Quantum Optical Description of Radiation by a Two-Level System in Strong Laser Fields

We develop a quantum optical description of radiation from a two-level system(TLS)in strong laser fields,which provides a clear insight into the final states of the TLS and the harmonics field.It is shown that there are two emission channels:the Rayleigh-like channel and the Raman-like channel,which correspond to the TLS ending up in the ground state and excited state after the emission,respectively.The numerical result shows that the harmonics are mainly produced by the Rayleigh-like channel.In addition,according to the coherence of emission among the emitters,the radiation is divided into coherent parts that result from the semi-classical dipole oscillation and incoherent parts that result from the quantum fluctuations of the dipole moment.In the weak field limits,the Rayleigh-like channel corresponds to the coherent parts,and the Raman-like channel corresponds to the incoherent parts.However,in strong laser fields,both channels contribute to coherent and incoherent radiation,and how much they contribute depends on the final excitation.By manipulating the laser field,we can make the Rayleigh-like channel produce either coherent or incoherent radiation.

Crystal-Momentum-Resolved Contributions to Harmonics in Laser-Driven Graphene

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.

Decoherence Effects of Terahertz Generation in Solids under Two-Color Femtosecond Laser Fields

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.

Multiband Dynamics of Extended Harmonic Generation in Solids under Ultraviolet Injection

Using one-dimensional semiconductor Bloch equations,we investigate the multiband dynamics of electrons in a cutoff extension scheme employing an infrared pulse with additional UV injection.An extended three-step model is firstly validated to play a dominant role in emitting harmonics in the second plateau.Surprisingly,further analysis employing the acceleration theorem shows that,though harmonics in both the primary and secondary present positive and negative chirps,the positive(negative) chirp in the first region is related to the so-called short(long) trajectory,while that in the second region is emitted through ’general’ trajectory,where electrons tunneling earlier and recombining earlier contribute significantly.The novel characteristics deepen the understanding of high harmonic generation in solids and may have great significance in attosecond science and reconstruction of band dispersion beyond the band edge.