What is Laser Ablation? Mass removal by coupling laser energy to a target material

Laser Ablation Fundamentals & Applications Samuel S. Mao Department of Mechanical Engineering University of California at Berkeley Advanced Energy Technology Department March 1, 25

Laser Ablation What is Laser Ablation? Mass removal by coupling laser energy to a target material

laser ablation Is it important? Film deposition * oxide/superconductor films * nanocrystals/nanotubes substrate material plume target Materials characterization * semiconductor doping profiling * solid state chemical analysis plasma lens mass spectrometer Micro structuring * direct wave guide writing * 3D micro fabrication target target microstructure optical spectrometer transparent solid

laser ablation Is it important? Film deposition * oxide/superconductor films * nanocrystals/nanotubes Materials characterization * semiconductor doping profiling * solid state chemical analysis 1 µm Micro structuring * direct wave guide writing * 3D micro fabrication

laser ablation Do we really understand? laser beam target plasma Laser ablation is still largely unexplored at the fundamental level. J. C. Miller & R. F. Haglund, Laser Ablation and Desorption (Academic, New York, 1998)

What is happening? Laser Ablation laser pulse target femtosecond picosecond nanosecond microsecond 1-15 s 1-12 s 1-9 s 1-6 s

Experiments - ultrafast imaging Pump-probe technique ablation laser beam pump beam probe beam delay time CCD? target imaging laser beam

1 fs = 1 15 s Femtosecond Time Scale laser pulse Ultrafast imaging - time dependent energy transfer air glass 1 µm electronic excitation e-h plasma C V self-focusing 1 fs,8 nm E = 3 µj

Femtosecond Time Scale Electron number density n e time/space dependence z laser pulse electron number density (1 19 cm -3 ) 5 5 5 5 5 5 5 n e ~ 1 19 cm -3 z (µm) fs 333 fs 667 fs 1 fs 1333 fs 1667 fs 2 fs 1 2 3 4 5 Transmittance (probe beam) I I Absorption coefficient α = ω τ nc ω 2 p 2 2 1+ ω τ Plasma frequency 2 nee m ε d p = = e αd e

Femtosecond Time Scale Peak electron number density n e - time dependence electron number density (1 19 cm -3 ) 5 fs 5 333 fs 5 667 fs 5 1 fs 5 1333 fs 5 1667 fs 5 2 fs 1 2 3 4 5 z (µm) N e, max (cm -3 ) 6x1 19 5x1 19 4x1 19 3x1 19 2x1 19 1x1 19 5 1 15 2 25 time (fs) no breakdown flattened peak electron number density

Femtosecond Time Scale laser pulse Fundamental processes Nonlinear optics Self-focusing - intensity dependence of refractive index positive refractive index change Nonlinear absorption Electronic excitation - interband absorption z C negative refractive index change V suppress self-focusing

Femtosecond Time Scale Propagation of electronic excitation in glass 1 fs,8 nm E = 3 µj z (µm) 5 45 4 35 3 25 2 15 1 5-5 25 µj 12 µj 6 µj 3 µj 12 µj v = 1.8x1 8 m/s n = 1.65-5 5 1 15 2 25 t (fs) [Appl. Phys. A 79, 1695 179 (24)]

What is happening? Laser Ablation laser pulse target femtosecond picosecond nanosecond microsecond

1ps= 1 12 s Picosecond Time Scale Picosecond imaging ablation laser pulse t = 5 ps t = 5 ps 5 µm (air) 5 µm target Cu (laser pulse: 35 ps, fluence: 6 J/cm 2 ) (laser pulse: 35 ps, fluence: 9 J/cm 2 )

Picosecond Time Scale Threshold behavior regime-1 plasma onset regime-2 25 5 µm ablation depth (µm) 2 15 1 5 1 2 4 J/cm 2 85 J/cm 2 11 J/cm 2 (laser pulse length 35 ps; pictures taken at 2 ps) 1 2 3 4 5 laser fluence (J/cm 2 ) Threshold for picosecond plasma formation same as the threshold for ablation efficiency reduction: ~ 85 J/cm 2 (laser fluence) ~ 1 12 W/cm 2 (power density) (threshold for direct laser-induced air breakdown: ~ 1 13 W/cm 2 )

Ultrafast interferometry Picosecond Time Scale t = 15 ps interference pattern z 5 µm target r

Electron number density Picosecond Time Scale 1.2x1 2 1.x1 2 z t = 15 ps N e (cm -3 ) air density 8.x1 19 6.x1 19 4.x1 19 2.x1 19. 5 1 15 2 25 Z (µm) A large electron number density! (close to target surface)

Longitudinal (z) expansion Picosecond Time Scale longitudinal plasma extent vs. time 5 4 z 5 µm z (µm) 3 2 1 (laser energy: 1 mj) 2 4 6 8 1 12 t (ps) longitudinal expansion is suppressed (t > 5 ps)!

Picosecond Time Scale Lateral (r) expansion (t > 5 ps: expansion only in lateral direction) lateral plasma radius vs. time 5 4 r (µm) 3 2 1 r 5 1 15 2 25 (laser energy: 1 mj) t (ps) lateral expansion follows a power law!

Picosecond Time Scale Energy deposition to picosecond plasma r (µm) 1 1 t 1/2 power law 1 1 1 1 r similarity relation (2D blast wave - line energy source) E ρ o 2 Distribution of laser energy (1%): 1 ~ 5% absorbed by the picosecond micro-plasma 5 µm 4 J/cm 2 time (ps) t 1/2 1. mj 7.5 mj 85 J/cm 2 11 J/cm 2 1/ 4 1/ 2 ~ 5% reaching target surface plasma onset t E: energy deposition density (laser axis) ρ : ambient gas (air) density ablation depth (µm) 25 15 5 reduced efficiency 1 2 3 4 5 laser fluence (J/cm 2 )

Picosecond Time Scale Theoretical model (laser-solid-gas interaction) laser beam electron ion (gas) atom (gas) gas (1atm) photon Cu atom Cu before after laser irradiation electron laser heating of target (metal) electron heating - absorption of laser energy lattice heating - electron-phonon collisions plasma development above target surface electron emission (seed) impact ionization of gas

1 ns = 1 9 s Nanosecond Time Scale Plasma evolution picosecond to nanosecond time dependence (35 ps, 7 mj) laser energy dependence (35 ps, 2 ns)

Plasma development Nanosecond Time Scale plasma advancement: ~ 1 6 cm/s ~ 1 µm every 1 ns (1 ps) laser pulse shock wave plasma vapor target solid

Nanosecond Time Scale Plasma shielding nanosecond laser Theory without plasma 1 Experiment 2 GW/cm 2 3 GW/cm 2 ablation depth (µm) 1.1 solid 1 2 3 4 5 6 7 8 9 11112 t (ns) 1 GW/cm 2 3 ns, 164 nm laser ablation of Si (single pulse) 1 1 laser fluence (J/cm 2 ) 25 ns, 248 nm laser ablation of Cu (single pulse, in air, 1 µm spot diameter)

1 µs = 1 6 s Microsecond Time Scale ) Plume evolution z below threshold 1 ns 64 ns 16 ns 76 ns 1.6 µs 4.9 µs 1 µm (3 ns, 1.8x11 W/cm2) z above threshold 5 ns 7 ns 2 ns 86 ns 1.3 µs 4.2 µs 1 µm (3 ns, 2.1x11 W/cm2) University of California at Berkeley Q

Threshold behavior Microsecond Time Scale ablation depth (µm) +2.. -2. -4. -6. 18 GW/cm 2 4.9 µs ablation depth (µm) 25 2 15 1 5 ablation depth (µm) +2.. -2. -4. -6. 21 GW/cm 2 4.2 µs 1 1 1 11 laser intensity (W/cm 2 )

Theoretical model Microsecond Time Scale superheated liquid layer ~ 1 ns solid ~ 1 ns solid ~ 1 µs solid Normal vaporization (Hertz-Knudsen equations) x t = βp m (2πmk ρ T) L exp[ k m 1 ( T 1/ 2 ev b B x= B b T ablation below threshold: normal evaporation Explosive boiling (heat diffusion T max ~ T c ) T T ρc = ( k ) + αi laser exp( αx) t x x 1 )] ablation above threshold: normal evaporation and explosive boiling ablation depth (µm) 2 15 1 5 experiment theory 1 9 1 1 1 11 laser intensity (W/cm 2 )

Fundamental processes Laser Ablation ps ns µs fs laser µs boiling droplets electronic plasma plasma vapor electronic liquid excitation heated zone ns ps radiation ionization (shock wave) vaporization convection melting ionization (photon) conduction solid fs absorption/excitation

Laser Ablation Applications 1. ns laser, 266 nm fs laser, 266 nm Zn/Cu ratio.5. 2 4 6 8 1 12 14 16 18 time (s) micro-analysis nano-material

Applications of Ultrafast Laser Ablation Ultrafast laser ablation (τ pulse < t thermal ) FEL capability! Non-thermal ablation regime ion fs laser - + E electron target Reduced dependence on thermal properties Reduced larger cluster particles generation

Applications of Ultrafast Laser Ablation Micro Analysis The problem of nanosecond laser ablation MS signal: Zn/Cu ratio 1. ns laser, 266 nm fs laser, 266 nm.5 ns fs. 2 4 6 8 1 12 14 16 18 time(s) Mass Spectrometry - Laser ablation of brass (CuZn alloy)

Micro-Analysis Application New magnetic film material (data storage applications) laser 1 1 depth profile (bulk material) 1 9 sputtering target depth profiling laser element counts (a.u.) 1 8 1 7 1 6 1 5 1 4 1 3 1 2.2.15 Fe Cu 1 2 3 4 5 time (s) surface profiling (film material) deposited film surface profiling Cu/Fe ratio.1.5. 5 1 time (s) 15

Applications of Ultrafast Laser Ablation Nano Material The problem of nanosecond laser ablation 5 µm 5 µm 1 µm Nanowires with large particles

Nano-Material Application Pulsed laser deposition ZnO nanowire growth gas fabrication chamber target Setup ultrafast laser substrate temperature control gas

Nano-Material Application Nanowire nanolaser Nanolaser spectra excitation source 266 nm (Nd:YAG) UV lasing (room temperature) intensity (a.u.) 3 25 2 15 1 5 above threshold below threshold ZnO nanowire ZnO nanowire: natural laser cavity ~ 1 nm sapphire substrate emission intensity (a.u.) 37 375 38 385 39 395 4 1 6 1 5 wavelength (nm) spontaneous lasing 1-3 1-2 1-1 1 1 1 excitation energy (mj) [Science 292 (21) 1897]

Acknowledgements U. S. Department of Energy Yanfeng Zhang Quanming Lu Peidong Yang Richard Russo Xianglei Mao