The spatial spectrum is obtained by using a lens to *bYU=RS
do Fourier transform and the far-field spectrum is recorded ]5QXiF8`
by a CCD camera positioned in its focal plane. *&Iv Eu
First, we summarize our results of on-axis excitation of a YHom9&A
single 2D gap soliton in Fig. 1. Different from previous tlD^"eq4:
experimental observations in which either the probe beam NY<qoV
was launched off-axis to match the edge of the first BZ [6] t^K Qv~
or its input phase or spectrum was engineered [11], we FO[ s;dmzu
demonstrate on-axis excitation of a 2D gap soliton without 3GKKC9C6
a priori phase or spectral engineering. In fact, we show 24 [cU