Collimated plasmon beam: nondiffracting versus linearly focused.

We worked out a new group of collimated plasmon beams by the means of in-plane diffraction with symmetric phase modulation. As the phase type changes from 1.8 to 1.0, the beam undergoes an interesting evolution from focusing to a straight line. Upon this, an intuitive diagram was proposed to elucidate the beam nature and answer the question of whether they are nondiffracting or linear focusing. Based on this diagram, we further achieved a highly designable scheme to modulate the beam intensity (e.g., "lossless" plasmon). Our finding holds remarkable generality and flexibility in beam engineering and would inspire more intriguing photonic designs.