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
U N I AK S S E LIV E R S T TÄ
SPP 1327, Aachen 5. Mai 2010
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
U N I AK S S E LIV E R S T TÄ
SPP 1327, Aachen 5. Mai 2010
Reminder: Relevant Time Scales for Laser Processingof Transparent Media (Dielectrics) with fs Pulses*
*From E. Mazur Tutorial @ CLEO 2009 Munich+ PROPAGATION
U N I AK S S E LIV E R S T TÄ
SPP 1327, Aachen 5. Mai 2010
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
U N I AK S S E LIV E R S T TÄ
SPP 1327, Aachen 5. Mai 2010
Basic Ionization Processes in Dielectrics to Reach Critical Electron Energy / Density for Ablation
MultiphotonIonization
Free CarrierAbsorption- Heating -
ImpactIonization
NEEDS INTENSITY NEEDS SEED ELECTRONS & TIME
U N I AK S S E LIV E R S T TÄ
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
U N I AK S S E LIV E R S T TÄ
SPP 1327, Aachen 5. Mai 2010
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:
U N I AK S S E LIV E R S T TÄ
SPP 1327, Aachen 5. Mai 2010
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
U N I AK S S E LIV E R S T TÄ
SPP 1327, Aachen 5. Mai 2010
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
U N I AK S S E LIV E R S T TÄ
SPP 1327, Aachen 5. Mai 2010
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
U N I AK S S E LIV E R S T TÄ
SPP 1327, Aachen 5. Mai 2010
Control Of Basic Ionization Processes
calculated with multiple rate equation: B. Rethfeld PRL 92, 187401 (2004).
U N I AK S S E LIV E R S T TÄ
SPP 1327, Aachen 5. Mai 2010
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
U N I AK S S E LIV E R S T TÄ
SPP 1327, Aachen 5. Mai 2010
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
U N I AK S S E LIV E R S T TÄ
SPP 1327, Aachen 5. Mai 2010
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
U N I AK S S E LIV E R S T TÄ
SPP 1327, Aachen 5. Mai 2010
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
U N I AK S S E LIV E R S T TÄ
SPP 1327, Aachen 5. Mai 2010
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)
U N I AK S S E LIV E R S T TÄ
SPP 1327, Aachen 5. Mai 2010
Changes of the complex refraction index
ReferenceProbe
Pump
Time
Propagationdirection
Time
ReferenceProbe
Pump
Time
Propagationdirection
Time
Optical Kerr-effect Generation of the free carriers
U N I AK S S E LIV E R S T TÄ
SPP 1327, Aachen 5. Mai 2010
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
U N I AK S S E LIV E R S T TÄ
SPP 1327, Aachen 5. Mai 2010
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
U N I AK S S E LIV E R S T TÄ
SPP 1327, Aachen 5. Mai 2010
Restructuring within Focal Area
cubic phase mask600 000 fs³
spacing 300 nmbetween positions
1/e² focusdiameter no evident material modification in focal area
U N I AK S S E LIV E R S T TÄ
SPP 1327, Aachen 5. Mai 2010
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
U N I AK S S E LIV E R S T TÄ
SPP 1327, Aachen 5. Mai 2010
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)
U N I AK S S E LIV E R S T TÄ
SPP 1327, Aachen 5. Mai 2010
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)
U N I AK S S E LIV E R S T TÄ
SPP 1327, Aachen 5. Mai 2010
Preliminary Results with Singleshot PolarizationShaped Pulses
960 fs shaped pulseLinear polarization
crossed polarizations
115 nm
110 nm
U N I AK S S E LIV E R S T TÄ
SPP 1327, Aachen 5. Mai 2010
Preliminary Results with Multishot Polarization ShapedPulses: Ripple Control
linear polarization
crossed polarization
circular polarization
Pol
. E-F
ield
multishot:100 shots / position
U N I AK S S E LIV E R S T TÄ
SPP 1327, Aachen 5. Mai 2010
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
U N I AK S S E LIV E R S T TÄ
SPP 1327, Aachen 5. Mai 2010
• 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