nano structuring
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Femtosecond Laser Structuring of
Metal Surfaces
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Lotus Effect
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Outline
• Introduction
• Metal Interactions with fs Pulses.
•Self-organised structures: LIPSS.
•Setup and results on Al
•
Next Steps
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Introduction: Micromachining with laser
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Two Temperature Model (TTM)
( )ee e i
T Q Q QC T T S
t x y z
)( ie
i
i T T t
T C
)exp(**)( z At I S
Femtosecond Laser Heating
Lattice
Electrons
Laser
Electrons
Lattice
Laser
Transfer of heat from electrons to material takes time.Electron and material temperature treated separately during “pulse-on” time.
Pulse is over before heat diffusion in material lattice can occur.
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Time for
equilibrium γX1016
W/m3 K Metal
7.7 ps 10 Cu 16.5 ps 3.6 Ag 27.4 ps 2.1 Au
Calculated time evolution of surface electron and
lattice temperature in Cu, Ag and Au.
J. Wang and C. Guo, J. Appl. Phys. 102, 053522 (2007)
Temperature Evolutions of Electrons and Lattice
Materials with long delay for
coupling of energy from electron
to lattice system melt more than
those with fast coupling
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Light Penetration in Metals
Laser light is strongly reflected, penetration is limited to small skin depth of evanescent wave.
Optical absorption depths for several materials over a range of wavelengthsD.R. Lide, CRC Handbook of Chemistry and Physics, 82nd edn. (CRC, Boca Raton, 2001)
2
c
c = light speed, = conductivity,
= permeability, = frequency
For metals, ~ 20 nm at 1 m
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Femtosecond Pulses Absorbed Within Skin Depth
fs laser:•Skin depth is small
•Thermal penetration within theskin depth.
Result:•fast ionization and evaporation
•ablation of the material beforesignificant heat conduction
(non- thermal ablation).
1
10
100
1000
Nanos econd Fem tos econd
D e
p t h ( n m )
Skin Depth
Thermal Depth
Thermal penetration depth t d T T
skin depth
2
c
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300 nm
LIPSS= Laser Induced periodic surface structures.
Self-organised structures
M.Huang et al, Opt. Express 16(23), 19354- (2008)
E E
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Origin of LIPSS
Laser light
polarization
Distortions
couple the light
with surface
plasmons
Distortions are
sources of wavesWaves propagates
in both directions
Λ
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Direct Interference
Origin of LIPSS
e e’+ie’’
<l
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800 nm
70 fs, 0.7 mJ/pluse, 1kHz
D=300 μm
Experimental Setup
Laser Processed Aluminum
Φ=50mm,thickness=5mm
Laser Processed Gold
30 m gold coated on Copper plate
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Colours on Aluminum
Uniform colour
Λ=570 nmE
Angle-dependant
colour13
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Application: Colorizing Metals by Nanostructuring
Marking on 316L Stainless steel
sampleAluminium samples
Y. Vorobyev and C. Guo.
Appl. Phys. Lett. 92, 041914 2008
B. Dusser, Z. Sagan, H. Soder, N. Faure, J.P.
Colombier, M. Jourlin, and E. Audouard.
Optics Express, Vol. 18, Issue 3, pp. 2913-2924 (2010)
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E=244 mJ/cm2 E=137 mJ/cm2 E=975 mJ/cm2
Polarisation
Laser scan
direction
100 μm 100 μm 100 μm
10 μm 10 μm 10 μm
Scanning step=25μm
Number of pass/line=1
Scanning speed=5 mm/s.
Laser Fluence Effect
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Enhanced Absorption Of Aluminium
Golden Al
Polished Al
Gray Al
Black Al
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Enhanced Absorption Of Aluminium
Influence on reflectivityFeature size
Light trapping due to multiple reflections enhances coupling into
the material.
>> l
Small features can successively scatter light, increasing the
effective optical path length and enhancing absorption.
≈ l
??
Antireflection effect of random surface textures in terms of
graded refractive index at air/solid interface.
Broadening of SPs absorption spectra induced by various sizes
and shapes.
<< l
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Ti:Sapphire
Oscillator Amplifier
Pump Laser Pump Laser
BS
NF
Chopper
Optical Delay line
-100ps to 2.5 ns
M1
M2
M3 L1
L2
L3
Shutter
WLC
PM1
PM1
Sample NF
Spectrometer
with CCD
Data acquisition
Computer
Femtosecond Laser system
l=800nm
l=800nm
l=340-700nm
Next step:Time-resolved reflectivity setup
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