Influence of Pulse Duration in the Pico-and Femtosecond Regime on the Absorptance and Specific Removal Rate

Influence of Pulse Duration in the Pico- and Femtosecond Regime on the Absorptance and Specific Removal Rate

S. Remund, M. Chaja, Y. Zhang and B. Neuenschwander

USP Laser marking on steel, Bay bridge SFO

https://doi.org/10.24451/arbor.9344 | downloaded: 14.2.2022

 Motivation

 Pulse Duration Experiments

 Double Pulse Experiments

 Result Comparison and Hypothesis

 Reflectivity Measurement

 Calorimetry

 Conclusion

Content

Motivation – Pulse Duration Experiments Copper DHP

 Machined squares

 Used Lasers:

 350fs to 3ps:

Satsuma HP2, Amplitude

λ=1030nm, f_rep=505kHz

w₀=17.2µm, M²=1.3

 10ps to 52ps:

Duetto, Time Bandwidth

λ=1064nm, f_rep=200kHz

w₀=13µm, M²=1.45

0.00 0.02 0.04 0.06 0.08 0.10 0.12 0.14 0.16 0.18 0.20

0.0 2.0 4.0 6.0 8.0

ΔV/(Δt*P av) / mm3 /(min*W)

Φ₀ / J/cm²

Copper DHP Pulse Duration

350fs Satsuma 1ps Satsuma 3ps Satsuma 10ps Duetto 20ps Duetto 27ps Duetto 52ps Duetto

[1] B. Jaeggi, B. Neuenschwander, S. Remund, T. Kramer.

100910J. 10.1117/12.2253696. (2017)

[1]

Motivation – Pulse Duration Experiments Copper DHP

 Increasing specific removal rate by decreasing pulse duration

 52ps to 3ps nearly 5x

 3ps to 350fs around 1.1x

0.00 0.02 0.04 0.06 0.08 0.10 0.12 0.14 0.16 0.18 0.20

0.0 2.0 4.0 6.0 8.0

ΔV/(Δt*P av) / mm3 /(min*W)

Φ₀ / J/cm²

Copper DHP Pulse Duration

350fs Satsuma 1ps Satsuma 3ps Satsuma 10ps Duetto 20ps Duetto 27ps Duetto 52ps Duetto

[1] B. Jaeggi, B. Neuenschwander, S. Remund, T. Kramer.

100910J. 10.1117/12.2253696. (2017)

[1]

Motivation – Pulse Duration Experiments Steel 1.4301

 Increasing specific removal rate by decreasing pulse duration

 52ps to 3ps nearly 10x

 3ps to 350fs around 1.2x

0.00 0.05 0.10 0.15 0.20 0.25 0.30

0.0 0.5 1.0 1.5 2.0 2.5 3.0

ΔV/(Δt*P av) / mm3 /(min*W)

Φ₀ / J/cm²

Steel 1.4301 Pulse Duration

350fs Satsuma 1ps Satsuma 3ps Satsuma 10ps Duetto 20ps Duetto 27ps Duetto

[1]

[1] B. Jaeggi, B. Neuenschwander, S. Remund, T. Kramer.

100910J. 10.1117/12.2253696. (2017)

Motivation – Pulse Duration and Removal Rate

 Shorter pulse durations lead to higher maximal specific removal rates for

both materials

 Cu: Rate for fs nearly constant

 Steel: Rate increases also for fs

0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5

0.1 1 10 100 1000 10000

Max. spec. Removal rate µm3 /µJ

Dt / ps

Removal rates from machined squares

Steel 1.4301 Copper DHP

Motivation – Two Pulse Burst Experiment

Results for Copper Results for Steel

[1]

[1]

[1] A. Michalowski, F. Bauer, T. Bauknecht. (2016). Schwarzwald Workshop IFSW

Motivation – Comparison of Experimental Results

 Inter-pulse delay for double pulse experiment (DP) and pulse duration (tp) show very similar behavior for copper and steel

 Common cause?

[2] F. Bauer. Grundlegende Untersuchungen zum Abtragen von Stahl mit ultrakurzen Laserpulsen.

Friedrich-Schiller-Universität Jena, 2018.

0.0 0.2 0.4 0.6 0.8 1.0 1.2

1.0E+02 1.0E+03 1.0E+04 1.0E+05 1.0E+06 1.0E+07 1.0E+08

Relative Efficency

Inter-Pulse Delay - Pulse Duration / fs

Inter-Pulse Delay and Pulse Duration Experiments

Steel Φ0=0.5J/cm2 Copper Φ0=0.47J/cm2 Steel Φ0=0.5J/cm2 Copper Φ0=1.4 J/cm2

[2]

 Hypothesis:

 Pulse duration dependency: Shielding of pulse by its own processes

 Double pulse experiments: Shielding of the second by the processes from the first pulse

 Could both results share the same cause -> Shielding

 Experimental approach:

 Reflectivity measurements of single pulses for varying pulse durations

 Calorimetric measurements for varying pulse durations

Motivation – Results and Hypothesis

 Laser beam guided by M1 through L1 onto sample

 Transmission of M1 focused by L2 onto PD1

 Reference signal

 Reflection from sample back on M1

 Transmission of back reflection guided by M2 through BPF and L3 onto PD2

 Back reflected signal

 Ratio indicates relative reflectivity

Reflectivity Measurement – Setup

Reflectivity Measurement – Setup

 Laser: Satsuma HP2, Amplitude Systèmes, λ= 1030nm,

f_L1=100mm -> w₀=13µm, M²=1.6

 M1&M2: HR1030/45 PW1025C, Laser Components GmbH

 PD1&PD2: DET10A2, Thorlabs GmbH, Si detector, 200-1100nm, 1ns rise

time

 BPF: Hard coated OD 4, 1025nm CWL, 50nm bandpass filter, Edmund Optics

 Oscilloscope: LeCroy waveRunner 104MXi, 1GHz, 10GS/s

 Signal integrated (Simpson method)

Reflectivity Measurement – Results Copper DHP

 Decreasing reflectivity by increasing fluence

 At Φ₀=4 J/cm² small

separation for 305fs to 5ps

 10ps nearly 10% reduced reflectivity

 Pulse duration

dependent reflection indicated for 10ps

 Higher threshold fluence due to single pulse

(incubation)

0.7 0.75 0.8 0.85 0.9 0.95 1

0 1 2 3 4 5

Relative Reflectivity

Fluence Φ₀ / J/cm²

Cu Pulse Duration Dependency

tp=305fs tp=700fs tp=1ps tp=3ps tp=5ps tp=10ps

Reflectivity Measurement – Results Steel 1.4301

 For 305fs to 1ps similar values

 3ps decreasing reflectivity after Φ₀=3 J/cm²

 5ps decreasing reflectivity after Φ₀=2 J/cm²

 10ps decreasing reflectivity already after Φ₀=1 J/cm²

 Pulse duration

dependency indicated from 3ps to 10ps

0.5 0.52 0.54 0.56 0.58 0.6 0.62 0.64 0.66 0.68 0.7

0 1 2 3 4 5 6 7

Relative Reflectivity

Fluence Φ₀ / J/cm²

Steel Pulse Duration Dependency

tp=305fs tp=700fs tp=1ps tp=3ps tp=5ps tp=10ps

T / °C

t / s

Sensor SIgnal

Calorimetry

 A part of the incoming energy is always converted to heat

 Sample is heated up, T measured with a PT1000

 Cooling after irradiation

T / °C

t / s Sensor SIgnal

Calorimetry

 A part of the incoming energy is always converted to heat

 Sample is heated up, T measured with a PT1000

 Cooling after irradiation

 From this curve the residual energy in the sample can be calculated [4]

Calorimetry – Setup

 A part of the incoming energy is always converted to heat

 Sample is heated up, T measured with a PT1000

 Cooling after irradiation

 From this curve the residual energy in the sample can be calculated [4]

 𝐸_{𝐻𝑒𝑎𝑡} respectively 𝜂_{𝐻𝑒𝑎𝑡} = 𝐸_{𝐻𝑒𝑎𝑡}/𝐸_𝑖𝑛 is measured

Calorimetry – Results Copper DHP

 Similar and nearly constant residual heat for all pulse durations

 High reflectivity of copper indicated

0.15 0.20 0.25 0.30

0 1 2 3 4 5 6 7

ηRes

Fluence Φ₀ / J/cm²

Residual Heat

300fs 1ps 3ps 5ps 10ps

Calorimetry – Results Copper DHP

 No evidence for major impact by pulse duration

 Exceptional high relative heating meaning residual heat much higher than absorptance

 H1: Plasma causes higher absorption ->

additional energy transfer

 H2: Fluence dependent absorption

0.5 1.0 1.5 2.0 2.5

0 1 2 3 4 5 6 7

ηRes/ηAbs

Fluence Φ₀ / J/cm²

Relative Heating

300fs 1ps 3ps 5ps 10ps

Calorimetry – Two Temperature Model

 Two phase change models for melting and evaporation under superheating

 Calculated for a temporal gaussian shaped pulse, rotation symmetric model -> Yiming Zhang

 TTM based on:

 S.Y. Wang, Y. Ren, C.W. Cheng, J.K.

Chen, D.Y. Tzou. Applied Surface Science,Volume 265, (2013)

 Y. Ren, J. K. Chena, Y. Zhang.

Journal of Applied Physics 110, 113102 (2011)

0.0 0.1 0.2 0.3 0.4 0.5

0 5 10 15

Absorptance

Fluence Φ₀ / J/cm²

Cu: Absorption vs. Peak Fluence

10ps laser system 260fs laser system

Calorimetry – Results Steel 1.4301

 Residual heat on steel for low fluences unusually high

 Fast decreasing toward 30%

 No evidence for impact by pulse duration

0.2 0.4 0.6 0.8 1.0

0 1 2 3 4 5 6

η Res

Fluence Φ₀ / J/cm²

Residual Heat

300fs 700fs 1ps 2ps 3ps 5ps 10ps

Calorimetry – Results Steel 1.4301

 Relative heating >1 at low fluences caused by high residual heat

 Trend follows curves from residual heat

0.2 0.4 0.6 0.8 1.0 1.2

0 1 2 3 4 5 6

η Res/η Abs

Fluence Φ₀ / J/cm²

Relative Heating

300fs 700fs 1ps 2ps 3ps 5ps 10ps

 Reflectivity measurement supports shielding hypothesis

 Pulse duration dependent intra-pulse shielding for steel

 For copper only reasonable difference with 10ps pulses

Both comparable with pulse duration and double pulse experiments

 Decreasing reflectivity of copper with increasing peak fluence compatible with TTM simulation

 However, reflectivity experiment cannot determine reason for decreasing reflectivity by increasing fluence

Conclusion

 No major evidence of pulse duration dependency indicated by calorimetry

 Evidence was not necessarily assumed

 Two unusual results from calorimetry

 Cu: residual heat higher than absorptance

1. Hypothesis: Plasma from ablation causes higher absorption –>

additional insert of energy

2. Hypothesis: Fluence dependent absorption -> TTM simulation

Pulse duration dependency compatible with simulation

No such behavior for steel or at least only for 10ps

 Steel: high residual heat for low fluences

No TTM simulation found for steel

Conclusion

Thank You for Your Attention