Seth L. Cousin

Spatiotemporal isolation of attosecond soft X-ray pulses in the water window.

Attosecond pulses at photon energies that cover the principal absorption edges of the building blocks of materials are a prerequisite for time-resolved probing of the triggering events leading to electronic dynamics such as exciton formation and annihilation. We demonstrate experimentally the isolation of individual attosecond pulses at the carbon K-shell edge (284 eV) in the soft X-ray water window with pulse duration below 400 as and with a bandwidth supporting a 30-as pulse duration. Our approach is based on spatiotemporal isolation of long-wavelength-driven harmonics and validates a straightforward and scalable approach for robust and reproducible attosecond pulse isolation.

0.5-keV Soft X-ray attosecond continua.

Attosecond light pulses in the extreme ultraviolet have drawn a great deal of attention due to their ability to interrogate electronic dynamics in real time. Nevertheless, to follow charge dynamics and excitations in materials, element selectivity is a prerequisite, which demands such pulses in the soft X-ray region, above 200 eV, to simultaneously cover several fundamental absorption edges of the constituents of the materials. Here, we experimentally demonstrate the exploitation of a transient phase matching regime to generate carrier envelope controlled soft X-ray supercontinua with pulse energies up to 2.9±0.1 pJ and a flux of (7.3±0.1) × 107 photons per second across the entire water window and attosecond pulses with 13 as transform limit. Our results herald attosecond science at the fundamental absorption edges of matter by bridging the gap between ultrafast temporal resolution and element specific probing.

All-PM coherent 2.05 µm Thulium/Holmium fiber frequency comb source at 100 MHz with up to 0.5 W average power and pulse duration down to 135 fs

We report on a dual output all-PM fiber laser system running at 100 MHz repetition rate offering coherent broadband and narrowband pulses centered at 2.05 µm with a spectral FWHM bandwidth of 60 nm and 1.5 nm at up to 360 mW and 500 mW, respectively. The broadband pulses are compressed down to 135 fs. The multi-stage double-clad amplifier based on Tm/Ho codoping is seeded by a supercontinuum light source, spanning from around 1 µm up to 2.4 µm.

Three-dimensional spatiotemporal pulse characterization with an acousto-optic pulse shaper and a Hartmann–Shack wavefront sensor

We demonstrate a simplified arrangement for spatiotemporal ultrashort pulse characterization called Hartmann-Shack assisted, multidimensional, shaper-based technique for electric-field reconstruction. It employs an acousto-optic pulse shaper in combination with a second-order nonlinear crystal and a Hartmann-Shack wavefront sensor. The shaper is used as a tunable bandpass filter, and the wavefronts and intensities of quasimonochromatic spectral slices of the pulse are obtained using the Hartmann-Shack wavefront sensor. The wavefronts and intensities of the spectral slices are related to one another using shaper-assisted frequency-resolved optical gating measurements, performed at particular points in the beam. This enables a three-dimensional reconstruction of the amplitude and phase of the pulse. We present some example pulse measurements and discuss the operating parameters of the device.

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.

Generation of high-fidelity few-cycle pulses at 2.1 μm via cross-polarized wave generation.

We demonstrate the generation of temporally clean few-cycle pulses at 2.1 μm by shortening of 6-optical-cycle pulses via cross-polarized wave (XPW) generation in BaF(2), CaF(2) and CVD-Diamond crystals. By combining spectra and single-shot third-order intensity cross-correlation traces in a novel Bayesian pulse retrieval technique, we measured pulse durations of 20 fs, corresponding to 2.8 optical cycles. Our results show that XPW generation in the infrared could provide a high-fidelity source of few-cycle pulses for strong-field physics applications. It could also serve as an injector for high-peak power ultrafast mid-IR wavelength parametric amplifiers.

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.

Attosecond dispersive soft X-ray absorption fine-structure spectroscopy in graphite

Phase transitions of solids and structural transformations of molecules are canonical examples of important photo-induced processes, whose underlying mechanisms largely elude our comprehension due to our inability to correlate electronic excitation with atomic position in real time. Here, we present a decisive step towards such new methodology based on water-window-covering (284 eV to 543 eV) attosecond soft X-ray pulses that can simultaneously access electronic and lattice parameters via dispersive X-ray absorption fine-structure (XAFS) spectroscopy. We validate attoXAFS with an identification of the {\sigma}* and {\pi}* orbital contributions to the density of states in graphite simultaneously with its lattices four characteristic bonding distances. This work demonstrates the concept of attoXAFS as a powerful real-time investigative tool which is equally applicable to gas-, liquid- and condensed phase.

Broadband 7 μm OPCPA pumped by a 2 μm picosecond Ho:YLF CPA system

We generate 260 μJ energy optical pulses at 7 μm with an OPCPA pumped by an optically synchronized 2 μm picosecond CPA system. The mid-IR pulses exhibit a bandwidth supporting sub-4 optical cycle duration.

Carbon K-edge NEXFAS spectroscopy from a table-top soft x-ray high harmonics source driven by sub-2-cycle, CEP stable, 1.85-μm 1-kHz laser pulses

A high-flux table-top coherent soft x-ray high harmonic generation source, driven by sub-2-cycle, 1.85 μm, 1 kHz laser pulses, is applied to a condensed phase sample revealing the carbon binding orbitals of polyimide.