  We propose and demonstrate, by simply inserting an air-hole in the waveguide
side-coupling with a microring resonator (MRR), the transmission spectrum
presents Fano lineshapes at all of the resonant modes. Measured from the
fabricated devices, Fano lineshapes with slope rates over 400 dB/nm and
extinction ratios over 20 dB are obtained. We ascribe it to the
air-hole-induced phase-shift between the discrete resonant modes of the MRR and
the continuum propagating mode of the bus-waveguide, which modifies their
interference lineshapes from symmetric Lorentzian to asymmetric Fano. From
devices with varied locations and diameters of the air-hole, different Fano
asymmetric parameters are extracted, verifying the air-hole-induced
phase-shifts. This air-hole-assisted waveguide-MRR structure for achieving Fano
resonance lineshapes has advantages of simple design, compact footprint, large
tolerance of fabrication errors, as well as broadband operation range. It has
great potentials to expand and improve performances of on-chip MRR-based
devices, including sensors, switchings and filters.
