The spatial spectrum is obtained by using a lens to 8*#$3e
do Fourier transform and the far-field spectrum is recorded Eld[z{n"
by a CCD camera positioned in its focal plane. dcq18~
First, we summarize our results of on-axis excitation of a 1$C?+H
single 2D gap soliton in Fig. 1. Different from previous :qB|~"9O
experimental observations in which either the probe beam ^LB]
was launched off-axis to match the edge of the first BZ [6] Wuk8&P3
or its input phase or spectrum was engineered [11], we {{M/=WqC
demonstrate on-axis excitation of a 2D gap soliton without :Ru8Nm
a priori phase or spectral engineering. In fact, we show w8 UUeF