Laser-Induced Surface Damage of Optical Materials: Absorption Sources, Initiation, Growth, and Mitigation

Transcription

1 Laser-Induced Surface Damage of Optical Materials: Absorption Sources, Initiation, Growth, and Mitigation 100 nm 1 mm S. Papernov and A. W. Schmid University of Rochester Laboratory for Laser Energetics Boulder Damage Symposium Boulder, CO 22 September 2008

2 Outline Surface damage versus bulk damage Surface damage initiated by continuous (nonlocal) absorption mechanisms of surface damage in metals and semiconductors single-pulse and multiple-pulse damage, laser-spot-size dependence Surface damage initiated by localized absorption sources of localized absorption at dielectric-material surfaces physics of localized defect-driven damage model glass systems with embedded artificial absorbers glass-surface-damage morphology and modified structure damage growth entrance versus exit surface damage surface damage by adsorbed metallic particles Improving damage resistance of bare glass surfaces laser conditioning and advanced surface processing damage mitigation

3 Higher susceptibility to laser damage of optical material surfaces, compared to bulk, stems from technological surface processing, different laser-energy deposition, and dissipation Chemo-mechanical surface processing (grinding, polishing), necessary to satisfy wavefront and roughness requirements, leads to structural modification (chemical content, subsurface cracks, embedded particles) of near-surface material. Such structural modification significantly reduces mechanical stability, enhances near-surface absorption, and therefore lowers damage resistance. Proximity of the free surface can lead to the enhancement of the local laser-field intensity through interference of the incident and reflected waves. Pulsed-energy deposition caused by localized absorption is accompanied by shock-wave (compressive-wave) formation. Reflection of shock wave from the free surface generates rarefaction wave with tensile stresses. Since tensile material strength is usually much lower then compressive, it might lead to early material failure. G8607

4 Metals and semiconductors show significant continuous absorption in the UV and visible portion of the spectrum Visible and infrared reflectance of certain metals Absorption of various semiconductors at 300 K G8549

5 Metal surface damage morphology evolution CO 2, 10.6 nm, 200 ns Diamond-turned Cu mirror, m = 1064 nm; 10 ns G8570

6 Plastic deformation (slip) represent the earliest stage of damage on metal surfaces Diamond-machined Cu m = 1064 nm, F = 3 J/cm 2 20 ns, N = 3000 Schematic presentation of slip G8571

7 Ripple pattern is universal damage morphology observed on metal, semiconductor, and dielectric surfaces Cu mirror m = 10.6 nm, 100 ns Cd film on SiO 2 m = 257 nm, cw Ripples resulted from the interference between the incident and scattered surface waves. Fringe spacing d á m, or m/n for H = 0 G8572 Ge, m = 1.06 nm, 20 ns KCl, m = 10.6 nm, 150 ns

8 Threshold laser spot-size dependence can be different for single- and multiple-pulse irradiation 22 nsec Single pulse N = 10 4 G8573 Single-pulse damage depends on the probability of finding the most susceptible defect. Multiple irradiation causes an accumulation of stress and plastic deformation (slip). This process depends on thermal gradient.

9 Femtosecond laser interaction with materials is very localized m = 780 nm, 150 fs, 0.7 mj, Polished copper m = 527 nm, 600 fs, 7.5 nj, Corning 0211 glass G8569

10 Subsurface damage (cracks and deformation) is a major source of absorption and laser-induced damage to 1.0 nm 1 to 100 nm 1 to 200 nm Polished layer Defect layer Deformed layer Defectfree bulk G8564

11 Thermal approach to localized absorber-driven damage is based on reaching critical temperature T C One-dimensional thermal-diffusion model The temperature of the matrix surrounding absorbing particle can be found from the heat equation: 2 2T/ 2t = Dd T The most dangerous absorber size is 2a = 3. 6 Dx with boundary condition at r = a (particle radius): ai(t) = Z4k( 2T/ 2r) + 4/ 3 tca( 2T/ 2t) r= a r= a where Particle Matrix a: absorptivity D: thermal diffusivity t: mass density k: thermal conductivity C: heat capacity DELPOR calculations of threshold F th (m,x,z) T C = T m Fth = 6.3 Tc k x a D Pulse-length scaling: F th ~ x G8555

12 Damage initiated by localized absorbers is probabilistic by its nature Probability of damage, P, may be expressed as follows: P(F 0 ) = 1 (F 0 /T) (d S/2), where F 0 is a maximum fluence, S is the beam area at e 2 level, T is the threshold, and d is the absorber surface density. The best fit of experimental P curves provides data on T and d. G8554

13 Surface craters are the main morphological features of the damaged material surface; numerical modeling of crater formation provides insight into the mechanisms of crater formation DYNA 2-D code m = 351 nm, x = 3 ns, F = 10 J/cm 2 Ceria particle, z = 100 nm, h = 300 nm Fracture propagation initiated by pulse laser heating of near-surface particles is one of the crater-formation scenarios DELPOR + 1-D code, electromagnetic and thermal effects HESIONE 2-D/3-D code, fracture propagation G8553 Crack propagation around a 1-nm-Al inclusion embedded in fused silica, m = 351 nm, 3-ns square pulse, 20 J/cm 2

14 Phenomenological theory of crater formation based on thermal-explosion mechanism makes it possible to avoid complex computations and provides crater-size estimates Main assumptions: reaching critical temperature in the vicinity of the absorbing defect leading to ionization of the surrounding matrix the absorbed energy is concentrated in a small fireball whose radius grows exponentially with laser fluence F: a = a 0 exp c, c ~ F At high laser fluences, the plasma ball diameter reaches a maximum value of the order of m. Crater radius (R/h d ) Lodging depth (h/h d ) Theory (Eq. 3) 1.2 Absorbed energy can be estimated as E = Frm 2 The main results of this model: crater radius R as a function of the burial depth h is: R 2 = h 2/3 (h d 4/3 h 4/3 ) For an explosion with fixed energy E, there is a depth h m for which the crater has maximum size Rm= 2hm; hm hd G8551

15 Introduction of experimental model glass systems with well-characterized artificial near-surface absorbers (gold nanoparticles) allowed to clarify several important features of the absorber driver damage Gold nanoparticles SiO 2 film SiO 2 film h 2 h 1 Main results: G8550 Fused-silica substrate (Corning 7980, cleaved glass) Even a few nm-sized gold particles can significantly reduce intrinsic UV, nanosecond-pulse damage thresholds of SiO 2 surface Irradiation at subthreshold fluences may cause dispersion of the absorbers and partial diffusion of gold into surrounding matrix as indicated by photothermal microscopy Comparison of energy absorbed by gold particle, E abs, (Mie theory), with energy required for crater formation, E CR, (AFM analysis), gave an unambiguous result: - E abs % E CR & absorption spreads beyond absorbing particle volume into surrounding matrix- direct evidence of the plasma ball formation

16 Three possible mechanisms of the fused-silica matrix conversion into the absorbing medium are identified I II III UV photons e T > T CR e e Blackbody radiation from heated defect E ho > E band gap Thermionic emission of electrons Band-gap collapse due to heat conduction from particle T CR ~ 2000 K G8552

17 Two types of damage-crater morphology are found in model systems; one is linked to melting and evaporation and the other is complemented by shock-wave generated fracture m = 355 nm, x = 3 ns F = 6 J/cm 2, 600-nm gold m = 351 nm, x = 0.5 ns, 19-nm gold Regular crater Complex crater Experiment Theory h = 2 nm h = 5 nm 0.5 h = 60 nm μm μm h = 190 nm Shockwave G8556

18 Fused-silica damage morphology reveals explosive melting, evaporation, and fracture mechanisms G8568

19 Morphology of craters produced in fused silica by UV-pulsed laser irradiation points to material modification being highly susceptible to damage growth G8560 Morphology crater diameter: 1 to ~100 nm, dependent on fluence, number of pulses, and surface type z/d aspect ratio ~0.23±0.05 molten silica core surrounded by a near-concentric fractured shell Cracks macro, micro, and nano in size cracks may be a key defect structure responsible for damage growth Compaction layer (~20% denser) on crater floor and wall, ~10 nm thick high concentration of ODC and highly strained region Point defects NBOHC, localized in fractured shell ODC, localized in molten core STE

20 Fused silica surface damage grows exponentially at the exit surface and linearly at the entrance surface Laser Plasma Plasma Comparison of growth on exit versus input at 10 J/cm 2 Laser G8559 Final microscope images, surface and side views, of a site grown on the input (a) and a site grown on the exit (b)

21 Entrance surface damage versus exit surface damage; the problem is not trivial Standard approach E 2 Eexit 2 entrance 2 4n = ] n + 1g 2 n > 1 & T < T exit entrance For thick (>20-mm) fused-silica samples, damage probability is strongly linked to pulse coherence G8574 The SBS process strongly contributes to entrance surface damage in the case of single-mode pulses In the case of multi-mode pulses, SBS is suppressed and exit surface damage is dominant

22 Entrance surface damage versus exit surface damage; the problem is not trivial (continued) I/I 0 (%) nm depth Front irradiation Back 40 irradiation Air 60 nm depth Distance from silica/air interface (nm) Particle diam/ location Threshold J/cm 2 Normalized threshold J/cm 2 Threshold ratio T front T back T n front T n back T n back /T n front 8 nm 60 nm, 1.14± ± ± ± ±0.11 E low 14 nm 120 nm, 0.63± ± ± ± ±0.28 E high Front irradiation Laser Air Plasma fireball Absorbing defect Plasma ball grows toward film surface Film Crater profile Substrate Plasma ball grows away from film surface Laser G8558 Back irradiation

23 Laser removal of highly absorbing particles from glass surfaces without damage is based on tuning the irradation regime Schematic of a sample containing 50-nm-sq Al dots Topographical images of the 50- and 5-nm-wide dots after laser irradiation at 15 J/cm 2, 1064 nm, 6.5 ns Images of three 50-nm-wide dots after irradiation 5 J/cm J/cm 2 ( 100) ( 10) 40 J/cm 2 ( 2) 15 J/cm J/cm 2 ( 100) The most important factor here: fluence level on first irradiation G J/cm J/cm 2 ( 100)

24 UV-laser conditioning is essential for extending the life of fused-silica optics; is thermal conditioning feasable? Conditioning laser 355 nm, 7.5 ns 3 Raster scans at 4 J/cm 2, 6 J/cm 2, and 8 J/cm 2 Thermal conditioning with 10.6-nm CO 2 lasers 355 nm, 7.5 ns test G8567

25 Fused silica advanced surface finishing and laser conditioning dramatically improve damage thresholds Damage density observed at 355 nm, 3 ns, on 15.2-cm diam by 1.0-cm-thick fused-silica samples polished by Zygo using conventional polishing techniques and final polish using MRF to remove 1 nm of material. MRF: how it works G8565

26 Successful mitigation of FS glass surface damage using a CO 2 laser has been demonstrated Damage sites with ~5-nm lateral size and 0.5-nm depth can be mitigated using a 10.6-nm CO 2 laser in the melting regime m = 10.6 nm Spot size = 125 nm t = 200 ms F ~ 90% T ablation T ablation ~ 35 kw/cm 2 G8563

27 Mitigation of large (up to 500-nm) damage sites with 10.6-nm CO 2 laser requires ablative regime 750 nm 850 nm m = 10.6 nm, 10 ns- cw, f = 5 KHz Max Mitigated Site Lateral size: 360 nm Depth: 60 nm Drawbacks Downstream intensification creates hot spots. G8562

28 Best mitigation results achieved with 4.6-nm CO 2 laser; long absorption length is a key factor Micrograph following 4.5 W step Micrograph following 5.5 W step m = 4.6 nm, 22 ns, f = 75 KHz Chart of intensification at positions downstream from the mitigation pit Micrograph following 6.0 W step Micrograph following 6.25 W step G8561 Max Mitigated Site Lateral size: 500 nm Depth: 200 nm m = 335 nm, 7.5 ns

29 KDP/DKDP surface damage can be successfully mitigated using a diamond tool Dimple aspect ratio length/width/depth = 30:6:1 G8566

30 Summary/Conclusions Significant progress has been made both in understanding the physics of laser-induced damage to optical material surfaces and increasing surface-damage resistance through more sophisticated technological processing. Recently developed techniques for surface laser conditioning, contamination removal, and damage mitigation significantly extend the lifetime of optical components on high-power laser systems. Further challenges, related to ever increasing laser-power densities, can be met only through better understanding of damage mechanisms. Progress in detailed theoretical modeling of extrinsically and intrinsically driven damage is currently hampered by a lack of information about material properties at elevated temperatures and pressures. Generation of such information is of crucial importance. An experimental challenge to overcome is the characterization of micro- and nanoscale absorbers distributed with very low densities. Dramatic improvement in sensitivity of existing (photothermal, for instance) techniques and the invention of new methods is required. G8608