Intense Near-infrared Laser Pulses Research Articles

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.

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.

Reverse wavefront engineering for remote generation of a near-infrared femtosecond Bessel beam filament in air

Precise control of the filamentation of an ultrashort and intense near-infrared laser pulse in air is crucial for many applications but remains challenging. By combining inverse design for shaping of a pulse at the output of a laser with simulation of the nonlinear propagation of the pulse in air, we numerically investigate the way to simultaneously control the length and position of a plasma filament at an arbitrary remote distance for the first time. An intermediate state, with a Bessel-Gauss beam profile, is introduced between the laser output and the filament. Through forward and backward propagations of the intermediate state, an end-to-end design of the initial laser pulse is achieved to match the desired filament characteristics. Our calculations show that a properly engineered torus pulse can project such a Bessel-Gauss beam at remote distance and generate a long plasma filament without requiring any feedback loop. We also proposed an experimental arrangement to project such an intermediate state and generate the corresponding filament remotely. The method can be further extended to other types of filaments or targets that may be reached from different intermediate states.

Efficient generation of mid-infrared few-cycle pulses by the intrapulse difference-frequency generation in YCOB.

Yttrium calcium oxyborate (YCOB) crystals have been widely applied for generating intense near-infrared laser pulses by optical parametric amplification. Here, we show that the YCOB crystals oriented in both the XZ and XY principal planes possess broadband phase-matching property of intrapulse difference-frequency generation in the mid-infrared region. Few-cycle pulses tunable from 2 to 4 µm are experimentally produced by using a 7.5-fs pump laser at 800 nm, in which the conversion efficiency can be as high as 2.5%. With a large-size crystal and high-power pump laser, intrapulse difference-frequency generation based on YCOB may provide a new route for directly producing intense few-cycle mid-infrared pulses.

Ionization and ultrafast hydrogen migration of methylamine in few-cycle intense near-infrared laser fields

Double ionization and ultrafast hydrogen migration of methylamine induced by a few-cycle intense near-infrared laser pulse are investigated by the coincidence momentum imaging method. By reference to the results obtained by DFT calculations, the peak profiles in the kinetic energy release distributions have been assigned to the singlet and triplet decomposition channels of CH3NH22+. The anisotropy in the angular distribution of the ejection direction of the fragment ions is interpreted based on the symmetries of the (7a’) HOMO orbital. By the pump–probe measurements, it is revealed that the ultrafast hydrogen migration in CH3NH2+ proceeds within ∼ 30 fs.

High harmonic generation in crystalline silicon irradiated by an intense ultrashort laser pulse

We investigate the high harmonic generation in bulk silicon irradiated by intense near-infrared laser pulses with pulse duration le 100 fs. For peak field strength of the applied laser is below 1 V/Å, the spectral intensity of the emitted harmonics follows the prediction of perturbative nonlinear optics—the frequency comb consists of a series of discrete peaks at odd harmonic orders. For a pulse duration longer than 30 fs and peak laser field strength exceeding 1 V/Å, non-perturbative effects and generation of even order harmonics occur. The appearance of even harmonics is due to optical rectification of the transmitted pulse, which includes weak quasi-DC component with electric field as low as 3 V/upmu m. In the strong coupling regime, when the peak field strength inside vacuum exceeds 1.5 V/Å, the laser creates dense breakdown plasma of electron–hole pairs, which in turn results in severe spectral broadening of the transmitted pulse. The harmonic spectrum superimposes onto a continuous background, the spectral width of individual harmonics is substantially broadened, and their central wavelength undergoes a blue shift that covers the spacing between adjacent harmonic orders.Graphic abstract

Femtosecond optical breakdown in silicon

We investigate photo-ionization, energy deposition, plasma formation and the ultrafast optical breakdown in crystalline silicon irradiated by intense near-infrared laser pulses with duration τ≤ 100 fs. The occurrence of high-intensity breakdown was established by the sudden increase of the absorbed laser energy inside the bulk, which corresponds to threshold energy fluence Φth> 1 J/cm2. The optical breakdown is accompanied by severe spectral broadening of the transmitted pulse. For the studied irradiation conditions, we find that the threshold fluence increases linearly with the increase of the pulse duration, while the corresponding laser intensity threshold decreases. The effect of the high plasma density on the stability of diamond lattice is also examined. For near threshold fluences, when about 5 % of valence electrons are promoted into the conduction band, the Si–Si bonds are softened and large Fermi degeneracy pressure arises (with pressure up to 100 kbar). The mechanical instability of the diamond lattice suggests that the large number of electron–hole pairs leads directly to ultrafast melting of the crystal structure.

Time-dependent optimized coupled-cluster method for multielectron dynamics. IV. Approximate consideration of the triple excitation amplitudes.

We present a cost-effective treatment of the triple excitation amplitudes in the time-dependent optimized coupled-cluster (TD-OCC) framework called TD-OCCDT(4) for studying intense laser-driven multielectron dynamics. It considers triple excitation amplitudes correct up to the fourth-order in many-body perturbation theory and achieves a computational scaling of O(N7), with N being the number of active orbital functions. This method is applied to the electron dynamics in Ne and Ar atoms exposed to an intense near-infrared laser pulse with various intensities. We benchmark our results against the TD complete-active-space self-consistent field (TD-CASSCF), TD-OCC with double and triple excitations (TD-OCCDT), TD-OCC with double excitations (TD-OCCD), and TD Hartree-Fock (TDHF) methods to understand how this approximate scheme performs in describing nonperturbatively nonlinear phenomena, such as field-induced ionization and high-harmonic generation. We find that the TD-OCCDT(4) method performs equally well as the TD-OCCDT method, almost perfectly reproducing the results of the fully correlated TD-CASSCF with a more favorable computational scaling.

Femtosecond Single Cycle Pulses Enhanced the Efficiency of High Order Harmonic Generation.

High-order harmonic generation is a nonlinear process that converts the gained energy during light-matter interaction into high-frequency radiation, thus resulting in the generation of coherent attosecond pulses in the XUV and soft x-ray regions. Here, we propose a control scheme for enhancing the efficiency of HHG process induced by an intense near-infrared (NIR) multi-cycle laser pulse. The scheme is based on introducing an infrared (IR) single-cycle pulse and exploiting its characteristic feature that manifests by a non-zero displacement effect to generate high-photon energy. The proposed scenario is numerically implemented on the basis of the time-dependent Schrödinger equation. In particular, we show that the combined pulses allow one to produce high-energy plateaus and that the harmonic cutoff is extended by a factor of 3 compared to the case with the NIR pulse alone. The emerged high-energy plateaus is understood as a result of a vast momentum transfer from the single-cycle field to the ionized electrons while travelling in the NIR field, thus leading to high-momentum electron recollisions. We also identify the role of the IR single-cycle field for controlling the directionality of the emitted electrons via the IR-field induced electron displacement effect. We further show that the emerged plateaus can be controlled by varying the relative carrier-envelope phase between the two pulses as well as the wavelengths. Our findings pave the way for an efficient control of light-matter interaction with the use of assisting femtosecond single-cycle fields.

Single-shot electron imaging of dopant-induced nanoplasmas

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.

Generation of visible-wavelength-tunable ultrashort laser pulses through femtosecond laser filamentation in air: strong external focusing effect

Wavelength-tunable ultrashort laser pulses in the visible spectral range are generated in a femtosecond laser filament in air through four-wave mixing of an intense 808-nm near-infrared (NIR) laser pulse and a weak tunable infrared (IR: 1.3–2.2 µm) laser pulse. We found that the external focusing geometries strongly influence the four-wave mixing efficiency in air filament. The measurements of the resultant four-wave mixing laser intensity reveal that changing both the tilting angle and the focal length of the focal lens can optimize the four-wave mixing signal. We interpret the process based on the strong influence of the external focusing on the plasma density and the volume of the filament during the filament formation.

THz generation by optical rectification of intense near-infrared pulses in organic crystal BNA.

Generation of terahertz radiation by optical rectification of intense near-infrared laser pulses in N-benzyl-2-methyl-4-nitroaniline (BNA) is investigated in detail by carrying out a complete characterization of the terahertz radiation. We studied the scaling of THz yield with pump pulse repetition rate and fluence which enabled us to predict the optimal operating conditions for BNA crystals at room temperature for 800 nm pump wavelength. Furthermore, recording the transmitted laser spectrum allowed us to calculate the nonlinear refractive index of BNA at 800 nm.

Blueshifts of high-order harmonic generation in crystalline silicon subjected to intense femtosecond near-infrared laser pulse

We present the generation of high order harmonics in crystalline silicon subjected to intense near-infrared 30fs laser pulses. The harmonic spectrum extends from the near infrared to the extreme ultraviolet spectral region. Depending on the pulsed laser intensity, we distinguish two regimes of harmonic generation: (i) perturbative regime: electron-hole pairs born during each half-cycle of the laser pulse via multiphoton and tunnel transitions are accelerated in the laser electric field and gain kinetic energy; the electron-hole pairs then recombine in the ground state by emitting a single high-energy photon. The resultant high harmonic spectrum consists of sharp peaks at odd harmonic orders. (ii) non-perturbative regime: the intensity of the harmonics increases, their spectral width broadens and the position of harmonics shifts to shorter wavelengths. The blueshifts of high harmonics in silicon are independent on the harmonic order which may be helpful in the design of continuously tunable XUV sources.

Ultrafast energy absorption and photoexcitation of bulk plasmon in crystalline silicon subjected to intense near-infrared ultrashort laser pulses

We investigate the non-linear response and energy absorption in bulk silicon irradiated by intense 12-fs near-infrared laser pulses. Depending on the laser intensity, we distinguish two regimes of non-linear absorption of the laser energy: for low intensities, energy deposition and photoionization involve perturbative three-photon transition through the direct bandgap of silicon. For laser intensities near and above 1014 W/cm2, corresponding to photocarrier density of order 1022 cm−3, we find that absorption at near-infrared wavelengths is greatly enhanced due to excitation of bulk plasmon resonance. In this regime, the energy transfer to electrons exceeds a few times the thermal melting threshold of Si. The optical reflectivity of the photoexcited solid is found in good qualitative agreement with existing experimental data. In particular, the model predicts that the main features of the reflectivity curve of photoexcited Si as a function of the laser fluence are determined by the competition between state and band filling associated with Pauli exclusion principle and Drude free-carrier response. The non-linear response of the photoexcited solid is also investigated for irradiation of silicon with a sequence of two strong and temporary non-overlapping pulses. The cumulative effect of the two pulses is non-additive in terms of deposited energy. Photoionization and energy absorption on the leading edge of the second pulse is greatly enhanced due to free carrier absorption.

Plasma channel formation in NIR laser-irradiated carrier gas from an aerosol nanoparticle injector

Aerosol nanoparticle injectors are fundamentally important for experiments where container-free sample handling is needed to study isolated nanoparticles. The injector consists of a nebuliser, a differential pumping unit, and an aerodynamic lens to create and deliver a focused particle beam to the interaction point inside a vacuum chamber. The tightest focus of the particle beam is close to the injector tip. The density of the focusing carrier gas is high at this point. We show here how this gas interacts with a near infrared laser pulse (800 nm wavelength, 120 fs pulse duration) at intensities approaching 1016 Wcm−2. We observe acceleration of gas ions to kinetic energies of 100s eV and study their energies as a function of the carrier gas density. Our results indicate that field ionisation by the intense near-infrared laser pulse opens up a plasma channel behind the laser pulse. The observations can be understood in terms of a Coulomb explosion of the created underdense plasma channel. The results can be used to estimate gas background in experiments with the injector and they open up opportunities for a new class of studies on electron and ion dynamics in nanoparticles surrounded by a low-density gas.

Single nanowire defined emission properties of ZnO nanowire arrays

We report on stimulated emission from vertically aligned, vapor transport grown, ZnO nanowire arrays, and pumped by three-photon absorption in intense near-infrared femtosecond laser pulses. In respect to single nanowires, arrays have the advantage of a higher light absorption and emission rate. The intensity and bandwidth of the emitted ultraviolet radiation as a function of the pump intensity is compared for nanowire arrays with different wire lengths, diameters, and spacing. The measured lasing thresholds for all arrays can be well described by the geometry of individual nanowire lasers, showing that coupling effects between the individual emitters in the arrays are negligible, even for the smallest 100 nm diameter wires with an average distance of 200 nm.

Characterization of 20-fs VUV pulses by plasma-mirror frequency-resolved optical gating.

We demonstrate that vacuum ultraviolet (VUV) pulses (λ∼160 nm) with 20-fs duration can be fully characterized by plasma-mirror frequency-resolved optical gating. Plasma is formed as an ultrafast optical switch on a fused silica surface by irradiation of an intense near-infrared (NIR) femtosecond laser pulse. We measure the spectra of the VUV pulses reflected off plasma as a function of the delay with respect to the NIR pulse and reconstruct it to simultaneously extract a nearly Fourier-transform limited VUV pulse shape and a time-dependent reflectivity of plasma using the least-squares generalized projections algorithm. We show that there is almost no spatial dependence of the VUV pulse shape, whereas the plasma mirror reflectivity strongly depends on the spatial region.

Electron correlation effects in enhanced ionization of diatomic molecules in near-infrared fields

We investigate electron correlation effects in internuclear-distance-dependent enhanced ionization of $\mathrm{H}_2$, $\mathrm{LiH}$, and $\mathrm{HF}$ molecules by intense near-infrared laser pulses using a 3D description of the systems with the time-dependent generalized-active-space configuration-interaction method. This method systematically incorporates electron-electron correlation of the quantum many-electron system under consideration. Our correlated description of diatomic molecules shows that enhanced ionization occurs at certain critical internuclear separations and electron correlation systematically improves the ionization probability in this process until convergence is reached. We demonstrate the failure of the single-active-electron and the configuration-interaction singles approximations to produce the correct internuclear position and probability of the strong-field enhanced-ionization process. We elucidate the role of low-lying electronic excited states in the enhanced ionization process of diatomic molecules. There is clear evidence that an accurate description of low-lying electronically excited states is important to describe the non-perturbative enhanced ionization phenomenon in the ultrashort intense near infrared laser pulses.

Subcycle dynamics of electron–hole pairs and high-harmonic generation in bulk diamond subjected to intense femtosecond laser pulse

We present calculation of photoexcitation and high-harmonic generation in bulk diamond induced by intense near-infrared laser pulse of photon energy 1.55 eV, time duration of 15 fs and peak field strength $F=$ 0.4 V/{\AA}. Depending on the laser polarization direction, the pulsed irradiation creates electron-hole pairs and transient density fluctuations in a localized region of the crystal momentum space. As a consequence energetic inter- and intra-band harmonics are generated with alternating phase during each half-cycle of the driving pulse. The corresponding inter- and intra-band currents are in definite phase relation with the laser pulse, reflecting the build-up of coherent superposition of population between valence and conduction bands, and the accelerated motion of charge carriers in their respective bands.

Low-Energy Electron Emission in the Strong-Field Ionization of Rare Gas Clusters.

Clusters and nanoparticles have been widely investigated to determine how plasmonic near fields influence the strong-field induced energetic electron emission from finite systems. We focus on the contrary, i.e., the slow electrons, and discuss a hitherto unidentified low-energy structure (LES) in the photoemission spectra of rare gas clusters in intense near-infrared laser pulses. For Ar and Kr clusters we find, besides field-driven fast electrons, a robust and nearly isotropic emission of electrons with <4 eV kinetic energies that dominates the total yield. Molecular dynamics simulations reveal a correlated few-body decay process involving quasifree electrons and multiply excited ions in the nonequilibrium nanoplasma that results in a dominant LES feature. Our results indicate that the LES emission occurs after significant nanoplasma expansion, and that it is a generic phenomenon in intense laser nanoparticle interactions, which is likely to influence the formation of highly charged ions.