Intense Infrared Pulses Research Articles

XUV Rectification Effect in the IR-Dressed Medium.

We show that the quasistatic dipole moment can be induced by a short extreme ultraviolet (XUV) pulse (XUV rectification effect) in atomic gas medium subjected to an intense infrared (IR) field (IR-dressed atoms). The general theory of the XUV rectification effect for a single IR-dressed atom is presented, which explicitly relates IR-modified polarizability of an atomic system in the XUV range with the induced quasistatic dipole moment. We illustrate general properties of the XUV rectification effect in an atomic system within the analytical zero-range potential model by presenting the dependence on the IR-field intensity and the time delay between XUV and IR pulses.

Second-harmonic generation of a short XUV pulse interacting with an IR-laser-dressed atom

The nonlinear response of an atomic system interacting with short extreme ultraviolet (XUV) and infrared (IR) pulses is considered within the adiabatic approach (for an IR field) and the perturbation theory (for an XUV pulse). The second-order perturbation theory in the XUV field is developed for the IR-dressed atom, and the laser-induced generation of a pulse with a doubled carrier XUV frequency is analyzed. We study the yield of harmonics in the frequency band of the generated doubled frequency XUV pulse as a function of the time delay between initial XUV and IR pulses. Our analytical and numerical results show that this dependence carries the time dependence of the intense IR pulse and can be utilized to retrieve the waveform of the IR pulse.

Contribution of the collective electron dynamics to the polarization response of an atom subjected to intense IR and weak XUV pulses.

We analyze the polarization response of a single Ne atom in an intense infrared (IR) laser field and weak extreme ultraviolet (XUV) isolated attosecond pulse (IAP). The analysis is based on the numerical solution of the time-dependent Kohn-Sham equations and the recently developed perturbation theory in the XUV field for an atom subjected to an intense IR field. In our numerical results, we observe a significant increase in the magnitude of the atomic polarization response at the frequencies near the carrier frequency of the IAP and associate it with XUV-induced collective dynamics contributing to the polarizability of Ne. The specific interference between IR- and XUV-induced channels is discussed, and its utilization for retrieving the phase of the generated harmonics in the IR field is suggested.

Optimization of a multi-TW few-cycle 1.7-µm source based on Type-I BBO dual-chirped optical parametric amplification.

This paper presents the optimization of a dual-chirped optical parametric amplification (DC-OPA) scheme for producing an ultrafast intense infrared (IR) pulse. By employing a total energy of 0.77 J Ti:sapphire pump laser and type-I BBO crystals, an IR pulse energy at the center wavelength of 1.7 µm exceeded 0.1 J using the optimized DC-OPA. By adjusting the injected seed spectrum and prism pair compressor with a gross throughput of over 70%, the 1.7-µm pulse was compressed to 31 fs, which resulted in a peak power of up to 2.3 TW. Based on the demonstration of the BBO type-I DC-OPA, we propose a novel OPA scheme called the "dual pump DC-OPA" for producing a high-energy IR pulse with a two-cycle duration.

Attosecond-pulse metrology based on high-order harmonic generation

An all-optical method to retrieve the temporal intensity profile of an extreme ultraviolet (XUV) attosecond pulse is proposed based on XUV-assisted high-order harmonic generation (HHG) by an intense infrared (IR) pulse. For a harmonic located on the XUV-induced high-energy plateau (beyond the IR HHG plateau), the measured harmonic yield as a function of the time delay between the XUV and IR pulses is shown to accurately map the temporal intensity profile of the XUV pulse. Single-color and two-color orthogonal, linearly polarized IR pulses are used to demonstrate the method.

Self-channelled high harmonic generation of water window soft x-rays

Owing to the increasing significance of high harmonic generation (HHG) as a tabletop coherent x-ray source and the coming of age of intense infrared (IR) lasers, the development of high brightness soft x-ray beamlines is gaining a lot of attention. We discuss the self-guided propagation of high energy IR pulses around 1.8 μm centre wavelength being loosely focused into a long, high-pressure gas cell. A bright x-ray beam with photon energies extending up to the oxygen K-edge at 543 eV is achieved with a flux of 2.9 × 103 photons/shot/1% bandwidth around the carbon K-edge (280 eV). We provide experimental and numerical evidence of an ionization steady state condition in the generation medium causing self-channelling and intensity clamping of the driving field. While the later limits the HHG cut-off energy for a given driving field wavelength, self-channelling increases the HHG flux through a longer, phase-matched, interaction length and provides a well-collimated HHG beam covering more than three octaves from <50 to 550 eV.

High-order harmonic generation of Li+ with combined infrared and extreme ultraviolet fields**Project supported by the National Natural Science Foundation of China (Grant Nos. 11465016, 11664035, 11764038, and 11765018) and the Foundation of Northwest Normal University, China (Grant No. NWNU-LKQN-17-1).

We investigate high-order harmonic generation (HHG) of Li+ ion driven by an intense infrared (IR) laser field in combination with a weak XUV pulse. To achieve this, we first construct an accurate single-active electron angular-momentum-dependent model potential of Li+ ion, by which the accurate singlet energy levels of Li+ for the ground state and excited states with higher quantum numbers can be obtained. Then, we solve numerically the three dimensional time-dependent Schrödinger equation of Li+ ion by means of the generalized pseudospectral method to obtain HHG. Our results show that the strength of assisted XUV is not amplified during the harmonic generation process, but the yield of HHG power spectrum in the whole plateau has a significant enhancement. Furthermore, the optimal phase delay between the IR and XUV pulses allows the production of ultrabroadband supercontinuum spectra. By superposing some harmonics, a strong new single 27-attosecond ultrashort pulse can be obtained.

Attosecond streaking with twisted X waves and intense infrared pulses

Attosecond streaking with twisted <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mi>X</mml:mi></mml:math> waves and intense infrared pulses

Interference structures in photoelectron spectra of atoms ionized by XUV pulses in the presence of a strong IR field

The dynamics of photoelectrons ionized by XUV pulses in the presence of a strong infrared (IR) field is theoretically investigated by solving the time-dependent Schr\odinger equation (TDSE). We study the ionization dynamics of He by an XUV pulse in the presence of a relatively strong IR laser field, which is different from the conventional attosecond streaking experiments where a rather weak IR field is applied. Comparing with the photoelectron spectra produced by the IR field only, we find the spectra ionized by the combined fields construct interesting interference structures for different XUV photon energies and pulse intensities. These features can be analyzed and explained by a quantum-trajectory theory based on the strong-field approximation (SFA). The SFA theory shows excellent agreement with the TDSE results, revealing that these structures originate from interferences among photoelectrons ionized by XUV pluses (streaked by the strong IR field) at different moments, and in some energy regions, together with the photoelectrons ionized by the intense IR pulse itself.

Laser-assisted-autoionization dynamics of helium resonances with single attosecond pulses

The strong coupling between two autoionizing states in helium is studied theoretically with the pump-probe scheme. An isolated 100-as XUV pulse is used to excite helium near the $2s2p{(}^{1}\phantom{\rule{0.16em}{0ex}}P)$ resonance state in the presence of an intense infrared (IR) laser. The laser field introduces strong coupling between $2s2p{(}^{1}\phantom{\rule{0.16em}{0ex}}P)$ and $2{p}^{2}{(}^{1}\phantom{\rule{0.16em}{0ex}}S)$ states. The IR also can ionize helium from both autoionizing states. By changing the time delay between the XUV and the IR pulses, we investigated the photoelectron spectra near the two resonances. The results are used to explain the recent experiment by Gilbertson et al. [Phys. Rev. Lett. 105, 263003 (2010)]. Using the same isolated attosecond pulse and a 540-nm laser, we also investigate the strong coupling between $2s2p{(}^{1}\phantom{\rule{0.16em}{0ex}}P)$ and $2{s}^{2}{(}^{1}\phantom{\rule{0.16em}{0ex}}S)$ by examining how the photoelectron spectra are modified versus the time delay and the possibility of observing Autler-Townes doublet in such experiments.

Spectral Features of the Interaction of Femtosecond Light Pulses of Different Frequencies near the Boundary of a Kerr Medium

The interaction of probe ultraviolet (UV) and intense infrared (IR) pump pulses with a duration of 150 fs in a thin 2-mm-long sample of fused silica has been studied theoretically and experimentally. The spectra of UV radiation at the output of the sample have been measured depending on the time delay between the pulses at the sample input. The maximum shifts of the spectrum, attaining up to 0.22 nm for an IR power density of 3 × 1011 W/cm2, have been observed under conditions of coincidence of the pulses at the sample input and output, which corresponds to a predominant interaction of the probe radiation with the leading and trailing edges of the pump pulse near the boundary of the medium. The observed dependences are interpreted as a manifestation of the cross-phase modulation due to the Kerr nonlinearity of the medium and the dispersion of the group velocities of the UV and IR pulses. The numerical simulations performed taking into account these effects agree well with the experimental data.

Photoluminescence Study of Erbium in Silicon with a Free-Electron Laser

The influence of intense infrared (IR) radiation in the range 7-17 μm on the photoluminescence (PL) of erbium in silicon has been investigated. To excite the PL a pulsed Nd:YAG laser operating in the visible with a wavelength of 532 nm has been used. The infrared beam was generated by a free-electron laser. In the experiment the intensity and decay kinetics of the low-temperature PL of Er-doped silicon were monitored as a function of the wavelength of the quenching beam and its delay with respect to the excitation pulse of the visible-light laser. The experiments show quenching of the PL by the IR pulse only at delays shorter than approximately 250 μs The result is interpreted as dissociation of the Er-related bound-exciton (BE) state whose effective lifetime is then estimated as approximately 100 μs. A special quenching feature for λ≈ 12.5 μm is detected indicating a possible back-transfer mechanism involving the excited Er state. For still longer delay times a small transient increase of Er PL is observed.