Chirp form selection to produce intense and broad harmonic spectra and attosecond pulses in the presence of single and superposition initial states

The chirp form selection for producing intense and broad high-order harmonic spectra has been investigated when the initial state is chosen to be the single or superposition states. It is found that, for the case of a single ground initial state, the down-chirp is much better for extending the harmonic cutoff with the stronger emission intensity. Moreover, the multi-color combined field is beneficial to produce the larger harmonic cutoff and higher harmonic intensity. After the control of laser waveform, the combination of 3-color down-chirps with a proper UV pulse is the best condition to obtain the intense X-ray spectral continuum and the isolated attosecond pulse. For the case of superposition initial state, both the up-chirp and down-chirp are beneficial to generate the high-intensity spectral regions. However, with the combination of multi-color field, only the harmonic cutoff can be further extended, and the harmonic intensity presents almost no changes for the superposition initial state case. Finally, by properly choosing the 3-color up-chirps or 3-color down-chirps combined pulses, the stronger intensity harmonic spectra covering the X-ray region can be obtained, which can produce the isolated pulses of 37 as.

Spontaneous polarization effects on solid high harmonic generation in ferroelectric lithium niobate crystals

High-order harmonic generation (HHG) in ferroelectric lithium niobate (LiNbO$_{3}$) is investigated theoretically by solving the semi-conductor Bloch equations. Because of the spontaneous polarization, even-order harmonics are produced in the HHG spectra of the LiNbO$_{3}$ crystal driven by a monochromatic multi-cycle 3300-nm laser. Our numerical calculation shows that they are originated from the suppression of one half-optical cycle HHG process in each cycle of the driving field due to the spontaneous polarization. We also illustrate that the spontaneous polarization will increase the harmonic yield and extend the maximally attainable cutoff energy at the same time. We further report that the carrier-envelope phase dependence of HHG spectra changes from a minimum period of $\pi$ rad to 2$\pi$ rad when the laser polarization direction is parallel/anti-parallel to the spontaneous polarization direction in LiNbO$_{3}$ crystal. This is promising to be utilized as an isolated attosecond pulse (IAP) gating mechanism. Moreover, the two-color relative phase dependence of HHG in LiNbO$_{3}$ is also investigated and shows broken inversion-symmetry.

Bayesian approach for the optimal control of high-order harmonics for the generation of ultrashort attosecond laser pulses

We present an efficient and powerful method to optimize the production of high-order harmonic generation (HHG) and ultrashort single attosecond laser pulses. The system under investigation is the helium atoms that are exposed to the combination of chirped two-color mid-IR laser field and its 34th harmonic. The time-dependent Schrödinger equation is solved accurately and efficiently by means of the time-dependent generalized pseudospectral method and the time-frequency spectrum is obtained by the wavelet transform. We extend the machine-learning based optimization method, called Bayesian optimization (BO), to optimize the incident laser pulse to generate ultrashort attosecond laser pulse successfully for the first time. It is shown that the intensity of HHG power spectrum from the plateau region to the cutoff is enhanced by the optimized laser field by several orders of magnitude. Further, an ultrafast isolated attosecond pulse of 10 attosecond can be generated efficiently by superposing the plateau harmonics.

Strong-field coherent control of isolated attosecond pulse generation

Attosecond science promises to reveal the most fundamental electronic dynamics occurring in matter and it can develop further by meeting two linked technological goals related to high-order harmonic sources: improved spectral tunability (allowing selectivity in addressing electronic transitions) and higher photon flux (permitting to measure low cross-section processes). New developments come through parametric waveform synthesis, which provides control over the shape of field transients, enabling the creation of highly-tunable isolated attosecond pulses via high-harmonic generation. Here we demonstrate that the first goal is fulfilled since central energy, spectral bandwidth/shape and temporal duration of isolated attosecond pulses can be controlled by shaping the laser waveform via two key parameters: the relative-phase between two halves of the multi-octave spanning spectrum, and the overall carrier-envelope phase. These results not only promise to expand the experimental possibilities in attosecond science, but also demonstrate coherent strong-field control of free-electron trajectories using tailored optical waveforms.

Generation of elliptical isolated attosecond pulse from oriented $\textrm{H}_{2}^{+}$ in a linearly polarized laser field

We investigate the ellipticity of the high-order harmonic generation from the oriented $\textrm{H}_{2}^{+}$ exposed to a linearly polarized laser field by numerically solving the two dimensional time-dependent Schr$\ddot{\textrm{o}}$dinger equation (2D TDSE). Numerical simulations show that the harmonic ellipticity is remarkably sensitive to the alignment angle. The harmonic spectrum is highly elliptically polarized at a specific alignment angle $\theta = 30^\circ $, which is insensitive to the variation of the laser parameters. The position of the harmonic intensity minima indicates the high ellipticity, which can be attributed to the two-center interference effect. The high ellipticity can be explained by the phase difference of the harmonics. The results that we obtain facilitates the synthesis of a highly elliptical isolated attosecond pulses with duration down to 65 as, which can be served as a powerful tool to explore the ultrafast dynamics of molecules and study chiral light-matter interaction.

A Custom-Tailored Multi-TW Optical Electric Field for Gigawatt Soft-X-Ray Isolated Attosecond Pulses

Since the first isolated attosecond pulse was demonstrated through high-order harmonics generation (HHG) in 2001, researchers’ interest in the ultrashort time region has expanded. However, one realizes a limitation for related research such as attosecond spectroscopy. The bottleneck is concluded to be the lack of a high-peak-power isolated attosecond pulse source. Therefore, currently, generating an intense attosecond pulse would be one of the highest priority goals. In this paper, we review our recent work of a TW-class parallel three-channel waveform synthesizer for generating a gigawatt-scale soft-X-ray isolated attosecond pulse (IAP) using HHG. By employing several stabilization methods, we have achieved a stable 50 mJ three-channel optical-waveform synthesizer with a peak power at the multi-TW level. This optical-waveform synthesizer is capable of creating a stable intense optical field for generating an intense continuum harmonic beam thanks to the successful stabilization of all the parameters. Furthermore, the precision control of shot-to-shot reproducible synthesized waveforms is achieved. Through the HHG process employing a loose-focusing geometry, an intense shot-to-shot stable supercontinuum (50–70 eV) is generated in an argon gas cell. This continuum spectrum supports an IAP with a transform-limited duration of 170 as and a submicrojoule pulse energy, which allows the generation of a GW-scale IAP. Another supercontinuum in the soft-X-ray region with higher photon energy of approximately 100–130 eV is also generated in neon gas from the synthesizer. The transform-limited pulse duration is 106 as. Thus, the enhancement of HHG output through optimized waveform synthesis is experimentally proved.

Generation of an isolated attosecond pulse from aligned molecules via laser synthesis

A laser synthesis scheme via the superposition of single-color and hyper-Gaussian pulses is proposed, and the synthesized laser pulse is adopted to interact with pre-aligned molecules for the production of an isolated attosecond pulse (IAP). The numerical results based on strong-field approximation disclose that the proposed scheme is efficient for the generation of an IAP under suitable hyper-Gaussian parameters, and the intensity of an IAP is sensitive to molecular pre-alignment angles, and thus an optimal set of hyper-Gaussian parameters and pre-alignment angles can support the strongest IAP.

Mid-Infrared Few-Cycle Pulse Generation and Amplification

In the past decade, mid-infrared (MIR) few-cycle lasers have attracted remarkable research efforts for their applications in strong-field physics, MIR spectroscopy, and bio-medical research. Here we present a review of MIR few-cycle pulse generation and amplification in the wavelength range spanning from 2 to ~20 μm. In the first section, a brief introduction on the importance of MIR ultrafast lasers and the corresponding methods of MIR few-cycle pulse generation is provided. In the second section, different nonlinear crystals including emerging non-oxide crystals, such as CdSiP2, ZnGeP2, GaSe, LiGaS2, and BaGa4Se7, as well as new periodically poled crystals such as OP-GaAs and OP-GaP are reviewed. Subsequently, in the third section, the various techniques for MIR few-cycle pulse generation and amplification including optical parametric amplification, optical parametric chirped-pulse amplification, and intra-pulse difference-frequency generation with all sorts of designs, pumped by miscellaneous lasers, and with various MIR output specifications in terms of pulse energy, average power, and pulse width are reviewed. In addition, high-energy MIR single-cycle pulses are ideal tools for isolated attosecond pulse generation, electron dynamic investigation, and tunneling ionization harness. Thus, in the fourth section, examples of state-of-the-art work in the field of MIR single-cycle pulse generation are reviewed and discussed. In the last section, prospects for MIR few-cycle lasers in strong-field physics, high-fidelity molecule detection, and cold tissue ablation applications are provided.