Laser-induced inner-shell excitations through direct electron re-collision versus indirect collision.

The dynamics and the decay processes of inner-shell excited atoms are of great interest in physics, chemistry, biology, and technology. The highly excited state decays very quickly through different channels, both radiative and non-radiative. It is therefore a long-standing goal to study such dynamics directly in the time domain. Using few-cycle infrared laser pulses, we investigated the excitation and ionization of inner-shell electrons through laser-induced electron re-collision with the original parent ions and measured the dependence of the emitted x-ray spectra on the intensity and ellipticity of the driving laser. These directly re-colliding electrons can be used as the initiating pump step in pump/probe experiments for studying core-hole dynamics at their natural temporal scale. In our experiment we found that the dependence of the x-ray emission spectrum on the laser intensity and polarization state varies distinctly for the two kinds of atomic systems. Relying on our data and numerical simulations, we explain this behavior by the presence of different excitation mechanisms that are contributing in different ratios to the respective overall x-ray emission yields. Direct re-collision excitation competes with indirect collisions with neighboring atoms by electrons having "drifted away" from the original parent ion.

Carrier-to-envelope phase-stable, mid-infrared, ultrashort pulses from a hybrid parametric generator: Cr:ZnSe laser amplifier system.

Our Cr:ZnSe laser amplifier, seeded by parametric difference mixing, produces 72fs long pulses at the central wavelength of ~2.37µm. The stability of the carrier-to-envelope phase of the amplified seed pulses, attained at the stage of their parametric generation, is preserved through 6 orders of magnitude of laser amplification.

Two-wavelength Tm:YLF/KGW external-cavity Raman laser at 2197 nm and 2263 nm.

This paper presents a KGW Raman laser with an external-cavity configuration at the 2 µm region. The Raman laser is pumped by an actively Q-switched Tm:YLF laser, especially designed for this purpose emitting at 1880 nm. Due to the KGW bi-axial properties, the Raman laser is able to lase separately at two different output lines, 2197 nm and 2263 nm. The output energies and pulse durations that were achieved for these two lines are 0.15 mJ/pulse at 21 ns and 0.4 mJ/pulse at 5.4 ns, respectively. To the best of our knowledge, this is the first time that the KGW crystal, which is well known for its wide use in shorter wavelengths, is demonstrated in a Raman laser in the 2 µm region. According to the achieved results and due to the KGW properties, it appears to be a suitable crystal for energy scaling and efficient Raman conversion in this spectral range. An estimation of the Raman gain coefficient for this wavelength is provided as well.

Parametric amplification in large-aperture diffusion-bonded periodically poled crystals.

With conventional poling techniques of pyroelectric crystals, the thickness of the periodically poled crystals is typically limited to 0.5-1 mm. Such a small aperture of the crystal limits the amount of energy/power that this device may deliver. Here we discuss diffusion bonding as an alternative method to achieve a wider periodically poled crystal, with virtually unlimited width. It is shown that the amplified signal preserved a good beam profile despite a possible phase shift between the stitched crystals. This technique may be extended to larger aperture optical parametric amplifiers and allows for high energy output from periodically poled crystals.

Actively Q-switched tunable narrow bandwidth milli-Joule level Tm:YLF laser.

A pulsed high energy and narrow bandwidth tunable Tm:YLF laser at the milli-Joule level is demonstrated. The spectral bandwidth was narrowed down to 0.15 nm FWHM, while 33 nm of tunability range between 1873 nm and 1906 nm was achieved using a pair of YAG Etalons. The laser was actively Q-switched using an acousto-optic modulator and mJ level pulse energy was measured along the whole tuning range at a repetition rate of 1 kHz. Up to 1.97 mJ of energy per pulse was achieved at a pulse duration of 37 ns at a wavelength of 1879 nm, corresponding to a peak-power of 53.2 kW and at a slope efficiency of 36 %. The combination of both high energy pulsed lasing and spectral tunability, while maintaining narrow bandwidth across the whole tunability range, enhances the laser abilities, which could enable new applications in the sensing, medical and material processing fields.

Ultrafast Excitation of an Inner-Shell Electron by Laser-Induced Electron Recollision.

Extreme ultraviolet attosecond pulses, generated by a process known as laser-induced electron recollision, are a key ingredient for attosecond metrology, providing a tool to precisely initiate and probe subfemtosecond dynamics in atoms, molecules, and solids. However, extending attosecond metrology to scrutinize the dynamics of the inner-shell electrons is a challenge, that is because of the lower efficiency in generating the required soft x-ray (ℏω>300 eV) attosecond bursts. A way around this problem is to use the recolliding electron to directly initiate the desired inner-shell process, instead of using the currently low flux x-ray attosecond sources. Such an excitation process occurs in a subfemtosecond time scale, and may provide the necessary "pump" step in a pump-probe experiment. Here we used a few cycle infrared (λ_{0}≈1800 nm) source and observed direct evidence for inner-shell excitations through the laser-induced electron recollision process. It is the first step toward time-resolved core-hole studies in the keV energy range with subfemtosecond time resolution.

Deciphering excited state evolution in halorhodopsin with stimulated emission pumping.

The primary photochemical dynamics of Hb. pharaonis Halorhodopsin (pHR) are investigated by femtosecond visible pump-near IR dump-hyperspectral probe spectroscopy. The efficiency of excited state depletion is deduced from transient changes in absorption, recorded with and without stimulated emission pumping (SEP), as a function of the dump delay. The concomitant reduction of photocycle population is assessed by probing the "K" intermediate difference spectrum. Results show that the cross section for stimulating emission is nearly constant throughout the fluorescent state lifetime. Probing "K" demonstrates that dumping produces a proportionate reduction in photocycle yields. We conclude that, despite its nonexponential internal conversion (IC) kinetics, the fluorescent state in pHR constitutes a single intermediate in the photocycle. This contrasts with conclusions drawn from the study of primary events in the related chloride pump from Hb. salinarum (sHR), believed to produce the "K" intermediate from a distinct short-lived subpopulation in the excited state. Our discoveries concerning internal conversion dynamics in pHR are discussed in light of recent expectations for similar excited state dynamics in both proteins.