Multi-color Laser Fields Research Articles

Interference-induced generation of a chirp-free short isolated attosecond pulse in the water window region with multicolor laser fields.

Compensating for the intrinsic attosecond chirp (atto-chirp) of wideband high-order harmonics in the water window region is a significant challenge, in order to obtain isolated attosecond pulses (IAPs) with a width of tens of attoseconds (as). Here, we propose to realize the generation of IAP with duration as short as 20 as, central energy of 365 eV, and bandwidth exceeding 150 eV from chirp-free high harmonics generated by a four-color driving laser, without the necessity for atto-chirp compensation with natural materials. Unlike any other gating methods that an IAP arises from only one electron ionization event, we take advantage of the interference between harmonic radiation produced by multiple ionizing events. We further demonstrate that such chirp-free short IAP survives after taking account of macroscopic propagation effects. Given that the synthesized multicolor laser field can also effectively increase the harmonic flux, this work provides a practical way for experiments to generate the broad bandwidth chirp-free IAPs in the water window region.

Terahertz pulse generation by multi-color laser fields with linear versus circular polarization: erratum

We present an erratum to our Letter [Opt. Lett. 46, 5906 (2021)10.1364/OL.442519]. This erratum corrects the caption of Fig. 2, which contains confusing information. This correction does not affect any of the results or the conclusions of the original Letter.

Generation of a quasi-chirp-free short isolated attosecond pulse from high-order harmonics by optimized multicolor laser fields

Generation of a quasi-chirp-free short isolated attosecond pulse from high-order harmonics by optimized multicolor laser fields

Three-dimensional analysis of THz radiation mechanisms in interaction of arbitrarily polarized multi-color laser pulses and obliquely magnetized plasma

Three-dimensional THz radiation generation via wave-mixing and photo-current mechanisms of multi-color femtosecond laser fields in obliquely magnetized plasma for various laser polarization states is considered. The main motive behind present study has been to investigate the development of more efficient and versatile THz radiation generation for a range of applications through a comprehensive mechanism describing the entire THz frequency band. Two and three-color laser waves interactions with plasma electrons results in a transverse nonlinear current density which causes THz radiation generation. Variation of THz electric field under different laser polarizations is examined analytically and numerically. The impacts of the applied oblique magnetic field and the field oblique angle on the THz electric field for both wave mixing and photo-current schemes are studied and result are compared. A method to implement THz radiation for special applications was developed showing that the external magnetic field is crucial in controlling plasma dynamics. The waveforms of THz angular distribution in forward direction are examined and the impact of different harmonics, various polarization states, external magnetic field, oblique angle of the applied external magnetic field, and the length of plasma on generated waveforms is considered. Based on the results of study, laser harmonics and applied external magnetic field increase electrons current density, leading to enhancement of radiation power without change in directionality diagram. These findings are in close agreement with experimental results.

Traveling-wave antenna model for terahertz radiation from laser-plasma interactions

Generation of terahertz (THz) wave from air plasma induced by femtosecond laser pulses with a single central frequency (the so-called "single-color") is one of the fundamental interactions between light and matter, and is also the basis of subsequent pumping schemes using two- or multi-color laser fields. Recently, more states of media beyond gas (e.g., atomic cluster and liquid) via photo-ionization have brought new experimental observations of THz radiation, which can no longer be simply attributed to the mainstream model of the transition-Cherenkov radiation (TCR), thus making the whole picture unclear. Here, we revisited the mechanism of this dynamic process in a new view of the traveling-wave antenna (TWA) model. By successfully reproducing the reported far-field THz radiation profiles from various plasma filament arrangements, the wide applicability of the TWA theory has been revealed. On the other hand in the microscopic view, we investigated the plasma oscillation during filamentation aiming at further bridging the plasma filament and the antenna. Accordingly, THz-plasma resonance has been theoretically and experimentally demonstrated as the elementary THz emitter, paving the way towards fully understanding this important single-color plasma based THz source.

Manipulating synthetic gauge fluxes via multicolor dressing of Rydberg-atom arrays

Arrays of highly excited Rydberg atoms can be used as powerful quantum simulation platforms. Here, we introduce an approach that makes it possible to implement fully controllable effective spin interactions in such systems. We show that optical Rydberg dressing with multicolor laser fields opens up distinct interaction channels that enable complete site-selective control of the induced interactions and favorable scaling with respect to decoherence. We apply this method to generate synthetic gauge fields for Rydberg excitations where the effective magnetic flux can be manipulated at the single-plaquette level by simply varying the phase of the local dressing field. The system can be mapped to a highly anisotropic Heisenberg model, and the resulting spin interaction opens the door for explorations of topological phenomena with nonlocal density interactions. A remarkable consequence of the interaction is the emergence of topologically protected long-range doublons, which exhibit strongly correlated motion in a chiral and robust manner.

Terahertz pulse generation by multi-color laser fields with linear versus circular polarization.

We study the influence of the polarization state of multi-color femtosecond laser pulses ionizing air or noble gases on the emitted terahertz radiation. A local-current model and plane wave evaluations predict a cross-over in the THz energy yields with increasing number of pump harmonics, for which circular laser polarization is more efficient for a few harmonics, and linear polarization is favorable for more than six pump colors. Comprehensive 3D numerical simulations of gas jet experiments confirm this property for singly and multiply ionized gases. Rotation of the THz polarization ellipse in the case of circular laser polarization is explained by phase shifts that may alter the phase angle between the harmonics.

Mechanism study of terahertz radiation regulation in a multi-color laser field

By establishing a theoretical model for terahertz (THz) generation from a laser-induced air plasma filament and the subsequent propagation process, we study the physical mechanisms of THz wave radiation regulation in the multi-color femtosecond laser field. It is proved that the total THz radiation in the far field is affected by three factors: the ionized electron density, transient current distribution along the laser filament, and coherent superposition between THz waves generated by each point along the filament. The results show that the wavelength, relative phase, energy ratio, pulse width, laser frequency ratio, and frequency combination of the multi-color laser are all correlated to their radiated THz energy with different changing laws. The proper regulation of a three-color filament can realize a 4 orders of magnitude increase in the generated THz pulse energy as compared with the conventional THz generation by a two-color filament. These results provide the theoretical basis for the enhancement of THz radiation energy, and they are of great significance for clarifying the physical process of laser filament.

Simulations on the single attosecond pulse generation by optimized multicolor laser field

The attosecond (as) pulses are of great significance to observe and control the ultrafast electron dynamics in atoms, molecules and condensed matter. By utilizing some sub-cycle gating methods, i.e., amplitude gating, polarization gating, and double optical gating, isolated attosecond pulses (IAP) have been generated experimentally from high-order harmonics generation (HHG) by the interaction of intense femtosecond laser fields and gas targets. At present, about 50-as IAPs are now available only in few laboratories in the world. Thus, searching for the ways to generate more short IAPs is still desirable and important for the strong field physics and attosecend science. The color gating is another potential technique generating ultrashort IAP, which could generate continuum high-order harmonics without using sub-cycle laser pulses. On the theoretical side, many schemes with two- and three-color combination field have been proposed to generate short IAP, and some works propose that in this way the IAP with width less than 10 as may be generated. However, these investigations are performed only on the single-atom level, and the real experimental conditions and strength of IAPs are not considered in their simulations. In this work, we perform a numerical experiment on the generation of strong IAP from HHG driven by multicolor driving fields. Unlike the previous studies, the macroscopic effects are included in the present simulations. Considering the detection range of spectrograph, the centering energy of IAP is confined as ~200 eV. To obtain single attosecond pulse with short width, a genetic algorithm is used to optimize waveforms of two- and three-color excitation laser pulses, which is determined by intensity, wavelength and carrier envelope phase for each component. We choose the fundamental pulse as Ti:sapphire laser field with the wavelength of 800 nm to ensure the intensity of the generated attosecond pulse, and the second or third component is not confined as the commensurate harmonics of fundamental pulse. In order to possess an acceptable computation time, the model of strong field approximation is adopted in the simulations of HHG. Our simulations show that by coherent synthesizing two or three multi-cycle pulses (the full width at half maximum of each laser field is 6 fs) we obtain sub-optical-cycle laser pulse, which can easily generate supercontinuum high-order harmonic spectra with wide bandwidth. Different from the single-color driving field, which produces the attosecond pulse train, the single attosecond pulses are generated from optimized laser fields. The width of IAP decreases with the increase of intensity of driving laser field. On the single atom level, our simulations show that about 50 as and 40 as IAP can be obtained by superimposing appropriate harmonics generated by optimized two- and three-color laser pulses, respectively. More importantly, after considering the propagation effects, about 50 as IAPs are obtained with spherical spatial filtering of harmonics in the far field, under reasonable conditions, i.e., the length and position of gas jet, gas pressure. With great advent of the ultrafast lasers technology, such as coherent waveform synthesizing, optical parametric amplification and optical parametric chirped-pulse amplification, this work gives helpful guidance for intense short IAP generation in the experiment.

Attosecond recorder of the polarization state of light

High harmonic generation in multi-color laser fields opens the opportunity of generating isolated attosecond pulses with high ellipticity. Such pulses hold the potential for time-resolving chiral electronic, magnetization, and spin dynamics at their natural timescale. However, this potential cannot be realized without characterizing the exact polarization state of light on the attosecond timescale. Here we propose and numerically demonstrate a complete solution of this problem. Our solution exploits the extrinsic two-dimensional chirality induced in an atom interacting with the chiral attosecond pulse and a linearly polarized infrared probe. The resulting asymmetry in the photoelectron spectra allows to reconstruct the complete polarization state of the attosecond pulse, including its possible time dependence. The challenging problem of distinguishing circularly polarized, partially polarized, or unpolarized pulses in the extreme ultraviolet range is also resolved. We expect this approach to become the core ingredient for attosecond measurements of chiral-sensitive processes in gas and condensed phase.

Quantum-mechanical elaboration for the description of low- and high-order harmonics generated by extended gas media: prospects to the efficiency enhancement in spatially modulated media

A simple model fully matching the description of the low- and high-order harmonic generation in extended media interacting with multicolor laser fields is proposed. The extended atomic media is modeled by a 1D chain of atoms, the number of atoms and the distance between them depend on the pressure of the gas and the length of the gas cell. The response of the individual atoms is calculated accurately in the frame of the non-perturbative theory where the driving field for each atom is calculated with account of dispersion properties of any multicolor field component. In spite of the simplicity of the proposed model it provides the detailed description of behaviour of harmonic spectra under variation of the gas pressure and medium length, it also predicts a scaling law for harmonic generation (an invariant). To demonstrate the wide range of applications of the model we have simulated the results of recent experiments dealing with spatially modulated media and obtained good coincidence between the numerical results and the experimental ones.

Compact and flexible harmonic generator and three-color synthesizer for femtosecond coherent control and time-resolved studies.

Intense, multi-color laser fields permit the control of the ionization of atoms and the steering of electron dynamics. Here, we present the efficient collinear creation of the second and third harmonic of a 790 nm femtosecond laser followed by a versatile field synthesizer for the three color fields' composition. Using the device, we investigate the strong-field ionization of neon by fields composed of the fundamental, and the second or third harmonic. The three-color device offers sufficient flexibility for the coherent control of strong-field processes and for time-resolved pump-probe studies.

Controlling of strong tunable THz emission with optimal incommensurate multi-color laser field

Based on the photocurrent model, we study terahertz (THz) emission from argon plasmas induced by incommensurate-frequency two- and three-color laser fields. In order to enhance the THz radiation at an arbitrary frequency efficiently, a genetic algorithm is applied to search for the optimum laser parameters. For the longer two-color field, our optimizations show that the THz tunability is mainly determined by two laser frequencies, which approximately meets the law Ω = 2ω1−ω2. However, for the shorter laser pulse, the tunability of the THz wave with lower frequency also depends on the relative phase. To control the tunable THz emission, we systematically investigated how to generate the stronger THz wave with the shorter spectrum width using the optimal synthesized waveform. We found that the THz intensity can be enhanced by about an order with three-color field compared with the two-color cases. We also show that the tunable single ultrashort THz pulses can be obtained by using an optimized 50-fs two-color pulse.

Selection of odd or even harmonics by a multi-color laser field with macroscopic phase-matching

We experimentally and theoretically demonstrate that the property (odd or even) of generated harmonics can be selected by manipulating the macroscopic phase-matching conditions based on a three-color laser field. Only odd or even harmonics can be made dominant by changing the focal position and adjusting the gas pressure. These results indicate that the odd-even property of the generated harmonics can be controlled by using the multi-color laser field with macroscopic phase-matching.

Efficient selection of a single harmonic emission using a multi-color laser field with an aperture-iris diaphragm

The efficient selection of an almost pure single harmonic emission (the 14th harmonic at 57 nm) from the harmonic comb has been experimentally achieved in an argon gas cell using a multi-color laser field with an aperture-iris diaphragm. When compared with the non-diaphragm case, the purity of the selected single harmonic emission (i.e. the contrast ratio) in the case of using the aperture-iris diaphragm is dramatically increased by approximately order of magnitude. Therefore, the modification of the multi-color laser field by using such an aperture-iris diaphragm before focusing is demonstrated to be an effective way of improving the phase-matching conditions for selective enhancement of the single harmonic emission.

Control of bandwidth and central wavelength of an enhanced extreme ultraviolet spectrum generated in shaped laser field

We theoretically investigate the enhancement of a selected spectral range in the high-order harmonic generation driven by the multi-color laser fields. The results show that the central wavelength of the enhanced narrow-bandwidth spectrum can be effectively controlled by adjusting the laser intensity and the time delay between laser pulses of different colors, due to the modified electron trajectory in the shaped laser field. Our approach can offer intense, bandwidth controllable single attosecond pulses for attosecond pump-probe experiments.

Field-free molecular orientation by a multicolor laser field

We theoretically study the field-free molecular orientation by a multicolor laser field with a superposition of the fundamental wave and its harmonics. It is shown that the molecular orientation will be up to a maximum value pumped by the four-color laser field at the same laser intensity, and the molecular orientation direction and its degree can be controlled by varying the carrier-envelope phase of the four-color laser field. It is also indicated that the molecular orientation induced by a four-color laser field can be tremendously enhanced by applying another in-phase or out-phase four-color laser field at the beginning of the rotational wave-packet rephasing or the end of the rotational wave-packet dephasing.

Nonlinear surface photoeffect in multi-color laser field: effect of the finite width of the surface potential barrier

Nonlinear surface photoeffect in multi-color laser field: effect of the finite width of the surface potential barrier

Generalized Floquet theoretical formulation of time-dependent density functional theory for many-electron systems in multicolor laser fields

We generalize the Floquet formulation of the time-dependent density . w . x functional theory TDDFT Telnov and Chu, Chem. Phys. Lett. 264, 466 1997 to the case of many-electron systems in multicolor or polychromatic time-dependent fields. It is shown that the time-dependent Kohn)Sham equations can be transformed into an equivalent time-independent infinite-dimensional Floquet Hamiltonian eigenvalue problem. For the case of bound-free transitions, we introduce the notion of complex density and present a non-Hermitian many-mode Floquet formulation of TDDFT. A procedure is presented for the determination of the complex quasi-energies and the calculation of the total and partial ionization rates from individual electron orbitals. The theory is illustrated by a case study of multiphoton ionization of He atoms in the presence of an intense 248 nm laser field and its third harmonic. Novel intensity- and phase-dependent behavior of multiphoton processes in two-color fields is reported.

MAKING LOCAL MODE VIBRATION LONG LIVED BY THE INTERACTION BETWEEN A STRONG MULTI-COLOR LASER FIELD AND MOLECULES

Trapping vibrational energy into one single bond is a key problem of bond-selective chemistry. We explored this possibility by means of restricting local mode wavepacket dephasing with multi-color laser field. The vibrational excitation and evolution of molecules in weak and strong multi-color laser fields are discussed by field-quantization method and semi-classical method. The results demonstrate that population can not be trapped in a local mode wave-packet using multi-color weak field, while it is possible to trap vibrational population between the ground state and local mode vibration by making use of power-broadening induced by strong laser field and the interference between two wave-packets excited by two strong laser of different frequencies.