Spatio-temporal characterization of intense few-cycle 2 µm pulses.

We present a variant of spatially encoded spectral shearing interferometry for measuring two-dimensional spatio-temporal slices of few-cycle pulses centered around 2 µm. We demonstrate experimentally that the device accurately retrieves the pulse-front tilt caused by angular dispersion of two-cycle pulses. We then use the technique to characterize 500-650 µJ pulses from a hollow fiber pulse compressor, with durations as short as 7.1 fs (1.3 optical cycles).

Comparison of high-order harmonic generation in uracil and thymine ablation plumes.

We present studies of high-order harmonic generation (HHG) in laser ablation plumes of the ribonucleic acid nucleobase uracil and its deoxyribonucleic acid counterpart thymine. Harmonics were generated using 780 nm, 30 fs and 1300 nm, 40 fs radiation upon ablation with 1064 nm, 10 ns or 780 nm, 160 ps pulses. Strong HHG signals were observed from uracil plumes with harmonics emitted with photon energies >55 eV. Results obtained in uracil plumes were compared with those from thymine, which did not yield signs of harmonic generation. The ablation plumes of the two compounds were examined by collection of the ablation debris on a silicon substrate placed in close proximity to the target and by time-of-flight mass spectrometry. From this evidence we conclude that the differences in HHG signal are due to the different fragmentation dynamics of the molecules in the plasma plume. These studies constitute the first attempt to analyse differences in structural properties of complex molecules through plasma ablation-induced HHG spectroscopy.

Control of light emission of quantum emitters coupled to silicon nanoantenna using cylindrical vector beams.

Light emission of europium (Eu3+) ions placed in the vicinity of optically resonant nanoantennas is usually controlled by tailoring the local density of photon states (LDOS). We show that the polarization and shape of the excitation beam can also be used to manipulate light emission, as azimuthally or radially polarized cylindrical vector beam offers to spatially shape the electric and magnetic fields, in addition to the effect of silicon nanorings (Si-NRs) used as nanoantennas. The photoluminescence (PL) mappings of the Eu3+ transitions and the Si phonon mappings are strongly dependent of both the excitation beam and the Si-NR dimensions. The experimental results of Raman scattering and photoluminescence are confirmed by numerical simulations of the near-field intensity in the Si nanoantenna and in the Eu3+-doped film, respectively. The branching ratios obtained from the experimental PL maps also reveal a redistribution of the electric and magnetic emission channels. Our results show that it could be possible to spatially control both electric and magnetic dipolar emission of Eu3+ ions by switching the laser beam polarization, hence the near field at the excitation wavelength, and the electric and magnetic LDOS at the emission wavelength. This paves the way for optimized geometries taking advantage of both excitation and emission processes.

Characterising and optimising impulsive molecular alignment in mixed gas samples.

Laser induced impulsive molecular alignment has been fully characterized in linear molecules by matching numerical simulations and experimental data of the corresponding rotational wavepacket in the frequency domain. A rigorous procedure for an accurate matching between simulation and experimental data is presented for the first time, making this a versatile technique for experiments where the molecular axis distribution is not directly accessible. Seeding small molecules in Ar as a carrier gas has then been employed to assist cooling and we systematically retrieve the molecule's rotational temperature and alignment distribution for different mixing ratios. For a total backing pressure of 2 bar it was found that seeding 10% N(2) in Ar results in the best cooling. Compared to pure N(2) the rotational temperature was reduced from 24 ± 2 K down to 9 ± 2 K. This leads to an improvement of the peak alignment distribution from = 0.60 to = 0.71. For the same mixing ratio CO(2) was cooled from 34 ± 3 K to 9 ± 1 K improving the alignment distribution from 0.48 to 0.64. In O(2) a cooling from 58 ± 2 K to 37 ± 4 K was observed, corresponding to an alignment distribution improvement from 0.49 to 0.58. The results demonstrate the wide applicability of the characterisation procedure and of seeded supersonic beams to optimise impulsive alignment of small molecules.

Evidence of depolarization and ellipticity of high harmonics driven by ultrashort bichromatic circularly polarized fields.

High harmonics generated by counter-rotating laser fields at the fundamental and second harmonic frequencies have raised important interest as a table-top source of circularly polarized ultrashort extreme-ultraviolet light. However, this emission has not yet been fully characterized: in particular it was assumed to be fully polarized, leading to an uncertainty on the effective harmonic ellipticity. Here we show, through simulations, that ultrashort driving fields and ultrafast medium ionization lead to a breaking of the dynamical symmetry of the interaction, and consequently to deviations from perfectly circular and fully polarized harmonics, already at the single-atom level. We perform the complete experimental characterization of the polarization state of high harmonics generated along that scheme, giving direct access to the ellipticity absolute value and sign, as well as the degree of polarization of individual harmonic orders. This study allows defining optimal generation conditions of fully circularly polarized harmonics for advanced studies of ultrafast dichroisms.