Few-cycle Mid-infrared Laser Research Articles

High-harmonic generation in liquids with few-cycle pulses: effect of laser-pulse duration on the cut-off energy.

High-harmonic generation (HHG) in liquids is opening new opportunities for attosecond light sources and attosecond time-resolved studies of dynamics in the liquid phase. In gas-phase HHG, few-cycle pulses are routinely used to create isolated attosecond pulses and to extend the cut-off energy. Here, we study the properties of HHG in liquids, including heavy water, ethanol and isopropanol, by continuously tuning the pulse duration of a mid-infrared driver from the multi- to the two-cycle regime. Similar to the gas phase, we observe the transition from discrete odd-order harmonics to continuous extreme-ultraviolet emission. However, the cut-off energy is shown to be entirely independent of the pulse duration. These observations are confirmed by ab-initio simulations of HHG in large liquid clusters. Our results support the notion that the cut-off energy is a fundamental property of the liquid, independent of the driving-pulse properties. Our work implies that few-cycle mid-infrared laser pulses are suitable drivers for generating isolated attosecond pulses from liquids and confirm the capability of high-harmonic spectroscopy to determine the mean-free paths of slow electrons in liquids.

Carrier-envelope-phase-dependent below-threshold harmonic generation in few-cycle mid-infrared laser fields.

We theoretically study the dependence of below-threshold harmonic generation (BTHG) of atoms on the carrier-envelope phase (CEP) driven by few-cycle mid-infrared laser pulses. The BTHG spectra can be accurately and efficiently calculated by solving the three-dimensional time-dependent Schrödinger equation using the time-dependent generalized pseudospectral method. We present the BTHG spectra as a function of the laser-field CEP. CEP-dependent enhancement or suppression occurred at low laser field intensities owing to the changes in the resonant effects associated with multiple quantum trajectories. However, the BTHG of atoms driven by high laser intensities is insensitive to the CEP. The synchrosqueezing time-frequency transform of the BTHG and extended semiclassical analysis are performed to elucidate the underlying physical mechanism.

Sub-50 fs pulses at 2050 nm from a picosecond Ho:YLF laser using a two-stage Kagome-fiber-based compressor

The high-energy few-cycle mid-infrared laser pulse beyond 2 μm is of immense importance for attosecond science and strong-field physics. However, the limited gain bandwidth of laser crystals such as Ho:YLF and Ho:YAG allows the generation of picosecond (ps) long pulses and, hence, makes it challenging to generate few-cycle pulse at 2 μm without utilizing an optical parametric chirped-pulse amplifier (OPCPA). Moreover, the exclusive use of the near-infrared wavelength has limited the generation of wavelengths beyond 4 μm (OPCPA). Furthermore, high harmonic generation (HHG) conversion efficiency reduces dramatically when driven by a long-wavelength laser. Novel schemes such as multi-color HHG have been proposed to enhance the harmonic flux. Therefore, it is highly desirable to generate few-cycle to femtosecond pulses from a 2 μm laser for driving these experiments. Here, we utilize two-stage nonlinear spectral broadening and pulse compression based on the Kagome-type hollow-core photonic crystal fiber (HC-PCF) to compress few-ps pulses to sub-50 fs from a Ho:YLF amplifier at 2 μm at 1 kHz repetition rate. We demonstrate both experimentally and numerically the compression of 3.3 ps at 140 μJ pulses to 48 fs at 11 μJ with focal intensity reaching 10 13 W / cm 2 . Thereby, this system can be used for driving HHG in solids at 2 μm. In the first stage, the pulses are spectrally broadened in Kagome fiber and compressed in a silicon-based prism compressor to 285 fs at a pulse energy of 90 μJ. In the second stage, the 285 fs pulse is self-compressed in air-filled HC-PCF. With fine-tuning of the group delay dispersion (GDD) externally in a 3 mm window, a compressed pulse of 48 fs is achieved. This leads to a 70-fold compression of the ps pulses at 2050 nm. We further used the sub-50 fs laser pulses to generate white light by focusing the pulse into a thin medium of YAG.

Few-cycle mid-infrared laser based on nonlinear self-compression in solid thin plates.

A few-cycle mid-infrared (MIR) laser is demonstrated via nonlinear self-compression in solid thin plates. In this novel solution, the anomalous material dispersion in the MIR band and the chirp induced by self-phase modulation are mutually compensated, which can achieve self-compression. Finally, with the 4 µm laser injection with 4.8 mJ/155 fs and few-cycle pulses with 3.44 mJ, 29.4 fs are generated with a high efficiency of 71.7%, and the system maintains very good spectral stability in 10 days. Compared with other post-compression methods, this self-compression technique has the advantages of high efficiency and robust and large energy expansion scale, which can be further extended to MIR lasers with other wavelengths and higher peak power.

Carrier-envelope-phase measurement of few-cycle mid-infrared laser pulses using high harmonic generation in ZnO.

High-harmonic generation (HHG) in crystals offers a simple, affordable and easily accessible route to carrier-envelope phase (CEP) measurements, which scales favorably towards longer wavelengths. We present measurements of HHG in ZnO using few-cycle pulses at 3.1µm. Thanks to the broad bandwidth of the driving laser pulses, spectral overlap between adjacent harmonic orders is achieved. The resulting spectral interference pattern provides access to the relative harmonic phase, and hence, the CEP.

Crystal-momentum-resolved contributions to high-order harmonic generation in solids

We analytically and numerically investigate the emission of high-order harmonic radiation from model solids by intense few-cycle midinfrared laser pulses. In single-active-electron approximation, we expand the active electron's wave function in a basis of adiabatic Houston states and describe the solid's electronic band structure in terms of an adjustable Kronig-Penney model potential. For high-order harmonic generation (HHG) from MgO crystals, we examine spectra from two-band and converged multiband numerical calculations. We discuss the characteristics of intra- and interband contributions to the HHG spectrum for computations including initial crystal momenta either from the $\mathrm{\ensuremath{\Gamma}}$ point at the center of the first Brioullin zone (BZ) only or from the entire first BZ. For sufficiently high intensities of the driving laser field, we find relevant contributions to HHG from the entire first BZ. Based on numerically calculated spectra, we scrutinize the cutoff harmonic orders as a function of the laser peak intensity and find good qualitative agreement with our analytical saddle-point-approximation predictions and published theoretical data.

Sub-cycle steering of the deprotonation of acetylene by intense few-cycle mid-infrared laser fields.

Directional breaking of the C-H/C-D molecular bond is manipulated in acetylene (C2H2) and deuterated acetylene (C2D2) by waveform controlled few-cycle mid-infrared laser pulses with a central wavelength around 1.6 μm at an intensity of about 8 × 1013 W/cm2. The directionality of the deprotonation of acetylene is controlled by changing the carrier-envelope phase (CEP). The CEP-control can be attributed to the laser-induced superposition of vibrational modes, which is sensitive to the sub-cycle evolution of the laser waveform. Our experiments and simulations indicate that near-resonant, intense mid-infrared pulses permit a higher degree of control of the directionality of the reaction compared to those obtained in near-infrared fields, in particular for the deuterated species.

Dissociation and Ionization of Quasi-Periodically Vibrating H2+ in Intense Few-Cycle Mid-Infrared Laser Fields

Using quantum mechanics calculations, we theoretically study the dissociation and ionization dynamics of the hydrogen-molecule ion in strong laser fields. Having prepared the nuclear wave packet of H2+ in a specific vibrational state, a pump laser is used to produce a vibrational excitation, leading to quasi-periodical vibration without ionization. Then, a time-delayed few-cycle laser is applied to trigger the dissociation or ionization of H2+. Both the time delay and the intensity of the probe laser alter the competition between dissociation and ionization. We also explore the dependence of kinetic-energy release spectra of fragments on the time delay, showing that the channels of above-threshold dissociation and below-threshold dissociation are opened and closed periodically. Also, dissociation from different channels is influenced by nuclear motion. The dissociation mechanism has been described in detail using the Floquet picture. This work provides a useful method for steering the electronic and nuclear dynamics of diatomic molecules in intense laser fields.

Correspondence of below-threshold high-order-harmonic generation and frustrated tunneling ionization

Among many of the nonlinear phenomena induced by strong laser pulses, two of the important processes are the harmonic generation and the creation of neutral atoms in the Rydberg states. We carry out a joint study of the below-threshold high-order-harmonic (BTH) generation and the production of low-lying Rydberg atoms driven by an intense few-cycle midinfrared laser pulse. Our results are based on the numerical solution to the three-dimensional time-dependent Schr\odinger equation within the single active electron approximation and a semiclassical simulation. The yields of BTH and low-lying Rydberg atoms are found to have a similar carrier envelope phase dependence. We find that both processes can be faithfully described semiclassically in the deep tunneling regime. The trajectory analysis shows that these two processes share the same series of trajectories and can be simultaneously manipulated by the driving pulse shape. Our finding bridges the below-threshold high-order-harmonic generation and the frustrated tunneling ionization.

Steering the electron in dissociatingH2+via manipulating two-state population dynamics by a weak low-frequency field

We propose a scheme to steer the electron localization in dissociating ${{\mathrm{H}}_{2}}^{+}$ by using the combination of a phase-stabilized few-cycle mid-infrared laser pulse and a low-frequency field. Via manipulating the population dynamics in the $1s{\ensuremath{\sigma}}_{g}$ and $2p{\ensuremath{\sigma}}_{u}$ states by the weak low-frequency field, the dissociative wave packets on these two degenerate states of opposite parities are managed to share the same kinetic energy and overlap in space. By adjusting the carrier-envelope phase of the mid-infrared pulse or simply changing the intensity of the low-frequency field, the electron localization induced by the spatial interference of the two dissociation pathways can be efficiently controlled.

Breakdown of the dipole approximation in strong-field ionization.

We report the breakdown of the electric dipole approximation in the long-wavelength limit in strong-field ionization with linearly polarized few-cycle mid-infrared laser pulses at intensities on the order of 10¹³ W/cm². Photoelectron momentum distributions were recorded by velocity map imaging and projected onto the beam propagation axis. We observe an increasing shift of the peak of this projection opposite to the beam propagation direction with increasing laser intensities. From a comparison with semiclassical simulations, we identify the combined action of the magnetic field of the laser pulse and the Coulomb potential as the origin of our observations.

Carrier-envelope phase effects on the spatial coherence of high-order harmonics.

In this work, we report the Carrier-Envelope Phase Effects on the Spatial Coherence of High-order Harmonics driven by phase-stabilized few-cycle mid-infrared laser pulses. The degree of coherence varies with carrier-envelope phase periodically with a period of π. At the same time, as the harmonic orders increase, the extreme points on the curve of coherence degree vs. carrier-envelope phase shift toward the direction of carrier-envelope phase increasing. Through theoretical analysis, we find that the ionization induced frequency chirp plays an important role in the Carrier-Envelope Phase Effects on the Spatial Coherence. This effect suggests a possible method to optimize the spatial coherence of harmonics by tuning the carrier-envelope phase of the driving field.

Energy transfer between few-cycle laser filaments in air

We report on the energy exchange between two filaments generated by few-cycle mid-infrared laser in air. The direction of energy transfer vs the relative time delay is demonstrated distinctly as oscillations with a period of optical cycle. In addition, benefitting from the broadband spectrum of few-cycle laser pulses, we confirm an unreported fine spectral structure of energy transfer when the relative time delay is changed. The simulation results obtained by using a generalized two-beam coupling model agree well with the experimental results. A simple analytical expression has been deduced to explain the fine structure of energy transfer.

Broadband water window supercontinuum generation with a tailored midinfrared laser pulse in asymmetric molecular gases

Theoretically, we investigate the high-order harmonic generation (HHG) from asymmetric molecules with a phase-stabilized few-cycle midinfrared laser pulse tailored by a much weaker 800-nm laser pulse. The results indicates that the ionization is almost restricted within half an optical cycle of the driving laser pulse and the energy difference between the photons emitted by the ejected electrons in this half optical cycle and its next half optical cycle is significantly enlarged. Then a 270eV water window supercontinuum is produced in the plateau of the high-order harmonic spectrum, from which an isolated water window 67 attoseconds pulse can be filtered directly. Moreover, this scheme can also be extended to the multi-cycle regime, enabling the generation of a broadband supercontinuum with higher photon energy and an intensity comparable to that of the lower-order harmonics.

Attosecond-chirp compensation with material dispersion to produce near transform-limited attosecond pulses

The duration of attosecond pulses, produced through high harmonic generation from gaseous atoms, was found to be much longer than the transform-limited duration. The inherent attosecond chirp existing in the temporal structure of the attosecond pulses was analysed to obtain a scaling factor in terms of laser intensity and wavelength. We explored the attosecond chirp compensation using various materials with negative group delay dispersion to cover different spectral ranges. From numerical simulations based on the time-dependent Schrödinger equation, we showed that 38 as isolated attosecond pulses could be produced by applying intense few-cycle mid-infrared laser pulses after compensating for the attosecond chirp in Xe.

Generation of isolated attosecond pulses in the far field by spatial filtering with an intense few-cycle mid-infrared laser

We report theoretical calculations of high-order harmonic generation (HHG) of Xe with the inclusion of multi-electron effects and macroscopic propagation of the fundamental and harmonic fields in an ionizing medium. By using the time-frequency analysis we show that the reshaping of the fundamental laser field is responsible for the continuum structure in the HHG spectra. We further suggest a method for obtaining an isolated attosecond pulse (IAP) by using a filter centered on axis to select the harmonics in the far field with different divergence. We also discuss the carrier-envelope-phase dependence of an IAP and the possibility to optimize the yield of the IAP. With the intense few-cycle mid-infrared lasers, this offers a possible method for generating isolated attosecond pulses.