Femtosecond ablation of silver with single and double pulses Ted Roberts, Anton du Plessis*, Lourens Botha

Transcription

1 Femtosecond ablation of silver with single and double pulses Ted Roberts, Anton du Plessis*, Lourens Botha CSIR National Laser Centre, Femtosecond Science group SAIP 29

2 Outline of talk Motivation of this study Timescales of laser ablation Single pulse femtosecond ablation of silver Double pulse femtosecond ablation of silver Conclusions Slide 2

3 Laser ablation is the process of removing material from a surface, by vapourisation or other means, by irradiating it with a laser beam. Why femtosecond laser ablation of silver? Contract research project to study the feasibility of measuring silver in fuel kernels by femtosecond Laser Induced Breakdown Spectroscopy Need to interpret the LIBS line intensities from the plasma formed from ablated material in terms of densities of silver Project to demonstrate ability to ablate well defined holes with a few µm diameter with the femtosecond laser Drilling of holes in silver foil to study scientifically the fs laser ablation of silver Slide 3

4 Timscales of laser ablation What physical processes go on during the laser pulse? Energy transferred from laser radiation to electrons by inverse Bremsstrahlung 1fs Electrons collide and thermalise so velocity distribution can be described by a temperature (T e ) 1fs Electrons transfer energy to the ions in the lattice by phonon collisions 1 to 2 ps Hot electrons diffuse into lattice Material is ejected from the lattice Plasma formation occurs 5 to 1ps few 1ps.2 to 2ns Incident radiation can be absorbed by plasma 5ns 13fs 1ns Slide 4

5 Experimental set-up for ablation of metal foils Transmitted signal d = t/n (nm/pulse) Intensity autocorrelator system Sample Vacuum chamber P(mW) F (Jcm -2 /pulse) Reference signal Femtosecond laser He-Ne Slide 5

6 Photos of femtosecond drilled holes in silver foil Increasing fluence Slide 6

7 Ablation results at low fluence Nolte et al 1997 found two ablation regimes: d = 1 Ln (F/.14) nm/pulse for F <.5 Jcm -2 and d = 8 Ln (F/.46) nm/pulse for F >.7 Jcm -2 Ablation depth (nm/pulse) Silver 2mm 28/4/9 1torr Copper Nolte et al 1997 Copper Nolte et al α -1 radiation penetration depth (nm) 8 electron diffusion depth (nm) Fluence (Jcm -2 /pulse) We do not clearly see the low fluence regime with drilling in foil Slide 7

8 Vacuum high fluence results 4 Ablation depth (nm/pulse) mm 28/4/9 1torr Pow er (mw) Ablation depth (nm/pulse) mm 28/4/9 1torr Best Ln fit Fluence (Jcm -2 /pulse) Slide 8

9 Double femtosecond pulse ablation (separation τ S ) The objective of the second pulse is to probe the conditions created by the first pulse (This is a pump probe experiment but on a macroscopic not molecular scale) 13fs x 3x1 8 m/s Hot electrons τ S 1ps τ S 1 ps τ S 1 ps τ S 1, ps Slide 9

10 Generation of double pulses separated by from ps to 3ns: ps τ S 6ps: Dazzler 6ps τ S 3ns: Michelson interferometer: Δt = 2Δx/c θ x x + Δx Specifications for Δt and θ are quite restrictive: Slide 1

11 Calibration of pulse separation from Michelson 3ns easy to check but 6ps end of range has to be calibrated with second harmonic intensity autocorrelator ( TOAD or FROG ) Intensity (rel) Pulse separation 5.49ns 3/2/29..E+ 5.E-9 1.E-8 1.5E-8 2.E-8 Intensity (rel) Time (s) Pulse separation.71ns.35ps. 3/2/29.E+ 5.E-9 1.E-8 1.5E-8 2.E-8 Time (s) Fast photodiode (pulse width.7ns) Can fit > 5x 13 fs pulses within this instrument width Second harmonic intensity autocorrelator (TOAD) Slide 11

12 Alignment of Michelson beams We drill holes of < 5μm diameter with a 2mm f.l. lens so need θ <.25 mrad. After sending beams up and down lab 1m they must coincide to <.25mm (Spiricon camera). Beam Profiles after sending Michelson beams up and down lab for 1m: 4mm Slide 12

13 Single and double pulse ablation of silver 1 Ablation depth (nm/pulse) /3/ Pulse separation (ps) Slide Jcm -2 Dazzler 1.6 Jcm -2 Michelson.8 Jcm -2 Michelson Single pulse 2 Separate pulses

14 1 Ablation rate (nm/pulse) 9 Double pulse 8 Single pulse Jcm Ablation rate (nm/pulse) Jcm -2 Double pulse Single pulse Pulse separation (ps) Pulse separation (ps) 14 Double pulse Ablation rate (nm/pulse) Single pulse Jcm Pulse separation (ps) Ablation rate (nm/pulse) Jcm -2 Double pulse Single pulse Ablation rate (nm/pulse) Ablation rate (nm/pulse) Slide 14 Double pulse 2 Single pulse Jcm Pulse separation (ps) Double pulse Jcm -2 Single pulse Ablation rate (nm/pulse) Pulse separation (ps) 12 1 Double pulse Single pulse Jcm Pulse separation (ps) Pulse separation (ps)

15 Pulse separation time for which double pulse ablation rate equals single pulse ablation rate 1 Cross over time (ps) 1 1 3/4/ Single pulse fluence (Jcm -2 ) Ablation depth (nm/pulse) mm 28/4/9 1torr Best fit Ln functions Fluence (Jcm -2 /pulse) Slide 15

16 Conclusions We have found a femtosecond laser ablation regime which is not described by a simple two-temperature plus vapourisation model Probing this regime with a second pulse shows significant differences as a function of pulse separation The sharp transition is probably due to a change in ablation mechanism rather than transport parameters Slide 16

17 Thank you for your 9 femtoseconds of attention! Slide 17

18 Air and vacuum ablation results 14 For P > 1mW (corresponding to F > 23 Jcm -2 or peak I > 2x1 14 Wcm -2 we find d decreases with increasing fluence due to air breakdown Ablation depth (nm/pulse) Vacuum 5torr Air Pow er (mw) Power loss (mw) 1 1 PM signal (mv) Laser power (mw) Laser power (mw) Slide 18