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arXiv:1904.00356 · Applied Optics

Phase-matched nonlinear wave-mixing processes in XUV region with multicolor lasers

Khoa Anh Tran, Khuong Ba Dinh, Peter Hannaford, Lap Van Dao

Published Mar 25, 2019• 10 citations
arXivHTMLPDF

Abstract

We report here experimental results of perturbative nonlinear optical wave-mixing processes in the extreme ultraviolet region by using two-color and three-color laser fields. Besides the usual odd-harmonic spectrum of high harmonic generation, new spectral components are observed when multiple incommensurate lasers (one driving plus one or two control field) interact with neutral krypton gas. To demonstrate the wave-mixing process underlying such an observation, we first couple the driving field with either the signal or the idler field of an optical parametric amplifier in the gaseous ensemble to generate certain mixing frequencies. The two control fields are then simultaneously combined with the driving field to produce broad and distinguishable mixing peaks that clearly reveal the contribution of each control laser. Finally, the variation of the intensity of the mixing waves with the intensity of each control field, the gas density, and the relative focus position is examined for signatures of phase-matched generation of the mixing fields in this spectral region.

DOI
10.1364/ao.58.002540

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We investigate generation of the third harmonic (TH; λ = 355 nm) signal by two-colour excitation (λ = 1064 nm and its second harmonic, λ = 532 nm) in argon gas, with emphasis on the influence of relative phases and intensities of the two-colour pump on the third-order nonlinear frequency conversion process. Perturbative nonlinear optics predicts that the TH signal will oscillate periodically with the relative phases of the two-colour driving laser fields due to the interference of TH signals from a direct third-harmonic-generation (THG) channel and a four-wave mixing (FWM) channel. For the first time, we show unequivocal experimental evidence of this effect. A modulation level as high as 0.35 is achieved by waveform control of the two-colour laser field. The modulation also offers a promising way to retrieve the relative phase value of the two-colour laser field.
The amplification of ultrashort extreme-ultraviolet (XUV) pulses in argon in high-order harmonic generation processes is studied by using the time-dependent Schr\"odinger equation in the spin-free one-active-electron and single-atom approximation. We consider a neutral argon atom initially in the valence 3p state and a sufficiently intense two-cycle driving infrared (IR) pulse for the atom to be mainly ionized after the first laser cycle. The correlated dynamics and transitions from the valence 3p to a virtual subvalence 3s state and in the ionized regions are examined by synchronizing a 1.5-fs XUV pulse to the IR pulse. The calculated single-atom gain spectrum (26--45 eV) agrees with recent experimental measurements. We discuss different channels that can be present in the gain process as a function of pulse parameters and through an analysis of the dynamics of the populations in terms of field-free eigenstates. When the XUV pulse is considered at the end of the driving IR field the amplification is due to contributions of stimulated recombination from excited Rydberg and low energy continuum states to the 3s and 3p states of argon. In regions where the IR field is intense, high energy and angular momentum states are populated and the medium can interact with the pulses through bound and continuum states involving parametric transitions, which is further confirmed by studying classical electron trajectories. We discuss how these parametric interactions might be suitable for amplification of photon energies far from the ionization threshold as observed in the experiments.
Summary form only given. In the visible region of the spectrum, nonlinear conversion of light from one frequency to another is efficient because of phase matching. The phase velocity of the fundamental and nonlinear signal is matched such that the generated nonlinear signal sums coherently throughout the entire interaction region. At visible frequencies, this is accomplished by appropriate orientation of an anisotropic crystal. Alternatively, quasi-phase matching (QPM) techniques, which restrict nonlinear light generation to specific spatial regions, can be used. Both of these techniques rely on propagation effects. In this work, we present four new results. First, we present two new phase matching techniques that can be applied uniquely to extreme nonlinear optics, to upshift light into the XUV using high harmonic generation (HHG). The first uses a type of phase matching that can occur within single atoms. The second demonstrates a new type of QPM in the XUV based on amplitude modulation. We also show that these new techniques require an electron trajectory to be adjusted by 25 attoseconds. Finally, we demonstrate excellent agreement between theory and experiment for the first time in learning control of high-order nonlinear processes.