The investigation of laser-matter interactions calls for ever shorter pulses as new
effects can thus be explored. With laser pulses consisting of only a few cycles of the
electric field, the phase of these electric field oscillations becomes important for many
applications.
In this thesis ultrafast laser sources are presented that provide few-cycle laser pulses
with controlled evolution of the electric field waveform. Firstly, a technique for phasestabilizing
ultra-broadband oscillators is discussed. With a simple setup it improves the
reproducibility of the phase by an order of magnitude compared to previously existing
methods.
In a further step, such a phase-stabilized oscillator was integrated into a chirped-pulse
amplifier. The preservation of phase-stability during amplification is ensured by
secondary phase detection. The phase-stabilized intense laser pulses from this system
were employed in a series of experiments that studied strong-field phenomena in a
time-resolved manner. For instance, the laser-induced tunneling of electrons from
atoms was studied on a sub-femtosecond timescale.
Additional evidence for the reproducibility of the electric field waveform of the laser
pulses is presented here: individual signatures of the electric field half-cycles were
found in photoelectron spectra from above-threshold ionization.
Frequency conversion of intense laser pulses by high-order harmonic generation is a
common way of producing coherent light in the extreme ultraviolet (XUV) spectral
region. Many attempts have been made to increase the low efficiency of this nonlinear
process, e.g. by quasi phase-matching. Here, high-harmonic generation from solid
surfaces under grazing incidence instead from a gas target is studied as higher
efficiencies are expected in this configuration.
Another approach to increasing the efficiency of high-harmonic generation is the
placing of the gas target in an enhancement resonator. Additionally, the production of
XUV photons happens at the full repetition rate of the seeding laser, i.e. in the region
of several tens to hundreds of megahertz. This high repetition rate enables the use of
the XUV light for high-precision optical frequency metrology with the frequency comb
technique. With such an arrangement, harmonics up to 15th order were produced. A
build-up cavity that stacks femtosecond laser pulses in a coherent manner to produce
intra-cavity pulse energies of more than ten microjoules at a repetition rate of ten
megahertz is presented here...
Table of Contents
1 Introduction
1.1 Time-resolved spectroscopy
1.2 Optical frequency metrology with frequency combs
2 Ultra-broadband oscillators
2.1 Few-cycle Kerr-lens mode-locked Ti:sapphire oscillator
2.2 Chirped mirror technology for dispersion control
2.3 The carrier-envelope phase of a mode-locked oscillator
2.3.1 Measurement of the frequency comb parameters
2.3.2 CE phase stabilization by difference frequency generation
2.3.3 Control of the frequency comb parameters
2.3.4 CE phase stability characterization
2.4 Long-cavity chirped-pulse oscillators
2.4.1 Double-pass post-amplifier
2.5 Conclusions
3 Few-cycle chirped-pulse amplifier systems
3.1 CE phase-stabilized chirped-pulse amplifier system
3.1.1 Origins of CE phase noise of amplified pulses
3.2 Conclusions
4 Femtosecond enhancement cavities
4.1 Passive optical resonators for femtosecond pulses
4.1.1 Dispersion control
4.1.2 Electronic feedback techniques
4.2 Vacuum enhancement cavity at 10 MHz repetition rate
4.3 Conclusions
5 Applications
5.1 Spectroscopy experiments with frequency combs
5.2 High-order harmonic generation
5.2.1 High harmonic generation from surfaces
5.2.2 High-order harmonic generation in an enhancement cavity
5.3 Above-threshold ionization
5.4 Conclusions
6 Outlook
A Appendix
A.1 Origin of the frequency comb
A.2 Offset frequency dependence on group and phase velocity
Research Objectives and Core Topics
The work aims to develop and refine ultrafast laser sources capable of generating few-cycle pulses with a precisely controlled electric field waveform. This control is crucial for probing and manipulating fundamental light-matter interaction processes at the sub-femtosecond and attosecond timescales, enabling high-precision spectroscopy and the investigation of strong-field phenomena.
- Stabilization of carrier-envelope (CE) phase in ultra-broadband oscillators using monolithic difference frequency generation.
- Integration of phase-stabilized oscillators into chirped-pulse amplifier (CPA) systems to maintain phase control for high-energy pulses.
- Development of femtosecond enhancement cavities for high-repetition-rate nonlinear frequency conversion.
- Application of these laser sources to high-order harmonic generation (HHG) from surfaces and gas targets.
- Experimental study of above-threshold ionization (ATI) to demonstrate CE phase-dependent electron dynamics.
Excerpt from the Book
2.3.2 CE phase stabilization by difference frequency generation
As an alternative to the microstructure fiber-based f-to-2f interferometer, a simple, yet highly effective scheme for stabilization of the CE phase is presented here. It avoids the downsides of the previously mentioned scheme, as it allows for CE phase stabilization directly in the usable laser output. Thus, the CE phase is controlled directly in the beam that is used for applications. Due to the moderate dispersion of the employed nonlinear medium, the transmitted laser pulses are re-compressible. As a consequence, the full laser power is used for inducing the nonlinear processes, resulting in an enhanced beating signal. Also, as no branching off for phase stabilization is needed, almost the entire laser power is available for application.
Furthermore, it relies on the integration of the interferometer into a single monolithic crystal, thereby obviating complex alignment-sensitive setups and improving the achievable CE phase stability. Improved spatial overlap between the two interfering waves, due to the absence of walk-off effects, results in an increased signal-to-noise ratio of the beating signal at fCEO. The absence of a microstructure fiber avoids instabilities (in amplitude and phase) associated with coupling into its tiny core.
Summary of Chapters
1 Introduction: Provides an overview of light-matter interactions and introduces the motivation for phase-stabilized ultrashort laser systems in spectroscopy and metrology.
2 Ultra-broadband oscillators: Details the design of Kerr-lens mode-locked Ti:sapphire oscillators and presents a monolithic technique for CE phase stabilization via difference frequency generation.
3 Few-cycle chirped-pulse amplifier systems: Discusses the integration of phase-stabilized oscillators into amplifier chains and analyzes the noise sources that impact phase stability during amplification.
4 Femtosecond enhancement cavities: Explores the use of passive optical resonators to increase pulse energy and average power for applications like high-order harmonic generation at high repetition rates.
5 Applications: Examines practical implementations of the developed laser sources, including frequency comb spectroscopy, high-order harmonic generation from surfaces, and above-threshold ionization experiments.
6 Outlook: Summarizes the technological progress and discusses future potential for scaling pulse energy and extending spectroscopic capabilities to new spectral ranges.
Keywords
Ultrafast laser, Few-cycle pulses, Carrier-envelope phase, CE phase stabilization, Ti:sapphire oscillator, Frequency comb, Chirped-pulse amplification, Enhancement cavities, High-order harmonic generation, Above-threshold ionization, Dispersion control, Nonlinear optics, Spectroscopy, Attosecond physics, Femtochemistry.
Frequently Asked Questions
What is the primary objective of this research?
The research focuses on the generation and stabilization of few-cycle ultrashort laser pulses with a controlled carrier-envelope (CE) phase to enable high-precision time-resolved spectroscopy and strong-field physics.
Which laser system serves as the foundation for these experiments?
The work primarily utilizes mode-locked Ti:sapphire laser oscillators, which are known for their broad spectral bandwidth and suitability for generating ultrashort pulses.
What is the importance of the carrier-envelope (CE) phase?
In ultrashort pulses, the CE phase determines the exact position of the electric field oscillations relative to the pulse envelope, which is critical for steering and observing processes on sub-femtosecond timescales.
How is the CE phase stabilized?
The author introduces a monolithic approach using difference frequency generation (DFG) in a nonlinear crystal, which avoids the complexities and instability issues of traditional fiber-based f-to-2f interferometers.
What is the role of enhancement cavities?
Enhancement cavities are used to increase the pulse energy and peak intensity of the laser pulses while maintaining a high repetition rate, which is necessary for high-efficiency nonlinear processes like high-order harmonic generation.
What are the key themes of the application chapter?
The applications focus on high-resolution frequency comb spectroscopy, the generation of extreme ultraviolet (XUV) light through high-order harmonic generation, and the study of photoelectron dynamics via above-threshold ionization.
Why are solid surfaces explored for high-order harmonic generation (HHG)?
Solid targets offer higher particle density compared to gases, holding the potential for increased conversion efficiency and lower vacuum requirements in the experiment.
How does the author characterize the quality of the CE phase stabilization?
Stability is characterized through both "in-loop" and "out-of-loop" measurements, using techniques such as the Allan variance to determine the long-term drift and timing jitter of the pulses.
- Quote paper
- Dr. Jens Rauschenberger (Author), 2007, Phase-stabilized Ultrashort Laser Systems for Spectroscopy, Munich, GRIN Verlag, https://www.grin.com/document/90151
