Probing the ladder spectrum arising from motion in a 2-D lattice driven by an in-plane constant force

The coherent interband dynamics of optically excited two-dimensional lateral surface superlattices driven by an in-plane static electric field has been investigated. The linear absorption, the spectrally-resolved pump-probe fourwave mixing signals and spatial coherent wavepacket evolution in the timedomain are obtained. When the rational condition Ex/Ey = p/q, with p, q prime to each other, is fullfilled, it is found that p peaks appear within the frequency interval ωBx = eExa/h̄ in both linear absorption and degenerate four-wave mixing signals. The coherent time evolution of the electron-hole pair wavepacket is that of a breathing mode, with the period 2πp/ωBx, These findings are consistent with the recent spectral results (Phys. Rev. Lett. 86, 3116), hence providing a method for probing the coherent dynamics of quantum particles in 2D lattices. PACS numbers: 78.47.+p, 73.50.Fq, 73.20.Dx Typeset using REVTEX 1