Two-color Femtosecond Laser Pulses Research Articles

High-efficiency and frequency-controllable terahertz pulses driven by two-color relativistic laser pulses

The two-color laser filamentation technique serves as a high-quality, laser-based strong-field terahertz source. However, the terahertz energy obtained from this method tends to saturate at high laser intensity, limiting its ability to fully exploit the advantage of the extremely strong field offered by relativistic lasers. Here, we proposed a novel approach based on photon deceleration in plasma to obtain controllable terahertz radiations with two-color relativistic femtosecond laser pulses. In our method, an 800-nm laser drives the plasma wave and controls the plasma density distribution, while the other 10.6-μm laser experiences a strong frequency downshift in the plasma wave and converts to a multi-cycle terahertz pulse, whose central frequency is adjustable by altering the parameters of laser and plasma. The conversion efficiency from the 10.6-μm laser to terahertz radiation can be over 12%, potentially unlocking novel applications in the terahertz domain.

Identifying photoelectron releasing order in strong-field dissociative ionization of H2.

Driven by intense laser fields, the outgoing photoelectrons in molecules possess a quiver motion, resulting in the rise of the effective ionization potential. The coupling of the field-dressed ionization potential with abundant molecular dynamics complicates the laser-molecule interactions. Here, we demonstrate an approach to resolve photoelectron releasing order in the dissociative and non-dissociative channels of multiphoton ionization driven by an orthogonally polarized two-color femtosecond laser pulse. The photoelectron kinetic energy releases and the regular nodes in the photoelectron angular distributions due to the participation of different continuum partial waves allow us to deduce the field-dressed ionization potential of various channels. It returns the ponderomotive energy experienced by the outgoing electron and reveals the corresponding photoionization instants within the laser pulse. Our results provide a route to explore the complex strong-field ionization dynamics of molecules using two-dimensional photoelectron momentum spectroscopy.

Numerical study of electron acceleration driven by two-color laser pulses

The use of two-color relativistic femtosecond laser pulses for large-amplitude wakefield excitation and electron acceleration to relativistic energies in very short distances is a promising candidate that has recently been investigated experimentally and numerically by using the particle-in-cell method. Here, we have numerically studied the evolution of a large-amplitude wakefield excited by two-color relativistic femtosecond laser pulses in an underdense plasma. Moreover, the effects of some physical parameters such as two-color pulse polarization and time delay on the wakefield, and the electron acceleration are investigated. The results show that the wakefield amplitude and the energy of the accelerated electrons can be controlled by these parameters. We compare some results with those obtained by applying single-color pulses with similar energy. According to the comparison results, by applying two-color laser pulses, a stronger wakefield and higher electron energies can be obtained. We also show that our results are in good agreement with the experimental data obtained earlier.

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.

Three dimensional orientation of small polyatomic molecules excited by two-color femtosecond pulses

We study the excitation of asymmetric-top (including chiral) molecules by two-color femtosecond laser pulses. In the cases of non-chiral asymmetric-top molecules excited by an orthogonally polarized two-color pulse, we demonstrate, classically and quantum mechanically, three-dimensional orientation. For chiral molecules, we show that the orientation induced by a cross-polarized two-color pulse is enantioselective along the laser propagation direction, namely, the two enantiomers are oriented in opposite directions. The classical and quantum simulations are in excellent agreement on the short time scale, whereas on the longer time scale, the enantioselective orientation exhibits quantum beats. These observations are qualitatively explained by analyzing the interaction potential between the two-color pulse and molecular (hyper-)polarizability. The prospects for using the enantioselective orientation for enantiomers’ separation is discussed.

Observation of rotational coherence in an excited state of CO.

The vacuum ultraviolet (VUV) radiation is generated in the strong-field-ionized CO molecules through 2+1 resonance excitation with two-color femtosecond laser pulses. When scanning the relative delay between two pump pulses, the rotational-resolved VUV radiations show periodic oscillations lasting as long as 500 ps. Fourier analysis reveals that these oscillations correspond to rotational beat frequencies of the A2Πi state of CO+, which is the result of multi-channel interference during the resonant excitation process. High resolution of Fourier transform spectra up to 0.067cm-1 allows us to obtain the fine energy levels of the A2Πi state. The theoretical calculation is in good agreement with the experimental observation. This work reveals the rotational coherence of the ionic excited state and shows the prospect of rotational coherence spectroscopy in measuring fine structures of molecular ions.

Quantum Interference Visibility Spectroscopy in Two-Color Photoemission from Tungsten Needle Tips.

When two-color femtosecond laser pulses interact with matter, electrons can be emitted through various multiphoton excitation pathways. Quantum interference between these pathways gives rise to a strong oscillation of the photoemitted electron current, experimentally characterized by its visibility. In this Letter, we demonstrate the two-color visibility spectroscopy of multiphoton photoemissions from a solid-state nanoemitter. We investigate the quantum pathway interference visibility over an almost octave-spanning wavelength range of the fundamental (ω) femtosecond laser pulses and their second harmonic (2ω). The photoemissions show a high visibility of 90% ± 5%, with a remarkably constant distribution. Furthermore, by varying the relative intensity ratio of the two colors, we find that we can vary the visibility between 0% and close to 100%. A simple but highly insightful theoretical model allows us to explain all observations, with excellent quantitative agreements. We expect this work to be universal to all kinds of photo-driven quantum interference, including quantum control in physics, chemistry, and quantum engineering.

Modeling the time-dependent electron dynamics in dielectric materials induced by two-color femtosecond laser pulses: Applications to material modifications

Controlling the electron dynamics during laser-matter interactions is a key factor to control the energy deposition and subsequent material modifications induced by femtosecond laser pulses. One way to achieve this goal is to use two-color femtosecond laser pulses. In this paper, the electron dynamics in dielectric materials induced by two-color femtosecond laser pulses is studied by solving dedicated optical Bloch equations. This model includes photo- and impact ionization, the laser heating of conduction electrons, their recombination to the valence band, and their collisions with phonons. The influence of photon energies, laser intensities, and pulse-to-pulse delay is analyzed. Depending on the interaction process, colors cooperate to excite electrons or drive them independently. For the given laser parameters, an optimal pulse-to-pulse delay is found which enhances significantly the energy deposition into the material, in agreement with experimental observations.

Spectral control of terahertz radiation from inhomogeneous plasma filaments by tailoring two-color laser beams.

Terahertz (THz) radiation from an inhomogeneous plasma filament generated by focusing two-color femtosecond laser pulses into argon gas filled in a chamber is investigated experimentally by tailoring the Gaussian pump laser beams with an iris, where broadband THz emission over 10 THz is produced. It is found that the collected far-field THz radiation includes not only coherent but also partial-coherent components of the THz waves, which are emitted from the different parts of the inhomogeneous plasma filament with different plasma densities, contributing correspondingly to the different frequencies of the THz spectrum. Our results suggest that the THz spectrum can be manipulated by controlling the plasma density distribution of the filaments.

Bright single-cycle terahertz source based on gas cells irradiated by two-color laser pulses

We study the excitation of electron currents in a transparent cell of sub-millimeter size filled by an atomic gas and illuminated by an intense two-color femtosecond laser pulse. The pulse consists of a strong fundamental component and its second harmonic of low intensity, both circularly polarized. We show that for a sufficiently small 20μm-size interaction volume the plasma oscillation excited by asymmetric ionization is almost spatially homogeneous. This coherent dipole plasma oscillation results in a remarkably efficient conversion of the electron energy into that of radiation emitted in the terahertz frequency domain. Simultaneously, strong quasi-static electric fields of maximal strength Em ≃ 10MV/cm are shown to exist inside the plasma during several hundred femtoseconds after the ionizing two-color laser pulse has gone.

A laser-plasma accelerator driven by two-color relativistic femtosecond laser pulses

A typical laser-plasma accelerator (LPA) is driven by a single, ultrarelativistic laser pulse from terawatt- or petawatt-class lasers. Recently, there has been some theoretical work on the use of copropagating two-color laser pulses (CTLP) for LPA research. Here, we demonstrate the first LPA driven by CTLP where we observed substantial electron energy enhancements. Those results have been further confirmed in a practical application, where the electrons are used in a bremsstrahlung-based positron generation configuration, which led to a considerable boost in the positron energy as well. Numerical simulations suggest that the trailing second harmonic relativistic laser pulse is capable of sustaining the acceleration structure for much longer distances after the preceding fundamental pulse is depleted in the plasma. Therefore, our work confirms the merits of driving LPAs by two-color pulses and paves the way toward a downsizing of LPAs, making their potential applications in science and technology extremely attractive and affordable.

Generation of Terahertz Radiation in Gas Media with Variable Structure when Focusing Two-Color Femtosecond Laser Pulses

The possibility of modulating the power of terahertz radiation generated in the laser plasma produced when focusing two-color femtosecond laser pulses in air, when using a gas jet directed to the plasma formation region, is demonstrated.

Vibrationally resolved above-threshold ionization in NO molecules by intense ultrafast two-color laser pulses: An experimental and theoretical study

We present above-threshold ionization (ATI) in the molecule nitric oxide, NO, by intense (${10}^{12}--{10}^{13}\phantom{\rule{0.28em}{0ex}}\mathrm{W}/{\mathrm{cm}}^{2}$) two-color femtosecond laser pulses. Vibrationally resolved coherent interference phenomena of multiphoton and multipathway ionization processes have been investigated by measuring experimentally and modeling theoretically the molecular photoionization yields. We find that the ATI photoelectron distribution of NO, initiated from degenerate electronic and vibrational states, depends on the relative phase of the two pulses, the kinetic energy, and emission angle of the photoelectrons. This dependence codes the information of interference effects of molecular electronic states. AC Stark effects modulate interference patterns in ATI spectra. Moreover, the ATI spectra exhibit peak energy dependencies that illustrate interference effects from coherent couplings between various ionization channels. These features reflect the molecular property and the multiple channel ionization interference effects in ATI processes. The results allow to explore coherent electron-vibration processes in ultrafast molecular photoionization.

Intensity modulated terahertz vortex wave generation in air plasma by two-color femtosecond laser pulses.

We investigate the generation of broadband terahertz (THz) pulses with phase singularity from air plasmas created by fundamental and second harmonic laser pulses. We show that when the second harmonic beam carries a vortex charge, the THz beam acquires a vortex structure as well. A generic feature of this THz vortex is that the intensity is modulated along the azimuthal angle, which can be attributed to the spatially varying relative phase difference between the two pump harmonics. Fully space- and time-resolved numerical simulations reveal that transverse instabilities of the pump further affect the emitted THz field along nonlinear propagation, which may produce additional singularities resulting in a rich vortex structure. The predicted intensity modulation is experimentally demonstrated with a thermal camera, in excellent agreement with simulation results. The presence of phase singularities in the experiment is revealed by astigmatic transformation of the beam using a cylindrical mirror.

Terahertz radiation from extended two-colour air filaments

We investigate the pulse shape and intensity of terahertz (THz) radiation generated in extended air filaments, which are produced by focusing on f /d = 42 and 83 of 1–2.3 mJ two-color femtosecond laser pulses. These consist of fundamental- and second-harmonic radiation. We show the intensity modulation caused by the screening of radiation from a part of the filament. This modulation is due to interference effects in the distributed plasma THz source and dependent on the initial phase shift between harmonics. We experimentally demonstrate the evolution of a THz pulse generated in different areas along plasma channel. We also perform spectral modulation of the emitted THz radiation by emphasizing distinct frequencies.

Field-free orientation of LiH molecule induced by two-color femtosecond and terahertz laser pulses

We theoretically study the field-free molecular orientation by the combination of a two-color femtosecond laser pulse and a time-delayed terahertz (THz) laser pulse, with LiH as the prototype molecule. It is shown that the molecular hyperpolarizability is a vital parameter in achieving molecular orientation according to the pre-excitation created by an intense two-color femtosecond laser pulse. The relative amplitude of the two-color femtosecond pulse is discussed. In addition, the center frequency of the THz laser pulse is chosen on the basis of the pre-excited population distributions. The carrier-envelope phase of the THz laser pulse as well as the delay time between the femtosecond laser pulse and THz laser pulse are adjusted to achieve a high orientation degree. In addition, the effect of temperature on molecular orientation is also discussed. Finally, according to the population distributions at room temperature, the parameters of the THz laser pulse are re-optimized, and the achieved maximal orientation degree is 0.221 at 300 K.

Terahertz spectroscopy from air plasmas created by two-color femtosecond laser pulses: The ALTESSE project

Terahertz pulses are very popular because of their numerous applications, for example in security. Located between microwaves and optical waves in the electromagnetic spectrum, their spectral domain can now be exploited for molecular spectroscopy using terahertz emission from plasmas formed by femtosecond laser pulses ionizing gases such as air. Downconversion of broadband optical spectra in a plasma produces intense radiation suitable for the detection of suspect materials remotely. The different physical mechanisms involved to create terahertz radiation by laser-matter interaction are reviewed. The new potentialities offered by intense ultrafast lasers allow the acquisition of unique spectral signatures characterizing various materials.

Coherent interference of molecular electronic states in NO by two-color femtosecond laser pulses

Quantum coherence interference between electronic states in molecules is an important factor for controlling electronic and nuclear dynamics in photoinduced physics and chemistry. We measure and model molecular angle-dependent photoionization yields to explore coherent interference dynamics between quasiequal energy electronic-vibrational states of the nitric oxide molecule, NO, by ultrafast phase-controlled two-color femtosecond laser pulses. We demonstrate by experiment and theory that the photoelectron angular distribution of NO, where two excited electronic states are coherently combined by ultrafast pulses, is a function of the relative phase of the pulses, and the photoelectron kinetic energy. The modification of photoelectron emission angular patterns encodes the information of the coherence via electronic state interference. The result allows access to phase-dependent state correlations and ultrafast vibrationic dynamics in molecules.

Field-free orientation of CO molecule by combining two-color femtosecond laser pulse with time-delayed positively chirped laser pulse

The orientation of CO molecule controlled by two-color femtosecond laser pulse and positively chirped laser pulse is investigated theoretically. Compared with the single two-color or positively chirped laser pulse, the combination of two laser pulses with an optimized delay time can greatly improve the degrees of the positive and negative molecular orientation, respectively. The effects of intensity of two-color femtosecond laser pulse on molecular orientation are discussed, and the molecular orientation can be achieved by a calculated intensity. Additionally, the delay time between two-color femtosecond laser pulse and positively chirped laser pulse can significantly enhance the molecular orientation. Furthermore, by varying the carrier frequency and the chirp value of the positively chirped laser pulse, it is shown that the molecular orientation can be changed to some degree. The positive and negative molecular orientation can also be manipulated by adjusting the relative amplitude of the second harmonic field with respect to the fundamental.

Generation and amplification of sub-THz radiation in a rare gases plasma formed by a two-color femtosecond laser pulse

A new approach to constructing the source of radiation in the sub-THz frequency range is discussed. It is based on the strong-field ionization of heavy rare gases with Ramsauer minimum in the transport cross-section by a two-color () femtosecond laser pulse. Then a four-photon nonlinear process ( are the frequencies from the spectral width of the pulse with frequency ω, and is the frequency from the spectral width of the second harmonic 2ω) with a transition to the initial state results in a low-frequency spontaneous emission that can be amplified in the strongly nonequilibrium laser plasma if the position of the photoelectron peaks is located in the region of growing energy transport cross-section.