New Experiments with Optical Lattices

Abstract

rly ~II~II~IIIII ~ . l twry of dectrical coi~drirtivity iii crystal latticcs hy I3loch aiid Zener [ I ] I(YI t,o the striking predirtion that a lioniogciicons static clcctric ficld iiiduces ail oscillatory rather thau u~ i i fo r~ r i notion of the electrons. l lcrr wc preseut Blocli oscillations o f atoiiis i i i I.he fundanieutal and iii the first excit.ccl cwxgy baiid of a periodic optical potential 121. ‘lliis poteiitial results from the light-shift of the grouud state of atoms illuminatr t l hy a laser stailding wave. The laser is detuncd far from any atomic resouancc so tliat spoiitancoiis emission can he neglected. Using one-dimensional Ranian laser cooling [3,4] we first prepare a gas of free atoms with a ~nomentum spread 61, = hk/4 in the direction of the standing wave, where hk i s tlic pliol.on ~nonrei~tum. The corresponding atomic coherence length h/6p extends over ~everal periods d = X/2 = 7 r / k of the optical lattice. I3y adiabatically switching on thc light potential this initial momentum distribution is turned into a statistical mixture of 131ocli stales i n the ground energy band with a quasi-rnomnrtu~n width 6y = 6 p / h much snrallcithan the width 2k of the Brillouin zone. We mimic a constant external force by inl.roducing a t.uiiable frequency difference 6 v ( t ) between the two counterpropagating laser waves rreal.ing the optical potential. For a linear variation io time of 6v( l ) a constant inerl.ial force 1.’ = m a = -mX$6v( t ) /2 is exerted 011 the atoms in the frame of the slanding wave. After a given evolution time, we abruptly switch off the light potential and inrasure thr momentum distribution of the Blocli sl.ate with a resolution of R k / l S . ‘I’lie nieaii atomic velocity displays oscillations with periods in the millisecond range and a~nplit~ndes between 0.7 and 0.3 UR for potential depths (io between 0.8 and 5.6 ER. where i>f? = h.k/7n is the recoil velocity and ER = (hk)’/2m the recoil energy. We measured positive and negative effective masses, differeut from the mass of the free cesium atom. Froiu the measured velocity curves we can reconstruct the shape of the fundamental energy band. Wr have also calculated numerically the band structure for our experimental values of [ io. The agreement with the experimental data is quite good. \4k have niadc investigations of atom acceleratiori in (.he case of deeper potentials up to r:, = 10EJ3. We could still observe Bloch states after more thau 40 oscillation periods. 111 t.lrc Iaboralory frame, this corresponds to a coherent mom en tun^ transfer of S O U . prodnring aii atomic beam with subrecoil momentum spread (h1;/4) in the beam direction.