Laser Breakdown of Dielectric Materials by Alan Heins Submitted in Partial Fulfillment of the ...

205 Pages·2013·4.68 MB·English

by  Alan Heins

Laser Breakdown of Dielectric Materials

by
Alan Heins

Submitted in Partial Fulfillment of the
Requirements for the Degree
Doctor of Philosophy

Supervised by Chunlei Guo

Institute of Optics
Arts, Sciences and Engineering
Edmund A. Hajim School of Engineering and Applied Sciences

University of Rochester
Rochester, New York

2013
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Biographical Sketch
The author was born in Park Ridge, Illinois. He attended the Massachusetts Institute of
Technology, and graduated with a Bachelor of Science in Physics in 2002. He began
doctoral studies in Optics at the University of Rochester in 2006. He pursued his research in
the interaction of strong laser fields with matter under the direction of Chunlei Guo.

The following publications were a result of work conducted during doctoral study:
A. Heins and C. Guo, “High-Intensity Femtosecond Laser Interactions with Gases and
Clusters,” p. 69-84 in Nanomaterials: Processing and Characterization with Lasers, 1st
Ed., edited by S. Singh, H. Zeng, C. Guo, and W. Cai
A. Heins and C. Guo, “High stability breakdown of noble gases with femtosecond laser
pulses,” Optics Letters 37, 599 (2012)
A. Heins and C. Guo, “Spatial mode cleaning in radically asymmetric strongly-focused laser
beams,” Appl. Phys. B, DOI: 10.1007/s00340-013-5483-5
A. Heins and C. Guo, “Shock-induced concentric rings in femtosecond laser ablation of
glass”, J. of Appl. Phys. 113, 223506 (2013) DOI: 10.1063/1.4810847
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Abstract
The breakdown of dielectric solids and transparent gases by high-intensity femtosecond
laser pulses are experimentally investigated. The plasma emission from atmospheric-
pressure gases is studied. The plasma emission strength is shown to exhibit much less shot-
to-shot variance than the plasma generated by longer-pulse lasers; this observation is
explained by the different breakdown mechanism active for very short pulses. The
breakdown plasma defocuses the pulse which creates it; this process is studied
experimentally and through a Finite-Difference Time-Domain (FDTD) simulation. The plasma
also causes a spectral blueshift in the generating pulse; the spectrum of the transmitted
pulse is investigated. Time-resolved measurements of the evolving plasma shape are
presented. The defocus, blueshift, and plasma shape experiments are all conducted for both
linear and circular polarization – and it is found that a circularly polarized pulse behaves
exactly like a linearly polarized pulse of lower intensity. The intensity shift is determined to be
1.36 times, and is consistent among all three experiments, but disagrees with the 1.5 times
shift seen in vacuum experiments. In spite of possessing a lower overall number of ions, the
plasma generated with a circular pulse is shown to emit more brightly than one produced
with a linear pulse. This is connected with theoretical descriptions of ionization which predict
higher electron kinetic energies following a circularly polarized breakdown pulse, and shows
that kinetic energy plays a major role in producing plasma emission. The role of impact
excitation is reaffirmed in a mixed-gas experiment, in which it is shown that a lower-
ionization-potential gas can increase emission from a higher-ionization-potential gas. In
dielectric solids, it is found that circular pulses produce a narrower but deeper crater. The
crater profiles are well fit by an atomic model which primarily considers lattice heating from
electron collisions. In both solids and gases, the role of dephasing of the electron motion –
induced either through collisions or continuous blueshifting of the pulse – is found to be the
critical factor in moving energy from the pulse to the material. Finally, the effects of the
shockwave generated when a solid is ablated in air are studied.
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Contributors and Funding Sources

This work was completed in the laboratory of Professor Chunlei Guo, and supervised by a
dissertation committee consisting of Professors Chunlei Guo of the Institute of Optics,
Professor Carlos Stroud of the Institue of Optics, Professor Nick Bigelow of the Department
of Physics, and Professor Chuang Ren of the Department of Mechanical Engineering. All
work for the dissertation was completed independently by the student. The work published
in this dissertation was funded by the US Air Force Office of Scientific Research.
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Table of Contents
Chapter 1: Introduction ....................................................................................................................... 1
Chapter 2: Laser System..................................................................................................................... 4
2.1 Oscillator 5
2.2 Amplifier 7
2.3 Pulse Measurement 18
Chapter 3: High Intensity Atomic and Molecular Physics ............................................................ 23
3.1 Tunneling Ionization 24
3.2 Multiphoton Ionization 29
3.3 Electron Energies 33
Chapter 4: Gas Breakdown ............................................................................................................... 40
4.1 Background 40
4.2 Spectra From Gas Breakdown 44
4.3 Breakdown Stability 60
4.4 Plasma Defocusing 66
4.5 Spectral Blueshifting 80
4.6 Plasma Imaging 88
4.7 Combining Defocusing, Blueshifting, and Plasma Luminous Yield 95
4.8 Additional Evidence for a Hot Plasma 101
4.9 Conclusion 105
Chapter 5: Surface Breakdown ...................................................................................................... 107
5.1 Background 107
5.2: Z-Scan 113
5.3: Crater Diameter and Ablation Threshold 120
5.4: Crater Depth and Energy Absorption 126
5.5 Conclusion 133
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Chapter 6: Air-Surface Interaction ................................................................................................. 134
6.1 Background 134
6.2 Concentric Rings 135
6.3: Model: Shockwave Reflection 141
6.4: Conclusion 152
Chapter 7 Conclusion ...................................................................................................................... 154
Bibliography ..................................................................................................................................... 156
Appendix A: Tunneling Models of Ionization ............................................................................... 174
Appendix B: Experimental Measurement of Ionization Rates .................................................... 183
Appendix C: Nonlinear Propagation of an Intense Pulse ........................................................... 190
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List of Figures and Tables
Figure 2-1: Diagram of complete amplifier. ............................................................................... 9
Figure 2-2: Impact of second (β ) and third (β ) order dispersion on a Gaussian pulse, initially
2 3
of 60 fs FWHM ................................................................................................................. 12
Figure 2-3: details of regenerative amplifier ............................................................................ 13
Figure 2-4: Compressor length scans for three different values of TOD. ............................... 22
Figure 3-1: Tunneling Ionization .............................................................................................. 25
Figure 3-2: Predicted ionization rates for the PPT and ADK models for N2 and O2 .............. 27
Figure 3-3: Heuristic model of multiphoton ionization. ............................................................ 32
Figure 3-4: Simpleman prediction for the kinetic energies of electrons created by a Gaussian
pulse ................................................................................................................................. 35
Figure 3-5: Simpleman prediction of kinetic energies for a parabolic pulse envelope of total
width 68 fs ........................................................................................................................ 37
Figure 4-1: A typical LIBS spectrum for argon ........................................................................ 45
Figure 4-2: Verifying the radiometric calibration of the Ocean Optics spectrometer .............. 45
Figure 4-3: Comparison of LIBS and arc spectra. ................................................................... 47
Figure 4-4: Nitrogen afterglow ................................................................................................. 48
Figure 4-5: Time evolution of the continuum component of the plasma emission for several
gases ................................................................................................................................ 51
Figure 4-6: Time evolution of the various spectral components ............................................. 52
Figure 4-7: Voigt fitting to the width of an excited oxygen atom triple line. ............................. 55
Figure 4-8: Attempted fit of Equation 4-2 to the xenon continuum, at two different times. ..... 59
Table 4-1: Stability (RSD) of the plasma emission of argon, krypton, and xenon under
femtosecond pulse breakdown.. ...................................................................................... 60
Figure 4-9: Multiphoton versus avalanche ionization. ............................................................. 63
Figure 4-10: Stability of plasma emission signals ................................................................... 65
Figure 4-11: Fluence (F) distribution for a pulse defocusing in air .......................................... 71
Figure 4-12: Center detail of Figure 4-11 ................................................................................ 72
Figure 4-13: Intensity distribution produced by the highest energy time slice ........................ 73
Figure 4-14: Oxygen ion density following a 0.5 mJ pulse focused with a 60 mm lens. ........ 74
Figure 4-15: Same as Figure 4-14, but covering nitrogen ions. .............................................. 75
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Figure 4-16: Comparison of ablation crater areas in vacuum and air on soda-lime glass for a
0.5 mJ, 63 fs pulse ........................................................................................................... 76
Figure 4-17: Energy lost to defocusing at a function of pulse energy in air. ........................... 79
Figure 4-18: Setup for the polarization gating experiment ...................................................... 83
Figure 4-19: Branching ratio versus wavelength for 0 – 5 waveplates (delay stages) ........... 84
Figure 4-20: Effect of third order dispersion on blueshift spectra ........................................... 87
Figure 4-21: Images of a 900 μJ pulse focused in air ............................................................. 89
Figure 4-22: Projections of the plasma ball onto the y axis, same pulse as Figure 4-21 ....... 90
Figure 4-23: Projections of the plasma ball onto the z axis, taken at t = 3 ns ........................ 91
Figure 4-24: Projections of the plasma ball onto the z axis, for varying pulse energy (vertical
axis) – linear polarization.. ............................................................................................... 92
Figure 4-25: Projections of the plasma ball onto the z axis, for varying pulse energy (vertical
axis) – circular polarization .............................................................................................. 93
Figure 4-26: Growth of plasma left “edge” with pulse energy ................................................. 94
Figure 4-27: Centroid of the spectral distribution for pulses which have passed through an air
focus. ................................................................................................................................ 96
Figure 4-28: Total luminescence yield of the plasma, taken in a 3 ns window around the
emission peak .................................................................................................................. 97
Figure 4-29: Same data as Figure 4-28, plotted on a linear scale .......................................... 99
Figure 4-30: Mixed gas emission ...........................................................................................102
Figure 4-31: Pulse train measurement ..................................................................................104
Figure 5-1: Reflectivities predicted by the Drude model for three different assumptions about
the electron collision rate ...............................................................................................112
Figure 5-2: Damage (thermal) and cratering for a 0.5 mJ pulse, focused by a 60 mm
achromatic lens onto a soda-lime microscope slide ......................................................114
Figure 5-3: Same as previous, but 530 μm before the geometric focus ...............................116
Figure 5-4: Continuation of Figures 5-2 and 5-3, at (a) z = -76 μm, (b) z = -25 μm, and (c) z =
25 μm .............................................................................................................................117
Figure 5-5: Astigmatism in passing through the focus ..........................................................118
Figure 5-6: Ablation of glass by a 0.5 mJ pulse 430 μm after the geometric focus ..............119
Figure 5-7: Measured crater areas in soda-lime glass as a function of z .............................122
Figure 5-8: Craters at z = -1 mm, taken for three different limiting aperture diameters ........123
Figure 5-9: Diameter of the craters in linear and circular polarization ..................................125
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Figure 5-10: Radially averaged crater profiles in linear polarization .....................................127
Figure 5-11: Similar to Figure 5-10, but with circular polarization .........................................128
Figure 5-12: Modeling the crater shape ................................................................................132
Figure 6-1: Damage spots obtained for an unclipped, 1 mJ, 90 fs, 19 GW peak power pulse
focused in air ..................................................................................................................136
Figure 6-2: Rings around the damage spot at z = 0 for 1 mJ pulses in air (a), argon (b), and
vacuum (c) ......................................................................................................................137
Figure 6-3: Damage spots created with a clipped “half moon” beam ...................................138
Figure 6-4: Effect of astigmatism on the ablation of glass in air ...........................................139
Figure 6-5: Evolution of beam with horizontal stripe removed, from z = -450 μm to z = 0 ...140
Figure 6-6: Evolution of the beam with a horizontal stripe removed, from z = +50 μm to z =
+550 μm .........................................................................................................................140
Figure 6-7: Pump-probe experiment with 0.2 mJ pulses, z = 0 ............................................142
Figure 6-8: Origin of the rings ................................................................................................143
Figure 6-9: Fluence flattening ................................................................................................146
Figure 6-10: Comparison of theory and experiment ..............................................................147
Figure 6-11: Damage spots in air with a much weaker (~1%) prepulse ...............................150
Figure 6-12: Pump-probe experiment at short and long delays ............................................151
Figure 6-13: Enhancing radial symmetry as a shockwave expands .....................................153
Figure B-1: Ion counting spectrometer ..................................................................................184
Figure B-2: Closeup of a microchannel plate, an ion amplifier .............................................187
Figure B-3: Diagram of a delay-line anode ............................................................................187
Figure B-4: dissociation of molecular hydrogen in a WATRIMS setup, p polarization .........188
Figure B-5: dissociation of molecular hydrogen in a WATRIMS setup, s polarization .........189
Figure C-1: Self-focusing of a beam ......................................................................................192
Figure C-2: Filamentation models .........................................................................................194
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Table of Symbols
β electric field absorption parameter
γ Keldysh (adiabicity) parameter
Δφ change in SOD with compressor length
2
Δφ change in TOD with compressor length
3
Δf full-width, half-maximum in frequency space
FWHM
Δλ full-width, half-maximum in wavelength
FWHM
Δt full-width, half-maximum pulse width in time
FWHM
ε permittivity; electric field (Appendix A only)
Θ electric field energy density
λ laser wavelength
ρ electron density
ρneutral neutral atom density
σ conductivity
σ DC conductivity
0
τ electron dephasing time
τ pulse half width (e-2 in intensity)
0
φ Carrier-Envelope Phase (CEP)
φ Second Order Disperson (SOD)
2
φ Third Order Dispersion (TOD)
3
ψ electric field phase (zero at peaks)
ψ hydrogen-like ground state
g
ψ vacuum state
p
ω laser angular frequency
ω plasma frequency
p
ω inverse tunneling time
tunnel
a atomic radius (5.292 x 10-11 m)
0
e electron charge (1.602 x 10-19 C)