20 research outputs found
Attosecond pulse shaping around a Cooper minimum
High harmonic generation (HHG) is used to measure the spectral phase of the
recombination dipole matrix element (RDM) in argon over a broad frequency range
that includes the 3p Cooper minimum (CM). The measured RDM phase agrees well
with predictions based on the scattering phases and amplitudes of the
interfering s- and d-channel contributions to the complementary photoionization
process. The reconstructed attosecond bursts that underlie the HHG process show
that the derivative of the RDM spectral phase, the group delay, does not have a
straight-forward interpretation as an emission time, in contrast to the usual
attochirp group delay. Instead, the rapid RDM phase variation caused by the CM
reshapes the attosecond bursts.Comment: 5 pages, 5 figure
Phase-matched extreme-ultraviolet frequency-comb generation
Laser-driven high-order harmonic generation (HHG) provides tabletop sources
of broadband extreme-ultraviolet (XUV) light with excellent spatial and
temporal coherence. These sources are typically operated at low repetition
rates, 100 kHz, where phase-matched frequency conversion into
the XUV is readily achieved. However, there are many applications that demand
the improved counting statistics or frequency-comb precision afforded by
operation at high repetition rates, > 10 MHz. Unfortunately, at such
high , phase matching is prevented by the accumulated steady-state
plasma in the generation volume, setting stringent limitations on the XUV
average power. Here, we use gas mixtures at high temperatures as the generation
medium to increase the translational velocity of the gas, thereby reducing the
steady-state plasma in the laser focus. This allows phase-matched XUV emission
inside a femtosecond enhancement cavity at a repetition rate of 77 MHz,
enabling a record generated power of 2 mW in a single harmonic order.
This power scaling opens up many demanding applications, including XUV
frequency-comb spectroscopy of few-electron atoms and ions for precision tests
of fundamental physical laws and constants.Comment: 9 pages, 4 figure
Noncollinear enhancement cavity for record-high out-coupling efficiency of an extreme-UV frequency comb
We demonstrate a femtosecond enhancement cavity with a crossed-beam geometry
for efficient generation and extraction of extreme-ultraviolet (XUV) frequency
combs at a 154 MHz repetition rate. We achieve a record-high out-coupled power
of 600 {\mu}W, directly usable for spectroscopy, at a wavelength of 97 nm. This
corresponds to a >60% out-coupling efficiency. The XUV power scaling and
generation efficiency are similar to that achieved with a single Gaussian-mode
fundamental beam inside a collinear enhancement cavity. The noncollinear
geometry also opens the door for the generation of isolated attosecond pulses
at >100 MHz repetition rate.Comment: 13 pages, 5 figure
A nozzle for high-density supersonic gas jets at elevated temperatures
We present the development of a gas nozzle providing high-density gas at elevated temperatures inside a vacuum environment. Fused silica is used as the nozzle material to allow the placement of the nozzle tip in close proximity to an intense, high-power laser beam, while minimizing the risk of sputtering nozzle tip material into the vacuum chamber. Elevating the gas temperature increases the gas-jet forward velocity, allowing us to replenish the gas volume in the laser-gas interaction region between consecutive laser shots. The nozzle accommodates a 50 μm opening hole from which a supersonic gas jet emerges. Heater wires are used to bring the nozzle temperature up to 730 °C, while a cooling unit ensures that the nozzle mount and the glued nozzle-to-mount connection is kept at a temperature below 50 °C. The presented nozzle design is used for high-order harmonic generation in hot gases using gas backing pressures of up to 124 bar
A nozzle for high-density supersonic gas jets at elevated temperatures
We present the development of a gas nozzle providing high-density gas at elevated temperatures inside a vacuum environment. Fused silica is used as the nozzle material to allow the placement of the nozzle tip in close proximity to an intense, high-power laser beam, while minimizing the risk of sputtering nozzle tip material into the vacuum chamber. Elevating the gas temperature increases the gas-jet forward velocity, allowing us to replenish the gas volume in the laser-gas interaction region between consecutive laser shots. The nozzle accommodates a 50 μm opening hole from which a supersonic gas jet emerges. Heater wires are used to bring the nozzle temperature up to 730 °C, while a cooling unit ensures that the nozzle mount and the glued nozzle-to-mount connection is kept at a temperature below 50 °C. The presented nozzle design is used for high-order harmonic generation in hot gases using gas backing pressures of up to 124 bars
Precise Access to the Molecular-Frame Complex Recombination Dipole through High-Harmonic Spectroscopy
International audienceWe report on spectral intensity and group delay measurements of the highest-occupied molecular-orbital (HOMO) recombination dipole moment of N2 in the molecular-frame using high harmonic spectroscopy. We take advantage of the long-wavelength 1.3 μm driving laser to isolate the HOMO in the near threshold region, 19–67 eV. The precision of our group delay measurements reveals previously unseen angleresolved spectral features associated with autoionizing resonances, and allows quantitative comparison with cutting-edge correlated 8-channel photoionization dipole moment calculations
Phase-matched extreme-ultraviolet frequency-comb generation
Extreme ultraviolet (XUV) laser radiation is commonly produced via high-harmonic generation (HHG) in gases. The lasers that drive this process typically operate at low pulse repetition rates (10 MHz) necessary. Unfortunately, at high repetition rates, plasma accumulates in the XUV generation region and prevents phase-matching, resulting in low HHG efficiency. We use high-temperature gas mixtures to increase the gas translational velocity, thus reduce plasma accumulation and facilitate phase-matching. We experimentally achieve phase-matched HHG at a repetition rate of 77 MHz, generating record power of ~2 mW at 97 nm and ~0.9 mW at 67 nm