Quasienergy resonance in a dynamicWannier-Stark ladder

A continuum effect of a dynamic Wannier-Stark ladder driven by a cw laser is examined in terms of an excess density of states (EDOS), corresponding to the lifetime of a resonance state [1]. It is mathematically shown that EDOS is governed by three different physical mechanisms, namely, the single-channel resonance mechanism, the multichannel nonresonance mechanism, and the multichannel resonance mechanism. The last mechanism becomes more important with increasing laser amplitude F$_{\mathrm{ac}}$. The effect of the interchannel interaction is maximized when the ratio of a Bloch frequency to a laser frequency, represented as $\eta $, equals unity. In the actual calculations based on the R-matrix Floquet theory, it is revealed that, in a large-F$_{\mathrm{ac}}$region, EDOS for $\eta =$ 1 shows a complicated spectral structure composed of a couple of newly growing peaks, in contrast to EDOS for $\eta =$ 3 which just shows a monotonic change of a single spectral peak. It is speculated that the pronounced feature of the former spectra is attributed to the Fano-like multichannel resonance mechanism, whereas the feature of the latter case is attributed to the multichannel nonresonance mechanism. [1] Y. Nemoto, K. Hino, and N. Maeshima, Phys. Rev. B 87, 205305 (2013).