The spatial spectrum is obtained by using a lens to +qDudGI
do Fourier transform and the far-field spectrum is recorded rS8/_'
by a CCD camera positioned in its focal plane. m03;'Nj'7#
First, we summarize our results of on-axis excitation of a .k Gg}
single 2D gap soliton in Fig. 1. Different from previous sDw&U?gUv
experimental observations in which either the probe beam -=QA{n
was launched off-axis to match the edge of the first BZ [6] lP\7=9rh^x
or its input phase or spectrum was engineered [11], we )Q&:$]
demonstrate on-axis excitation of a 2D gap soliton without q>o1kTI
a priori phase or spectral engineering. In fact, we show FdzsWm