Collapse of Fractal Structure in a Dynamic Fractional Stark Ladder Driven by an Intense THz Laser

We examine a dynamic Wannier-Stark ladder in biased semiconductor superlattices driven by a monochromatic THz laser with a fractional matching ratio $\eta $; this is the ratio of a Bloch frequency$_{\mathrm{\thinspace }}$to a laser frequency. This type of a dynamic Wanneir-Stark ladder is termed a dynamic fractional Stark ladder (DFSL). It is known that a DFSL shows fractal structure in quasienergy levels as a function of 1/$\eta $ based on a tight-binding model [1]. However, with the increase in intensity of an irradiated laser, it is found that photon-assisted tunneling causes single-channel shape-resonance with quasienergy shift and width, making inaccurate the above-mentioned fractal structure [2]; the channel means a photon sideband of DFSL. In present study, we investigate the resonance structure of DFSL involving stronger interchannel interactions relevant to ac-Zener tunneling by applying a multichannel scattering based on the R-matrix Floquet theory. The obtained results show that conspicuous redshift of resonance spectral peaks and the associated resonance decay, concluding the collapse of the fractal structure characteristic of DFSL. [1]X. -G. Zhao, et al., Phys. Lett. A 202, 297 (1995). [2]T. Karasawa, et al., Solid State Comm 151, 392 (2011).