special Publication 1969–1989, SPIE vols.1989–2003. CD-Rom version of collected papers from1969 to 1998 available: Washington: SPIE.

Guenther KH, Ebert J, Kiesel E, et al. (1984) 1.06-m laserdamage of thin film optical coatings: a round-robinexperiment involving various pulse lengths and beamdiameters. Applied Optics 23: 3743.

ISO 11254 (2001) Test methods for laser induced damagethreshold of optical surfaces. Part 1: 1 on 1-test, 2000,Part 2: S on 1 test.

ISO 11254-3 (2002) Laser and laser-related equipment –Determination of laser-induced damage threshold ofoptical surfaces – Part 3: Assurance of laser powerhandling capabilities. Draft Standard. InternationalOrganization of Standardisation.

Jasapara J, Nampoothiri AVV, Rudolph W, Ristau D andWelsch E (2001) Femtosecond laser pulse inducedbreakdown in dielectric thin films. Physical Review B63(4): 045117.

Puttick K, Holm R, Ristau D, et al. (1997) Continuouswave CO2 laser induced damage thresholds in optical

Table 3 Laser-induced damage threshold values of selected

optical coating systems for laser applications (Types: HR: high

reflectingmirror,AR: antireflective coating, th: thermalevaporation,

IBS: ion beam sputtering)

Laser system

wavelength

Type Laser-induced damage

threshold ISO 11254

193 nm, ArF-excimer AR/th 1–2 J/cm2 (1 on 1, 20 ns)

HR/th 2–4 J/cm2 (1 on 1, 20 ns)

248 nm, KrF-excimer AR/th 10 J/cm2 (1 on 1, 30 ns)

HR/th .20 J/cm2 (1 on 1, 30 ns)

HR/IBS .3 J/cm2 (1 on 1, 30 ns)

1,064 mm, Nd:YAG AR/th .60 J/cm2 (12 ns, 0,25 mm)

HR/th .100 J/cm2 (12 ns, 0,25 mm)

HR/IBS .80 J/cm2 (12 ns, 0,25 mm)

10,6 mm, CO2-Laser AR/th .20 J/cm2 (100 ns, 1,4 mm)

.2 kJ/cm2 (1,2 ms, 250 mm)

.3 kW/mm (cw, 100 mm)

HR/th .25 J/cm2 (100 ns, 1,4 mm)

.2 kJ/cm2 (1,2 ms, 250 mm)


components, Proceedings of the Symposium on LaserInduced Damage in Optical Materials. SPIE vol. 3244,188–197.

Rudolph W, Mero M, Liu J, et al. (2003) Femtosecondpulse damage and pre-damage behavior of dielectricthin films, Proceedings of the 34th Annual Symposiumon Optical Materials for High Power Lasers. SPIE vol.4932, 202–215.

Walker TW, Vaidyanathan A, Guenther AH and Nielsen P(1979) Impurity breakdown in thin films, Pro-ceedings of the Symposium on Laser Induced Damagein Optical Materials. NBS Special Publication, No.568,479–495.

Wood R (1990) Laser Damage in Optical Materials.Bristol and New York: Adam Hilger. Reprint withcorrections.

Optical Black Surfaces

S M Pompea, National Optical AstronomyObservatory, Tucson, AZ, USA

S H McCall, Stellar Optics Research InternationalCorporation (SORIC), Thornhill, ON, Canada

q 2005, Elsevier Ltd. All Rights Reserved.

Introduction

Black surfaces play an important role in many, if notmost, optical systems. Optical instruments andsystems of all types often rely on black surfaces tohelp minimize the effect that stray or off-axis light canhave in degrading optical system performance. Thestrategic selection and placement of appropriateblack surfaces can often limit significant detrimentalstray light effects in an optical systems, therebydramatically improving system performance. Thehigh emissivity of black surfaces also gives them animportant role in the design and construction of blackbodies, calibration surfaces, and radiator surfaces.

The characterization and selection of black sur-faces is an important field of optics and optical systemdesign. However, the selection of black surfaces is aspecialized undertaking requiring careful study toensure the proper selection of particular surfaces foreach optical system. A black surface that works wellin one application may not be at all appropriate for adifferent application, where different systems per-formance goals are desired. The wavelength ofoperation of the system is an important considerationsince a surface which has low reflectance at onewavelength band may have different reflective proper-ties at another wavelength or range of wavelengths.Specialized measuring devices called goniophot-ometers are often used to characterize the reflectanceand scatter of black surfaces for a given wavelengthor set of wavelengths. Goniophotometric measure-ments can be used to create a specialized functionaldescription of reflectance and scatter off a surface,called the Bidirectional Reflectance DistributionFunction (BRDF) or Bidirectional Scatter Distri-bution Function (BSDF). In practice, having the

BRDF or BSDF of a surface is extremely valuablefor surface selection and is more useful than otherreflectance and scatter measurement measurementsor descriptions. Comparing the measured BRDF orBSDF of several surfaces helps to distinguish thegeneral optical properties of these surfaces and allowsa direct comparison of the value of these differentsurfaces in an optical system as they contribute tooverall system performance.

The BRDF distinguishes, at a particular wave-length, how a surface specularly reflects or how itscatters light in different directions when incidentlight from a particular direction relative to the surface(at a given angle of incidence) interacts with thesurface. By understanding the BRDF of a surface, theoptical designer can understand how the surfacebehaves in a given system location when illuminatedfrom a specific direction. The BRDF of a black surfaceis also important because it can be used as amathematical function in stray light analysis pro-grams to predict system performance. The BRDF andother properties of surfaces can be input into straylight codes so that the propagation of light from blacksurfaces can be modeled through Monte Carlo-basedpropagation models. Using these models, the straylight can be directed through the placement of blacksurfaces into directions less likely to degrade systemperformance. Thus a measure of stray light controlcan be gained through absorption by black surfacesor by redirection of the stray light into less criticaldirections.

Black surfaces may be considered a subset of a classof surfaces known as spectrally selective surfaces. Theterm ‘spectrally selective surface’ indicates that thespectral properties of many surfaces are differentwhen examined in various spectral regions. Forapplications such as the design of blackbodies,space radiators, and baffles, surfaces with uniquespectral emissive and reflection properties are needed.Large databases of spectrally selective surfaces areavailable to allow the optical designer flexibility inchoosing surfaces for applications where specificoptical reflectance and optical scatter properties

OPTICAL COATINGS / Optical Black Surfaces 349

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