Two-color Laser Research Articles

Magnetic-induced conversion between electric quadrupole radiation and quasi dipole radiation at THz band

Based on particle-in-cell simulation and theoretical analysis, we demonstrate the possibility of the realization of the electric quadrupole radiation (EQR) and the quasi dipole radiation (QDR) modes at the THz band from the electrostatic oscillation of the electrons driven by the two-color lasers inside the plasma. Interestingly, an implantation of the external magnetic field will induce the flexible conversion between the EQR mode and the QDR mode. The emission angle, radiation frequency, and radiation field strength of the two radiation modes can be highly modulated by adjusting the intensity of the magnetic field, the frequency of the laser pulses, and the density of the plasma. This provides a new and practical way toward the generation and conversion of the multiple radiation modes, greatly extending the potential for optical manipulation in magnetized plasma.

Bayesian optimal control of the ultrashort circularly polarized attosecond pulse generation by two-color polarization gating.

We present ab initio simulations of optimal control of high-order-harmonic generation spectra that enable the synthesis of a circularly polarized 53-attosecond pulse in a single Helium atom response. The Bayesian optimization is used to achieve control of a two-color polarization gating laser waveform such that a series of harmonics in the plateau region are phase-matched, which can be used for attosecond pulse synthesis. To find the underlying mechanisms for generating these harmonics, we perform a wavelet analysis for the induced dipole moment in acceleration form, and compare the time-energy representation with the quantum paths extracted from the semiclassical calculation. We found that these coherent harmonics are excited along the short trajectories. The proposed method has the potential to migrate to laboratories for generation of isolated circularly polarized ultrashort attosecond pulses.

Ionization of the CO molecule in intense short two-color laser pulses

We present numerical results for the CO single ionization in the intense short two-color laser pulses consisting of a fundamental laser field and its second harmonic. Using a time-dependent Hartree–Fock approach, the ionization probabilities have been computed for different choices of the fundamental field intensity, intensity ratio and relative phase difference between the two colors. When the fundamental field intensity runs from 1×1013 to 2×1014 W/cm2, the dominant contribution of the highest occupied molecular orbital to the CO two-color ionization is emphasized. The contribution of the inner valence orbitals can be manipulated by varying the relative phase and the intensity ratio. Present numerical results reveal a higher CO ionization probability when laser electric field points from the C nucleus towards the O nucleus.

Control of high-order harmonic generation in solids by orthogonally polarized laser fields

We theoretically investigate the high-order harmonic generation (HHG) from ZnO crystals driven by orthogonally polarized two-color (OTC) laser fields. By solving the semiconductor Bloch equation (SBE) in two dimensions, it is found that the HHG yields show a high sensitivity to the relative phase between the two fields. By changing the laser intensity, the yields of HHG show two maxima in the high-order region. To reveal the physical origin of these phenomena, we analyze the behavior of the electron-hole pair in the real and reciprocal spaces by the recollision model, respectively. It could qualitatively interpret the results by SBE simulations. Our analysis demonstrates that the OTC laser fields could control the electron-hole dynamics and HHG processes efficiently.

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.

Investigations on synthesis, growth, crystal structure, thermal, Dielectric and Terahertz Transmission properties of Organic NLO Crystal : (2-(2-hydroxy-3-methoxystyryl)-1-methylquinolinium-4-methylbenzenesulfonate (O-HMQ)

We investigate an organic nonlinear optical (NLO) crystal, (2-(2-hydroxy-3-methoxystyryl)-1-methylquinolinium-4-methylbenzenesulfonate (O-HMQ) suitable for Terahertz applications. The present study elaborates the synthesis, crystal structure, thermal stability and dielectric and transmission properties of O-HMQ crystal at terahertz frequencies. O-HMQ is successfully synthesized in our Laboratory. H1-NMR and FTIR results revealed the molecular structure. O-HMQ single crystals of size 7 × 3 × 3 mm3 are grown from methanol solution by solvent evaporation technique. Single crystal X-ray diffraction data shows that O-HMQ belongs to a monoclinic crystal structure with space group C 2/c. The melting point analysis shows a phase transition around 72°C and a mesophase. The temperature-dependent dielectric study has been carried out for different frequencies ranging from 1kHz to 1 MHz proved the phase transition. The real part of the dielectric constant (εrʹ), imaginary part (εi″) and dielectric loss (tanδ) are estimated. Further, we demonstrate Terahertz (THz) transmission characteristics of our as-grown O-HMQ single crystal using 50 fs ultra-short two-color laser plasma source in a broad spectral range of 0.1–50 THz, proving its suitability for THz related applications.

High-order harmonic generation by planar polyatomic molecules exposed to an orthogonally polarized two-color laser field

The molecular strong-field approximation is employed to study high-order harmonic generation by linear and planar polyatomic molecules exposed to an orthogonally polarized two-color laser field, which consists of two orthogonal linearly polarized components with commensurable frequencies. For such a driving field, we find that the harmonic emission rate and the shape of the spectrum strongly depend on the laser-field parameters including the relative phase and the ratio of the intensities of the two components. The values of the relative phase that correspond to the optimal harmonic emission rate, as well as the cutoff position, can be assessed using a classical model. The possible production of an isolated attosecond pulse is investigated. For suitable symmetry of the laser field an attosecond pulse train with only one attosecond pulse per cycle can be generated. Depending on the frequencies of the two field components, the molecular symmetry properties and the orientation of the molecule with respect to the field, the even harmonics can be absent from the spectrum, which can be used to determine the molecular orientation. The emitted harmonics are elliptically polarized and their ellipticity depends on the molecular orientation.

Phase delay effect of nonsequential double ionization of Ar atoms in a 45°cross-linearly-polarized two-color laser field

We theoretically investigate the nonsequential double ionization (NSDI) of Ar atoms by a three-dimensional (3D) classical ensemble method in a 45∘ cross-linearly-polarized two-color (CTC) laser field. Our results show that the phase delay of the synthesized two-color laser field can influence the NSDI process. By tracing the classical trajectories of the NSDI process, we find that the single-recollision (SR) and the double-recollision (DR) both occur. Whether the recollision processes happen once or twice, the NSDI predominantly experience the recollision-induced excitation with subsequent ionization (RESI) channel. And the phase delay of the CTC pulse could influence the probability of the double ionization and the energy transfer in NSDI process. This provides a method to obtain the information about the dynamics of the recollision process.

Enhance terahertz radiation and its polarization- control with two paralleled filaments pumped by two-color femtosecond laser fields.

We present experimentally an obvious enhancement of the terahertz (THz) radiation with two paralleled filaments pumped by two-color laser fields for a full use of a high laser power, compared with single filament. By mapping the 3-dimensional electric trajectories of generated THz fields with a (111) ZnTe crystal, we observe that the total THz polarization from two filaments can be manipulated by varying the time delay between the two orthogonally polarized pumps, which agrees well with the simulations under the photocurrent model. Notably, the power and spectrum of the THz field almost keep unchanged while manipulating the ellipticity of the THz polarization, which is important for a polarization-controllable THz source.

Two-color x-ray free-electron laser by photocathode laser emittance spoiler

A simple and noninvasive method based on a laser emittance spoiler allows one to produce two-color x rays in free-electron lasers with high repetition rate and high flexibility.

Two-color coherent control in photoemission from gold needle tips

We demonstrate coherent control of photoemission from a gold needle tip using a two-color laser field. The relative phase between a fundamental field and its second harmonic imprints a strong modulation on the emitted photocurrent with up to 96.5% visibility. The visibility as a function of the second harmonic intensity can be described by three interfering quantum pathways. Increasing the bias voltage applied to the tip reduces the maximum achievable visibility and modifies the weights of the involved pathways. Simulations based on the time-dependent Schrödinger equation reproduce the characteristic cooperative signal and its dependence on the second harmonic intensity, which further confirms the involvement of three emission pathways.

Coherent Control of High Harmonic Generation Driven by Metal Nanotip PhotoemissionSupported by the National Key Research and Development Program of China (Grant Nos. 2019YFA0307700 and 2016YFA0401100), the National Natural Science Foundation of China (Grant Nos. 11774215, 11674209, 91950101, 11947243, 11334009, 11425414, and 11947080), Sino-German Mobility Programme (Grant No. M-0031), Department of Education of Guangdong Province (Grant No. 2018KCXTD011), High Level

Steering ultrafast electron dynamics with well-controlled laser fields is very important for generation of intense supercontinuum radiation. It can be achieved through coherent control of the symmetry of the interaction between strong-field laser fields and a metal nanotip. We employ a scheme of two-color laser pulses combined with a weak static field to realize the control of a single quantum path to generate high harmonic generation from a single solid-state nanoemitter. Moreover, a smooth and ultrabroad supercontinuum in the extreme ultraviolet region is obtained, which can produce a single attosecond pulse. Our findings are beneficial for efficient generation of isolated sub-100 as XUV pulses from solid-state sources.

Generation of necklace-shaped high harmonics in a two-color vortex field.

We numerically studied gas high-harmonic generation in a two-color vortex laser field using the non-adiabatic Lewenstein model. Macroscopic responses were calculated by numerically solving the three-dimensional propagation equation in cylindrical coordinates. It was confirmed that unique high-harmonic signals with necklace-like shapes exhibit orbital angular momentum (OAM). The azimuthally distributed necklace harmonics exhibit periodic modulation as a function of laser frequency and topological charges of the driving field. Phase investigation showed that the OAM of the necklace harmonics is attributable to the tuning of the relative intensity of the two driving pulses. These findings provide a new dimension for high-harmonic manipulation in the vortex field. The two-color vortex field is the first scheme proposed for manipulating the intensity profile of high harmonics.

Enhancement of the photocurrents injected in gapped graphene by the orthogonally polarized two-color laser field.

We theoretically investigate the photocurrents injected in gapped graphene by the orthogonally polarized two-color laser field. Depending on the relative phase, the photocurrents can be coherently controlled by deforming the electron trajectory in the reciprocal space. Under the same field strength, the peak photocurrent in the orthogonally polarized two-color field is about 20 times larger than that for linearly polarized light, and about 3.6 times for elliptically polarized light. The enhancement of the photocurrent can be attributed to an obvious asymmetric distribution of the real population in the reciprocal space, which is sensitive to the waveform of the laser field and related to the quantum interference between the electron trajectories. Our work provides a noncontact method to effectively enhance the injected current in graphene.

Elliptical high-order harmonic generation from H2+ driven by orthogonally polarized two-color laser fields

We theoretically investigate the elliptical high-order harmonic generation from the H${}_{2}^{+}$ molecule driven by the orthogonally polarized two-color laser field by numerically solving the two-dimensional time-dependent $\mathrm{Schr}\stackrel{\ifmmode \ddot{}\else \"{}\fi{}}{\text{o}}\mathrm{dinger}$ equation. The results show that the odd-order harmonics in the $x$ component and the even-order harmonics in the $y$ component can be generated with the specific alignment angle $\ensuremath{\theta}={0}^{\ensuremath{\circ}}$ for the different intensity ratios of the laser fields. The harmonic intensity distribution and ellipticity distribution at the different intensity ratios of the laser fields and the alignment angles differ significantly from each other. The result also shows that the ellipticity-tunable high-order harmonics can be generated through adjusting the intensity ratio of the laser fields and the alignment angles. Further analyses show that the amplitude ratio and the phase difference of the $x$ and $y$ components of the harmonics are the origin of the ellipticity of the harmonics in general. The two-center interference effect can also affect the ellipticity of the harmonics. The results show that the ellipticity of the harmonics can be used as a potential tool to probe the position of the minimum. The results we obtained can be useful for the optimization of elliptically polarized XUV radiation generation.

Strong-field ionization of heteronuclear diatomic molecules using an orthogonally polarized two-color laser field

We apply the improved molecular strong-field approximation to investigate high-order above-threshold ionization (HATI) of heteronuclear diatomic molecules by an orthogonally polarized two-color (OTC) laser field. The OTC field consists of two linearly polarized components with frequencies $r\ensuremath{\omega}$ and $s\ensuremath{\omega}$, where $r$ and $s$ are integers, and $\ensuremath{\omega}$ is the fundamental frequency. The molecule is aligned in the OTC laser field polarization plane. We show that the photoelectron momentum distribution obeys one reflection symmetry which is valid for arbitrary values of the relative phase between the OTC field components in the case when $r+s$ is odd. For molecules oriented along the polarization axis ${z}_{L}$ of the $r\ensuremath{\omega}$ component (${\ensuremath{\theta}}_{L}={0}^{\ensuremath{\circ}}$) and $r$ even and $s$ odd, the HATI spectrum exhibits the reflection symmetry with respect to the ${z}_{L}$ axis. When the molecular orientation is along the ${x}_{L}$ axis, which is perpendicular to the polarization axis of the $r\ensuremath{\omega}$ component (${\ensuremath{\theta}}_{L}={90}^{\ensuremath{\circ}}$), the spectrum exhibits the reflection symmetry with respect to the ${x}_{L}$ axis for $r$ odd and $s$ even. In addition, we analyze the asymmetry in the photoelectron spectra of heteronuclear molecules by comparing them with the photoelectron spectra obtained ionizing a homonuclear diatomic molecule. We also explore the influence of the shift of the relative phase by ${180}^{\ensuremath{\circ}}$ on the HATI spectra. We explain some characteristics of the obtained HATI spectra using a generalization of the classical two-dimensional simple man's model which includes ionization probabilities calculated using the imaginary-time method. Finally, we analyze the interference minima for different heteronuclear diatomic molecules and for particular values of the emission angle, laser-field parameters, and internuclear distance. These minima are well fitted with the curve obtained using the derived condition for the two-center destructive interference minima.

Spectral caustics of high-order harmonics in one-dimensional periodic crystals.

We theoretically investigate the spectral caustics of high-order harmonics in solids. We analyze the one-dimensional model of high-order harmonic generation (HHG) in solids and find that apart from the caustics originating from the van Hove singularities in the energy band structure, another kind of catastrophe enhancement also emerges in solids when the different branches of electron-hole trajectories generating high-order harmonics coalesce into a single branch. We solve the time-dependent Schrödinger equation in terms of the periodic potential and demonstrate the control of this kind of singularity in HHG with the aid of two-color laser fields. The diffraction patterns of the harmonic spectrum near the caustics agree well with the interband electron-hole recombination trajectories predicted by the semiconductor semiclassical equation. This work is expected to improve our understanding of the HHG dynamics in solids and enable us to manipulate the harmonic spectrum by adjusting the driving field parameters.

Electronic Spectroscopy and Photoionization of LiMg.

Dimers consisting of an alkali metal bound to an alkaline earth metal are of interest from the perspectives of their bonding characteristics and their potential for being laser cooled to ultracold temperatures. There have been experimental and theoretical studies of many of these species, but spectroscopic data for LiMg and the LiMg+ cation are sparse. In this study, rotationally resolved electronic spectra for LiMg are presented. The ground state is confirmed to be X12Σ+ and observations of low-lying electronically excited states are reported for the first time. Reexamination of transitions in the near-UV spectral range indicates that previous band assignments should be revised. Two-color laser excitation techniques were used to determine an ionization energy of 4.7695(4) eV. This value is 1.2 eV below the previously reported experimental estimate. Vibrationally resolved spectra were obtained for LiMg+, yielding molecular constants that were consistent with a substantial strengthening of the bond on ionization.

Manipulating frustrated double ionization by orthogonal two-color laser pulses

Using a three-dimensional classical ensemble method, we theoretically investigated frustrated double ionization (FDI) of atoms driven by the orthogonal two-color (OTC) laser pulses. The numerical results show that the FDI yields could be controlled by changing the relative phase of the OTC fields. Moreover, the relative contribution of the two pathways of FDI, corresponding to the processes wherein the firstly or the secondly ionized electron is finally recaptured when the laser field is over, could also be manipulated by changing the relative phase of the OTC fields. The underlying dynamics for this relative-phase dependence of the FDI pathways is revealed by back-tracing the classical trajectories of the FDI events. Interestingly, the energy distributions of the highly excited states of the atom ions in the FDI events also display a sensitive relative phase dependence.