The spatial spectrum is obtained by using a lens to /iG7MC\`
do Fourier transform and the far-field spectrum is recorded 4TZ cc|B5
by a CCD camera positioned in its focal plane. cP rwW6
First, we summarize our results of on-axis excitation of a p$XKlg&
single 2D gap soliton in Fig. 1. Different from previous /pykW_`/-
experimental observations in which either the probe beam wk @,wOt
was launched off-axis to match the edge of the first BZ [6] i=#F)AD^5#
or its input phase or spectrum was engineered [11], we J'Sm0
demonstrate on-axis excitation of a 2D gap soliton without 3i=Iu0
a priori phase or spectral engineering. In fact, we show v* ~3Z1