F. Calegari

Millijoule-level phase-stabilized few-optical-cycle infrared parametric source.

Ultrabroadband self-phase-stabilized near-IR pulses have been generated by difference-frequency generation of a filament broadened supercontinuum followed by two-stage optical parametric amplification. Pulses with energy up to 1.2 mJ and duration down to 17 fs are demonstrated. These characteristics make such a source suited as a driver for high-order harmonic generation and isolated attosecond pulse production.

High-energy, few-optical-cycle pulses at 1.5 µm with passive carrier-envelope phase stabilization

We report on a source of ultrabroadband self-phase-stabilized near-IR pulses by difference-frequency generation of a hollow-fiber broadened supercontinuum followed by two-stage optical parametric amplification. We demonstrate energies up to 200 microJ with 15 fs pulse width, making this source suited as a driver for attosecond pulse generation.

Efficient continuum generation exceeding 200 eV by intense ultrashort two-color driver

A temporal gating on the high-order harmonic emission process is achieved using an intense 20 fs, 1.45 microm pulse (IR) in combination with an intense 13 fs, 800 nm pulse [visible (VIS)]. Exploiting this two-color gating scheme, a coherent continuous emission extending up to 160 eV using Ar gas and 200 eV using Ne gas is efficiently generated. The IR pulse contributes to significantly extending the harmonic emission to higher photon energies, whereas the VIS pulse improves the conversion efficiency of the process. These results indicate the possibility to produce bright attosecond pulses approaching the soft X spectral region.

Characterization of a high-energy self-phase-stabilized near-infrared parametric source

We present a broadband self-phase-stabilized near-IR source based on difference frequency generation (DFG) of a filament broadened supercontinuum followed by a two-stage optical parametric amplifier. We demonstrate pulses with energy up to 1.2 mJ and duration down to 17 fs. We theoretically study the process of DFG and investigate the carrier-envelope phase (CEP) dependence on the driving pulse parameters. We find that robust CEP stability is possible even with fluctuations in the phase and intensity of the DFG seed pulse. These characteristics make this parametric source suitable as a driver for high-order harmonic generation and isolated attosecond pulse production.

Elemental sensitivity in soft x-ray imaging with a laser-plasma source and a color center detector

Elemental sensitivity in soft x-ray imaging of thin foils with known thickness is observed using an ultrafast laser-plasma source and a LiF crystal as detector. Measurements are well reproduced by a simple theoretical model. This technique can be exploited for high spatial resolution, wide field of view imaging in the soft x-ray region, and it is suitable for the characterization of thin objects with thicknesses ranging from hundreds down to tens of nanometers.

Table-top soft x-ray imaging of nanometric films

Profiles of nanometric aluminum and parylene foils have been characterized by soft x-ray contact imaging using a laser-plasma source and a LiF crystal as detector. Due to the characteristic emission of this source in a 2π angle, it was possible to obtain the sample image in a wider field of view with respect to coherent sources. LiF crystal is a cheap and robust imaging detector for soft x-ray radiation, that allows one to get high spatial resolution images of thin films with thickness from hundreds down to a few tens of nanometers.

Observation of autoionization dynamics and sub-cycle quantum beating in electronic molecular wave packets

The coherent interaction with ultrashort light pulses is a powerful strategy for monitoring and controlling the dynamics of wave packets in all states of matter. As light presents an oscillation period of a few femtoseconds (T = 2.6 fs in the near infrared spectral range), an external optical field can induce changes in a medium on the sub-cycle timescale, i.e. in a few hundred attoseconds. In this work, we resolve the dynamics of autoionizing states on the femtosecond timescale and observe the sub-cycle evolution of a coherent electronic wave packet in a diatomic molecule, exploiting a tunable ultrashort extreme ultraviolet pulse and a synchronized infrared field. The experimental observations are based on measuring the variations of the extreme ultraviolet radiation transmitted through the molecular gas. The different mechanisms contributing to the wave packet dynamics are investigated through theoretical simulations and a simple three level model. The method is general and can be extended to the investigation of more complex systems.

Complete analog control of the carrier-envelope-phase of a high-power laser amplifier

In this work we demonstrate the development of a complete analog feedback loop for the control of the carrier-envelope phase (CEP) of a high-average power (20 W) laser operating at 10 kHz repetition rate. The proposed method combines a detection scheme working on a single-shot basis at the full-repetition-rate of the laser system with a fast actuator based either on an acousto-optic or on an electro-optic crystal. The feedback loop is used to correct the CEP fluctuations introduced by the amplification process demonstrating a CEP residual noise of 320 mrad measured on a single-shot basis. The comparison with a feedback loop operating at a lower sampling rate indicates an improvement up to 45% in the residual noise. The measurement of the CEP drift for different integration times clearly evidences the importance of the single-shot characterization of the residual CEP drift. The demonstrated scheme could be efficiently applied for systems approaching the 100 kHz repetition rate regime.

Molecular rotovibrational dynamics excited in optical filamentation.

The rotovibrational dynamics excited by optical filamentation in molecular gases is studied in the temporal domain. Two time-delayed replicas of the same laser pulse have been used to generate a first filament, for the rotovibrational excitation of the sample, and a second collinear filament probing the Raman dynamics. The Fermi doublet structure in CO(2) as well as the very fast stretching mode of H(2) were clearly resolved.

Phase-Contrast Imaging of Nanostructures by Soft X Rays from a Femtosecond-Laser Plasma

The possibility of phase-contrast imaging of nanostructures has been analyzed with the use of a femtosecond-laser plasma as a spatially coherent soft x-ray source and a LiF crystal as an x-ray detector having both the submicron spatial resolution in a wide field of view and a high contrast. It is demonstrated that the spatial coherence length of radiation in the wavelength range 1–13 nm at a distance of 30 cm from the femtosecond-laser plasma source is ≃1.5 μm. The achieved spatial coherence of the source is sufficient to obtain high-quality phase-contrast x-ray images of foils with various chemical compositions and a thickness of ≃100 nm.