Alexander Grün

Optical amplification in the near-infrared in gas-filled hollow-core fibers

We demonstrate efficient optical parametric amplification and generation in a gas-filled hollow-core fiber of near-infrared pulses, peaked at 1.4 microm wavelength, with 5 microJ energy and 45 fs duration at the fiber output. Numerical simulations confirm that the OPA is phase matched through excitation of higher-order fiber modes.

Wavelength Dependence of the Suppressed Ionization of Molecules in Strong Laser Fields

We study ionization of molecules by an intense laser field over a broad wavelength regime, ranging from 0.8 to 1.5 μm experimentally and from 0.6 to 10 μm theoretically. A reaction microscope is combined with an optical parametric amplifier to achieve ionization yields in the near-infrared wavelength regime. Calculations are done using the strong-field S-matrix theory and agreement is found between experiment and theory, showing that ionization of many molecules is suppressed compared to the ionization of atoms with identical ionization potentials at near-infrared wavelengths at around 0.8 μm, but not at longest wavelengths (10 μm). This is due to interference effects in the electron emission that are effective at low photoelectron energies but tend to average out at higher energies. We observe the transition between suppression and nonsuppression of molecular ionization in the near-infrared wavelength regime (1-5 μm).

Accuracy measurements and improvement for complete characterization of optical pulses from nonlinear processes via multiple spectral-shearing interferometry.

We demonstrate that multiple spectral-shearing interferometry increases the precision and accuracy of measurements of the spectral phase of a complex pulse (time-bandwidth product = 125) arising from self-phase modulation in a gas filled capillary. We verify that the measured interferometric phase is accurate to 0.1 rad across the full bandwidth by checking the consistency between the spectral phases of each individual shear measurement. The accuracy of extracting pulse parameters (group delay dispersion, pulse duration and peak intensity) for single shear measurements were verified to better than 10% by comparison with the multishear reconstruction. High order space-time coupling is quantified across a single transverse dimension, verifying the suitability of such pulses for use in strong field experiments.

Few-cycle pulse characterization with an acousto-optic pulse shaper

An acousto-optic pulse shaper has been used to characterize few-cycle pulses generated in a hollow-core fiber. A grism pair precompensates for the dispersion of the acousto-optic crystal, allowing the full pulse-shaping window to be used for replica generation rather than self-compensation. A 9.4 fs pulse was measured, the shortest ever measured with an acousto-optic pulse shaper, to our knowledge.

Three-wave mixing mediated femtosecond pulse compression in β-barium borate

Nonlinear pulse compression mediated by three-wave mixing is demonstrated for ultrashort Ti:sapphire pulses in a type II phase-matched β-barium borate (BBO) crystal using noncollinear geometry. 170 μJ pulses at 800 nm with a pulse duration of 74 fs are compressed at their sum frequency to 32 fs with 55 μJ of pulse energy. Experiments and computer simulations demonstrate the potential of sum-frequency pulse compression to match the group velocities of the interacting waves to crystals that were initially not considered in the context of nonlinear pulse compression.

On the Wavelength Dependence of the Suppressed Ionization of Molecules in Strong Laser Fields

We study ionization of molecules and atoms with same IP by intense laser field from 0.6-10 μm. A trend from ionization-suppression to non-suppression is found for many molecules as a function of wavelength.

Scalable collinear BiB 3 O 6 OPA for few-cycle CEP stable pulses at 2.1 µm

Few-cycle pulses centered at long wavelengths with stable carrier envelope phase are increasingly sought after for a range of strong-field physics and spectroscopic applications. The generation of such pulses at central wavelengths up to 1.8 µm is well-known [1], however at longer wavelengths non-collinear optical parametric amplifiers must be used to maintain bandwidth. Using the idler from such a non-collinear interaction always implies an angularly dispersed pulse, requiring complicated and lossy compensation strategies [2] which limit scalability to higher energies.

Optical parametric amplification in the NIR in a gaseous medium by use of a hollow fibre

We demonstrate efficient parametric generation of ≫ 4 µJ, 80 fs pulses centered at 1:3 µm in a gas-filled hollow fibre. The measured bandwidth supports few cycle, passively CEP-locked infrared pulses for EUV and attosecond physics.

Near-infrared optical parametric amplification in a gas-filled hollow fibre

Nonlinear optical interactions in gas-filled hollow fibres are currently widely employed for the generation of intense, few-cycle laser pulses[1], while shifting the center wavelength of such few-cycle pulses toward longer wavelengths is of great importance for the generation of extreme ultra-violet radiation [2].