Optical components for high power and ultra-short laser illumination Laser damage issues. Laurent GALLAIS

Transcription

1 Optical components for high power and ultra-short laser illumination Laser damage issues Laurent GALLAIS

2 High power, ultrashort illumination Ultrashort 1fs=10-15s In 10fs: light travels 3µm In 1s: earth-moon distance High power Commercial laser peak power: GW, can reach TW Scientific laser systems: few PW Projects: Power EW Peak power Energy Time wikipedia

3 Laser damage Irreversible modification of a material submitted to laser irradiation Result of the coupling of laser light with material Energy Absorption Transfer Dissipation Subject is studied since the advent of lasers*,** *M. Hercher, Laser-induced damage in transparent media, JOSA (1964). **C. Giuliano, Laser-induced damage to transparent dielectric materials, JOSA (1964).

4 Outline of the presentation Introduction Some Issues of laser damage of optical components ILM team and research topics in this field High power laser matter interactions Basics of laser / matter interactions & damage effects Ultrashort high power laser / matter interactions Optical components for high power and ultra-short laser illumination Laser damage resistance of optical materials Optical interference filters Diffractive optics Influence of operational parameters Conclusions

5 Introduction Advent of fs lasers has open new opportunities and research fields for scientific, industrial and medical applications. Peak power and energy in laser systems is limited by the damage threshold of the optics in the laser chain Examples of current topics where optical resistance is an issue: High power commercial laser systems Lasers for spatial applications Fusion class lasers : LMJ, NIF, HIPER Lasers for high-field physics : ELI, Apollon, Vulcan Pulse compression grating for PetaWatt class laser (Apollon) ChemCam experiment on CURIOSITY (NASA/CNES) Commercial TW laser (Amplitude) 5

6 Introduction Examples of European projects The Extreme Light Infrastructure aims to host the most intense lasers world-wide 4 sites: Prague (Czech Republic), Szeged (Hungary) and Magurele (Romania)... <1fs, 200 PW 6

7 Introduction Examples of European projects The HIPER project The European High Power laser Energy Research facility, is dedicated to demonstrating laser driven fusion as a future energy source. 7

8 Introduction Some issues : Complexity of fs laser systems and optical components: Several wavelengths Different pulse durations High repetition rate Many materials and components Pockels cell Pump laser OUT IN Faraday rotator Amplifier Example of regenerative ps pulse amplifier 8

9 Introduction Some results of laser interaction with optical components... Understanding the mechanisms responsible for optical damage may allow higher-damage-threshold optics to be designed and manufactured for laser systems 9

10 Introduction Objectives of the lecture Introduction to the physical mechanisms responsible for the damage of optics Focus on the ultrashort regime Design of laser resistant optical components Some open questions in the field and research topics 10

11 Outline of the presentation Introduction Some Issues of laser damage of optical components ILM team and research topics in this field High power laser matter interactions Basics of laser / matter interactions & damage effects Ultrashort high power laser / matter interactions Optical components for high power and ultra-short laser illumination Laser damage resistance of optical materials Optical interference filters Diffractive optics Influence of operational parameters

12 ILM team of the Institut Fresnel Laser Matter Interactions Experimental and theoretical work to study the physics of laser damage phenomena Progress in the design, manufacturing of optical components for high power applications Laser processing 12

13 Outline of the presentation Introduction Some Issues of laser damage of optical components ILM team and research topics in this field High power laser matter interactions Basics of laser / matter interactions & damage effects Ultrashort high power laser / matter interactions Optical components for high power and ultra-short laser illumination Laser damage resistance of optical materials Optical interference filters Diffractive optics Influence of operational parameters

14 Physical mechanisms The degradation of optical materials is the results of several successive mechanisms Absorption of the energy Absorption fs Localized / intrinsic Linear / non-linear Energy transfer During / after irradiation Electrons / lattice Conduction, radiation Increase of temperature Thermo-mechanical stress Melting, vaporization Thermal runaway Ionisation of the material Plasma formation Mechanical effects Shock wave propagation ps Thermal effects ns Mechanical effects, hydrodynamics µs time 14

15 Physical mechanisms Physical mechanisms depend on material and irradiation conditions Material Laser Wavelenght Pulse duration Fluence (J/cm²) Intensity (W/cm²) Peak power (W) Repetition rate Optical properties absorption non-linear properties Electronic band structure Thermal properties Melting point Heat conduction and heat capacity Mechanical properties Environment Atmosphere and pressure Temperature Contamination issues 15

16 Physical mechanims These physical mechanisms have different time scale *S.K. Sundaram and E. Mazur, Nature Materials 1, 217 (2002) 16

17 Case of continuous irradiation, µs / ms pulses Interaction is thermo-mechanical Laser irradiation creates a heat source at the surface/in the volume of the component Absorption of the energy is driven by the absorption coefficient: = 4 n 2 S r, z, t = 1 R I r,t e z W / m3 n =n 1 n 2 = 1 /2 Heat propagates through conduction Evolution of temperature in space and time is obtained by solving the heat equation T C K T =S t Temperature increases up to the point where: Melting or sublimation of the material occurs Thermal runaway process Thermo-mechanical stress leading to cracks and failure Thermo-activated chemical reactions... 17

18 Case of ns pulses Interaction is a combination of photo-electronic, thermal, plasma, mechanical effects Damage is initiated by defects embedded in the optical materials Absorbing defect Absorbtion delocalisation Plasma Shockwave Damage Pulse duration hν Danileiko et al., Sov. J. Quant. Elec., 1978 e - Grua et al., Phys. Rev B,2003 T>Tcrit. Saito et al., Phys. Rev B,2000 Bude et al., Laser damage symposium

19 Case of ns pulses Simulations Surface Volume Bonneau et al., Appl. Phys. B

20 Case of ns pulses Observations Bulk silica Gallais et al., Appl. Opt Silica surface Bertussi et al., Opt. Express

21 Outline of the presentation Introduction Some Issues of laser damage of optical components ILM team and research topics in this field High power laser matter interactions Basics of laser / matter interactions & damage effects Ultrashort high power laser / matter interactions Optical components for high power and ultra-short laser illumination Laser damage resistance of optical materials Optical interference filters Diffractive optics Influence of operational parameters

22 Case of ultrashort pulses Ultrashort laser/matter interactions On a laser matter interaction point of view: Pulse is ultrashort when the pulse duration is << time needed for heat to transfer in the material There is no thermal equilibrium during the pulse Transition between short and long pulses: ~10ps B. Stuart et al., Phys. Rev. B,

23 Case of ultrashort interaction Mechanism in 2 separate steps: Energy deposition and transfer Response of the material Energy deposition Energy deposition on the electrons Initial free electrons in metals Free electrons created by ionisation in dielectrics Electrons thermalization, diffusion in the material Energy transfer Energy transfer from electrons to ions (Te=Ti after few ps) Response of the material Energy transport and dissipation Phase change Hydrodynamics motion Damage by differrent mechanisms (thermal or mechanical) depending on the deposited energy 23

24 Absorption in the case of metals Optical properties of metals Response of free electrons Drude model in first approximation 2 p Ne 2 =1 =1 0 m 2 Free electrons in the material strongly absorb the energy (Joule heating) 'hot' electrons Skin depth is ~10 to 100nm Thermalization of the electron gas is very fast: te-e~1fs Electronic temperature (Te) established in ~10fs Energy of the electrons to be transfered to the lattice te_i~1-100ps frozen/cold ions during the pulse Te>>Ti during the first ps 24

25 2 Temperatures model for the case of metals 2T model* description of transition phenomena in a nonequilibrium electron gas and a lattice under subpicosecond laser irradiation *Anisimov et al., JETP, 1974 Gold film irradited at 0.85J/cm², 500fs, 800nm Metal film (Pt) irradited at 0.3J/cm², 500fs, 1030nm 25

26 Ionisation and free electron generation in dielectrics Transparent material: no linear absorption of laser light Non-linear absorption mechanisms to deposit laser energy into the material Promotion of electrons from valence to conduction band Different possible mechanisms E(eV) Conduction band Valence band k Band structure of Al2O3 Simplified band structure (parabolic bands) 26

27 Ionisation and free electron generation in dielectrics Photo-ionization E Conduction band 2 regimes: multiphoton or tunnelling Multiphoton ionization Direct excitation of the e- by the laser field Number of photons: smallest k that sastifies kep>eg PI rate depends strongly on intensity The rate of photo-ionization is : Eg Valence band σk : multiphoton absorption coefficient for absorption of k photons ±2 k-1 2k 1-k σk ~ 10.(10 ) cm s Probability for k photon transition: Example: k Silica (Eg~9eV), Ti:Sa laser (1.5eV) Pk=NVσk[F/(Epτ)] τ => 6 photons absorption 27 k

28 Ionisation and free electron generation in dielectrics Photoionization : Tunnelling ionization regime Strong E-field of the laser suppress the Coulomb well that binds a valence electron to its parent atom Bound electron tunnels through the short barrier and becomes free E=0 *Mezel et al., Phys. Plasmas 17, E 28

29 Ionisation and free electron generation in dielectrics Photoionization: MPI and tunnel regimes The Keldysh theory* is mainly used to described and compute the PI rates. Assumptions: Monochromatic wave, parobolic bands Moderate E-field value high frequency *L.V. Keldysh,Soviet Physics JETP vol. 20, High E-field value Low frequency 29

30 Ionisation and free electron generation in dielectrics Avalanche Ionization : Free carrier absorption followed by impact ionization Conduction band E E E>Eg phonon Valence band k k 30

31 Ionisation and free electron generation in dielectrics Avalanche Ionization: Free carrier absorption followed by impact ionization Drude model applied to the description of electrons heating Absorption cross section of an electron: Avalanche ionization rate: Characteristics: Exponential growth Seed electrons are needed Time is needed for the free electron population to grow More complex and realistic models available in the litterature Sequential absorption and thermalization* Temporal dynamics of energy distribution**... *B. Rethfeld, PRL, 2004 **Kaiser et al., PRB 61,

32 Free electron generation in dielectrics: summary S. Guizard, A. Mouskeftaras

33 Free electron generation in dielectrics Rate Equation (SRE) First approximation ionization model Contribution of the different ionization mechanisms can be evaluated: Native and photo-activated electronic defects can facilitate the ionisation process 33

34 Metal-like behaviour of dielectrics Free electrons modifies the optical response of the material N can reach values of cm-3 Such electronic densities strongly affect the dielectric function of the material under irradiation The electronic density dependence of the refractive index can be described by the Drude model of free electron gas. Under irradiation material acquires a metal-like behaviour Stong coupling of laser energy deposition Critical electronic density (~1021cm-3) ω Laser =ω Plasma = ρ el e 2 ε 0 m* At this point it can be considered (almost) as a metal... 34

35 Dynamics of the excitation process Example of recent results Based on Time-resolved digital holographic microscopy* T. Balciunas et al., Opt. Lett. 33, 2008 Camera Probe: 515nm / 25fs pulse Objective Pump: 1030nm / 300fs pulse Reference Ta2O5 film Intensity Phase Delay -100fs 0fs 100fs 200fs 400fs 800fs 1.6ps 3.2ps 10ps 100ps 500ps 1ns 35

36 Damage mechanisms: charge effects Coulomb explosion Excess of + charges Electrostatic repulsion High density of charges Highly charged ions Non thermal effect Molecular Dynamics Simulations *Stoian et al, PRB 62, 2000 Z. Jurek et al 2004 Europhys. Lett

37 Damage mechanisms: thermal effects Energy transfers after the energy deposition process From electrons to ions Te=Ti, thermal equilibrium From the absorbing area to the surroundings Electron diffusion (<ps) Thermal conduction (ion-ion) Phase transitions Melting, vaporization Sublimation Thermal explosion Roth et al 2012, inside vol. 10,

38 Damage mechanisms: mechanical effects Rapid heating of the material will generate shock waves Compression and rarefaction waves (Gpa) Temporal scale is >1ns heating time is shorter than the time needed for the material to expand in response to the thermoelastic stresses. The relaxation of the laser-induced stresses can lead to cavitation and disruption of a liquid surface region mechanical fracture material ejection due to the laser-induced stresses NaCl crystal Metal film 38

39 Damage mechanisms Main mechanism is dependent on irradiation and material parameters Example of mixture materials (HfO2/SiO2) 39

40 Outline of the presentation Introduction Some Issues of laser damage of optical components ILM team and research topics in this field High power laser matter interactions Basics of laser / matter interactions & damage effects Ultrashort high power laser / matter interactions Optical components for high power and ultra-short laser illumination Laser damage resistance of optical materials Optical interference filters Diffractive optics Influence of operational parameters Conclusions

41 Materials and components for high power fs lasers Context : Ultrafast high power laser Specific and complex components are requested Laser damage resistance, Bandwidth, dispersion, large dimensions, vacuum,... Oscillator Compression Experiment Stretching Amplification 41

42 Materials and components for high power fs lasers Specific multilayer coatings required for ultrafast high power laser Example : Requirements of mirrors for the Apollon laser R>99.5% Bandwith : nm AOI : 45 (P polarisation) GDD : <30fs² LIDT : >1J/cm² Large optics (215x350mm), vacuum 10PW 42

43 Outline of the presentation Introduction Some Issues of laser damage of optical components ILM team and research topics in this field High power laser matter interactions Basics of laser / matter interactions & damage effects Ultrashort high power laser / matter interactions Optical components for high power and ultra-short laser illumination Laser damage resistance of optical materials Optical interference filters Diffractive optics Influence of operational parameters Conclusions

44 Laser damage resistance of optical materials How to measure optical resistance? IF system for the metrology of laser damage effects 3ps-70fs ; 10Hz-100kHz 1030 / 515 / 343nm BS HR HR L CCD HR HR HR SSA LASER Sh HR W HeNe LASER Mangote et al., Rev. Sci. Instrum. 83, 2012 L BS ND HR BS P Py1 BS z S F M y x Py2 44

45 Laser damage resistance of optical materials LIDT is deterministic Example: measurement on a HfO2 single layer coating Single pulses 30 sites / energy ΔF/F=+/-0.6% 3,01 J/cm² 3,07 J/cm² 3,14 J/cm² 3,23 J/cm² 45

46 Laser damage resistance of optical materials Case of oxide films: measurements 4.5 Damage threshold : fs -1030nm 3.5 SiO 2 Nb2O5 ZrO 2 Ta2O 5 Al2O3 HfO2 Sc2O 3 Linear fit Param eters from [1] Error bars from [1] Threshold (J/cm²) Bandgap: Tauc-Lorentz model Bandgap (ev) 8 9 [1] Mero et al., Phys. Rev. B 71, 2005 LIDT=(a+bEg)*t^0.3 46

47 Laser damage resistance of optical materials Case of oxide films: simulations S io 2 Nb 2 O5 Z ro 2 T a 2O 5 A l2 O 3 HfO 2 S c 2O 3 S im u la tion S im u la tion w ith m + /-20% Threshold (J/cm²) Mangote et al., Opt. Lett.37, B an dg ap (e V) 47

48 Laser damage resistance of optical materials Damage is linked to intrinsic properties of the materials These properties can be different depending on the deposition conditions Examples : HfO2 films tested at I.F. 5 different manufacturers, 12 different deposition process pulse 10 pulses 100 pulses 1000 pulses LIDT (J/cm²) Sample 48

49 Laser damage resistance of optical materials Case of bulk materials

50 Laser damage resistance of optical materials Comparison with bulk materials SiO2 CaF2 MgF2 Al2O3 KBr ZnS Ge Pt Si NaCl ZnSe

51 Laser damage resistance of optical materials Case of hybrid materials Mangote et al., Opt. Lett.37, 2012

52 Outline of the presentation Introduction Some Issues of laser damage of optical components ILM team and research topics in this field High power laser matter interactions Basics of laser / matter interactions & damage effects Ultrashort high power laser / matter interactions Optical components for high power and ultra-short laser illumination Laser damage resistance of optical materials Optical interference filters Diffractive optics Influence of operational parameters Conclusions

53 Optical interference filters Principles of Optical Interfence Coatings Specificity of Optical Interfence Coatings Stratified system Interference effects Different materials Properties dependence with the deposition parameters 53

54 Optical interference filters Optical interference filters in ultrafast laser systems AR, HR, Polarizers, Pulse Stretching or compression Manage GDD and large spectral bandwidth of fs pulses Often the weakest part of the laser system... local intensity enhancements due to interference effects Exemple of a dispersive mirror: *V. Pervak et al.,opt. Express 16,

55 Optical interference filters Example E ² distribution Electronic density... Electronic densitiy (cm-3)... E/Einc ² Gallais et al., Appl. Phys. Lett. 97,

56 Optical interference filters Example... E ² distribution Electronic density... Electronic densitiy (cm-3)... E/Einc ² Gallais et al., Appl. Phys. Lett. 97,

57 Optical interference filters Example... E ² distribution Electronic density... Electronic densitiy (cm-3)... E/Einc ² Gallais et al., Appl. Phys. Lett. 97,

58 Optical interference filters Example... E ² distribution Electronic density... Electronic densitiy (cm-3)... E/Einc ² Gallais et al., Appl. Phys. Lett. 97,

59 Optical interference filters Example... E ² distribution Electronic density... Electronic densitiy (cm-3)... E/Einc ² Gallais et al., Appl. Phys. Lett. 97,

60 Optical interference filters Example... E ² distribution Electronic density... Electronic densitiy (cm-3)... E/Einc ² Gallais et al., Appl. Phys. Lett. 97,

61 Optical interference filters Example... E ² distribution Electronic density... Electronic densitiy (cm-3)... E/Einc ² Gallais et al., Appl. Phys. Lett. 97,

62 Optical interference filters Example... E ² distribution Electronic density... Electronic densitiy (cm-3)... E/Einc ² Gallais et al., Appl. Phys. Lett. 97,

63 Optical interference filters Example... E ² distribution Electronic density... Electronic densitiy (cm-3)... E/Einc ² Gallais et al., Appl. Phys. Lett. 97,

64 Optical interference filters Example... E ² distribution Electronic density... Electronic densitiy (cm-3)... E/Einc ² Gallais et al., Appl. Phys. Lett. 97,

65 Optical interference filters Knowing the E field distribution in the stack allows the prediction of the LIDT of the stack from the single layers LIDT Example: Comparison single layer LIDT / multilayer stack LIDT 4 different HfO2 single layers, tested with AOI 0 4 different mirrors with HfO2 in the design, AOI 45, S polarisation Air SiO2 HfO Mirror LIDT (J/cm²) pulse 1000 pulse R-on-1 tests LIDT(mirror)=LIDT(HfO 2)/ HfO 2 single layer LIDT 65

66 Optical interference filters High LIDT interference filters High bandgap materials Optimize design to reduce intensity in the weakest film Jupe et al., Laser damage symposium, SPIE 6403,

67 Optical interference filters Comparative study R-max (99,5%) at 800nm, tested at 200 fs : C.J. Stolz et al., «Thin Film Femtosecond Laser Damage Competition», Laser Damage Symposium, 2010

68 Optical interference filters Mixed Metal Multilayer Dielectrics (MMLD) Interest I Large spectral bandwidth Low dispersion Low number of layer Problem t Absorbing metalfs,film ps Ex : Subtrate/(metal)/(SiO2/HfO2)N/HfO2 x Air 0 Substrate... z H L Metal 68

69 Outline of the presentation Introduction Some Issues of laser damage of optical components ILM team and research topics in this field High power laser matter interactions Basics of laser / matter interactions & damage effects Ultrashort high power laser / matter interactions Optical components for high power and ultra-short laser illumination Laser damage resistance of optical materials Optical interference filters Diffractive optics Influence of operational parameters Conclusions

70 Diffractive optics Gratings are used for pulse compression in high power laser chains High diffraction efficiency Spectral range Minimize wavefront distorsion High LIDT 70

71 Diffractive optics Gratings for pulse compression MLD or MMLD gratings Perry et al.,optics Letters 20, 940 (1995) Bonod et al.,optics Communications 260 (2006) 71

72 Diffractive optics Gratings for pulse compression: Examples of very large gratings for Petawatt pulse compression LLNL Plymouth gratings LLE 72

73 Outline of the presentation Introduction Some Issues of laser damage of optical components ILM team and research topics in this field High power laser matter interactions Basics of laser / matter interactions & damage effects Ultrashort high power laser / matter interactions Optical components for high power and ultra-short laser illumination Laser damage resistance of optical materials Optical interference filters Diffractive optics Influence of operational parameters Conclusions

74 Pulse duration 2 Threshold fluence (J/cm ) 10 Bulk materials Optical thin films 1 10 B. Stuart et al., Phys. Rev. B, 1996 SiO2 Al2O3 HfO2 Ta2O5 TiO2 100 Pulse duration (fs) 1000 M. Mero et al., Phys. Rev. B,

75 Wavelength Wavelength Photo-ionization rate increase with the decrease of λ Impact ionization is also dependent on λ Bulk SiO2 and CaF2 (150fs) Thin film TiO2 (800nm, 130fs) T. Jia et al., Phys. Rev. B 73, 2006 M. Jupe et al., Optics Express 17,

76 Multiple pulses Photo-induced defects Electronic defects in HfO2 Coating HfO2 (800nm) L.Emmert et al., J. Appl. Phys A. Foster et al., Phys. Rev. B

77 Multiple pulses Thermal effects Cooling after a pulse : Air Optical window At high frequency the material has not enough time to cool down to the ambient between 2 pulses 77

78 Outline of the presentation Introduction Some Issues of laser damage of optical components ILM team and research topics in this field High power laser matter interactions Basics of laser / matter interactions & damage effects Ultrashort high power laser / matter interactions Optical components for high power and ultra-short laser illumination Laser damage resistance of optical materials Optical interference filters Diffractive optics Influence of operational parameters Conclusions

79 Conclusions The basic ultrashort laser interaction with optical materials have been described Band structure / electronic properties of the material and E-field value drive the process There are some intrinsic limits on the resistance of materials In optical components E-field enhancement need to be managed to improve LIDT Macroscopic defect than can amplified the E-field can also have some effects Following the physical origin of damage effects, scaling laws can be applied to compare and report LIDT to one application LIDT=f(t), LIDT=f(n), LIDT=f(Eg),... On fundamental and practical aspects, the physics of damage effect is still the subject of many investigations and follows the development of lasers and laser applications 79

80 Thank you for your attention 80