We investigate the crystal-momentum-resolved contributions to high-order harmonic generation in laser-driven graphene by semi-conductor Bloch equations in the velocity gauge.It is shown that each harmonic is generated...We investigate the crystal-momentum-resolved contributions to high-order harmonic generation in laser-driven graphene by semi-conductor Bloch equations in the velocity gauge.It is shown that each harmonic is generated by electrons with the specific initial crystal momentum.The higher harmonics are primarily contributed by the electrons of larger initial crystal momentum because they possess larger instantaneous energies during the intraband motion.Particularly,we observe circular interference fringes in the crystal-momentum-resolved harmonics spectrum,which result from the inter-cycle interference of harmonic generation.These circular fringes will disappear if the inter-cycle interference is disrupted by the strong dephasing effect.Our findings can help to better analyze the mechanism of high harmonics in graphene.展开更多
We theoretically investigate terahertz emission from solid materials pumped by intense two-color femtosecond laser field in the presence of decoherence effects.Quantum-mechanical simulations are based on the length ga...We theoretically investigate terahertz emission from solid materials pumped by intense two-color femtosecond laser field in the presence of decoherence effects.Quantum-mechanical simulations are based on the length gauge semiconductor Bloch equations describing the optical excitation and decoherence with phenomenological dephasing and depopulation times.Contributions of interband and intraband mechanisms are identified in time domain,and the latter has dominated THz generation in solid-state systems.It is found that dephasing is crucial for enhancing asymmetric intraband current and deduced that solid-state materials with short dephasing time and long depopulation time would be optimal selection for strong-field terahertz generation experiments.展开更多
Quantum interference occurs frequently in the interaction of laser radiation with materials,leading to a series of fascinating effects such as lasing without inversion,electromagnetically induced transparency,Fano res...Quantum interference occurs frequently in the interaction of laser radiation with materials,leading to a series of fascinating effects such as lasing without inversion,electromagnetically induced transparency,Fano resonance,etc.Such quantum interference effects are mostly enabled by single-photon resonance with transitions in the matter,regardless of how many optical frequencies are involved.Here,we report on quantum interference driven by multiple photons in the emission spectroscopy of nitrogen ions that are resonantly pumped by ultrafast infrared laser pulses.In the spectral domain,Fano resonance is observed in the emission spectrum,where a laser-assisted dynamic Stark effect creates the continuum.In the time domain,the fast-evolving emission is measured,revealing the nature of free-induction decay arising from quantum radiation and molecular cooperativity.These findings clarify the mechanism of coherent emission of nitrogen ions pumped with mid-infrared pump laser and are found to be universal.The present work opens a route to explore the important role of quantum interference during the interaction of intense laser pulses with materials near multiple photon resonance.展开更多
Understanding the evolution of molecular electronic structures is the key to explore and control photochemical reactions and photobiological processes.Subjected to strong laser fields,electronic holes are formed upon ...Understanding the evolution of molecular electronic structures is the key to explore and control photochemical reactions and photobiological processes.Subjected to strong laser fields,electronic holes are formed upon ionization and evolve in the attosecond timescale.It is crucial to probe the electronic dynamics in real time with attosecond-temporal and atomic-spatial precision.Here,we present molecular attosecond interferometry that enables the in situ manipulation of holes in carbon dioxide molecules via the interferometry of the phase-locked electrons(propagating in opposite directions)of a laser-triggered rotational wave packet.The joint measurement on high-harmonic and terahertz spectroscopy(HATS)provides a unique tool for understanding electron dynamics from picoseconds to attoseconds.The optimum phases of two-color pulses for controlling the electron wave packet are precisely determined owing to the robust reference provided with the terahertz pulse generation.It is noteworthy that the contribution of HOMO-1 and HOMO-2 increases reflecting the deformation of the hole as the harmonic order increases.Our method can be applied to study hole dynamics of complex molecules and electron correlations during the strong-field process.The threefold control through molecular alignment,laser polarization,and the two-color pulse phase delay allows the precise manipulation of the transient hole paving the way for new advances in attochemistry.展开更多
基金the National Key R&D Program of China(Grant No.2019YFA0307703)the National Natural Science Foundation of China(Grant Nos.12234020 and 12274384)the Major Research Plan of the National Natural Science Foundation of China(Grant No.91850201)。
文摘We investigate the crystal-momentum-resolved contributions to high-order harmonic generation in laser-driven graphene by semi-conductor Bloch equations in the velocity gauge.It is shown that each harmonic is generated by electrons with the specific initial crystal momentum.The higher harmonics are primarily contributed by the electrons of larger initial crystal momentum because they possess larger instantaneous energies during the intraband motion.Particularly,we observe circular interference fringes in the crystal-momentum-resolved harmonics spectrum,which result from the inter-cycle interference of harmonic generation.These circular fringes will disappear if the inter-cycle interference is disrupted by the strong dephasing effect.Our findings can help to better analyze the mechanism of high harmonics in graphene.
基金supported by the National Key Research and Development Program of China(Grant No.2019YFA0307704)the National Natural Science Foundation of China(Grant Nos.91850201,U1830206,11974426,11974425,11874425,11774428,and 12074431)。
文摘We theoretically investigate terahertz emission from solid materials pumped by intense two-color femtosecond laser field in the presence of decoherence effects.Quantum-mechanical simulations are based on the length gauge semiconductor Bloch equations describing the optical excitation and decoherence with phenomenological dephasing and depopulation times.Contributions of interband and intraband mechanisms are identified in time domain,and the latter has dominated THz generation in solid-state systems.It is found that dephasing is crucial for enhancing asymmetric intraband current and deduced that solid-state materials with short dephasing time and long depopulation time would be optimal selection for strong-field terahertz generation experiments.
基金supported in part by the National Natural Science Foundation of China(Grant Nos.12034013,12234020,12204308,12174011,and 12104380)the Shanghai Science and Technology Commission(Grant No.22ZR1444100)+1 种基金the Early Career Scheme(No.9048216)NSFC/RGC Collaborative Research Scheme(No.9054901)from the Research Grants Council of Hong Kong.
文摘Quantum interference occurs frequently in the interaction of laser radiation with materials,leading to a series of fascinating effects such as lasing without inversion,electromagnetically induced transparency,Fano resonance,etc.Such quantum interference effects are mostly enabled by single-photon resonance with transitions in the matter,regardless of how many optical frequencies are involved.Here,we report on quantum interference driven by multiple photons in the emission spectroscopy of nitrogen ions that are resonantly pumped by ultrafast infrared laser pulses.In the spectral domain,Fano resonance is observed in the emission spectrum,where a laser-assisted dynamic Stark effect creates the continuum.In the time domain,the fast-evolving emission is measured,revealing the nature of free-induction decay arising from quantum radiation and molecular cooperativity.These findings clarify the mechanism of coherent emission of nitrogen ions pumped with mid-infrared pump laser and are found to be universal.The present work opens a route to explore the important role of quantum interference during the interaction of intense laser pulses with materials near multiple photon resonance.
基金supported by the National Key Research and Development Program of China(Grant Nos.2019YFA0307703,2019YFA0307700,and 2016YFA0401100)the Major Research Plan of the National Natural Science Foundation of China(Grant No.91850201)the National Natural Science Foundation of China(Grant Nos.11804388,11874066,11904400,U1830206,and 11974426).
文摘Understanding the evolution of molecular electronic structures is the key to explore and control photochemical reactions and photobiological processes.Subjected to strong laser fields,electronic holes are formed upon ionization and evolve in the attosecond timescale.It is crucial to probe the electronic dynamics in real time with attosecond-temporal and atomic-spatial precision.Here,we present molecular attosecond interferometry that enables the in situ manipulation of holes in carbon dioxide molecules via the interferometry of the phase-locked electrons(propagating in opposite directions)of a laser-triggered rotational wave packet.The joint measurement on high-harmonic and terahertz spectroscopy(HATS)provides a unique tool for understanding electron dynamics from picoseconds to attoseconds.The optimum phases of two-color pulses for controlling the electron wave packet are precisely determined owing to the robust reference provided with the terahertz pulse generation.It is noteworthy that the contribution of HOMO-1 and HOMO-2 increases reflecting the deformation of the hole as the harmonic order increases.Our method can be applied to study hole dynamics of complex molecules and electron correlations during the strong-field process.The threefold control through molecular alignment,laser polarization,and the two-color pulse phase delay allows the precise manipulation of the transient hole paving the way for new advances in attochemistry.
