Manifestation of anomalous Floquet states with longevity in dynamic fractional Stark ladder with high AC electric fields

We examine a resonance structure of Floquet state in dynamic fractional Stark ladder (DFSL) realized in biased semiconductor superlattices driven by a terahertz cw laser on the basis of the R-matrix Floquet theory. To do this, we calculate an excess density of state ρ(ex)(E) corresponding to lifetime of the Floquet state with a fractional matching ratio η, where η is the ratio of a Bloch frequency ΩB to a laser frequency ω, namely, η=ΩB/ω. The results for η=3/2 demonstrate the appearance of discernibly large peaks associated with Floquet states with longevity in a region of relatively high laser-intensity. The underlying physics is discussed in terms of an analytical expression of ρ(ex)(E) and the associated Green function in which ponderomotive couplings are included in a non-perturbative way.