Coherent sources of XUV radiation : soft X-ray lasers and high-order harmonic generation

407 Pages·2005·7.187 MB·English

by  Pierre Jaeglé

Pierre Jaegle´
Coherent Sources
of XUV Radiation
Soft X-Ray Lasers and High-Order
Harmonic Generation
With332Illustrations
PierreJaegle´
DirecteurdeRechercheEme´riteauCentreNationaldelaRechercheScientifique
X-RayMatterInteractionLaboratory(LIXAM)
Universite´Paris-Sud
91405Orsay
France
LibraryofCongressControlNumber:2005934933
ISBN10:0-387-23007-6
ISBN13:978-0387-23007-8
Printedonacid-freepaper.
(cid:1)C 2006SpringerScience+BusinessMedia,Inc.
Allrightsreserved.Thisworkmaynotbetranslatedorcopiedinwholeorinpartwithoutthewritten
permission of the publisher (Springer Science+Business Media, Inc., 233 Spring Street, New York,
NY10013,USA),exceptforbriefexcerptsinconnectionwithreviewsorscholarlyanalysis.Usein
connectionwithanyformofinformationstorageandretrieval,electronicadaptation,computersoftware,
orbysimilarordissimilarmethodologynowknownorhereafterdevelopedisforbidden.
Theuseinthispublicationoftradenames,trademarks,servicemarksandsimilarterms,eveniftheyare
notidentifiedassuch,isnottobetakenasanexpressionofopinionastowhetherornottheyaresubject
toproprietaryrights.
PrintedintheUnitedStatesofAmerica. (TB/EB)
9 8 7 6 5 4 3 2 1
springeronline.com
To Rachel
Acknowledgments
I am grateful to my colleagues Clary Mo¨ller and Pierre Dhez for their help
in the preparation of this book. The dynamism of the X-Ray Laser Research
GroupofLIXAMhasprovidedgreatencouragementinthecompletionofthis
work.
Contents
Part I Introduction to Coherent Extreme-Ultraviolet and Soft
X-Ray Sources
1 Short Survey of XUV Emission Mechanisms and Sources............... 3
1.1 Radiation Transfer Through Matter, Opacity, and Gain......... 7
1.2 Transfer Equation, Absorption, and Gain............................. 8
1.3 Profile Functions.............................................................. 13
1.4 Line Narrowing................................................................ 14
1.5 Atomic Level Population Densities...................................... 17
1.6 Source Brightness and Number of Photons per Mode............. 19
2 XUV Optics........................................................................... 23
2.1 XUV Optical Constants.................................................... 23
2.2 Absorption, Reflection, and Refraction of XUV Radiation...... 28
2.3 Grazing Incidence Optics................................................... 34
2.4 Multilayer Mirrors............................................................ 36
3 Coherent XUV Radiation Beams............................................... 41
3.1 Interferences and Degree of Coherence................................. 41
3.2 Modes of Free Radiation Field............................................ 48
3.3 Three Ways of Producing Coherent XUV Radiation Beams.... 52
References.............................................................................. 55
Part II State of the Art and Prospect of X-Ray Lasers
4 Beginnings............................................................................. 59
4.1 Experiments.................................................................... 59
4.2 Pumping Mechanisms....................................................... 63
x Contents
5 General Features of X-Ray Lasers.............................................. 69
5.1 Survey of Laser-Produced Plasma Physics............................ 69
5.1.1 Main Parameters Related to Plasma Expansion........... 69
5.1.2 Atomic Physics in the Plasma Corona........................ 72
5.2 X-Ray Laser Configurations............................................... 74
5.2.1 X-Ray Lasers Pumped by Lasers............................... 74
5.2.2 Multiple Target Systems.......................................... 78
5.2.3 Optics for the Production of Line Focused Plasmas...... 80
5.2.4 Capillary-Discharge XUV Laser................................ 83
5.2.5 XUV Laser Cavity Issues......................................... 84
5.3 Diagnostics of X-Ray Laser Media...................................... 88
5.3.1 Plasma Imaging...................................................... 88
5.3.2 Temperature and Density Diagnostics........................ 92
6 Propagation of XUV Laser Beams.............................................. 99
6.1 Beam Refraction.............................................................. 101
6.2 From Small-Signal Gain to Saturation................................. 111
6.3 Coherence Building.......................................................... 120
6.4 Coherence Measurements................................................... 129
6.4.1 Coherence Characterization...................................... 129
6.4.2 Interferometric Methods.......................................... 130
6.4.3 Diffractometry........................................................ 133
7 Saturated XUV Lasers............................................................. 137
7.1 Gain Predictions for the Collisional-Excitation Pumping
Scheme........................................................................... 139
7.2 Single Pump-Pulse of Nanosecond Duration......................... 144
7.2.1 Ne-Like Selenium Laser........................................... 144
7.2.2 Ne-Like Ge Laser (Saturation, Coherence, Polarization) 147
7.2.3 Ne-Like Yttrium Laser............................................. 160
7.2.4 Ne-Like Silver Laser................................................ 162
7.2.5 Ni-Like Ion Lasers................................................... 162
7.3 Pumping with Prepulses.................................................... 163
7.3.1 General Characteristics of Prepulse Influence
on Pumping........................................................... 164
7.3.2 Prepulsed Ne-Like Zinc Laser................................... 171
7.3.3 Prepulsed Ne-Like Germanium Laser......................... 176
7.3.4 Ne-Like Lasers with Low Z Elements......................... 178
7.3.5 Prepulsed Ni-Like Lasers: Sn, Sm, Dy, Pd, Ag............. 182
7.4 Transient Collisional Excitation (TCE) Scheme of Pumping.... 193
7.4.1 Traveling Wave Implementation................................ 196
7.4.2 TCE Ne-Like Titanium Laser (32.63 nm)................... 201
7.4.3 TCE Ne-Like Iron Laser (25.5 nm)............................ 205
7.4.4 TCE Ni-Like Tin Laser (11.9 nm)............................. 205
7.4.5 TCE Ni-Like Germanium Laser (19.6 nm).................. 207
Contents xi
7.4.6 TCE Ni-Like Molybdenum Laser (18.9 nm)................ 208
7.4.7 TCE Ni-Like Silver Laser (13.9 nm)........................... 211
7.5 Fast Capillary Discharge X-Ray Laser................................. 215
7.5.1 Discharge Characteristics......................................... 216
7.5.2 Small-Signal Gain, Saturation, and Output of the
Ne-Like Argon Laser............................................... 218
7.5.3 Coherence.............................................................. 222
7.5.4 New Lasing Materials.............................................. 223
7.6 Optical-Field-Ionization Lasers........................................... 224
8 Recombination Lasers.............................................................. 235
8.1 Long Pump Pulses............................................................ 235
8.1.1 Hydrogen-Like Ions................................................. 236
8.1.2 Lithium-Like Ions................................................... 244
8.1.3 Gain–Length Product Limitation.............................. 250
8.2 Short and Ultrashort Pump Pulses...................................... 251
9 Schemes for Future Soft X-Ray Lasers........................................ 255
9.1 Inner Shell Photopumping................................................. 255
9.2 Free Electron Lasers......................................................... 256
References.............................................................................. 257
Part III High Harmonic Generation
10 Introduction.......................................................................... 277
11 Survey of the Theoretical Background....................................... 281
11.1 Atoms in Strong Field.................................................... 281
11.2 Phase-Matching............................................................ 286
12 General Characteristics of High-Order Harmonic Emission........... 291
12.1 Coherence.................................................................... 291
12.1.1 Coherence Control............................................ 291
12.1.2 Spatial Coherence Measurements........................ 294
12.1.3 Temporal Coherence......................................... 300
12.2 Conversion Efficiency..................................................... 303
12.2.1 Scaling Law in the Plateau Region...................... 304
12.2.2 Influence of Atomic Density............................... 305
12.2.3 Influence of the Length of the Pumped Medium.... 308
12.2.4 Influence of the Diameter of Apertured Beam....... 311
12.2.5 Phase-Matching by Wave Guiding....................... 311
12.2.6 Emitters of Complex Structure: Molecules,
Clusters, Solid–Vacuum Interfaces....................... 316
xii Contents
12.2.7 Two-Color High Harmonic Generation................. 331
12.2.8 Tunability....................................................... 339
References.................................................................. 344
Part IV A Survey of Coherent XUV Sources Applications
13 Introduction.......................................................................... 353
13.1 Interferometry of Laser-Created Plasma........................... 354
13.2 Interferometry and Shadography of Exploding Wire
Plasma........................................................................ 357
13.3 Reflectometry of Solid Materials...................................... 359
14 Time-Resolution About 100 Picoseconds................................... 361
14.1 Characterization of Dense Plasmas.................................. 361
14.1.1 Density Measurements up to (cid:1)1021
Electrons cm−3................................................ 361
14.1.2 Colliding Plasmas............................................. 363
14.1.3 Soft X-Ray Radiographic Probing of
Laser-Irradiated Thin Si Foils............................. 364
14.2 Atomic Physics............................................................. 366
14.2.1 Lifetime Measurement of Excited He States.......... 366
14.2.2 Absolute Photo-Ionization Cross-Section
of Excited He-States......................................... 367
14.3 Material Properties....................................................... 369
14.3.1 Snapshots of Intense Electric Field Effects
on Metal Surface.............................................. 369
14.3.2 CsI Crystal Luminescence Induced by
Very Intense XUV Flux..................................... 373
14.4 Production of Highly Focused XUV Beams....................... 374
14.4.1 Method of Wave Front Characterization............... 375
14.4.2 Measurement of the Spatial Intensity Distribution
of a Soft X-Ray Laser Beam............................... 376
15 Time-Resolution About One Picosecond.................................... 379
15.1 Picosecond X-Ray Laser Interferometry............................ 379
15.2 Material Probe at the Picosecond Scale............................ 381
15.2.1 Study of the Surface Domain–Structure
of Ferroelectric BaTiO ..................................... 381
3
15.2.2 Time-Resolved Measurement of Material
Scintillation..................................................... 383
15.2.3 Single-Shot Probe of Photoelectron Emission........ 384
Contents xiii
16 Subfemtosecond Time-Resolution............................................. 387
16.1 Frequency-Domain Interferometry Applied
to Electron-Density Measurements................................... 387
16.2 Generation of Attosecond Pulses..................................... 391
17 Future Prospects................................................................... 395
17.1 Nonlinear XUV Optics................................................... 395
17.2 Microlithography........................................................... 395
17.3 Biological Applications................................................... 396
References............................................................................ 397
Index......................................................................................... 403
1. Short Survey of XUV Emission
Mechanisms and Sources
Theradiationrangeofphotonenergycomprisedbetween,letussay,20eVand
600 eV is known by the names of extreme ultraviolet, soft X-ray or XUV ra-
diation.Thisfluctuatingdesignationreflectsthevariousoriginsofaradiation
pattern which, in other respects, exhibits singular optical properties marking
itofffromfar-ultraviolet,ononeside,andX-rays,ontheother.Thepenetra-
tion length of XUV radiation is very small in almost all materials—generally
lessthan1µm.Thereispracticallynoreflectiononopticalsurfacesundernor-
mal incidence and, in addition, wavelength is too large to make use of Bragg
reflection on natural crystals. These peculiarities made the development of
instrumentationforXUVstudiesmoredifficultthanforothersspectralfields,
afactwhichhasbeenprejudicialtotheadvanceofXUVresearch.Duringthe
last decades important progress, especially in XUV optics, became possible
with the help of synchrotron radiation sources.
AsillustratedinFigure1.1,theenergyofthetransitionsoccurringbetween
twoupperelectronicshellsofatoms,remainsofaconstantorderofmagnitude
as the atomic number increases. For instance, the L –K transition of boron
II
(Z =5), the M –L transition of silicon (Z =14), the N –M transition of
I II I II
zinc (Z =30), the O −N transition of tin (Z =50), all have an energy of
I II
theorderof100eV,whichcorrespondstoawavelengthof∼12nm.Therefore
B atoms ionized in K-shell, Si atoms ionized in L-shell, Zn atoms ionized in
M-shell,andsoon,willbesourcesofsoftX-rayemission(seeforinstance[1]).
Onecanaskthequestionwhetherthisprocesscanprovideexcitationden-
sities such that population inversions might appear between lower and upper
levels. This would open the way to neutral atom photo-pumped XUV lasers.
But attempts at implementing X-ray photo-pumping have not yet succeeded
since a very large X-ray flux is necessary to pump the lasing material, while
the small aperture of XUV optics makes it difficult to concentrate the flux on
the target.
XUV radiation is also emitted by electron transitions between the optical
levelsofmultichargedions.Theword“optical”referstotheemptylevelsabove
the ground state, that is to say the levels otherwise involved in the visible or