materialbearbeitung von dielektrika auf der nanometerskala

U N I AK S S E LIV E R S T TÄ

SPP 1327, Aachen 5. Mai 2010

Materialbearbeitung von Dielektrika auf der Nanometerskala

mit zeitlich asymmetrisch geformtenFemtosekundenlaserpulsen und

polarisationsgeformten Femtosekundenlaserpulsen

Prof. Dr. Thomas Baumert / PD Dr. Matthias WollenhauptLars Englert, Dirk Otto, Jens Köhler, Cristian Sarpe, Jutta Mildner, Nadine Götte,

Tillmann Kalas, Alexander Horn

Universität Kassel, Institut für Physik und CINSaTHeinrich-Plett-Str. 40

D-34132 Kassel



Outline• Reminder: Basic Ionization Processes in Dielectrics and

Control of Ionization Processes Nanostructures

• Goals:

1) Further optimization Frequency asymmetry vs temporal asymmetry / No control with temporal symmetric pulse shapes / Simulations

2) Analysis of processes Direct measurement of e-density

3) Direct writing Restructuring within focal area

4) Nanostructures via polarization shaping PolarizationShaping and Zeptosecond Temporal Phase Resolution



Reminder: Relevant Time Scales for Laser Processingof Transparent Media (Dielectrics) with fs Pulses*

*From E. Mazur Tutorial @ CLEO 2009 Munich+ PROPAGATION



Reminder: Basic Ionization Processes in Dielectrics to Reach Critical Electron Energy / Density for Ablation

A theoretical model based on electron production via multiphoton ionization, Joule heating, and collisional(avalanche) ionization is in good agreement withexperimental results



Basic Ionization Processes in Dielectrics to Reach Critical Electron Energy / Density for Ablation

MultiphotonIonization

Free CarrierAbsorption- Heating -

ImpactIonization

NEEDS INTENSITY NEEDS SEED ELECTRONS & TIME


SPP 1327, Aachen 5. Mai 2010Crosscorrelation of tailored pulses on target, φ3 = 600 000 fs3

Control of Basic Ionization Processes viaTemporally Asymmetric Femtosecond Pulses

Generation of seed electrons via MPIwell below damage threshold for short pulse ablation

i.e. strong spatial confinement

Heating and electron impact ionizationto reach critical energy / density for ablation

Cubic phase



Crosscorrelation of cubic phase shaped pulses on target

(Cubic phase shaped pulses from 35 fs bwl. pulses)

50 000 fs3

200 000 fs3

+ 600 000 fs3

- 600 000 fs3

cubic phase:



Experimental set up

femtosecond laser

Pulseshaper &delay stage

microscope

sample

piezo positioner

35fs, 800nm, 1kHz, 500mW

Spectrometer &ICCD Camera

Post Mortem Analysis via

SEM

AFM

NA = 0.5; 1/e2 diameter: 1.4 µm; 1/e2 length: 9.1 µm

Energy

Z



77 nJ; 2σ = 50 fs

Reduction In Structure Size Via Pulseshaping

Reduction is one order of magnitude below diffraction limit!

71 nJ; 2σ = 960 fs 110 nJ; 2σ = 960 fs

3 µm

diffractionlimit

(1400 nm)

100 nm

AFM

Opt. Express 15 (2007); Appl. Phys. A 92 (2008)

SAME FOCUS CONDITIONSSAME FLUENCESAME SPECTRUM



Thresholds and Structure Sizes as Function of Fluence

positive cubicnegative cubic

Opt. Express 15 (2007); Appl. Phys. A 92 (2008) in coop. with B. Rethfeld

SEM

unshaped pulses

SEM

substructure diameter

AFM

For temporally asymmetric pulse shapes: i.) strong threshold dependency i.e. control of ionizationii.) “robust” nanostructure size



Control Of Basic Ionization Processes

calculated with multiple rate equation: B. Rethfeld PRL 92, 187401 (2004).





• Goals:







AFM

Quadratic chirped pulses leading to temporal symmetricenvelopes but temporal asymmetric frequencies

downchirped

Quadratic phase 220( ) ( )

2!φφ ω ω ω= −

f=+/- 10 000 fs2 corresponds to 0.7 ps statistical pulse length

upchirped

i.) no controlii.) larger structures at threshold



Preliminary: Simulation of radial electron densitydistribution over focus spot

unshaped 35 fs pulse (35 nJ) pos. cubic shaped 960 fs pulse (75 nJ)

Simulation: generation of free electrons with simple model ofMulti Photon Ionization ( ) and Avalanche ( ) and Recombination

focal beamprofile





• Goals:







The principle of spectral interference* method

ReferenceProbe

Pump

Time axe

tΔ

Propagationdirection

Spectrum

Grating

Spectrometer

Spectrometer slit

Detection (CCD)

* Also named „frequency domain interferometry“See for ex.: Audebert et al, PRL, 73, 1990 (1994), V. V. Temnov et al. PRL 97, 237403 (2006)



Changes of the complex refraction index

ReferenceProbe

Pump

Time


Time

ReferenceProbe

Pump

Time


Time

Optical Kerr-effect Generation of the free carriers



Our experimental setupExperimental setup used for time-resolved spectral interferometry. Laser parameters: 40 fs pulses, 1 kHz repetition rate, 785 nm central wavelength. Pump beam: 0-20 µJ; beam waist (1/e²): 30.5 µm. Probe and reference beams: 2x0.1µJ, 400nm, beam waist 120µm; . Water jet thickness: 95µm.

1900fs delay





• Goals:







Restructuring within Focal Area

cubic phase mask600 000 fs³

spacing 300 nmbetween positions

1/e² focusdiameter no evident material modification in focal area





• Goals:







Our polarization pulse shaper setupKey features:

• 2 x 640 pixel LC modulator(Jenoptik)

• Polarization pulse shapingor Phase & Amplitude shaping

• High spectral resolution:0.16 nm/pixel @ 800 nm

• Large temporal window:> 10 ps

• Volume Phase Holographic Gratings1840 lines/mm

Appl. Phys. B 95, 245 (2009)



Shaping Capabilities

Time [fs]

PolarizationExample for designed and

characterized pulse in interaction region

Amplitude and PhaseGenerating changes in temporal

phase with zeptosecondprecision

0

+π/2

+π

E(t)

χ(t)



Preliminary Results with Singleshot PolarizationShaped Pulses

960 fs shaped pulseLinear polarization

crossed polarizations

115 nm

110 nm



Preliminary Results with Multishot Polarization ShapedPulses: Ripple Control

linear polarization

crossed polarization

circular polarization

Pol

. E-F

ield

multishot:100 shots / position



Eng. Fract. Mech. In print (2010)

Three related projects that might be of interestLIBS resolution below 2 µm

and spectrochemicalSensitivity: TiAl

Commercial confocal laser

scanningmicroscope

NA = 0.7; lateral PSF 460 nm0.85 x diffraction

λ

SPM Microscopy

Appl. Phys. Lett 78 (2005)

short pulse< 80 fs

long pulse> 1 ps

Length: 6000 nm, Width: 100 nm, Depth: 4 nm (APL 95 (2009))

Structures via near fields



• Reminder: Basic Ionization Processes in Dielectrics and Control of Ionization Processes Nanostructures

• Goals:





materialbearbeitung von dielektrika auf der nanometerskala

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