  Manipulating the spectral line shape exhibits great potential in realizing
active optical circuits with switching, sensing, and modulation capabilities.
Exploring unusual line shapes, such as Fano resonance and electromagnetically
induced transparency (EIT), has attracted substantial interest. Conventional
methods of engineering the spectral line shape have limited tunability and face
challenges in multiplexing different spectral line shapes. Here, we propose and
numerically demonstrate a new mechanism to tailor the transmission line shape
almost at will by exploiting the interference of frequency-dependent chiral
dipolar states in two helix particles sitting above a dielectric waveguide. We
show that, by tuning the polarization of the chiral dipoles and exploiting
transverse spin-orbit interaction, one can control the asymmetric
Pancharatnam-Berry geometric phase for the excited guided waves propagating in
opposite directions. The interference of the guided waves respectively excited
by the two particles can give rise to transmissions with various line shapes,
including Lorentzian-like, antiresonance-like, Fano-like, and EIT-like line
shapes, which carry an intriguing property of line shape-momentum locking,
i.e., the transmissions in opposite directions have different line shapes. Our
findings open new possibilities for multiplexed and multifunctional
nanophotonic designs with unprecedented capability of spectral-line shaping.
The proposed structures can be conveniently integrated with optical circuits
for on-chip applications.
