Extreme-ultraviolet coherent pulse amplification in argon

The amplification of ultrashort extreme-ultraviolet (XUV) pulses in argon in high-order harmonic generation processes is studied by using the time-dependent Schrödinger equation in the spin-free one-active-electron and single-atom approximation. We consider a neutral argon atom initially in the valence 3p state and a sufficiently intense two-cycle driving infrared (IR) pulse for the atom to be mainly ionized after the first laser cycle. The correlated dynamics and transitions from the valence 3p to a virtual subvalence 3s state and in the ionized regions are examined by synchronizing a 1.5-fs XUV pulse to the IR pulse. The calculated single-atom gain spectrum (26–45 eV) agrees with recent experimental measurements. We discuss different channels that can be present in the gain process as a function of pulse parameters and through an analysis of the dynamics of the populations in terms of field-free eigenstates. When the XUV pulse is considered at the end of the driving IR field the amplification is due to contributions of stimulated recombination from excited Rydberg and low energy continuum states to the 3s and 3p states of argon. In regions where the IR field is intense, high energy and angular momentum states are populated and the medium can interact with the pulses through bound and continuum states involving parametric transitions, which is further confirmed by studying classical electron trajectories. We discuss how these parametric interactions might be suitable for amplification of photon energies far from the ionization threshold as observed in the experiments.Peer ReviewedPostprint (author's final draft

Cathode Sheath Thickness of a Microhollow Cathode Discharge Plasma In Argon High Gas Pressures

In a glow discharge, the sheath region that is formed around the cathode surface has a decisive effect on the generation of plasmas. In order to investigate the sheath structure in an atmospheric pressure plasma, we developed a microhollow cathode discharge (MHCD) device. The MHCD device had a cathode diameter of 0.5 mm and its length of 2.0 mm. The discharge was operated at a discharge voltage and current of -220 V and 15 mA, respectively, up to 20 kPa of He-Ar mixtures. We carried out the visible/UV emission spectroscopy, which enabled us to understand the characteristics in the cathode sheath. It was found that two dimensional emission images attributed to Ar+ ion and neutral atom showed significantly different behavior with increasing gas pressure. By comparing the results obtained by an ionizing sheath theory with experimental ones, the detail of the sheath structure is clarified

Core-level attosecond transient absorption spectroscopy of laser-dressed solid films of Si and Zr

We investigated experimentally as well as theoretically the ultrafast response of the wave function of the conduction band (CB) of Si and Zr to a near-infrared laser field using extreme ultraviolet (XUV) absorption spectroscopy in the spectral range of 80–220 eV. The measured dynamics of the XUV transmission demonstrates that the wave function of the CB follows the electric field of the dressing laser pulse. In these terms, laser dressing was earlier mainly studied on gases. Measurements with two-femtosecond and 200-attosecond temporal steps were performed in the vicinity of the SiL2,3 edge near 100 eV, the SiL1 edge near 150 eV, and the ZrM4,5 edge near 180 eV. The observed changes were dependent on the core states being excited by the XUV probe pulse. At the 2p to CB transitions of Si, the XUV transmission increased via the effect of the dressing laser pulse, while at the 2s to CB transition of Si and the 3d to CB transition of Zr, the XUV transmission decreased. Furthermore, beats between the transition from 2p1/2 and 2p3/2 levels of Si and from 3d3/2 and 3d5/2 levels of Zr were observed with 20.7 fs and 3.6 fs periods.Peer ReviewedPostprint (author's final draft

Spectroscopic Observation of He Arcjet Plasma Expanding Through a Converging and Diverging Slit Nozzle

An arcjet plasma generator with a converging and diverging slit nozzle was constructed. This plasma source allowed us to directly observe the arc plasma in the discharge section, which provided useful information about a transition from ionizing thermal plasma to recombining phase. Spatial distributions of the electron temperature and density in the rectangular shaped anode nozzle were evaluated by visible emission spectroscopy. The temperature and density for a discharge current of 20 A were determined to be 0.18 eV and 3.7×1013 cm-3, respectively, at the nozzle throat. The values on the jet axis were compared with those calculated by the gas dynamic theory on one dimensional slit nozzle

Attosecond sublevel beating and nonlinear dressing on the 3d-to-5p and 3p-to-5s core-transitions at 91.3 eV and 210.4 eV in krypton

© 2017 Optical Society of America. Users may use, reuse, and build upon the article, or use the article for text or data mining, so long as such uses are for non-commercial purposes and appropriate attribution is maintained. All other rights are reserved.Applying extreme ultraviolet (XUV) transient absorption spectroscopy, the dynamics of the two laser dressed transitions 3d5/2-to-5p3/2 and 3p3/2-to-5s1/2 at photon energies of 91.3 eV and 210.4 eV were examined with attosecond temporal resolution. The dressing process was modeled with density matrix equations which are found to describe very accurately both the experimentally observed transmission dynamics and the linear and nonlinear dressing oscillations at 0.75 PHz and 1.5 PHz frequencies. Furthermore, using Fourier transform XUV spectroscopy, quantum beats from the 3d5/2-3d3/2 and 3p3/2-3p1/2 sublevels at 0.3 PHz and 2.0 PHz were experimentally identified and resolved.Peer ReviewedPostprint (published version

Mass Spectrometry Study of Reactive Species in a Microhollow Cathode Discharge in He+H2O Mixtures

Reactive species created by a microhollow cathode discharge (MHCD) in He+H2O mixtures were investigated at 160 Torr using the molecular beam mass spectrometry. Ion currents of He+, HeH+, O+, OH+, H+(H2O), and H+(H2O)2 were measured as functions of H2O relative concentration (0.24 - 14%) and discharge current (5 - 15 mA). When the concentration exceeds 6%, most of the ions were decreased, but H+(H2O)2 (cluster ion) significantly increased at discharge current of 5 mA, whose ion current was very low compared with those of other ions. With increasing the discharge current, the cluster ion showed a sharp decrease, while the other ions were almost constant. These features were reasonably explained by the increase of the gas temperature and the plasma rarefaction due to the Joule heating of the working gas

Millimeter-wave Wireless LAN and its Extension toward 5G Heterogeneous Networks

Millimeter-wave (mmw) frequency bands, especially 60 GHz unlicensed band, are considered as a promising solution for gigabit short range wireless communication systems. IEEE standard 802.11ad, also known as WiGig, is standardized for the usage of the 60 GHz unlicensed band for wireless local area networks (WLANs). By using this mmw WLAN, multi-Gbps rate can be achieved to support bandwidth-intensive multimedia applications. Exhaustive search along with beamforming (BF) is usually used to overcome 60 GHz channel propagation loss and accomplish data transmissions in such mmw WLANs. Because of its short range transmission with a high susceptibility to path blocking, multiple number of mmw access points (APs) should be used to fully cover a typical target environment for future high capacity multi-Gbps WLANs. Therefore, coordination among mmw APs is highly needed to overcome packet collisions resulting from un-coordinated exhaustive search BF and to increase the total capacity of mmw WLANs. In this paper, we firstly give the current status of mmw WLANs with our developed WiGig AP prototype. Then, we highlight the great need for coordinated transmissions among mmw APs as a key enabler for future high capacity mmw WLANs. Two different types of coordinated mmw WLAN architecture are introduced. One is the distributed antenna type architecture to realize centralized coordination, while the other is an autonomous coordination with the assistance of legacy Wi-Fi signaling. Moreover, two heterogeneous network (HetNet) architectures are also introduced to efficiently extend the coordinated mmw WLANs to be used for future 5th Generation (5G) cellular networks.Comment: 18 pages, 24 figures, accepted, invited paper

Electron spectra of xenon clusters irradiated with a laser-driven plasma soft-x-ray laser pulse

Xenon clusters were irradiated with plasma soft-x-ray laser pulses (having a wavelength of 13.9 nm, time duration of 7 ps, and intensities of up to 10 GW/cm2). The laser photon energy was high enough to photoionize 4d core electrons. The cross section is large due to a giant resonance. The interaction was investigated by measuring the electron energy spectra. The photoelectron spectra for small clusters indicate that the spectral width due to the 4d hole significantly broadens with increasing cluster size. For larger clusters, the electron energy spectra evolve into a Maxwell-Boltzmann distribution, as a strongly coupled cluster nanoplasma is generated

Emission spectroscopy of a microhollow cathode discharge plasma in helium-water gas mixtures

A dc microhollow cathode discharge (MHCD) plasma was generated inflowing helium gas containing water vapor. The cathode hole diameters were 0.3, 0.7, 1.0, and 2.0 mm, each with a length of 2.0 mm. Emission spectroscopy was carried out to investigate the discharge mode and to determine the plasma parameters. For the 0.3-mm cathode, stable MHCDs in an abnormal glow mode existed at pressures up to 100 kPa, whereas for larger diameters, a plasma was not generated at atmospheric pressure. An analysis of the lineshapes relevant to He at 667.8 nm and to Hα at 656.3 nm implied an electron density and gas temperature of 2 × 1014 cm-3 and 1100 K, respectively, for a 100-kPa discharge in the negative glow region. The dependence of the OH band, and Hα intensities on the discharge current exhibited different behaviors. Specifically, the OH spectrum had a maximum intensity at a certain current, while the H atom intensity kept increasing with the discharge current. This observation implies that a high concentration of OH radicals results in quenching, leading to the production of H atoms via the reaction OH + e- → O + H + e-