Nano Structuring

8/3/2019 Nano Structuring

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Femtosecond Laser Structuring of

Metal Surfaces

1



Lotus Effect

2



Outline

• Introduction

• Metal Interactions with fs Pulses.

•Self-organised structures: LIPSS.

•Setup and results on Al

•

Next Steps

3



Introduction: Micromachining with laser

4



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.

5



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

6



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

7



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

8



300 nm

LIPSS= Laser Induced periodic surface structures.

Self-organised structures

M.Huang et al, Opt. Express 16(23), 19354- (2008)

E E

9



Origin of LIPSS

Laser light

polarization

Distortions

couple the light

with surface

plasmons

Distortions are

sources of wavesWaves propagates

in both directions

Λ

10



Direct Interference

Origin of LIPSS

e e’+ie’’

<l

11



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

12



Colours on Aluminum

Uniform colour

Λ=570 nmE

Angle-dependant

colour13



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)

14



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

15



Enhanced Absorption Of Aluminium

Golden Al

Polished Al

Gray Al

Black Al

16



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

17



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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Nano Structuring