Intense Femtosecond Laser Pulses Research Articles

Amplification characteristics of N<i/>2+ lasing at multiple wavelengths generated by polarization-modulated femtosecond laser fields

Multiple gain routes complicate the amplification behaviors of N2+ lasing. A direct comparison of the amplification processes of various lasing lines of N2+ is still lacking to date, mainly because the efficient generation of different lasing lines requires different experimental conditions. In this work, to overcome the limitation, we use an intense polarization-modulated femtosecond laser pulse to simultaneously produce high-intensity N2+ lasing signals at 391 nm and 428 nm, permitting us readily performing their time-domain characterizations. Our results show that the primary amplification of 428-nm lasing is always retarded by a few picoseconds with respect to the probe pulse while the amplification of 391-nm lasing can be basically completed within the probe pulse duration at a relatively high gas pressure, and besides, the time duration of the former sustains several times longer than that of the latter. These observations indicate the non-negligible contribution of the initial electronic coherence established by the pump laser, apart from the external triggering pulse. For the 391-nm lasing, the seed amplification and superradiance can fiercely interplay within the probe pulse duration. However, for the 428-nm lasing, its amplification is in essence Raman-assisted superfluorescence due to the lacking of the initial electronic coherence and a direct triggering pulse. Our findings not only shed light on the physics of N2+ lasing but also promote the relevant studies on the collective emissions in the cascaded multi-level molecular ion system.

Interference mechanism of plasma self-organization in transparent dielectrics under the intense femtosecond laser pulse exposure

A new mechanism of plasma self-organization in transparent dielectrics with wide bandgap exposed to the intense tightly focused laser radiation was revealed, which causes the generation of 3D periodic ring structures with subwavelength period both along the laser pulse propagation and in the radial direction. The mechanism involves formation of dense plasma burst in the pre-focal region that provides efficient scattering of the incident wave. The interference of a plane incident laser wave in the focal region and a divergent reflected one will form the standing wave pattern with local minima and maxima of laser field both in the direction of the incident wave propagation and perpendicular to it producing the ring patterns of effective ionization regions in the dielectric volume. Analytical and numerical simulations of the process of laser wave scattering on a near-spherical plasma object with dimensions both smaller and larger than the laser radiation wavelength are performed to verify the proposed model.

Pulse repetition-rate effect on the critical power for self-focusing of femtosecond laser in air

The femtosecond laser filamentation is of significant interest due to its remarkable characteristics, and determining the critical power of self-focusing is essential for the process of filamentation. In this work, the critical power for self-focusing of intense femtosecond laser pulses at different repetition rates is experimentally measured according to the focus-shift method. A bimodal fitting method is proposed to more accurately determine the self-focusing critical power. It is found that the self-focusing critical power decreases as the laser repetition rate increases. A numerical simulation of the filamentation process based on the modified nonlinear Schrödinger equation effectively explains the experimental results obtained. This work provides valuable insights for the generation and application of high repetition rate femtosecond laser filamentation.

The Role of Momentum Partitioning in Covariance Ion Imaging Analysis.

We present results from a covariance ion imaging study, which employs extensive filtering, on the relationship between fragment momenta to gain deeper insight into photofragmentation dynamics. A new data analysis approach is introduced that considers the momentum partitioning between the fragments of the breakup of a molecular polycation to disentangle concurrent fragmentation channels, which yield the same ion species. We exploit this approach to examine the momentum exchange relationship between the products, which provides direct insight into the dynamics of molecular fragmentation. We apply these techniques to extensively characterize the dissociation of 1-iodopropane and 2-iodopropane dications prepared by site-selective ionization of the iodine atom using extreme ultraviolet intense femtosecond laser pulses with a photon energy of 95 eV. Our assignments are supported by classical simulations, using parameters largely obtained directly from the experimental data.

Two-Dimensional Projected-Momentum Covariance Mapping for Coulomb Explosion Imaging.

We introduce projected-momentum covariance mapping, an extension of recoil-frame covariance mapping for 2D ion imaging studies. By considering the two-dimensional projection of the ion momenta as recorded by the detector, one opens the door to a complex suite of analysis tools adapted from three-dimensional momentum imaging studies. This includes the use of different frames of reference to unravel the dynamics of fragmentation and the application of fragment momentum constraints to isolate specific fragmentation channels. The technique is demonstrated on data from a two-dimensional ion imaging study of the Coulomb explosion of the cis and trans isomers of 1,2-dichloroethene, following strong-field ionization by an intense near-infrared femtosecond laser pulse. Classical simulations are used to guide the interpretation of projected-momentum covariance maps. The results offer a detailed insight into the distinct Coulomb explosion dynamics for this pair of isomers and lay the groundwork for future time-resolved studies of photoisomerization dynamics in this molecular system.

Structure determination of alkali trimers on helium nanodroplets through laser-induced Coulomb explosion.

Alkali trimers, Ak3, located on the surface of He nanodroplets are triply ionized following multiphoton absorption from an intense femtosecond laser pulse, leading to fragmentation into three correlated Ak+ ions. Combining the information from threefold covariance analysis of the emission direction of the fragment ions and their kinetic energy distributions P(Ekin), we find that Na3, K3, and Rb3 have an equilateral triangular structure, corresponding to that of the lowest lying quartet state A2'4, and determine the equilibrium bond distance Req(Na3) = 4.65 ± 0.15Å, Req(K3) = 5.03 ± 0.18Å, and Req(Rb3) = 5.45 ± 0.22Å. For K3 and Rb3, these values agree well with existing theoretical calculations, while for Na3, the value is 0.2-0.3Å larger than the existing theoretical results. The discrepancy is ascribed to a minor internuclear motion of Na3 during the ionization process. In addition, we determine the distribution of internuclear distances P(R) under the assumption of fixed bond angles. The results are compared to the square of the internuclear wave function |Ψ(R)|2.

Self-assembly plasmonic gold nanoribbons on few-layer PtSe2 under femtosecond laser irradiation

Functional two-dimensional (2D) materials have been extensively explored for a wide range of applications such as energy generation, low-power computing, and biosensing. In this work, we present an approach involving the integration of six-layer platinum diselenide (PtSe2) as an interlayer between the thin gold film and SiO2 substrate to induce spontaneous formation of plasmonic nanostructures (nanoribbons) on the upper gold film (∼8 nm) under 1030 nm femtosecond laser irradiation. The formation of periodic nanostructures is attributed to the periodic energy deposition that occurs in the PtSe2 layers under intense femtosecond laser pulses. Notably, the self-assembled gold nanostructures exhibit a distinctive polarization-dependent plasmonic response in the near-infrared spectral region and could be directly fabricated in a centimeter scale within several minutes. This straightforward method for self-assembling plasmonic nanostructures using layered materials may expand the utility of functional 2D materials and advance the cost-effective and large-area fabrication of plasmonic thin-film nanostructures in a simplified manner.

Size reduction and water dispersibility improvement of hexagonal boron nitride particles by femtosecond laser irradiation in water

Controlling the size and surface state of inorganic particles, which strongly influence their dispersibility in solvents, is important for diverse applications. Intense femtosecond laser pulses can induce plasma formation in material–dispersed solvents and interact with both materials and solvents. In this study, femtosecond laser pulses are employed to modify hexagonal boron nitride (hBN) particles dispersed in water, with the aim of evaluating the effects of the femtosecond laser process on the size reduction and surface modification of hBN particles. Shadowgraph imaging reveals the formation of the reactive environment in hBN–dispersed water, resulting from the ionization of water molecules which leads to the generation of OH radicals. Evaluation of the hBN particle sizes suggests an overall reduction from 160 to 110 nm after 60 min of irradiation and the generation of nanodots between 5 and 10 nm in size. In addition, it is confirmed that the number of particles with higher zeta potentials increases after the samples are laser-irradiated, suggesting a change in the surface state. Consequently, the duration of hBN particle dispersion in water is significantly increased, with an improvement of at least one order of magnitude, for the laser-irradiated samples. This study presents a demonstration of the formation of a reaction field that affects hBN particles in size and dispersibility in water.

Energy-dependent photoion angular distributions in two-body Coulomb explosions of molecules.

We experimentally study two-body Coulomb explosions of CO2, O2, and CH3Cl molecules in intense femtosecond laser pulses. We observe an obvious variation in the ionic angular distribution of the fragments with respect to the kinetic energy releases (KERs). Using a classical model based on abinitio potential energy curves, we find that the dependence of the ionic angular distribution on the KER is relevant to the fact that the accurate potential energy deviates significantly from the value determined by applying the Coulomb interaction approximation at a relatively small internuclear distance of the molecule. We show that the KER-dependent ionic angular distribution provides an effective way to determine the critical internuclear distance at which the Coulomb interaction approximation holds or breaks down without relying on the knowledge of the accurate potential energy curves.

Tailored mesoscopic plasma accelerates electrons exploiting parametric instability

Laser plasma electron acceleration from the interaction of an intense femtosecond laser pulse with an isolated microparticle surrounded by a low-density gas is studied here. Experiments presented here show that optimized plasma tailoring by introducing a pre-pulse boosts parametric instabilities to produce MeV electron energies and generates electron temperatures as large as 200 keV with the total charge being as high as 350 fC/shot/sr, even at a laser intensity of a few times 1016 Wcm−2. Corroborated by particle-in-cell simulations, these measurements reveal that two plasmon decay in the vicinity of the microparticle is the main contributor to hot electron generation.

Ultrafast dynamics of fluorene initiated by highly intense laser fields.

We present an investigation of the ultrafast dynamics of the polycyclic aromatic hydrocarbon fluorene initiated by an intense femtosecond near-infrared laser pulse (810 nm) and probed by a weak visible pulse (405 nm). Using a multichannel detection scheme (mass spectra, electron and ion velocity-map imaging), we provide a full disentanglement of the complex dynamics of the vibronically excited parent molecule, its excited ionic states, and fragments. We observed various channels resulting from the strong-field ionization regime. In particular, we observed the formation of the unstable tetracation of fluorene, above-threshold ionization features in the photoelectron spectra, and evidence of ubiquitous secondary fragmentation. We produced a global fit of all observed time-dependent photoelectron and photoion channels. This global fit includes four parent ions extracted from the mass spectra, 15 kinetic-energy-resolved ionic fragments extracted from ion velocity map imaging, and five photoelectron channels obtained from electron velocity map imaging. The fit allowed for the extraction of 60 lifetimes of various metastable photoinduced intermediates.

Enhancement of plasmonic coupling on Si metallized with intense femtosecond laser pulses

Using a pump–probe technique, the reflectivity of a silicon grating surface irradiated with intense femtosecond (fs) laser pulses was measured as a function of the incidence angle and the delay time between pulses. After irradiating the surface with an intense s-polarized, 400 nm, 300 fs laser pulse, the reflectivity measured with a weak p-polarized, 800 nm, 100 fs laser pulse exhibited an abrupt decrease for an incidence angle of ~ 24°. The depth of the dip was greatest for a delay time of 0.6–10 ps, for which the reflectivity around the dip was highest. The surface was also found to be ablated most strongly for the conditions causing the deepest dip for a delay time of 5–10 ps. Surface plasmon polaritons (SPPs) on silicon metallized by the intense pulse are resonantly excited by the subsequent pulse, and the strong coherent coupling between the subsequent pulse and SPPs excited on the molten Si surface produced by high-density free electrons induces strong surface ablation due to the intense plasmonic near-field. The results clearly show that fs pulses can be used to significantly modulate the nature of nonmetallic materials and could possibly serve as a basic tool for the excitation of SPPs on nonmetallic materials using ultrafast laser–matter interactions.

XUV fluorescence as a probe of laser-induced helium nanoplasma dynamics

XUV fluorescence spectroscopy provides information on energy absorption and dissipation processes taking place in the interaction of helium clusters with intense femtosecond laser pulses. The present experimental results complement the physical picture derived from previous electron and ion spectroscopic studies of the generated helium nanoplasma. Here, the broadband XUV fluorescence emission from high-lying Rydberg states that covers the spectral region from at 53.0 eV all the way to photon energies corresponding to the ionization potential of He+ ions at 54.4 eV is observed directly. The cluster size-dependent population of these states in the expanding nanoplasma follows the well-known bottleneck model. The results support previous findings and highlight the important role of Rydberg states in the energetics and dynamics of laser-generated nanoplasma.

High-order harmonic generation, attosecond pulse, and non-sequential double ionization in the helium atom under high-intensity femtosecond laser pulses

High-order harmonic generation (HHG), attosecond pulse train (APT), isolated attosecond pulse (IAP), and non-sequential double ionization (NSDI) in the Helium atom under intense femtosecond laser pulses are computed using the time-dependent Schrodinger equation (TDSE) in one dimension (1D). By taking into account the electron–electron and electron-nucleus interactions, as well as determining the Helium atom’s ground state wave function through the imaginary time propagation (ITP) method, we are able to observe the widely recognized ‘knee structure’ in the ionization probability of the Helium atom as a function of intensity within an ionization boundary condition model. The results are in good agreement with the experimental data reported by (Walker et al (1994) Phys. Rev. Lett. 73, 1227).

Amplification of light pulses with orbital angular momentum (OAM) in nitrogen ions lasing.

Nitrogen ions pumped by intense femtosecond laser pulses give rise to optical amplification in the ultraviolet range. Here, we demonstrated that a seed light pulse carrying orbital angular momentum (OAM) can be significantly amplified in nitrogen plasma excited by a Gaussian femtosecond laser pulse. With the topological charge of ℓ = ±1, we observed an energy amplification of the seed light pulse by two orders of magnitude, while the amplified pulse carries the same OAM as the incident seed pulse. Moreover, we show that a spatial misalignment of the plasma amplifier with the OAM seed beam leads to an amplified emission of Gaussian mode without OAM, due to the special spatial profile of the OAM seed pulse that presents a donut-shaped intensity distribution. Utilizing this misalignment, we can implement an optical switch that toggles the output signal between Gaussian mode and OAM mode. This work not only certifies the phase transfer from the seed light to the amplified signal, but also highlights the important role of spatial overlap of the donut-shaped seed beam with the gain region of the nitrogen plasma for the achievement of OAM beam amplification.

Ultrathin GeTe Crystal in a Strong Femtosecond Laser Field: Manifestation of a Quantum Size Effect

The behavior of a thin-film GeTe crystal induced by intense femtosecond laser pulses (lambda = 0.8{kern 1pt} μm) has been studied using a pulsed electron diffractometer. The sample is an annealed 20-nm GeTe film on a copper grid with a carbon coating. It has been found that laser ablation results in the formation of an ultrathin GeTe crystal (assumingly, GeTe monolayer) with a high radiation resistance. Possible reasons for the detected nanosize effect are discussed.

Control of Spectral and Polarization Properties of Quasiunipolar Terahertz Pulses in Strongly Nonequilibrium Magnetized Plasma Channels

The possibility to control both spectral and polarization properties of seed THz pulses in strongly nonequilibrium elongated magnetized plasma channels formed via intense UV femtosecond laser pulses in nitrogen (air) is analyzed. The physical mechanism of THz pulse control is based on cyclotron resonance, which can strongly reconstruct electrodynamical plasma features and, in particular, its ability to amplify the radiation of different spectral bands and polarization states. In particular, the formation of quasiunipolar pulses with a non-zero electric area and a specific polarization state is discussed. This study is based on the self-consistent solution of the kinetic Boltzmann equation for the electron velocity distribution function (EVDF) in the plasma channel and the second-order wave equation for THz pulse propagation.

Intense widely controlled terahertz radiation from laser-driven wires

Irradiation of a thin metallic wire with an intense femtosecond laser pulse creates a strong discharge wave that travels as a narrow pulse along the wire surface. This traveling discharge efficiently emits secondary radiation with spectral characteristics that are mostly defined by the wire geometry. Several examples of designs are considered here in the context of generation of intense terahertz radiation with controllable characteristics for various scientific and technological applications. The proposed setup may be easily realized, and it has the merits of robustness, versatility, and high conversion efficiency (reaching several percent) of laser energy to terahertz radiation.

Strong-field ionization and AC-Stark shifted Rydberg states in OCS

We present theoretical results for intensity-dependent above-threshold ionization (ATI) spectra from oriented OCS molecules probed by intense femtosecond laser pulses with wavelengths of 800 and 400 nm. The calculations were performed using the time-dependent Schrödinger equation within the single-active-electron approximation and including multielectron polarization effects. The results capture some of the experimental findings (Yu et al 2017 J. Phys. B: At. Mol. Opt. Phys. 50 235602), in particular when considering the sensitivity of the ATI spectra on the molecular orientation. We identify characteristic features in the ATI spectra which correspond to resonant multiphoton ionization via highly-excited Rydberg states are captured by the theory.

Isomeric Excitation of ^{229}Th in Laser-Heated Clusters.

We consider theoretically isomeric excitation of ^{229}Th in a laser-heated cluster. A ^{229}Th cluster is first radiated by an intense femtosecond laser pulse, causing ionization of the constituting atoms. The cluster will then survive for a time on the order of 1ps, during which the electrons collide with the nuclei repeatedly and excite them to the isomeric state. Two mechanisms are responsible for the isomeric excitation: nuclear excitation by electron capture (NEEC) and nuclear excitation by inelastic electron scattering (NEIES). By changing the laser intensity, one can tune between NEEC and NEIES continuously. This laser-heated-cluster scheme not only provides a highly efficient means for isomeric excitation, but also provides an approach for the confirmation of the NEEC process, which has been predicted for over forty years without conclusive experimental verifications.