Parametric Waveform Synthesis: a scalable approach to generate sub-cycle optical transients

The availability of electromagnetic pulses with controllable field waveform and extremely short duration, even below a single optical cycle, is imperative to fully harness strong-field processes and to gain insight into ultrafast light-driven mechanisms occurring in the attosecond time-domain. The recently demonstrated parametric waveform synthesis (PWS) introduces an energy-, power- and spectrum-scalable method to generate non-sinusoidal sub-cycle optical waveforms by coherently combining different phase-stable pulses attained via optical parametric amplifiers. Significant technological developments have been addressed to overcome the stability issues related to PWS and to obtain an effective and reliable waveform control system. Here we present the main ingredients enabling PWS technology. The design choices concerning the optical, mechanical and electronic setups are justified by analytical/numerical modeling and benchmarked by experimental observations. In its present incarnation, the PWS technology enables the generation of field-controllable mJ-level few-femtosecond pulses spanning the visible to infrared range.Comment: 34 page

THz coupler design for electron accelerators

Elektromagnetische Strahlung im THz Bereich war bis vor einigen Jahren ein nur schwer zugänglicher Frequenzbereich. Im Übergang von der Mikrowellenstrahlung zu dem optischen Bereich mangelte es an ver- fügbaren Strahlungsquellen der Frequenz 0, 1 bis 10 THz. Die zunehmende Entwicklung und Optimierung von Strahlungsquellen im THz Bereich ermöglicht deren Einsatz in Beschleuniger- und Spektroskopiean- wendungen. Verbreitete Technologien sind die Backward-Wave Oszillatoren (BWO), optische Mischer oder Quantenkaskadenlaser [14]. Der Einsatz von THz-Strahlung in der Beschleunigerphysik führt zu einer Re- duzierung der Beschleunigungsstrecken durch den höheren Feldgradienten im Vergleich zur Radiofrequenz, die in herkömmlichen Beschleunigerröhren genutzt wird. Elektronenbeschleuniger und Freie-Elektronen- Laser werden in Zukunft in Tischgröße konstruiert werden können. In einem Gemeinschaftsprojekt der "Ultrafast Optics and X-Rays Division" um Prof. Dr. Franz X. Kärtner am "Center for Free-Electron Laser Science" (CFEL) in Hamburg und dem MIT in Cambridge, MA wird die "Coherent Ultrabright Inverse Compton Scattering X-ray Source" (CUBIX) entwickelt. Die acht Meter lange Anordnung, dargestellt in Abbildung 1, besteht aus einer Elektronenquelle, einem Linearbeschleuniger, einer Fokussiereinheit zur Gewährleistung der longitudinalen Periodizität des Elektronenstrahls und einer Kammer zur Erzeugung der Strahlung durch inverse Compton-Streuung. Die CUBIX Source sieht die Verwendung von THz Strahlung zur Elektronenbeschleunigung und eventuell für die inverse Compton-Streuung vor.Das Ziel der folgenden Arbeit ist die Entwicklung und Untersuchung möglicher Wellenleiter-Koppler zur Bündelung der THz-Strahlung in die Apparatur. Die THz Strahlung von ca. 450 GH z ensteht durch den Effekt der optischen Gleichrichtung in einem kryogen gekühltem Lithiumniobat Kristall durch Bestrahlung mit ultrakurzen Laserimpulsen [5]. Der Durchmesser des Kristalls beträgt drei Millimeter. Die Beschleuni- gungsröhre misst 0, 82 mm im Durchmesser und ist zur Anpssung der Phasengeschwindigkeit der elektro- magnetischen Welle an die Elektronengeschwindigkeit mit einer 60 µm dicken Schicht aus Quarz überzo- gen, dargestellt in Abbildung 5. Für den Koppler wird ein konzentrisches, trichterförmiges Profil gewählt das die Strahlung aus dem Vakuum in den Hohlleiter bündelt. Bei der Entwicklung galt es eine hohe Trans- mission der eingekoppelten Energie durch die Minimierung von Reflexionen zu erreichen. Die Berechnun- gen der Koppler wurden numerisch mit CST Microwave Studio (MWS) durchgeführt. Die kommerzielle Software CST MWS basiert auf einem am DESY entwickelten Code zur Lösung der Maxwell Gleichungen durch die finite Integrationsmethode in fein aufgelösten Gitterzellen. Es wird in Abschnitt 7 gezeigt, dass eine sich zum Anfang verjüngende Schicht aus Quarz im Koppler die besten Ergebnisse erzielt. Nach Ende der Beschleunigungsstrecke kann die Welle über einen Ausgangskoppler aus dem Hohlleiter geführt werden um Irritationen im elektromagnetischen Feld zu vermeiden. Mit einer Energietransmission von über 90 % und einem sauberen Gauß-Puls innerhalb der Beschleunigungsstrecke kann die THz-Strahlung effizient zurElektronenbeschleunigung genutzt werden

Ultrabroadband spectral phase measurements by two-dimensional spectral shearing interferometry (2DSI)

This work presents an implementation and optimization of a pulse characterization technique for ultrashort laser pulses. Namely the technique of two-dimensional spectral shearinginterferometry (2DSI) is set up with the development a full automatic MATLAB code to acquire, store and reconstruct the complex pulse in frequency and time domain. The performance of the retrieval code itself is tested with experimental consistency checks and simulations. Prominent characterization techniques such as SPIDER and FROG are presented and compared in a discussion. A Ti:sapphire laser oscillator with a Fourier-transform-limited FWHM duration of 4 fs serves as a source for the first application of our device. Compression of the pulse withdouble chirped mirrors (DCMs) and BaF2 wedges yields a measured pulse duration of 4.2 fs at FWHM. Within the frame of this thesis, a (noncollinear) optical parametric amplifier (OPA) isbuilt. The spectrum spans from 1.2 mm to 2 mm and supports a Fourier-transform-limited FWHM duration of 8.2 fs. The uncompressed pulse is fully characterized and found to have a smooth phase that should be compressible rather evenly. We found that the apparatus presented here gives reliable results from low intensity pulses in the visible spectrum up to high intensity ones in the infrared regime and can be considered as an easy to use, high precision and flexible pulse characterization technique

Strong-Field Physics with a High Energy Waveform Synthesizer

The motivation to control and temporarily isolate a single quantum mechanical event like an electronic transition has driven the quest for ever shorter light pulses. Controlling a highly intense electric field in the visible to infrared wavelength range on a sub-cycle time scale becomes feasible with the concept of the parametric waveform synthesizer. High harmonic generation (HHG) is explored as a primary application with the aim to generate isolated attosecond pulses and high photon energies in the ”Water Window” spectral region from 284 eV to 533 eV. The tight synchronization of pulses from optical parametric amplifiers (OPAs) spanning from 650 nm to 2200 nm is achieved with long-term synthesized waveform stability. To work out and monitor the ideal synthesis parameters, a set of characterization methods are developed in the time and spatial domain.The ability to control sub-cycle fields is displayed via the large variety of XUV and soft X-ray continua as generated via HHG covering photon energies starting from 30 eV all the way upentering the ”Water Window”. The peculiarity of sub-cycle infrared driving pulses allows togenerate isolated attosecond pulses a priori without further gating techniques. The phase-stablecontrol of the driver pulse allows to shape the harmonic continua in bandwidth and central en-ergy.Characterization of both the generated isolated attosecond pulses and the synthesized infrareddriving field is performed in an attosecond streaking experiment yielding reproducible and com-pressed XUV pulses down to 80 as FWHM, including the retrieval of an ≈ 2 octaves spanningdriver pulse.A partial redesign of the vacuum apparatus to aim for high-pressure phase-matching conditionsinside the HHG gas nozzle enabled the generation of soft X-ray continua up to 450 eV. The con-tinuous spectra in neon and helium are investigated as a function of the driving waveform andmacroscopic parameters. A comparison of the pulse energies at soft X-ray bandwidths oncegenerated with the infrared OPA (two-cycle pulse) and once generated with the synthesizedpulse (sub-cycle), shows an up to 5-fold flux increase. The absolute maximum pulse energy ismeasured to 1 pJ integrated from 200 eV onwards.Finally, the design of an experimental chamber, an updated toroidal mirror and a soft X-Rayspectrometer prepare for future attosecond transient absorption experiments

Controlled HHG with a Sub-Cycle mJ-Level Parametric Waveform Synthesizer

We present high-harmonic generation (HHG) driven with a sub-cycle mJ-levelparametric waveform synthesizer. The HHG yield and spectral shape can be controlled byvarying the carrier-envelope phase and the relative phase in the synthesizer channels

Millijoule-level sub-cycle pulses from two channels of a parallel parametric waveform synthesizer

We report on an optical synthesis of two compressed channels from our parametric waveform synthesizer, leading to a 0.6 mJ 3.4 fs pulse (3.2 fs transform limited) with a central wavelength of 1.8 µn, corresponding to 0.6 optical cycles

Extreme UV Spectral Broadening in Hollow Core Fiber

We demonstrate extreme spectral broadening of third harmonic pulses from a 35-fs Ti:Sapphire laser using cross-phase modulation with the fundamental in Neon filled hollow core fibers. The potential for high energy sub-2-fs UV-pulses is explored

Millijoule-Level Sub-Cycle Pulses from Two Channelsof a Parallel Parametric Waveform Synthesizer

We report on an optical synthesis of two compressed channels from our parametric waveformsynthesizer, leading to a 0:6 mJ 4:4 fs pulse (3:6 fs transform limited) with a central wavelengthof 1:8 mm, corresponding to 0:74-cycles