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We present single-shot electron velocity-map images of nanoplasmas generated from doped helium nanodroplets and neon clusters by intense near-infrared and mid-infrared laser pulses. We report a large variety of signal types, most crucially depending on the cluster size. The common feature is a two-component distribution for each single-cluster event: A bright inner part with nearly circular shape corresponding to electron energies up to a few eV, surrounded by an extended background of more energetic electrons. The total counts and energy of the electrons in the inner part are strongly correlated and follow a simple power-law dependence. Deviations from the circular shape of the inner electrons observed for neon clusters and large helium nanodroplets indicate non-spherical shapes of the neutral clusters. The dependence of the measured electron energies on the extraction voltage of the spectrometer indicates that the evolution of the nanoplasma is significantly affected by the presence of an external electric field. This conjecture is confirmed by molecular dynamics simulations, which reproduce the salient features of the experimental electron spectra., Comment: 12 pages,12 figures, submitted to New Journal of Physics: Article reference: NJP-113197

Above-threshold ionization spectra from cesium are measured as a function of the carrier-envelope phase (CEP) using laser pulses centered at $3.1\text{ }\text{ }\ensuremath{\mu}\mathrm{m}$ wavelength. The directional asymmetry in the energy spectra of backscattered electrons oscillates three times, rather than once, as the CEP is changed from 0 to $2\ensuremath{\pi}$. Using the improved strong-field approximation, we show that the unusual behavior arises from the interference of few quantum orbits. We discuss the conditions for observing the high-order CEP dependence, and draw an analogy with time-domain holography with electron wave packets.

Laser induced x-ray fluorescence were observed against laser polarization ellipticity. While emission from krypton peaks at linear polarization, a signature of recollision, emission from neon shows opposite trend. We attribute it to two competing processes.

Spectral broadening of 3.2 µm pulses is experimentally demonstrated at 8.2 W average power output through nonlinear propagation in thin crystal plates. Sub-two-cycle compression was achieved with a bulk compressor and custom-made dispersive mirrors.

High-harmonic generation (HHG) in crystals offers a simple, affordable and easily accessible route to carrier-envelope phase (CEP) measurements, which scales favorably towards longer wavelengths. We present measurements of HHG in ZnO using few-cycle pulses at 3.1µm. Thanks to the broad bandwidth of the driving laser pulses, spectral overlap between adjacent harmonic orders is achieved. The resulting spectral interference pattern provides access to the relative harmonic phase, and hence, the CEP.

In the past decade ultrafast mid-IR laser sources have experienced fast growing, due to the fact that many applications benefit from long wavelength driving pulses. For instance the cut-off energy for high harmonic generation can be extended to the keV range [1], however the conversion efficiency varies as ∼λ−6 [2]. Consequently, the generation of few-tens of mJ mid-IR driving pulses is essential together with high repetition rate in order to compensate the low efficiency in strong field experiments. Commercially available ytterbium-based thin-disk pump lasers are now capable of delivering 200 mJ pulses at 1 kHz repetition rate [3], and it is expected that the average power of these pump lasers will be promoted to the 2 kW range soon. These are ideal sources for pumping mid-IR chirped pulse optical parametric amplifier (OPCPA) systems, thus making it possible to produce multi-mJ pulses in the mid-IR range beyond 1 kHz repetition rate.

In recent years attosecond science motivated the development of laser systems, which provide millijoule energy level few-cycle pulses. These experiments rely on a field with stable amplitude and carrier envelope phase (CEP) to provide reproducible results [1]. As a consequence, diagnostic devices, which are capable of single-shot CEP measurement and pulse tagging, became a must-have equipment in today's laser systems. The self-referenced interferometry such as f-to-2f [2] are one of the most typical way to characterize the shift of the CEP in a single­shot manner. The highest achievable recording rate is mostly limited by the measurement times of the optical spectroscopes, which can only reach 10 kHz even with fast detector array.

This paper presents a single-shot technique for measuring CEP. The Temporal dispersion based One-shot Ultrafast Carrier envelope phase Analysis method (TOUCAN) is an arbitrary repetition rate single-shot CEP drift measurement technique based on dispersive Fourier transformations and has been experimentally tested at 100 kHz. TOUCAN was validated by a direct comparison of decimated data with an independent traditional CEP drift measurement technique. The impact of a temporal jitter on the CEP drift measurement is investigated and a new mitigation technique is shown to produce high accuracy jitter-free CEP drift extraction. (C) 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement

A 10 kHz KTA-based OPCPA system was numerically investigated with exceptionally short 43 mJ signal and 10 mJ idler output pulses. Thermal limitations of the amplifier were thoroughly analyzed with special care on power stages.

We demonstrate an OPCPA driven thin plate compression in the mid-infrared region at 100 kHz with 7 W average output power. The resulting pulses are close to two cycle long and CEP stable.

The interpretation of experimental data from novel mid-infrared few-cycle laser sources requires an understanding of ionization mechanisms and knowledge about related ion yields. Experimental studies have indicated sequential double ionization as the dominant process above 1014 W cm−2. These results contradict a recent prediction that in this spectral region, non-sequential processes dominate the double ionization of xenon up to intensities of about 1015 W cm−2. In either case, the ratio of doubly to singly charged xenon yield reported in previous studies has been limited to a few percent, indicating a regime well below the onset of saturation of the double ionization process. We present an experimental study of double ionization of xenon and krypton atoms exposed to intense near four-cycle pulses at 3.2 μm. Our experiments rely on the ion microscopy technique, which facilitates the detection of ions originating from a restricted region within the interaction volume, thereby reducing the impact of focal averaging. Our measurements suggest that at intensities of close to 1.2 × 1014 W cm−2, double ionization of xenon and krypton is already significantly saturated. In particular, we find a doubly to singly charged yield ratio of about 75 percent for xenon and 25 percent for krypton. We compare our results with the predictions of different models accounting for the effects of volume averaging and focal geometry. We find that in the deeply saturated regime of our experiment, the Perelomov–Popov–Terentyev theory significantly underestimates the observed double ionization yield.

Output pulses of a 100 kHz mid-infrared OPCPA system are post-compressed from 4.7 cycles down to 2.3 cycles by using a combination of a dielectric and a semiconductor crystal in a hybrid thin plate setup. Efficient spectral broadening is demonstrated with 11 W average input power. After compression the output power reached 6.8 W with exceptional CEP and energy stability for a several hours. The post-compressed pulses were carefully characterized in both temporal and spatial domains, resulting in 2.3-cycle temporal duration at 3.1 μ m central wavelength with a temporal Strehl ratio of 0.73 and a spatial Strehl ratio of 0.97. Thermal limitations due to multiphoton absorption of semiconductors present at this power level are explored by temperature measurements, which are supported by detailed numerical simulations. Upscaling for higher average powers was also investigated.

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. (C) 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement

A novel copolymer poly(thiophene-2,5-diyl-2,5-di-n-octyloxycarbonyl-1,4-phenylene), denoted as P33, is introduced as potential material for photovoltaics, polymer light-emitting diodes, and/or organic transistors. P33 dissolved in chloroform is investigated by steady-state absorption, linear/non-linear fluorescence spectroscopies and time-resolved fluorescence spectroscopy. Molar extinction coefficient, fluorescence quantum yield, and singlet fluorescence lifetime of P33 are determined to be 18,315 M−1 cm−1, 0.4, and 810 ps, respectively. The P33 fluorescence fast components of decay times are 1.2 ps, 2.0 ps, and 0.5 ps for increasing wavelengths of 480 nm, 500 nm, and 520 nm, respectively. The fast component is attributed to a transport of nearly instantaneously formed excitons to localized states known as downhill energy transfer. Additionally multi-photon excited fluorescence is observed for pumping with wavelengths of 800 nm and 1200 nm. Two-photon absorption cross-section is determined to be 6.9 GM. These spectroscopic studies provide basic fluorescence characteristics of the novel thiophene copolymer P33.

The solvation and confinement of coumarin C153 within supramolecular host/guest complexes based on β-cyclodextrin (β-CD) and 6-deoxy-6-thio-β-cyclodextrin (β-CD-SH) in water are studied by fluorescence spectroscopy. For β-CD/C153, the 1:1 complex is proposed, and for β-CD-SH/C153 both the 1:1 and 2:1 complexes are believed to be formed. The 2:1 β-CD-SH/C153 complex has an association constant of 4.2×10(5) M(-1) and a C153 population of 82 %, which are interestingly high values, indicating that the proposed β-CD-SH dimers structure are connected by covalent disulfide bonds; this is supported by mass spectrometry. Solvation related to fast hydrogen-bond rearrangement as a part of fluorescence relaxation is determined by the ultrafast components of time-resolved spectroscopy to be 3 and 7 ps for the 1:1 β-CD/C153 and 2:1 β-CD-SH/C153 complexes, respectively.

In this theoretical work, the performances of ultrafast, carrier-envelope-phase-stable (CEP), mid-IR optical parametric chirped pulse amplification (OPCPA) systems were compared in two different amplification schemes. In the “idler scheme” the mid-IR pulse (idler) is produced in the first difference frequency generation stage of the OPCPA chain and then amplified in the following stages. In the “signal scheme,” the supercontinuum seed is amplified in the OPCPA chain and the mid-IR pulse is generated in the last amplifier stage. According to our results, the idler has higher energy and better energy stability in the idler scheme, while in the signal scheme the signal has the better characteristics in energy and stability. The CEP noise due to the pump intensity fluctuations was found to be lower in the signal scheme. Additionally, chirp optimization revealed that the peak power of the compressed idler and signal pulses at the output of the system is considerably higher if the chirp of the input signal pulse is positive.

Supercontinuum pulses covering the range from 1100 to 1700 nm with energies >1.0 mJ and excellent beam quality are generated via nonlinear spectral broadening of Cr:forsterite (1240 nm, 110 fs) pulses in pressurized molecular nitrogen. Our spectra, which extend over more than half an octave, offer an attractive alternative to intense few-cycle pulse synthesis in the 1–2 μm range and lend themselves as an important add-on to Cr:forsterite laser technologies.