Fast and selective inter-band transfer of ultracold atoms in bichromatic lattices permitting Dirac points

Abstract

An experimental group at Beijing[Yueyang Zhai, ${\it et. al.}$, Phys. Rev. A ${\bf 87}$, 063638 (2013)] introduced the method of standing-wave pulse sequence for efficiently preparing ultracold bosonic atoms into a specific excited band in a 1-dimensional optical lattice. Here, we report a theoretical extension of their work to the problem of 1-dimensional bichromatic superlattice. We find that varying the lattice parameters leads to the so-called Dirac point where a pair of excited bands crosses. This paper thus discusses ${\it simultaneously}$ the efficient excitation of the wave packet to the proximity of the Dirac point and its subsequent dynamics in the force field of a parabolic trap. With the aid of a toy model, we theoretically unravel the mechanism of the efficient preparation, and then numerically explore optimal pulse-sequence parameters for a realistic situation. We find an optimized sequence of a bichromatic optical lattice that excites more than 99% of the atoms to the 1st and 2nd excited bands within 100 $\mu$s without the harmonic trap. Our main finding is that the system permitting the Dirac point possesses a region of parameters where the excited energy bands become nearly parabolic, conducive to robust coherence and isochronicity. We also provide an appropriate data set for future experimentation, including effects of the atom-atom interaction by way of the mean-field nonlinear term.