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

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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 four-wave mixing signals and spatial coherent wavepacket evolution in the time-domain 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/ℏ 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.