Multicolor Pulses Research Articles

Bidirectional Cascaded Superfluorescent Lasing in Air Enabled by Resonant Third Harmonic Photon Exchange from Nitrogen to Argon.

Cavity-free lasing in atmospheric air has stimulated intense research toward a fundamental understanding of underlying physical mechanisms. In this Letter, we identify a new mechanism-a third-harmonic photon mediated resonant energy transfer pathway leading to population inversion in argon via an initial three-photon excitation of nitrogen molecules irradiated by intense 261nm pulses-that enables bidirectional two-color cascaded lasing in atmospheric air. By making pump-probe measurements, we conclusively show that such cascaded lasing results from superfluorescence rather than amplified spontaneous emission. Such cascaded lasing with the capability of producing bidirectional multicolor coherent pulses opens additional possibilities for remote sensing applications.

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.

Multicolor and supercontinuum radiation generation in terahertz and mid-infrared ranges due to the gas ionization by two-color chirped laser pulses

A method for generating multicolor phase-stable pulses tunable in a wide range from terahertz (THz) to mid-infrared (mid-IR) is proposed. It is based on gas ionization by a laser pulse containing a linearly chirped fundamental field and several double-frequency components with different time delays. Our closed-form analytical formulas and numerical calculations show that the generated current density contains unchirped components at several detuning frequencies determined by the difference between the instantaneous frequencies of the additional laser components and twice the instantaneous frequency of the fundamental field, as well as at all triple combination detuning frequencies. The generated frequencies can be changed by varying the time delay or the chirp value. The generated pulse durations are determined by the gas ionization duration, which is much shorter than the laser pulse duration and can be varied in a wide range by changing the laser pulse duration or intensity. Under a sufficiently short ionization duration, the peak widths at the generated frequencies increase greatly, forming a supercontinuum extending from THz to the mid-IR range.

Investigation of the multielectron dynamics of CO in intense laser fields by the effective potential analysis of natural orbitals

We simulated the multielectron dynamics of CO in multicolor pulses of a near-IR fundamental ω and harmonics up to 4ω by using the multiconfiguration time-dependent Hartree-Fock method. The dynamics were analyzed in terms of effective potentials that govern the time evolution of molecular orbitals. The effective potential for the highest occupied orbital has a hump barrier responsible for anisotropic ionization of CO and the shape of a hump significantly changes at different internuclear distances. The electronic response became nonadiabatic when a 4ω-field was included, resulting in the enhancement in ionization and the extension of the high-order harmonic generation cutoff.

Effect of laser focus in two-color synthesized waveform on generation of soft x-ray high harmonics

Synthesis of multi-color laser pulses has been developed as a promising way to improve low conversion efficiency of high-order harmonic generation (HHG). Here we systematically study the effect of laser focus in a two-color waveform on generation of macroscopic HHG in soft x-rays. We find that the dependence of HHG yields on laser focus at low or high gas pressure is sensitive to the characteristics of single-atom harmonic response, in which “short”- or “long”-trajectory emissions can be selectively controlled by changing the waveform of two-color synthesized laser pulse. We uncover the phase-matching mechanism of HHG in the gas medium by examining the propagation of the two-color waveform and the evolution of time-frequency emissions of high-harmonic field. We further reveal that the nonlinear effects, such as geometric phase, atomic dispersion, and plasma defocusing, are responsible for modification of two-color waveform upon propagation. This work can be used to find better macroscopic conditions for generating soft x-ray HHG by employing two-color optimized waveforms.

Raman-enhanced multicolor pulses in dissipative soliton mode-locked ring-cavity fiber laser based on GIMF-SA

The stable multicolor complex with dissipative solitons (DS) and Raman pulses (RP) is experimentally demonstrated in the mode-locked Yb-doped ring-cavity fiber laser based on a graded-index multimode fiber (GIMF) as a saturable absorber (SA). The central wavelength of DS and RP are 1032.01 nm and 1078.08 nm, respectively. By wavelength-dividing the output pulse, it is found that the pulse width and period of DS and RP are consistent with the total pulse, but the DS pulse intensity is larger than that of RP. The single-to-noise ratio (SNR) of DS is about 55 dB, which is large than that of the complex pulse. We further improve the stability of the mode-locked state in the cavity by separating the RP out of the cavity. The experimental results obtained by two methods of separating pulses indicates that the stimulated Raman scattering (SRS) effect destabilizes the DS mode-locked fiber laser and doesn’t disrupt the single pulse generation regime.

Optimal generation of delay-controlled few-cycle pulses for high harmonic generation in solids

Delay-controlled two-color, few-cycle pulses are powerful tools for ultrafast nonlinear optics. In this Letter, 35-fs, 800 nm pulses were injected into a noble-gas-filled hollow-core fiber to obtain over-octave spectra (450–1000 nm) and were divided into two parts for dispersion management by a Mach–Zehnder–type interferometer. Two few-cycle pulses with pulse widths of 9.3 and 4.5 fs were generated in the long-wavelength side and the short-wavelength side, respectively. The temporal profiles were measured as the function of the different delay between the two pulses. The shortest 3.6 fs, 0.75 mJ near-single-cycle pulses were synthesized at an optimal delay. The delay-controlled high-harmonic generation in MgO was experimentally demonstrated leading to twofold enhancement of high-order harmonic (HH) yields at 10.3 eV and the extension of HH frequency under time-delay modulation. This method provides an extensive way for manipulating delay-controlled multi-color pulses, which can be used for controlling ionization dynamics in extreme nonlinear optics. We believe that it will be a powerful tool for ultrafast science.

Publisher’s Note: “Passive, controllable generation of energetic multi-color pulses via spatial mode re-organizations in optical fibers” [APL Photonics 6, 126109 (2021)

Publisher’s Note: “Passive, controllable generation of energetic multi-color pulses via spatial mode re-organizations in optical fibers” [APL Photonics 6, 126109 (2021)

Passive, controllable generation of energetic multi-color pulses via spatial mode re-organizations in optical fibers

Multi-color energetic pulses of fs pulse width are of interest to applications such as mid-IR wavelength generation via difference frequency generation, nonlinear microscopy, and high harmonic generation. Traditional fiber-based methods to create such sources suffer from group velocity dispersion, which prevents different colors of a mode to be temporally coincident, as well as the timing jitter due to usual perturbations, such as changes in the refractive index of the fiber or slight variations of the pump power. We employ a multi-mode system, which allows group velocity matching of different modes over a Raman shift, hereby circumventing both of these issues by allowing the ultrafast Raman scattering mechanism soliton-self mode conversion (SSMC) to create multiple colors of distinct spatial modes from a single pump pulse. We experimentally demonstrate fiber-based generation of three- and four-color time-synchronized pulses of nJ-level pulse energies and ∼100 fs pulse widths, where the former span ∼25 THz and the latter ∼35 THz.

Electron beam shaping via laser heater temporal shaping

Active longitudinal beam optics can help FEL facilities achieve cutting edge performance by optimizing the beam to: produce multi-color pulses, suppress caustics, or support attosecond lasing. As the next generation of superconducting accelerators comes online, there is a need to find new elements which can both operate at high beam power and which offer multiplexing capabilities at Mhz repetition rate. Laser heater shaping promises to satisfy both criteria by imparting a programmable slice-energy spread on a shot-by-shot basis. We use a simple kinetic analysis to show how control of the slice energy spread translates into control of the bunch current profile, and then we present a collection of start-to-end simulations at LCLS-II in order to illustrate the technique.

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.

Raman-scattering-assistant large energy dissipative soliton and multicolor coherent noise-like pulse complex in an Yb-doped fiber laser.

In this Letter, we have demonstrated the generation of dissipative solitons (DSs) or multi-wavelength noise-like pulses (NLPs) directly from a common linear Yb-doped fiber laser in the presence of stimulated Raman scattering (SRS). For the DSs, the pulse energy of the solitons with a pulse width of 74.2 ps reaches 21.2 nJ. For the NLPs, the generation of the main NLP (1032 nm) together with the first-order Raman NLP (1080 nm) is realized. The narrow peak of the double-scale autocorrelation trace is characterized by quasi-periodic beat pulses with a pulse beating of 40.6 fs and a pulse separation of 79 fs, indicating that the generated solitons at dual wavelengths are mutually coherent. Furthermore, a three-color stable NLP complex with a broader spectrum is also obtained. The results contribute to an in-depth understanding of nonlinear dynamics and ultrafast physics.

Morphology-Controllable Ultrafast Fiber Lasers Based on Intracavity Manipulation of Transverse Modes

The performances of mode-locked fiber lasers rest with the design and configuration of the cavity, and their temporal morphologies are rarely linked with the modulation of transverse modes. Here, we demonstrate a morphology-controllable ultrafast fiber laser based on the intracavity manipulation of transverse modes, capable of producing dual-color pulses, narrowband picosecond pulses, and broadband femtosecond pulses. Unlike traditional multicolor pulses with unequal group velocities, the dual-color pulse operates in a synchronous state, displaying a multipeak structure where the modulation period relies on the group-delay difference of two modes in the fiber. Simulation results fully reproduce the experimental observations and show that the laser follows the minimum-loss principle with pulse morphologies depending on the mode-interference-induced filtering effect. This work connects transverse-mode modulation with pulse morphology, providing a stable and cost-effective laser source for terahertz-wave generation and nonlinear spectroscopy.

Enhancing terahertz radiation from femtosecond laser filaments using local gas density modulation

We present a method to enhance the terahertz (THz) wave radiation from a femtosecond laser-induced plasma filament by controlling the local gas density within the filament. We develop a theoretical model for THz generation from a laser-induced air plasma filament and the subsequent propagation process, to account for a varying local gas density. By adjusting the local gas density along the filament, the transient current distribution along the filament and the resulting coherent superposition of terahertz waves can be controlled. The location of the gas jet nozzle and the relative phase between multicolor light fields both affect the transient current distribution and thus the strength of the generated THz field. Compared with the conventional terahertz generation by a two-color filament in a homogeneous gas, a three-color filament can realize an increase by 6.12 times in the generated THz pulse energy, with optimized local gas density modulation. Our results suggest that the THz amplification via local gas density modulation can be further improved with well-designed multicolor pulses.

Generation of coherent multicolor noise-like pulse complex in Yb-doped fiber laser mode-locked by GIMF-SA

We have demonstrated the generation of multicolor noise-like pulse complex in a passively Yb-doped mode-locked fiber laser based on a single mode-graded index multimode-single mode fiber (SMF-GIMF-SMF) device as the saturable absorber (SA). The stimulated Raman scattering (SRS) effect leads to the cascaded generation of the main noise-like pulse (NLP) at 1028.8 nm together with the noise like Raman pulse (RP) at 1076.1 nm. The generated dual wavelength pulses demonstrate the unique properties of mutually synchronization and coherence. The autocorrelation traces show that each of the synchronously mode-locked pulses exhibits a double-scale structure with a narrow peak which consists of a train of quasi-periodic beat pulses with a 35.7 fs pulse width and a pulse separation of about 77.2 fs. The total output power reaches 102.5 mW with 34% of it belonging to the RP. And furthermore, by separating the two pulses with spectral filters, the modulation fringes cannot be observed anymore. These results indicate that the Raman component participates in the mode-locking operation as a 'signal' instead of 'noise'. Such a coherent Raman pulse source provides a novel platform for numerous applications, such as frequency comb spectroscopy and so on.

Excitation of molecular rotational levels by unipolar subcycle pulses

We study theoretically the action of subcycle unipolar pulses on the rotational dynamics of polar molecules. We show that efficient excitation of molecular rotational resonances by subcycle pump pulses with respect to bipolar single-cycle one is achieved, provided that duration of excitation pulses is much smaller than the rotational period of the molecule. The excitation probability for different rotational levels is analysed in dependence on the parameters of molecules and pump pulses. Remarkably, our analysis revealed the possibility of the almost single level excitation and the formation of the population inversion in the rotational levels excited by unipolar pulses, in spite of the nonresonant interaction and ultrabroad spectrum of subcycle pulses. This approach is entirely different from the traditional resonant excitation techniques with long quasi-monochromatic pulses tuned to the molecular transition. And from the other methods, when multi-color laser pulses or single-cycle THz pulses are used for control of the molecular properties. This opens new avenues in applications related to control of molecular systems and lasing creation, since it allows efficient ultrafast control over different rotational levels.

Generation of Terahertz Radiation by Multicolor Ionizing Pulses

Analytical formulas have been obtained for the optimal waveform of an ionizing pulse and corresponding maximum density of residual terahertz currents. The found analytical solutions can be implemented with multicolor femtosecond pulses containing the field at the fundamental frequency and several of its lowest harmonics. The amplitudes of odd harmonics in the field optimal for the generation of terahertz radiation are larger than those for neighboring even harmonics. The maximum residual current density increases with an increase in the number of harmonics, in the wavelength of the fundamental harmonic, and in the ionization potential of gas particles and with a decrease in the duration of the ionizing pulse. This maximum residual current density approaches limiting values for saturated ionization when few-cycle pulses are used.

Analysis of THz generation by multicolor laser pulses with various frequency ratios

Terahertz (THz) generation in a gas medium is simulated by quantum calculations with multicolor intense laser pulses of various frequency ratios. By correlating THz radiation with the above-threshold-ionization photoelectron spectrum, we confirm that our previously proposed free-free transition model for the generation of THz radiation is also applicable to multicolor pulses. THz photon emissions can proceed through transitions between continuous states with similar energies that have opposite parities. This mechanism predicts that THz waves can be efficiently generated at special two-color laser frequency ratios when the multiphoton mixing condition is satisfied. Applying this model to multicolor fields (sawtooth wave shape), we provide a quantum-mechanical interpretation for the reason for THz radiation enhancement. A scheme combining multiple lasers to raise THz radiation satisfying multiphoton mixing conditions is also proposed.

Enhanced high-order harmonic generation in semiconductors by excitation with multicolor pulses

We investigate high-order harmonic generation in ZnO driven by linearly polarized multi-color pulses. It is shown that the intensities of the harmonics in the plateau region can be enhanced by two to three orders of magnitude when driven by two- or three-color fields as compared with the single-color pulse excitation. By analyzing the time-dependent population in the conduction band as function of both the initial and the moving crystal momenta, we demonstrate that this remarkable enhancement originates from the intraband preacceleration of electrons from their initial momenta to the top of the valence band where interband excitation takes place. We show that this preacceleration strongly increases the population in the conduction band and correspondingly the intensities of high harmonics in the plateau region. Our results confirm the very recently proposed four-step model for high harmonic generation in semiconductors [Phys. Rev. Lett. 122, 193901 (2019)] and provide an effective way to enhance the intensities of harmonics in the plateau region which has significant implications for the generation of bright attosecond light sources from semiconductors.

Broad bandwidth dual-wavelength fiber laser simultaneously delivering stretched pulse and dissipative soliton.

We numerically and experimentally demonstrate the generation of broad bandwidth mode-locked dual-wavelength pulses with diverse-pattern from a dispersion managed erbium-doped (Er-doped) fiber laser. The two-peak gain profile of the Er-doped fiber is shown to have advantages in achieving broadband dual-wavelength pulses compared to a comb filter in our cavity. Our obtained bandwidths of 24 nm and 11.5 nm represent the broadest achieved in an Er-doped dual-wavelength fiber laser to date. In addition, the weak third-order dispersion (TOD) of the fibers facilitates two dispersion-pattern pulses (one stretched pulse and one dissipative soliton) generated in the near zero dispersion regime. Our results provide a convenient, effective way to obtain such sources for potential applications, such as in dual-comb metrology and multicolor pulses in nonlinear microscopy.