Analysis of high-harmonic generation in terms of complex Floquet spectral analysis

摘要

Recent developments on intense laser sources is opening a new field ofoptical sciences. An intense coherent light beam strongly interacting with thematter causes a coherent motion of a particle, forming a strongly dressedexcited particle. A photon emission from this dressed excited particle is astrong nonlinear process causing high-harmonic generation(HHG), where theperturbation analysis is broken down. In this work, we study a coherent photonemission from a strongly dressed excited atom in terms of complex spectralanalysis in the extended Floquet-Hilbert-space. We have obtained theeigenstates of the total Hamiltonian with use of Feshbach-Brilloiun-Wignerprojection method. In this extended space, the eigenstates of the totalHamiltonian consisting of the radiation field and the atom system have complexeigenvalues whose imaginary part represents the decay rate. Time evolution ofthe system is represented by the complex eigenvector expansion so that thecorrelation dynamics between the photon and the atom is fully taken intoaccount. The HHG is interpreted as the irreversible spontaneous photon emissiondue to the resonance singularity in terms of the multiple Floquet states thatare generated by periodic external field. We have found that the interferencebetween the emitted photons over the different Floquet states causes spatialpulse emission correlated with the decay process of the excited atom.