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

    )( iei

    i TTt

    TC

    )exp(**)( zAtIS

    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

    equilibriumX1016

    W/m3 KMetal

    7.7 ps10Cu16.5 ps3.6Ag27.4 ps2.1Au

    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 ofevanescent 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

    De

    pth(nm)

    Skin Depth

    Thermal Depth

    Thermal penetration depth td TT

    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)

    EE

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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

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    Direct Interference

    Origin of LIPSS

    e e+ie

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    800 nm

    70 fs, 0.7 mJ/pluse, 1kHz

    D=300 m

    Experimental Setup

    Laser Processed Aluminum

    =50mm,thickness=5mm

    Laser Processed Gold

    30 m gold coated on Copper plate

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    Colours on Aluminum

    Uniform colour

    =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/cm2E=137 mJ/cm2 E=975 mJ/cm2

    Polarisation

    Laser scan

    direction

    100 m 100 m 100 m

    10 m 10 m 10 m

    Scanning step=25m

    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.

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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

    M3L1

    L2

    L3

    Shutter

    WLC

    PM1

    PM1

    SampleNF

    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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