  As an analogue of electromagnetically induced transparency (EIT),
plasmon-induced transparency (PIT) has been realized both in plasmonic
metamaterial and waveguide structures. Via near-field coupling within unit
cells, PIT with broadband could be produced by plasmonic metamaterials, which,
however, has not been realized in on-chip plasmonic waveguide structures. Here,
we introduce broadband PIT based on a plasmonic metal-insulator-metal (MIM)
waveguide system. Utilizing the direct coupling structure, PIT emerges based on
an easy-fabricated structure without gap. By tuning coupling distance, the
transparent window can be continuously tuned from narrow- to broadband. Such
device is promising for on-chip applications on sensing, filtering and slow
light over a broad frequency range.
