Carrier-envelope phase stability of hollow-fibers used for high-energy, few-cycle pulse generation

We investigated the carrier-envelope phase (CEP) stability of a hollow-fiber setup used for high-energy, few-cycle pulse generation. Saturation of the output pulse energy is observed at 0.6 mJ for a 260 um inner-diameter, 1 m long fiber, statically filled with neon, with the pressure adjusted to achieve an output spectrum capable of supporting sub-4fs pulses. The maximum output pulse energy can be increased to 0.8mJ by using either differential pumping, or circularly polarized input pulses. We observe the onset of an ionization-induced CEP instability, which does not increase beyond an input pulse energy of 1.25 mJ due to losses in the fiber caused by ionization. There is no significant difference in the CEP stability with differential pumping compared to static-fill, demonstrating that gas flow in differentially pumped fibers does not degrade the CEP stabilization.Comment: 4 pages, 4 figure

Electron population dynamics in resonant non-linear x-ray absorption in nickel at a free-electron laser

Free-electron lasers provide bright, ultrashort, and monochromatic x-ray pulses, enabling novel spectroscopic measurements not only with femtosecond temporal resolution: The high fluence of their x-ray pulses can also easily enter the regime of the non-linear x-ray–matter interaction. Entering this regime necessitates a rigorous analysis and reliable prediction of the relevant non-linear processes for future experiment designs. Here, we show non-linear changes in the L3-edge absorption of metallic nickel thin films, measured with fluences up to 60 J/cm2. We present a simple but predictive rate model that quantitatively describes spectral changes based on the evolution of electronic populations within the pulse duration. Despite its simplicity, the model reaches good agreement with experimental results over more than three orders of magnitude in fluence, while providing a straightforward understanding of the interplay of physical processes driving the non-linear changes. Our findings provide important insights for the design and evaluation of future high-fluence free-electron laser experiments and contribute to the understanding of non-linear electron dynamics in x-ray absorption processes in solids at the femtosecond timescale

Generation and application of ultrashort laser pulses in attosecond science

In this thesis, I describe the development of a sub-4 fs few-cycle laser system at Imperial College London used to generate and characterise the first single attosecond (1 as = 10-18s) pulses in the UK. Phase-stabilised few-cycle laser pulses were generated using a hollow fibre system with a chirped mirror compression setup. The pulse was fully characterised using frequency-resolved optical gating (FROG) and spectral phase interferometry for direct electric field reconstruction in a spatially encoded filter arrangement (SEA-F-SPIDER). A pulse duration of 3.5 fs was measured with an argon filled hollow fibre. These phase stabilised Infra-Red (IR) pulses were used to generate a continuous spectrum of high harmonics in the Extreme Ultraviolet (XUV) originating from a single half-cycle of the driving field. Using subsequent spectral filtering, a single attosecond pulse was generated. The isolated XUV pulse was characterised using an atomic streaking camera and a pulse duration of ~260 as was retrieved using FROG for complete reconstruction of attosecond bursts (FROG-CRAB). In an experiment conducted at the Rutherford Appleton Laboratory, high harmonics were generated using a two-colour field with an energetic beam at 1300nm and a weak second harmonic orthogonally polarized to the fundamental. By changing the phase between the two fields, a deep modulation of the harmonic yield is seen and an enhancement of one order of magnitude compared to the single colour field with the same energy is observed.EThOS - Electronic Theses Online ServiceGBUnited Kingdo

Sub-4-fs laser pulse characterization by spatially resolved spectral shearing interferometry and attosecond streaking

We demonstrate the generation of high-energy sub-2-cycle laser pulses generated through hollow core fibre pulse compression. We demonstrate their full characterization with two independent methods. For all-optical characterization in amplitude and spectral phase, we employ spatially encoded arrangement spectral phase interferometry for direct electric-field reconstruction using spectrally filtered ancilla beams to characterize the sub-4-fs pulses with spatial resolution. For field-sensitive pulse characterization, we generate isolated attosecond pulses around 93 eV. The attosecond pulse as well as the infrared few-cycle pulse is characterized in amplitude and phase using the frequency resolved optical gating for complete reconstruction of the attosecond bursts technique. We find good agreement between the two methods

Characterization of high-intensity sub-4-fs laser pulses using spatially encoded spectral shearing interferometry

We report on the full amplitude and phase characterization of high-intensity few-cycle laser pulses generated in a single-stage hollow core fiber system with subsequent compression by ultrabroadband chirped mirrors. We use a spatially-encoded arrangement (SEA) spectral phase interferometry for direct electric field reconstruction (SPIDER) with spectral filters for ancilla generation to characterize the sub- 4 fs pulses with spatial resolution

Lateral shearing interferometry of high-harmonic wavefronts.

We present a technique for frequency-resolved wavefront characterization of high harmonics based on lateral shearing interferometry. Tilted replicas of the driving laser pulse are produced by a Mach-Zehnder interferometer, producing separate focii in the target. The interference of the resulting harmonics on a flat-field extreme ultraviolet spectrometer yields the spatial phase derivative. A comprehensive set of spatial profiles, resolved by harmonic order, validate the technique and reveal the interplay of single-atom and macroscopic effects

Comparative studies of resonance enhancement of harmonic radiation in indium plasma using multicycle and few-cycle pulses

Enhanced single harmonic generation is analyzed in indium laser ablation plasmas at excitation conditions of multicycle (30 fs) and few-cycle (3.5 fs) pulses. We demonstrate the strong influence of pulse duration, on the emission spectra from the indium plasma. For few-cycle pulses, the enhanced emissions do not coincide with the expected harmonic wavelengths, which is the case for multicycle pulses. We test the coherent properties of an enhanced emission around 20 eV using polarization and double-slit interference techniques. We also characterize the dynamics of the emissions from the indium plasma by tuning the laser pulse duration. A theoretical analysis is presented to describe the indium plasma emission upon excitation by few-cycle pulses. © 2013 American Physical Society.Peer Reviewe