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We consider directed quantum transport in one-dimensional, periodic tight-binding lattices that are driven by unbiased, but asymmetric driving forces which are composed of a fundamental and its second harmonic (harmonic mixing signal). The reduction to a single-band dynamics then yields a formally exactly integrable dynamics. An asymmetric drive generally breaks time-reversal symmetry without causing any dissipation. For an initially localized particle with zero momentum no finite coherent current emerges; in contrast, a directed current occurs, however, when prepared with an initial coherence. The presence of phase fluctuations yields a decaying current, thereby rendering the nondissipative currents a transient phenomenon. In contrast, the presence of quantum dissipation yields at all temperatures a finite asymptotic current independent of preparation.

A convenient nanotechnique is used to place analyte molecules between closely spaced silver-capped Si nanowires for investigating surface-enhanced Raman scattering (SERS). It is revealed that the SERS intensity (or sensitivity) is closely related to the etching time used to prepare the Si nanowires from wafer. As the etching leaves the nominal spacing between the nanowires unaffected, the observed effect can be explained based on different gaps between the Ag particles due to the different lengths of the Si nanowires. Large SERS intensity for short etching times can be elucidated in terms of the rigidity of the nanowires and the smaller SERS intensities for longer etching times can be explained by considering the bending of nanowires and the agglomeration of the Ag caps due to gravity and van der Waals forces.

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