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Springer Theses Recognizing Outstanding Ph.D. Research Marcus Seidel A New Generation of High-Power, Waveform Controlled, Few-Cycle Light Sources Springer Theses Recognizing Outstanding Ph.D. Research Aims and Scope The series “Springer Theses” brings together a selection of the very best Ph.D. theses from around the world and across the physical sciences. Nominated and endorsed by two recognized specialists, each published volume has been selected foritsscientificexcellenceandthehighimpactofitscontentsforthepertinentfield of research. For greater accessibility to non-specialists, the published versions includeanextendedintroduction,aswellasaforewordbythestudent’ssupervisor explainingthespecialrelevanceoftheworkforthefield.Asawhole,theserieswill provide a valuable resource both for newcomers to the research fields described, and for other scientists seeking detailed background information on special questions. Finally, it provides an accredited documentation of the valuable contributions made by today’s younger generation of scientists. Theses are accepted into the series by invited nomination only and must fulfill all of the following criteria (cid:129) They must be written in good English. (cid:129) ThetopicshouldfallwithintheconfinesofChemistry,Physics,EarthSciences, Engineeringandrelatedinterdisciplinary fields such asMaterials,Nanoscience, Chemical Engineering, Complex Systems and Biophysics. (cid:129) The work reported in the thesis must represent a significant scientific advance. (cid:129) Ifthethesisincludespreviouslypublishedmaterial,permissiontoreproducethis must be gained from the respective copyright holder. (cid:129) They must have been examined and passed during the 12 months prior to nomination. (cid:129) Each thesis should include a foreword by the supervisor outlining the signifi- cance of its content. (cid:129) The theses should have a clearly defined structure including an introduction accessible to scientists not expert in that particular field. More information about this series at http://www.springer.com/series/8790 Marcus Seidel A New Generation of High-Power, Waveform Controlled, Few-Cycle Light Sources Doctoral Thesis accepted by the Max Planck Institute of Quantum Optics, Garching, Germany 123 Author Supervisor Dr. Marcus Seidel Prof. Dr. FerencKrausz Institut deScience etd’Ingénierie MaxPlanckInstitute ofQuantum Optics Supramoléculaires Garching,Germany Strasbourg, France ISSN 2190-5053 ISSN 2190-5061 (electronic) SpringerTheses ISBN978-3-030-10790-1 ISBN978-3-030-10791-8 (eBook) https://doi.org/10.1007/978-3-030-10791-8 LibraryofCongressControlNumber:2018966118 ©SpringerNatureSwitzerlandAG2019 Thisworkissubjecttocopyright.AllrightsarereservedbythePublisher,whetherthewholeorpart of the material is concerned, specifically the rights of translation, reprinting, reuse of illustrations, recitation, broadcasting, reproduction on microfilms or in any other physical way, and transmission orinformationstorageandretrieval,electronicadaptation,computersoftware,orbysimilarordissimilar methodologynowknownorhereafterdeveloped. The use of general descriptive names, registered names, trademarks, service marks, etc. in this publicationdoesnotimply,evenintheabsenceofaspecificstatement,thatsuchnamesareexemptfrom therelevantprotectivelawsandregulationsandthereforefreeforgeneraluse. The publisher, the authors, and the editorsare safeto assume that the adviceand informationin this book are believed to be true and accurate at the date of publication. Neither the publisher nor the authorsortheeditorsgiveawarranty,expressorimplied,withrespecttothematerialcontainedhereinor for any errors or omissions that may have been made. The publisher remains neutral with regard to jurisdictionalclaimsinpublishedmapsandinstitutionalaffiliations. ThisSpringerimprintispublishedbytheregisteredcompanySpringerNatureSwitzerlandAG Theregisteredcompanyaddressis:Gewerbestrasse11,6330Cham,Switzerland ’ Supervisor s Foreword Exploitingtheuniquepropertiesofcoherentlaserlighthasbecomeessentialinour modernsociety.High-speedInternetisenabledbyopticallyencodeddatapackages. Laser material processingis, for instance, indispensableintheautomotiveindustry or in display manufacturing. Medical applications such as eye surgeries, cancer diagnosis, and treatment have been established. Yet, the development of laser technology, promising new exciting applications, is ongoing with no end in sight. One of the unique properties of coherent light is its compressibility to extreme timescales of femto- (10−15 s) or even attoseconds (10−18 s), resulting in two outstanding properties: First, ultrashort laser pulses are capable of tracing all motionsofelementarymatteroutsidethenucleusinrealtime.Second,laserpulses can reach power levels of large-scale power plants (partially even orders of mag- nitudemore)foranultrashorttimeandthusareabletoprovidethehighestdegreeof control over molecules, atoms, and electrons. Althoughfemtosecondpulsesweredemonstrated inthe1970sforthe firsttime, their wide applicability was enabled by the progress in the development of solid-state lasers. In particular, the demonstration of the Kerr-lens mode-locked Ti:sapphire oscillator in 1990 set a milestone for ultrafast laser development. Whereas Ti:sapphire lasers have enabled full control over the optical waveform, lightpulsegenerationwithdurationsofaboutasinglecarrierwavecycleandoctave spanning bandwidths as well as attosecond pulse formation via high harmonic generation, it remains elusive to trigger these spectacular phenomena at least a million times per second. Here, the “new generation of high-power, waveform controlled few-cycle light sources” comes into play. Peak and average power-scalable diode-pumped solid- state lasers, e.g. thin-disk lasers, can nowadays readily deliver up to kilowatts of optical average power. However, the available gain media are only capable of generating pulses with at best about 100 fs of duration, being far away from the few-cycle limit where full light field control becomes important and strong field effects are attainable. v vi Supervisor’sForeword This dissertation demonstrates ways of combining the best of both worlds. Marcus Seidel achieved few-cycle pulse generation and waveform control, prop- erties of the Ti:sapphire technology, from a high-power Kerr-lens mode-locked thin-disk laser oscillator. The workfirst brieflylooks backtomore than 50 years ofultrashortpulse laser generation. By doing so, methods and goals of the technical development are highlighted, setting the road map for 5 years of Ph.D. work. At the same time, an introduction for newcomers to the field, which points out general ideas of the research subject, is provided. In the experimental chapters, three major achieve- ments are described: First, the implementation of an amplification-free multi-watt few-cycle pulse source that can be carrier-envelope-phase stabilized yielding full controlovertheopticalwaveformisreported.Second,power-scalablemethodsare demonstratedwhichpresenttheprospectofwaveformcontrolledfew-cyclesources with hundreds of watts average power. Third, frequency conversion to the mid-infraredspectralregionisaccomplishedatunprecedentedhighaveragepowers. This presents an excellent example for applications of the new powerful ultra- short laser pulse sources. Experiments with low yield, for instance frequency conversion to the ultraviolet or mid-infrared, will strongly benefit from the sig- nificantlyincreasedpowerlevelsoftheultrafastnear-infrareddrivingsources.This in turn promises novel applications in both fundamental and applied sciences. For instance, my research group is exploring ways to employ this advanced laser technology in early cancer diagnosis. In this respect, Marcus Seidel’s Ph.D. thesis presents a very good reference for laser scientists working on the forefront of ultrafast optics as well as graduate students who are new to the field. Additionally, the reported results will have an impact on multi-disciplinary research topics where short pulse lasers serve as essential instruments. Garching, Germany Prof. Dr. Ferenc Krausz December 2018 Abstract With the advent of peak and average power-scalable femtosecond lasers, in par- ticular mode-locked thin-disk oscillators, the need for equally scalable pulse compression, carrier-envelope-phase stabilization, and frequency conversion schemes arose. These techniques have been routinely applied to lower average power ultrafast lasers, for instance, the widely used Ti:sapphire based ones. But they had to be reinvented for cutting-edge femtosecond sources with 100 W-level average and 10 MW-level peak powers. This dissertation presents how pulses emitted from a 45-W-average power mode-lockedthin-diskoscillatorwerecompressedforthefirsttimetoadurationof onlyafewopticalcycles.Pulsesasshortas7.7fswereattainedfromtwosequential spectral broadening and chirped mirror pulse compression stages. The same light source was also the first thin-disk oscillator, and simultaneously the first oscillator with an average power of more than 10 W, which was carrier-envelope-phase stabilized. Two stabilization methods are presented: The first one utilized intracavity loss modulation by means of an acousto-optic modu- lator. This resulted in 125 mrad in-loop and 270 mrad out-of-loop residual phase noise. The second one employed pump diode current modulation by means of an auxiliary power supply. This approach yielded a 390 mrad residual in-loop phase noise. Whereas the presented carrier-envelope-phase stabilization schemes are power-scalable,thescalability oftheinitial few-cycle pulse generation approachis restricted by the damage threshold of solid-core photonic crystal fiber. The thesis reports in detail on the limitations of these fibers with respect to maximally transmittablepeakpowersandattainablespectralbroadeningfactors.Moreover,an alternative approach utilizing hollow-core Kagomé-type photonic crystal fibers is demonstrated. A double-stage broadening and compression setup yielded pulse durations of only 9.1 fs, but also showed a significant intensity noise increase in comparison with the thin-disk oscillator output. Therefore, spectral broadening in bulk crystals was studied. By exploiting the optical Kerr effect, spectra with Fourier transform limits of 15 fs were achieved, opening the perspective for all solid-state spectral broadening in bulk material. vii viii Abstract Simulation results for a sequence of thin Kerr media predict a good power effi- ciency of the method. Furthermore, an experimental realization of pulse compres- sionfrom190fspulseswith90Waveragepowerto30fspulseswith70Waverage power in self-defocusing BBO crystals is reported. The presented comprehensive study on spectral broadening and pulse compression techniques paves the way to few-cyclepulsegeneration athundredsofMWpeakpowerandhundredsofWatts average power. Eventually, the dissertation addresses the issue of transferring broadband, powerful spectra to a wavelength region with a huge variety of characteristic molecular absorptions—the mid-infrared. Frequency down-conversion via optical parametricamplificationresultedinradiationwithupto5Wat4.1lmand1.3Wat 8.5 lm, corresponding to an order-of-magnitude average power increase for compact femtosecond light sources operating at wavelengths longer than 5 lm. In addition to the power measurements, both wavelength tunability and super- continuum generation by means of cascaded quadratic nonlinearities are reported, resulting in overall spectral coverage from 1.6 to 11 lm with power spectral densities exceeding 1 lW/cm−1 over the entire range. The pulse compression and carrier-envelope-phase stabilization schemes demonstrated in this dissertation will serve as fundamental techniques for the fur- ther development of a new generation of waveform-controlled few-cycle pulse lasers which are capable of triggering extreme nonlinear effects at unprecedented average powers and repetition rates. The multi-octave spanning, mid-infrared femtosecond source offers exciting opportunities for molecular fingerprinting, in particular by means of frequency up-conversion and field-sensitive techniques as well as frequency comb spectroscopy. Preface The year 2018 was special in the history of femtosecond laser technology. On December 10, the Nobel Prize in Physics was awarded to Gérard Mourou and his former Ph.D. student Donna Strickland “for their method of generating high-intensity, ultrashort optical pulses.” In their joint paper from 1985, the sci- entists proposed and demonstrated the concept of chirped pulse amplification. By means of this still very common optical amplification method, Strickland and Mourou successfully tackled an ever-present dilemma in femtosecond laser tech- nology: Owing to the compression of light energy to ultrashort time scales, extremelyhighinstantaneousopticalpowerscanreadilybeachievedandnonlinear effects in any material are triggered. On the one hand, these nonlinearities are essential for femtosecond pulse applications. On the other hand, they are detri- mental for femtosecond pulse generation as they induce distortions of the laser beam or even material damage. Today, the lasers employing Strickland’s and Mourou’s invention have found a large variety of applications in fundamental science and industry. This demonstrates the significance of femtosecond laser developmentwhichisstillasdemandedasinthe1980s,andtheolddilemmaisstill challenged—yet on a significantly higher level. The title of this dissertation: “A new generation of high-power, waveform controlled,few-cyclelightsources”,alreadyindicatesthatscalingupopticalpower hasnotlostitstopicality.WhereasStricklandandMourouachievedabreakthrough in increasing instantaneous power, the currently developed femtosecond laser technology aims for combining both high instantaneous and high average power. This poses new challenges on retaining unique ultrashort light pulse properties at anyopticalpowerlevel. Suchpropertiesarethecompressionoflightenergy down toafewelectricfieldcyclesandtheprecisecontrolofthelightfieldwaveform.Ina nutshell,thiswasthesubjectoftheresearchIconductedbetweenAugust2012and June 2017 at the Laboratory for Attosecond Physics in the group of Prof. Ferenc Krausz. The first version of this thesis was submitted in February 2018. Only minor changes of the text have been made since then. Consequently, some novel devel- opments might be missing in Sect. 1.2 (History) or Chap. 5 (Outlook). The large ix

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