Fig. 1. The model of two-dimensional（2D）triangular lattice PC flat lens with scatterer-size gradient：The red arrow represents the light path of plane wave focusing，and the black arrow represents the light path of point source imaging

Fig. 2. （a）TM polarization band structure of common photonic crystal（CPC）with r=0.29µm（black），0.34µm（red）and 0.39µm（blue）.（b）TE polarization band structure of CPC with r=0.29µm（black），0.39µm（red）and 0.49µm（blue）. Dot dash line，dot line and solid line represent the first，second and third bands respectively. The EFL of the second TM polarization band of CPC：（c）r=0.29µm，（d）r=0.34µm，（e）r=0.39µm. The EFL of TE polarization band of CPC：（f）r=0.39µm（the second band），（g）r=0.44µm（the third band），（h）r=0.49µm（the third band）.（i）The red，purple and black solid lines represent（c），（d）and（e）EFL corresponding to the incident light source frequency，respectively.（j）The purple and black solid lines represent（g）and（h）EFL corresponding to the incident light source frequency，respectively. Because（f）has no negative refraction，it is no longer studied separately. The blue dot line is EFL of light cone. The black arrow is an incident beam，incident from the air into the PC. The red arrow is the propagation direction of the beam in the PC，that is，the refracted beam.（i）and（j）are only schematic diagrams^［35-36］.

Fig. 3. Take λ=3.303µm90.84THz as an example. The field distribution of spherical wave stimulated by point source incident on the PC flat lens with scatterer-size gradient：（a）TM polarization mode.（b）TE polarization mode. The field distribution of plane wave incident on the PC flat lens with scatterer-size gradient：（c）TM polarization mode.（d）TE polarization mode. In TM and TE polarization modes，the field power of point source（PS）imaging and plane wave（PW）focusing：（e）axis plane（f）image plane and focal plane. STM and STE represent the field power of point source imaging in TM and TE polarization modes respectively. WTM and WTE represent the field power of plane wave focusing in TM and TE polarization modes respectively.