We investigate harmonic generation of ultrashort, picosecond pump laser pulses toward the vacuum-ultraviolet spectral regime, enhanced by preparation of dressed states in media of rare gas atoms. A control laser, with appropriately matched frequency and/or peak intensity serves to generate dressed states with large, tunable Autler–Townes splittings in the THz range. The energetic position of the dressed states is laser-controlled to match multiple pump laser photons. This enables resonance enhancement of harmonic generation also for typically fixed-frequency and off-resonant pump laser pulses. Moreover, the tunable Autler–Townes splittings also serve to compensate and even exploit inevitable Stark shifts of atomic resonances, which otherwise reduce the nonlinear response of the medium. We provide convincing experimental data to demonstrate the concept in argon and xenon, yielding enhancements of harmonic generation by 1–2 orders of magnitude.
We investigate harmonic generation of ultrashort, picosecond pump laser pulses toward the vacuum-ultraviolet spectral regime, enhanced by preparation of dressed states in media of rare gas atoms. A control laser, with appropriately matched frequency and/or peak intensity serves to generate dressed states with large, tunable Autler–Townes splittings in the THz range. The energetic position of the dressed states is laser-controlled to match multiple pump laser photons. This enables resonance enhancement of harmonic generation also for typically fixed-frequency and off-resonant pump laser pulses. Moreover, the tunable Autler–Townes splittings also serve to compensate and even exploit inevitable Stark shifts of atomic resonances, which otherwise reduce the nonlinear response of the medium. We provide convincing experimental data to demonstrate the concept in argon and xenon, yielding enhancements of harmonic generation by 1–2 orders of magnitude.
In this work, we thoroughly investigated frequency conversion towards ultrashort picosecond (ps) laser pulses in the vacuum ultraviolet (VUV) spectral regime via nonlinear optics in coherently-driven quantum systems at high intensities. We applied approaches of adiabatic quantum dynamics relying on coherent light-matter interactions, which modify the "bare" eigenstates of a quantum system to "dressed" states and exploit slow (i.e., adiabatic) population evolutions via such dressed states. This enables enhancement of typically low-order, low-intensity nonlinear optics (at MW/cm², GW/cm²), where the light field perturbs the atomic level structure only weakly. On the other hand, aiming at nonlinear optics at higher orders, e.g., high-harmonic generation, strong light fields are applied (100 TW/cm², PW/cm²), which modify the Coulomb potential of the atom strongly. In this case, resonances play no role anymore and adiabatic quantum dynamics via atomic resonances will not work. With this work, we demonstrated the implementation of adiabatic quantum dynamics and coherently prepared media for nonlinear optics in an intermediate intensity regime (TW/cm²), i.e., at Keldysh parameters approaching γ≈1. The fundamental question was, whether the high intensities still permit us to drive and exploit coherent-adiabatic dynamics via dressed states for enhancement of frequency conversion processes. We investigated two approaches to enhance the frequency conversion yield applying nonlinear optics in coherently-driven quantum systems (rare gases). In the first approach, we resonantly enhanced harmonic generation via dressed states with large Autler-Townes splittings. With this approach, we demonstrated how to make atomic resonances available for an initially far detuned fixed-frequency pump laser and showed a compensation for inevitable, perturbing Stark shifts. In the second approach, we investigated frequency conversion, enhanced by coherent population return (CPR) and preparation of maximal atomic coherences at high intensities. With this approach, we showed how to maximally benefit from an atomic resonance via adiabatically driven frequency conversion, i.e., by detuning the strong pump laser slightly from the resonance.
We study the photoelectron spectra of hydrogen and helium ionized by intense ultrafast two-color XUV laser pulses by numerically solving the time-dependent Schrödinger equation.The photon energy of the pump pulse is tuned to the 1s-2p excitation energy, inducing Rabi oscillations between these bound states and forming an Autler-Townes (AT) doublet of dressed states.When ionized by a time-delayed ultrafast XUV pulse with duration shorter than the Rabi oscillation period, we find novel and drastically enhanced AT splittings in the photoionization spectrum.We identify their origin in terms of the temporal interference of two attosecond wave packets whose spectral distribution is controlled by the interplay between Rabi frequency and bandwidth of the probe pulse.These time-dependent features in the spectra can be much larger than the standard AT spacing induced by the pump pulse alone and can be controlled by the time delay and the duration of the probe pulse.
Abstract We investigated the Autler–Townes splitting in photoelectron spectra of molecules steered by ultrashort laser pulses using the time-dependent wave-packet method. Structure of the Autler–Townes splitting was presented to analyze the information of a selective population of the dressed states. It was found that population transfer process, structure of photoelectron spectrum and pattern of Autler–Townes splitting can be controlled by adjusting the intensity, wavelength and delay time of laser pulses.
We present experimental data on adiabatically driven frequency conversion with ultrashort (picosecond) laser pulses towards the vacuum-ultraviolet regime, enhanced by coherent population return and preparation of maximal atomic coherences. We generate the sum frequency of an intense pump pulse and a probe pulse via a two-photon resonance in xenon. When we slightly detune the pump laser from two-photon resonance, the atomic populations are adiabatically driven forth and back from the atomic ground state to an excited state. This coherent population return (CPR) prepares the medium in a transient state of maximal atomic coherence, which enhances frequency mixing with a probe laser. We thoroughly study the variation of CPR-enhanced frequency conversion with experimental parameters and compare CPR with conventional, resonantly enhanced frequency conversion. In particular, we investigate the temporal evolution of the atomic coherence and the effect of inevitable ac Stark shifts. We find that ac Stark shifts induce an asymmetry in the spectral characteristics of CPR, which nevertheless permits enhanced frequency conversion. For the case of resonantly enhanced frequency conversion, we show that the atomic coherences are maintained for several tens of picoseconds after the pump pulse, which permits time-delayed frequency conversion. Moreover, we analyze the pressure dependence of the atomic coherence' lifetime and observe a free induction decay at the two-photon coherence with the generation of a second harmonic field. These findings shall push applications of adiabatic light-matter interactions also to the regime of nonlinear optics, driven by intense laser pulses.
We present experimental data on the coherent control of frequency conversion toward the vacuum-ultraviolet regime in xenon atoms, applying intense ultrashort picoseconds (ps) radiation pulses. We report on quantum interference between two resonantly enhanced frequency conversion processes (i.e., fifth harmonic generation and four-wave mixing). The conversion processes are driven by a fundamental (ps) laser pulse at 530 nm and the phase-locked second harmonic at 265 nm. Both fifth harmonic generation and four-wave mixing yield vacuum-ultraviolet radiation at 106 nm. The two frequency conversion pathways interfere with each other---either constructively or destructively, depending on the relative phase of the driving laser pulses. Thus, by variation of the relative phase, we substantially enhance or reduce the conversion efficiency (i.e., the yield of generated vacuum-ultraviolet radiation).
We present a bound-continuum ionization model combined with numerical simulations to investigate the asymmetric features of Autler-Townes (AT) splitting in the ionization spectra of hydrogen atoms, induced by a spectrally phase-modulated ultrashort pulse. This model incorporates a resonance-enhanced two-photon ionization process, enabling transitions from the ground electronic state to the continuum state via the first excited electronic state through the absorption of two photons. By comparing our model predictions with exact numerical simulations based on time-dependent electronic wave-packet calculations, we demonstrate that the characteristics of the AT doublets, shaped by the spectral phase, emerge from the competing effects of selective population of dressed states and the transition amplitude to the continuum. In addition, through a perturbative analysis of the photoelectronic spectrum and the application of the Magnus expansion, we reveal a correlation between the number of AT fringes, resonant states, and pulse area, while drawing insights from bound states. This work enhances our understanding of how the spectral phase can be utilized to explore bound-state effects in ultrafast ionization dynamics.