Intense Femtosecond Laser Research Articles

Bismuth antiphase domain wall: A three-dimensional manifestation of the Su-Schrieffer-Heeger model

The Su, Schrieffer and Heeger (SSH) model, describing the soliton excitations in polyacetylene due to the formation of antiphase domain walls (DW) from the alternating bond pattern, has served as a paradigmatic example of one-dimensional (1D) chiral topological insulators. While the SSH model has been realized in photonic and plasmonic systems, there have been limited analogues in three-dimensional (3D) electronic systems, especially regarding the formation of antiphase DWs. Here, we propose that pristine bulk Bi, in which the dimerization of $(111)$ atomic layers renders alternating covalent and van der Waals bonding within and between successive $(111)$ bilayers, respectively, serves as a 3D analogue of the SSH model. First, we confirm that the two dimerized Bi structures belong to different Zak phases of 0 and $\pi$ by considering the parity eigenvalues and Wannier charge centers, while the previously reported bulk topological phases of Bi remain invariant under the dimerization reversal. Next, we demonstrate the existence of topologically non-trivial $(111)$ and trivial $(11\bar{2})$ DWs in which the number of in-gap DW states (ignoring spin) is odd and even respectively, and show how this controls the interlinking of the Zak phases of the two adjacent domains. Finally, we derive general criteria specifying when a DW of arbitrary orientation exhibits a $\pi$ Zak phase based on the flip of parity eigenvalues. An experimental realization of dimerization in Bi and the formation of DWs may be achieved via intense femtosecond laser excitations that can alter the interatomic forces and bond lengths.

Angle-Resolved Time-of-Flight Electron Spectrometer Designed for Femtosecond Laser-Assisted Electron Scattering and Diffraction

We developed an apparatus for measuring kinetic energy and two-dimensional angular distributions of femtosecond laser-assisted electron scattering (LAES) signals with a high detection efficiency, consisting of a photocathode-type ultrashort pulsed electron gun, a gas injection nozzle, an angle-resolved time-of-flight analyzer, and a time-and-position sensitive electron detector. We also established an analysis method for obtaining the kinetic energy and two-dimensional angular distributions of scattered electrons from raw data of their flight times and the detected positions at the detector recorded using the newly developed apparatus. From the measurement of the LAES processes of Ar atoms in a femtosecond near-infrared intense laser field, we obtained a two-dimensional angular distribution image of the LAES signals and showed that the detection efficiency of the LAES signals was raised by a factor of 40 compared with that achieved before in 2010.

Dynamics of many-body fragmentation of carbon dioxide dimer tetravalent ions produced by intense femtosecond laser fields

We study experimentally the three-body Coulomb explosion dynamics of carbon dioxide dimer <inline-formula><tex-math id="M5">\begin{document}${\rm{(CO_2)}}_{2}^{4+}$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="18-20230699_M5.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="18-20230699_M5.png"/></alternatives></inline-formula> ions produced by intense femtosecond laser field. The three-dimensional momentum vectors as well as kinetic energy are measured for the correlated fragmental ions in a cold-target recoil-ion momentum spectrometer (COLTRIMS). Carbon dioxide dimer is produced during the supersonic expansion of <inline-formula><tex-math id="M6">\begin{document}${\rm{(CO_2)_2}}$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="18-20230699_M6.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="18-20230699_M6.png"/></alternatives></inline-formula> gas from a 30 μm nozzle with 10 bar backing pressure. The linearly polarized laser pulses with a pulse duration (full width at half maximum of the peak intensity) of 25 fs, a central wavelength of 790 nm, a repetition rate of 10 kHz, and peak laser intensities on the order of <inline-formula><tex-math id="M8">\begin{document}${\rm{8 \times10^{14}}}\;{\rm{W/cm^2}}$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="18-20230699_M8.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="18-20230699_M8.png"/></alternatives></inline-formula> are produced by a femtosecond Ti:sapphire multipass amplification system. We concentrate on the three-particle breakup channel <inline-formula><tex-math id="M10">\begin{document}${\rm{(CO_2)_2^{4+}}} \rightarrow {\rm{CO}}_{2}^{2+}+{\rm{CO^+}}+ {\rm{O^+}}$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="18-20230699_M10.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="18-20230699_M10.png"/></alternatives></inline-formula>. The two-particle breakup channels, <inline-formula><tex-math id="M15">\begin{document}${\rm{(CO_2)_2^{4+}}} \rightarrow {\rm{CO}}_{2}^{2+}+ {\rm{CO_{2}}^{2+}}$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="18-20230699_M15.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="18-20230699_M15.png"/></alternatives></inline-formula> and <inline-formula><tex-math id="M19">\begin{document}${\rm{CO_2^{2+}}\rightarrow CO^++O^+}$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="18-20230699_M19.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="18-20230699_M19.png"/></alternatives></inline-formula>, are selected as well for reference. The fragmental ions are guided by a homogenous electric field of 60 V/cm toward microchannel plates position-sensitive detector. The time of flight (TOF) and position of the fragmental ions are recorded to reconstruct their three-dimensional momenta. By designing some constraints to filter the experimental data, we select the data from different dissociative channels. The results demonstrate that the three-body Coulomb explosion of <inline-formula><tex-math id="M20">\begin{document}${\rm{(CO_2)}}_{2}^{4+}$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="18-20230699_M20.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="18-20230699_M20.png"/></alternatives></inline-formula> ions break into <inline-formula><tex-math id="M21">\begin{document}${\rm{CO}}_{2}^{2+}+{\rm{CO}}^++{\rm{O}}^+$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="18-20230699_M21.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="18-20230699_M21.png"/></alternatives></inline-formula> through two mechanisms: sequential fragmentation and non-sequential fragmentation, in which the sequential fragmentation channel is dominant. These three fragmental ions are produced almost instantaneously in a single dynamic process for the non-sequential fragmentation channel but stepwise for the sequential fragmentation. In the first step, the weak van der Waals bond breaks, <inline-formula><tex-math id="M22">\begin{document}${\rm{(CO_2)}}_{2}^{4+}$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="18-20230699_M22.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="18-20230699_M22.png"/></alternatives></inline-formula> dissociates into two <inline-formula><tex-math id="M23">\begin{document}${\rm{CO}}_{2}^{2+}$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="18-20230699_M23.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="18-20230699_M23.png"/></alternatives></inline-formula> ions; and then one of the C=O covalent bonds of <inline-formula><tex-math id="M24">\begin{document}${\rm{CO}}_{2}^{2+}$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="18-20230699_M24.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="18-20230699_M24.png"/></alternatives></inline-formula> breaks up, the <inline-formula><tex-math id="M25">\begin{document}${\rm{CO}}_{2}^{2+}$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="18-20230699_M25.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="18-20230699_M25.png"/></alternatives></inline-formula> ion breaks into <inline-formula><tex-math id="M26">\begin{document}${\rm{CO^+}}$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="18-20230699_M26.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="18-20230699_M26.png"/></alternatives></inline-formula> and <inline-formula><tex-math id="M27">\begin{document}${\rm{O^+}}$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="18-20230699_M27.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="18-20230699_M27.png"/></alternatives></inline-formula>. The time interval between the two steps is longer than the rotational period of the intermediate <inline-formula><tex-math id="M28">\begin{document}${\rm{CO}}_{2}^{2+}$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="18-20230699_M28.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="18-20230699_M28.png"/></alternatives></inline-formula> ions, which is demonstrated by the circle structure exhibited in the Newton diagram. We find that the sequential fragmentation channel plays a dominant role in the three-body Coulomb explosion of <inline-formula><tex-math id="M29">\begin{document}${\rm{(CO_2)}}_{2}^{4+}$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="18-20230699_M29.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="18-20230699_M29.png"/></alternatives></inline-formula> ions in comparison of the event ratio of the two fragmentation channels.

Intense and Superflat White Laser with 700-nm 3-dB Bandwidth and 1-mJ Pulse Energy Enabling Single-Shot Subpicosecond Pulse Laser Spectroscopy.

An optical spectrometer is a basic spectral instrument that probes microscopic physical and chemical properties of macroscopic objects but generally suffers from difficulty in broadband time-resolved measurement. In this work, we report the creation of ultrabroadband white-light laser with a 3-dB bandwidth covering 385 to 1,080 nm, pulse energy of 1.07 mJ, and pulse duration of several hundred femtoseconds by passing 3-mJ pulse energy, 50-fs pulse duration Ti:Sapphire pulse laser through a cascaded fused silica plate and chirped periodically poled lithium niobate crystal. We utilize this unprecedented superflat, ultrabroadband, and intense femtosecond laser light source to build a single-shot (i.e., single-pulse) subpicosecond pulse laser ultraviolet-visible-near-infrared spectrometer and successfully measure various atomic and molecular absorption spectra. The single-shot ultrafast spectrometer may open up a frontier to monitor simultaneously the ultrafast dynamics of multiple physical and chemical processes in various microscopic systems.

Спектры оптических гармоник 2 и 3/2 при субрелятивистской лазерно-плазменной генерации рентгеновского излучения

The dependence of the visible spectrum (harmonics 2 and 3/2) and the output of Cu K-alpha X-ray radiation on the energy and duration of subrelativistic intensity femtosecond laser pulse acting on a copper foil is investigated. It was found that for secondary radiation, the determining factor is the energy density, and not the intensity and duration of the pulse. The power-law dependencies of the secondary output radiation on the duration and the pulse energy have a significantly different indicator. The tendencies of changing the shape of the spectrum for regions dominated by various mechanisms of plasma heating are revealed.

Generation and control of localized terahertz fields in photoemitted electron plasmas.

Dense micron-sized electron plasmas, such as those generated upon irradiation of nanostructured metallic surfaces by intense femtosecond laser pulses, constitute a rich playground to study light-matter interactions, many-body phenomena, and out-of-equilibrium charge dynamics. Besides their fundamental interest, laser-induced plasmas hold great potential for the generation of localized terahertz radiation pulses. However, the underlying mechanisms ruling the formation and evolution of such plasmas are not yet well understood. Here, we develop a comprehensive microscopic theory to predictably describe the spatiotemporal dynamics of laser-pulse-induced plasmas. Through detailed analysis of electron emission, metal screening, and plasma cloud interactions, we investigate the spatial, temporal, and spectral characteristics of the so-generated terahertz fields, which can be extensively controlled through the metal morphology and the illumination conditions. We further describe the interaction with femtosecond electron beams to explain recent ultrafast electron microscopy experiments, whereby the position and temporal dependence of the observed electron acceleration permits assessing the associated terahertz field. Besides its potential application to the design of low-frequency light sources, our work contributes fundamental insight into the generation and dynamics of micron-scale electron plasmas and their interaction with ultrafast electron pulses.

Influence of a pinhole diameter on the experimental determination of critical power for femtosecond filamentation in air.

Filamentation of intense femtosecond laser pulses in optical media has attracted great attention due to its various unique characteristics and potential applications. It is an important task to determine the critical power for the filamentation especially in many applications, which can be obtained by evaluating the transmitted pulse energy through a pinhole located in the filamentation region as a function of input laser energy. The pinhole diameter is very crucial to the measurement. However, there is no report on the experimental determination of critical power for filamentation in air by using the pinhole method and the influence of the pinhole diameter on the determination. In this paper, we numerically and experimentally investigate the influence of pinhole diameter on the determination of the filamentation critical power. The obtained critical power tends to a reasonable value as the decrease of the pinhole diameter, because the transmitted energy through the pinhole with a smaller diameter is more sensitive to the change of energy distribution in the beam cross section during the beginning process of filamentation. Under our experimental condition, the pinhole diameter as small as ∼50 µm is applicable to be used to determine the critical power for filamentation of femtosecond laser pulses in air.

Enhanced energy deposition and carrier generation in silicon induced by two-color intense femtosecond laser pulses

We theoretically investigate the optical energy absorption of crystalline silicon subject to dual-color femtosecond laser pulses, using the time-dependent density functional theory (TDDFT). We employ the modified Becke-Johnson (mBJ) exchange-correlation potential which reproduces the experimental direct bandgap energy $E_g$. We consider situations where the one color is in the ultraviolet (UV) range above $E_g$ and the other in the infrared (IR) range below it. The energy deposition is examined as a function of mixing ratio $\eta$ of the two colors with the total pulse energy conserved. Energy transfer from the laser pulse to the electronic system in silicon is dramatically enhanced by simultaneous dual-color irradiation and maximized at $\eta\sim 0.5$. Increased is the number of generated carriers, not the absorbed energy per carrier. The effect is more efficient for lower IR photon energy, or equivalently, larger vector-potential amplitude. As the underlying mechanism is identified the interplay between intraband electron motion in the {\it valence} band (before excitation) driven by the IR component and resonant valence-to-conduction interband excitation (carrier injection) induced by the UV component. The former increases excitable electrons which pass through the $k$ points of resonant transitions. The effect of different multiphoton absorption paths or intraband motion of carriers generated in the conduction band play a minor role.

Different helicity dependency of all-optical magnetization switching in GdFeCo films with optical interference layer

We observed helicity-dependent all-optical magnetization switching (HD-AOS) phenomena in GdFeCo films, which have equivalent magnetic properties but different optical properties due to optical interference layer thickness. Consequently, we found that these films have different properties of HD-AOS with the duality relation between magnetization direction and light helicity in light absorption, which generally means magnetic circular dichroism (MCD). Therefore, we also evaluated the effective absorption of the same GdFeCo samples which have similar intrinsic MCD by CW laser and intense femtosecond laser pulses. We confirmed that the contribution of this duality relation is much smaller in the absolute value of absorption. Furthermore, the magnitude and the sign of the duality relation change by the interference layer’s thickness. As a result, we concluded that the duality relation in HD-AOS is derived from the duality relation in absorption. Furthermore, its duality relation can be changed effectively using multiple interference effects.

Photoionization and Photofragmentation Dynamics of I2 in Intense Laser Fields: A Velocity-Map Imaging Study.

The photoionization and photofragmentation dynamics of I2 in intense femtosecond near-infrared laser fields were studied using velocity-map imaging of cations, electrons, and anions. A series of photofragmentation pathways originating from different cationic electronic states were observed following single ionization, leading to I+ fragments with distinct kinetic energies, which could not be resolved in previous studies. Photoelectron spectra indicate that these high-lying dissociative states are primarily produced through nonresonant ionization from several molecular orbitals (MO) of the neutral. The photoelectron spectra also show clear signatures of resonant ionization pathways (Freeman resonances) to low-lying bound ionic states via Rydberg states of the neutral moiety. To investigate the role of these Rydberg states further, we imaged anionic products (I-) formed through ion-pair dissociations of neutral molecules excited to these Rydberg states by the intense femtosecond laser pulse. Collectively, these results shed significant new light on the complex dynamics of I2 molecules in intense laser fields and on the important role of neutral Rydberg states in a full description of strong-field phenomena in molecules.

Ripple formation with intense Gaussian femtosecond laser pulses close to the damage threshold

The formation of laser-induced periodic surface structures (LIPSS or ripples) is a topic that has been investigated for almost 60 years. More recently with the advent of ultrashort laser pulses this subject has regained interest, in particular, due to interaction regimes that have not been present so far. Consequently a lot of work has been done in that field, especially with comprehensive experimental and theoretical investigations of the scaling of ripple parameters on laser pulse duration, wavelength, applied fluence, shot number and so on. However, there are still a lot of questions. The present work addresses an important issue on that subject. In particular, ripple formation is investigated at high laser intensity, namely at an intensity sufficiently large to generate a femtosecond-laser induced plasma. Thus ripple formation occurs close to damage threshold. Experimental results and theoretical discussion of ripple formation and the interrelation to laser pulse energy deposition, energy transport and sample damage originating from the optical interaction and additional thermal effects, respectively, are discussed. Most important, a reduction of ripple formation threshold with laser intensity and fluence, respectively, has been observed which is associated by a super-linear increase of the ripple area. The scaling of this reduction with laser fluence obtained from theoretical estimates is in good agreement with the experimental data.

Molybdenum Blues with Tunable Light Absorption Synthesized by Femtosecond Laser Irradiation of Molybdenum Trioxide in Water/Ethanol Mixtures

AbstractMolybdenum blues (MBs) have attracted increasing attention due to their tunable structures and properties, which make them applicable in numerous fields. In this work, a novel strategy to synthesize MB nanorings by irradiating MoO3 suspended in water/ethanol mixtures with intense femtosecond laser pulses is demonstrated. It is found that the MoO3 can be dissolved in the water during laser irradiation to form molybdic acid, which provides an acidic environment for the formation of MB. Concentrations of ethanol as low as 1% result in the formation of MB and by adjusting the concentration of ethanol in the solvent, the absorption band can be tuned due to the modification of MoVOMoVI entities. The MB synthesized in 30% ethanol appears the darkest blue and assembly of vesicles 115 nm in size is observed. At high concentrations of ethanol (>70%), HXMoO3 and MoO3‐X are preferentially formed instead of MB. The photothermal conversion efficiency of the MBs synthesized in 1% and 30% ethanol is above 40%.

Dissociative multiple ionization of carbon dioxide dimers in intense femtosecond laser fields

Molecular dimers have attracted much attention in the study of molecular structure and dynamics due to their complex interactions involving both weak van der Waals and strong covalent bonds. We investigate the dissociative ionization of carbon dioxide dimers exposed to intense femtosecond laser fields. The angular distributions of ionic fragments of the breakup channels ${({\mathrm{CO}}_{2})}_{2}{}^{2+}\ensuremath{\rightarrow}{\mathrm{CO}}_{2}^{+}+{\mathrm{CO}}_{2}^{+}$ and ${({\mathrm{CO}}_{2})}_{2}{}^{3+}\ensuremath{\rightarrow}{\mathrm{CO}}_{2}{}^{2+}+{\mathrm{CO}}_{2}^{+}$ strongly depend on the laser intensity. Simulations based on time-dependent density-functional theory reproduce the experimental observations qualitatively and show that the angular distribution of fragments is determined by the angle-dependent orbital ionization probability and the relative contributions of different orbitals, both of which are intensity sensitive. By comparing the ionization of the dimer with ${\mathrm{CO}}_{2}$ monomer, we find that the weak van der Waals bond and molecular geometry in the dimer play considerable roles. This work extends significantly earlier studies of simple linear covalent bond molecules.

Cascading shocks and heated electrons probability distributions on the surface and inside silicon target irradiated by femto-second laser pulse

The femtosecond laser effects on shock cascades at the surface as well as inside the silicon target are studied by using the two temperature method (TTM) for simulating the non-equilibrium system. The laser wavelength is varied from 500 to 1500 nm and significant modifications are noted in the shock cascades, an electron temperature probability distributions, cumulative probability distributions, and electron-lattice equilibration temperature. It is found that the highest and lowest electron temperatures occurred for 500 and 1500 nm lasers respectively. To explain silicon heating sensitivity to the interacting laser wavelength, fundamental parameters including reflection coefficient, absorption, extinction coefficients, refractive index, specific heat, thermal conductivity, electron-hole collision frequency, and lattice-electron coupling coefficient are comprehensively studied. The temperature distributions included non-Gaussian tails and cumulative probability distributions included shock-like jumps at distinctly different temperatures for considered five types of laser wavelengths. The findings of this study can be helpful for understanding silicon crystals interaction with an ultra-short and an intense femtosecond laser pulses.

Dissociative Ionization and Coulomb Explosion of CHBrCl2 in Intense Near-Infrared Femtosecond Laser Fields

We experimentally demonstrate the dissociative photoionization of CHBrCl2 molecules in a femtosecond laser field by time-of-flight mass spectrum and dc-slice imaging technology. The results suggest that the low kinetic energy components are from the dissociative ionization process of single-charged molecular ions. The angular distribution of fragment Cl+ ions can be attributed to the features of dissociative state and molecular configuration, and that of Br+ ions results from the electronic wave-packet evolution and combination of the multi-dissociation processes. The high kinetic energy components are from the Coulomb explosion of multi-charged molecular ions, and the error of the C-Br distance involved in the Coulomb explosion can be explained by the movement of the effective charge center of the polyatomic molecule.

Melting of Titanium by a Shock Wave Generated by an Intense Femtosecond Laser Pulse

Laser shock peening with ultrashort laser pulses has been studied by hydrodynamic and atomistic simulations, as well as experimentally. It has been shown that, in contrast to traditional nanosecond pulses, ultrashort laser pulses allow one to increase the produced pressures by two or three orders of magnitude from 1–10 GPa to 1000 GPa (1 TPa). The physics of phenomena changes fundamentally because shock waves generating pressures exceeding the bulk modulus of a metal melt it. It has been shown for the first time that the shock melting depth at pressures about 1 TPa is an order of magnitude larger than the thickness of the melt layer caused by heat conduction. The appearance, propagation, and damping of a melting shock wave in titanium have been studied. The damping of the shock wave makes it possible to modify the surface layer, where the melting regime changes from a fast one in the shock jump to a slow propagation of the melting front in the unloading tail behind the shock wave. It has been shown experimentally that the ultrafast crystallization of the melt forms a solid layer with a structure strongly different from that before the action. The measured depth of this layer is in good agreement with the calculation.

Decoherence Effects of Terahertz Generation in Solids under Two-Color Femtosecond Laser Fields

We theoretically investigate terahertz emission from solid materials pumped by intense two-color femtosecond laser field in the presence of decoherence effects. Quantum-mechanical simulations are based on the length gauge semiconductor Bloch equations describing the optical excitation and decoherence with phenomenological dephasing and depopulation times. Contributions of interband and intraband mechanisms are identified in time domain, and the latter has dominated THz generation in solid-state systems. It is found that dephasing is crucial for enhancing asymmetric intraband current and deduced that solid-state materials with short dephasing time and long depopulation time would be optimal selection for strong-field terahertz generation experiments.

Control of the excited-to-ionized atoms ratio in a dense gas in the wake of an intense femtosecond laser pulse.

We demonstrate the sensitivity of the plasma composition in the filament wake channel in a dense gas to the temporal shape of the driving femtosecond laser pulse. During the pulse, the electrons released via strong-field ionization and driven by oscillating laser field are actively engaged in collisional processes with neutral neighbor atoms, including inverse Bremsstrahlung, impact ionization, and collisional excitation. By the end of the pulse, these collisional processes produce considerable numbers of additional free electrons (or ionized atoms) and excited atoms, and these contents of the filament wake channel determine its subsequent evolution dynamics. Addressing the case of high-pressure argon gas and using a kinetic model of these competing collisional processes, we explore the sensitivity of the resulting excited-to-ionized atoms number density ratio to the envelope shape of the driving laser pulse. By considering several families of pulses, we show that asymmetric pulse envelopes skewed toward the earlier time allow for efficient control of the ratio of excited atoms to ionized atoms. The pulse-shape control of the plasma composition in the immediate wake of the laser pulse projects into control of the wake channel evolution and of the associated transient electronic and optical nonlinearities.

Experimental observation of optically generated unipolar electromagnetic precursors.

It was recently predicted [Phys. Rev. A95(6), 063817 (2017) 10.1103/PhysRevA.95.063817] that an intense femtosecond laser pulse propagating in an electro-optic crystal and producing free carriers via multiphoton ionization can generate a unipolar electromagnetic precursor propagating ahead of the laser pulse. Here we report the experimental observation of this phenomenon in a GaP crystal excited by an amplified Ti:sapphire laser.

Optical Harmonics Generation under the Interaction of Intense (up to 1014 W/cm2) Mid-Infrared Femtosecond Laser Radiation of a Fe:ZnSe Laser System with a Dense Laminar Gas Jet

Optical Harmonics Generation under the Interaction of Intense (up to 1014 W/cm2) Mid-Infrared Femtosecond Laser Radiation of a Fe:ZnSe Laser System with a Dense Laminar Gas Jet