1. Physics on femtoscale L. Nánai Univ. of Szeged, TTIK, Dept. of Exp. Phys. H-6720 SZEGED DÓM t 9 Summer School on Optics June 7-10, 2015 Siófok (H) 1

2. Fundamentals of laser-matter interactions Classical model Optical characteristic of materials Linear and nonlinear optics Material processing (general) Melting, damage, vaporization, surface, plasma formation Heat treatment Cleaing Laser ablation Laser induced surface patterning Gas and liquid phase processes Role of pulse duration (thermal vs photoinduced processes) 2

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4. General tasks of laser-matter interaction Surface modifications include oxidation/nitridation of metals, surface doping, etc. PLA: pulsed laser ablation PLD: pulsed laser deposition LA: laser annealing LC: laser cleaning LIS: laser-induced isotope separation/IR- laser photochemistry MPA (MPI): Multiphoton absorption (ionization) LSDW (LSCW): laser-supported detonation (combustion) LCVD (LCLD): laser-induced chemical vapour (liquid)-phase deposition LEC: laser-induced electrochemical plating/etching RED/OX: long pulse or cw CO 2 lase- induced reduction/oxidation 4

5. Interaction processes depend on many parameters The most important ones are: Laser Wavelength Pulse length Fluence (energy density) MaterialsOptical properties Surface structure Thermal characteristics Generally, the most important effect is heating, which can lead to phase transformation. 5

6. Light absorption Absorption in a medium with refraction index n  n 1 + in 2 is given by the Beer-Lambert lawI(z) = I 0 e -  z where I 0 is the light intensity at z = 0 and I(z) is the intensiy at depth z. The attenuation coefficient is  = -(1/I)(dI/dz)=2  n 2 /c=4  n 2 / Penetration depth is given by  -1 For UV radiation: dielectrics   1 cm -1 For metals and semiconductors  = (2-3)x10 6 cm -1 6

7. Light absorption THE PARTITION OF THE ABSORBED ENERGY IS NOT THERMAL AT FIRST LASER LIGHT PRODUCES: PARTICLE EXCESS ENERGY EXCITATION ENERGY OF BOUND ELECTRONS KINETIC ENERGY OF FREE ELECTRONS EXCESS PHONONS 7

8. Heat propagation Heath conduction equation Wherek=KV/c p heat diffusivity K: thermal conductivity I a =(1-R)I o : unreflected part of the incident irradiance 8

9. Free carriers FREE-CARRIER GENERATION IS THE MOST IMPORTANT SELF-INDUCED COUPLING EFFECT IN NON-METALS. THE EFFECTS OF FREE CARRIERS IS TO REDUCE THE REAL PART AND TO INCREASE THE IMAGINARY PART OF n. THIS INCREASES THE ABSORPTION COEFFICIENT  = 4  n 2 / 9

10. Free carriers IN SEMICONDUCTORS HOLES AND ELECTRONS ARE IN EQUAL NUMBERS. IT IS CONVENIENT TO TREAT THEM TOGETHER AND TO WRITE  =  0 +N eh  eh WHERE  0 is the lattice absorption coefficient N eh = N e = N h  eh = absorpt. cross section of a carrier pair  eh scales with 2, making the free-carrier absorption mainly relevant for infrared beams. 10

11. Evaporation and plasma formation EVAPORATION USUALLY OCCURS FROM A LIQUID. PHASE EQUILIBRIUM BETWEEN A MELT AND ITS VAPOR REQUIRES EQUALITIES OF THE FREE ENERGIES. SINCE VAPORS ARE COMPRESSIBLE, THE EQUILIBRIUM CONDITIONS DEPEND ON p TOO. THE CHANGE OF FREE ENERGY WITH T CAN BE WRITTEN AS IT MUST HOLD FOR EACH PHASE INDIVIDUALLY. AT EQUILIBRIUM THE PRESSURE INSIDE THE LIQUID AND THE VAPOR MUST BE THE SAME 11

12. Interaction of ultrashort laser pulses with solids Optical parameters n and k Role in excitation of electronic subsystem linear and nonlinear response of target Energy transfer – electromagnetic field e-e „thermalization” transfer to ionic subsystem role of „time scale” 12

13. Solids in Very Intense Laser Field Linear optics: validity of the superposition theory Nonlinear optics: at high enough intensities Limit (classical): (for H) 13

14. Optical properties and  (  ) For semiconductors: In solids:  (  ): electric dipole approximation 14

15. Metals Linear optics if I< 10 15 W/cm 2 Free carrier absorption (through inverse Bremsstrahlung)  =(5-10)*10 -5 cm -1 up to  -1 =10-20 nm - role of ballistic transport (v b ~ 10 6 m/s for 100 fs ~ 100-200 nm) - role of density of states (DOS) at Fermi level ballistic transport of non-thermalized electrons and diffusive transport of thermalized electrons Heat transport from electronic subsystem to initially cold lattice 15

16. Two temperature models (TTM) Nolte model: T p - excitation radius T l - relaxation radius Where C e, k e and C l, k l are the lattice and electron capacity and thermal conductivity, S(z,t) is the laser heating source term. 16

17.  e =C e /g and  l =C l /g are the electron cooling and lattice heating times  eq Caracteristic timescale of the thermal equilibration T eq final equilibration temperature 17

18. Comparison between nickel and gold surface temperatures dynamics, in the same conditions as previous fig. Temporal distribution of electron T e and lattice T l surface temperature for a copper target irradiated by a 120 fs, 800 nm pulse at a laser intensity I 0 =5x10 12 W/cm 2 18

19. Semiconductors One-photon excitation yielding electron-hole pairs (interband transition) if h < E g multiphoton abs. ~ I N Free carrier absorption Impact ionization Relaxation by several channels: - radiative - nonradiative 19

20. TTM for semiconductors: U o =C o T o and U a =C a T a are the optical and acoustic phonon energy gis the coupling coefficient  is the free carrier absorption coefficient 20

21. Dielectrics multiphoton transition avalanche ionization Two photon absorption 21

22. Temperature evolution The temperature, T, at a depth, x, below the surface of a material hit by an ultrashort laser pulse is governed by the Quantum Heat Transport equation: Two-part solution : Ballistic: Diffusive: v: thermal pulse propagation speed m: heat carrier mass  p : laser pulse duration  : mv 2 /2ħ H: Heaviside’s step function 22

23. Evolution of Carrier Densities The densities of the electrons and holes created when an ultrashort laser pulse hits a semiconductor were calculated using the following equation: I 0 : peak laser intensity (adjusted to get max carrier density of 10 18 1/cm 3 ) c: speed of lightn: index of refraction  p : FWHM pulse duration (Gaussian pulse)  absorption constant R: reflectivity (0.286)D: diffusivity  l : electron-hole pair lifetime 23

24. Dember Electric Field The diffusivity of the electrons is about 20 times larger than for the holes. The electrons diffuse faster into the material than the holes causing the net charge density to be positive near the surface and negative deeper into the material. This produces a strong „Dember” electric field that can be calculated as follows: e: elementary charge  0 : permittivity of free space  r : dielectric constant  h : density of holes  e : density of electrons 24

25. Solutions to the QHT equation for a 50 attosecond laser pulse 25

26. Solutions to the QHT equation for a 20 femtosecond laser pulse 26

27. Calculated density of holes (a) and electrons (b) created by a 50 as laser pulse 27

28. Density of holes (dark line) and electrons (light line) at the surface (up) and a depth of 500 nm below the surface (right) for a 20 fs laser pulse 28

29. Density of holes (dark line) and electrons (light line) at the surface (up) and a depth of 250 nm below the surface (right) for a 50 as laser pulse 29

30. Dember electric field versus depth in material for 20 fs laser pulses Dember electric field versus depth in material for 50 as laser pulses 30

31. Summary of temperature Evolution For a 50-as pulse: - Ballistic solution dominates - A short thermal pulse propagates into the medium For a 20-fs pulse: - Diffusive solution dominates - Temperature at the surface is high for a period of time approximately equal to the laser pulse duration 31

32. Structural Changes -Thermal model: rapid Equilibration of hot el-ns with lattice heating up to melting temperature (ns, ps) -Plasma model: destabilization of the covalent bonds due to electronic exitation (slow rate of phonon emission) (fs) 32

33. Pump and Probe measurements Ti:sapphire laser Output power: 400 mW Rep.rate: 90 MHz Energy/pulse: 4.4 nJ Central wavelength: 820 nm (FWHM: 43 nm) AOM freq.: 25 kHz Pump power: 25 mW Probe power: 2.5 mW 33

34. Reflectivity at different sample orienteation normalized 34

35. Fourier transformations of the scans 35

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38. Reflectivity at different TeO 2 sample orienteation 38

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40. Thank You For Your Attention ! 40