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Coherent frequency conversion towards ultrashort VUV pulses enhanced by dressed states and adiabatic quantum dynamics

Fabian Cipura

Published Jan 1, 2020• 0 citations
Publisher

Abstract

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.

DOI
10.25534/tuprints-00012868

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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 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 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 present an experimental demonstration of sum-frequency generation toward tunable picosecond, broadband radiation pulses in the extreme-ultraviolet regime, supported by adiabatically prepared atomic coherences. We drive a two-photon transition in a dense medium of xenon atoms with a long (nanosecond) pump laser pulse at the Fourier-transform-limited bandwidth and with a wavelength of 225 nm. The frequency of the pump pulse is slightly detuned from exact two-photon resonance. In this configuration, the medium is adiabatically driven by a process of coherent population return. The adiabatic passage process generates a maximal coherent superposition of two quantum states with large energy spacing. An additional, short (picosecond) probe laser pulse at a wavelength of 540 nm beats with the maximal atomic coherence and generates a short (picosecond) signal radiation pulse at 93 nm. As compared to conventional (diabatic) frequency conversion, the adiabatically driven maximal atomic coherence yields enhanced conversion efficiency and significantly enhanced stability.
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 experimental data on quantum interferences in resonantly enhanced frequency up-conversion towards the vacuum-ultraviolet spectral regime. The process is driven in xenon atoms by ultrashort (picosecond) laser pulses. We use two simultaneous frequency conversion pathways via an excited intermediate state, i.e., fifth harmonic generation of the fundamental wavelength and four-wave mixing of the fundamental and two photons of its second harmonic wavelength. Both conversion pathways yield radiation at 102 nm. The two pathways interfere, depending on the relative phase of the fundamental and second harmonic. By appropriate choice of the phase we get constructive interference (resulting in increased conversion efficiency) or destructive interference (resulting in reduced conversion efficiency). The total conversion yield shows very pronounced constructive and destructive quantum interference with a visibility of roughly 90%. A stable and highly accurate phase control setup enables such strong quantum interferences for more than 260 oscillation cycles. In an extension of the experiment, simultaneously to frequency conversion we also monitor laser-induced fluorescence as a measure for the excitation probability to the excited intermediate state. Also, in the excitation probability we observe strong quantum interferences. As an interesting feature, a small phase lag occurs between the quantum interference patterns of frequency conversion and population transfer. This is due to an additional atomic phase acquired during frequency conversion.
For decades, most research of high harmonic generation (HHG) considered matter as quantum but light as classical. Recently, HHG driven by quantum states of light such as bright squeezed vacuum was predicted to reach beyond the classical HHG cutoff. Moreover, in squeezed coherent illumination, it was shown that the underlying dynamics are significantly modified by the photon statistics effective force. Here we show that HHG driven by quantum light results in quantum high harmonics. We derive a formula for the quantum state of the high harmonics, when driven by arbitrary quantum light states, and then explore specific cases of experimental relevance. Specifically, for a moderately squeezed pump, HHG driven by squeezed coherent light results in squeezed high harmonics. Harmonic squeezing is optimized by syncing ionization times with the pump's squeezing phase. Beyond this regime, as pump squeezing is increased, the harmonics initially acquire squeezed thermal photon statistics, and then occupy an intricate quantum state which strongly depends on the semiclassical nonlinear response function of the interacting system. Our results pave the way for generation of squeezed extreme-ultraviolet ultrashort pulses, and more generally, quantum frequency conversion into previously inaccessible spectral ranges, which may enable ultrasensitive attosecond metrology. Published by the American Physical Society 2024