Abstract Coherent control uses interference between quantum pathways to enhance or suppress transitions to a target quantum state. In stimulated Raman excitation, such control can be achieved through interference between two indistinguishable pathways connecting the same initial and final states, with the relative phase between the pathways modulating the population transfer to the final state. Extending this strategy to the extreme ultraviolet (XUV) and X-ray regimes would enable control of ultrafast electronic dynamics and site-selective core excitations. However, Raman coupling at short wavelengths competes with photoionization and intermediate-state decay, whereas intense fields with stable and adjustable relative phases remain difficult to produce. Here we use two-colour pulses from a seeded coherent free-electron laser to excite helium from its ground state to a bound metastable state via stimulated XUV Raman excitation. We implement phase-coherent control by creating two interfering Raman pathways through the ionization continuum and modulate the transferred population by varying their relative phases. Blocking one pathway removes the modulation, confirming its interferometric origin, and time-dependent Schrödinger equation calculations reproduce the measured phase dependence. Our results establish phase-controlled stimulated Raman excitation in the XUV regime, providing a strategy for the coherent control of atomic and molecular processes at short wavelengths.Abstract Coherent control uses interference between quantum pathways to enhance or suppress transitions to a target quantum state. In stimulated Raman excitation, such control can be achieved through interference between two indistinguishable pathways connecting the same initial and final states, with the relative phase between the pathways modulating the population transfer to the final state. Extending this strategy to the extreme ultraviolet (XUV) and X-ray regimes would enable control of ultrafast electronic dynamics and site-selective core excitations. However, Raman coupling at short wavelengths competes with photoionization and intermediate-state decay, whereas intense fields with stable and adjustable relative phases remain difficult to produce. Here we use two-colour pulses from a seeded coherent free-electron laser to excite helium from its ground state to a bound metastable state via stimulated XUV Raman excitation. We implement phase-coherent control by creating two interfering Raman pathways through the ionization continuum and modulate the transferred population by varying their relative phases. Blocking one pathway removes the modulation, confirming its interferometric origin, and time-dependent Schrödinger equation calculations reproduce the measured phase dependence. Our results establish phase-controlled stimulated Raman excitation in the XUV regime, providing a strategy for the coherent control of atomic and molecular processes at short wavelengths.

Nature PhotonicsOct 9, 20261 citation

Coherent Pulse Propagation

WORLD SCIENTIFIC eBooksFeb 1, 20230 citations

Rabi oscillations in extreme ultraviolet ionization of atomic argon

Martin Flögel, Judith Durá, Bernd Schütte +3 more

We demonstrate Rabi oscillations in nonlinear ionization of argon by an intense femtosecond extreme ultraviolet (XUV) laser field produced by high-harmonic generation. We monitor the formation of $\mathrm{A}{\mathrm{r}}^{2+}$ as a function of the time delay between the XUV pulse and an additional near-infrared (NIR) femtosecond laser pulse, and show that the population of an $\mathrm{A}{\mathrm{r}}^{+*}$ intermediate resonance exhibits strong modulations both due to an NIR laser-induced Stark shift and XUV-induced Rabi cycling between the ground state of $\mathrm{A}{\mathrm{r}}^{+}$ and the $\mathrm{A}{\mathrm{r}}^{+*}$ excited state. Our experiment represents a direct experimental observation of a Rabi-cycling process in the XUV regime.We demonstrate Rabi oscillations in nonlinear ionization of argon by an intense femtosecond extreme ultraviolet (XUV) laser field produced by high-harmonic generation. We monitor the formation of $\mathrm{A}{\mathrm{r}}^{2+}$ as a function of the time delay between the XUV pulse and an additional near-infrared (NIR) femtosecond laser pulse, and show that the population of an $\mathrm{A}{\mathrm{r}}^{+*}$ intermediate resonance exhibits strong modulations both due to an NIR laser-induced Stark shift and XUV-induced Rabi cycling between the ground state of $\mathrm{A}{\mathrm{r}}^{+}$ and the $\mathrm{A}{\mathrm{r}}^{+*}$ excited state. Our experiment represents a direct experimental observation of a Rabi-cycling process in the XUV regime.

Physical Review AFeb 10, 201722 citations

Modification of laser-induced state in atomic attosecond transient absorption by the XUV pulse pair

Xuhong Li, Guanglu Yuan, Xiangyu Tang +4 more

Abstract Attosecond transient absorption (ATA) has been developed as an all-optical technique for probing electron dynamics in matter. Here we present a scheme that can modify the laser-induced state and the corresponding ATA spectrum via excitation by a pair of XUV attosecond pulses and by a time-delayed mid-infrared (MIR) laser probe. Different from the scheme of the electronic excitation by a single XUV attosecond pulse, the application of a pair of XUV pulses provides extra degrees of freedom, such as the time delay and the intensity ratio between two XUV pulses, which make it possible to adjust the pump process, resulting in the modification of the ATA spectrum. We show that by varying the time delay between the two XUV pulses, the population of the dark state and the ATA spectrum of the laser-induced state have periodic modulations. We also demonstrate that the peak of the ATA spectrum of the laser-induced state appears at a fixed time delay between the XUV pair and the MIR laser when the intensity ratio is large, and it changes with the time delay when the intensity ratio is small, which can be related to either one of two peaks in the population of the dark state.Abstract Attosecond transient absorption (ATA) has been developed as an all-optical technique for probing electron dynamics in matter. Here we present a scheme that can modify the laser-induced state and the corresponding ATA spectrum via excitation by a pair of XUV attosecond pulses and by a time-delayed mid-infrared (MIR) laser probe. Different from the scheme of the electronic excitation by a single XUV attosecond pulse, the application of a pair of XUV pulses provides extra degrees of freedom, such as the time delay and the intensity ratio between two XUV pulses, which make it possible to adjust the pump process, resulting in the modification of the ATA spectrum. We show that by varying the time delay between the two XUV pulses, the population of the dark state and the ATA spectrum of the laser-induced state have periodic modulations. We also demonstrate that the peak of the ATA spectrum of the laser-induced state appears at a fixed time delay between the XUV pair and the MIR laser when the intensity ratio is large, and it changes with the time delay when the intensity ratio is small, which can be related to either one of two peaks in the population of the dark state.

Communications in Theoretical PhysicsOct 19, 20222 citations

We demonstrate Rabi oscillations in nonlinear ionization of argon by an intense femtosecond extreme ultraviolet (XUV) laser field produced by high-harmonic generation. We monitor the formation of

\mathrm{A}{\mathrm{r}}^{2+}

as a function of the time delay between the XUV pulse and an additional near-infrared (NIR) femtosecond laser pulse, and show that the population of an

\mathrm{A}{\mathrm{r}}^{+*}

intermediate resonance exhibits strong modulations both due to an NIR laser-induced Stark shift and XUV-induced Rabi cycling between the ground state of

\mathrm{A}{\mathrm{r}}^{+}

and the

\mathrm{A}{\mathrm{r}}^{+*}

excited state. Our experiment represents a direct experimental observation of a Rabi-cycling process in the XUV regime.