Quasi‑bound states in the continuum (q‑BIC) supported by metasurface enable resonant modes with ultra‑high quality factors, making them promising for nanophotonic applications. Manipulating the polarization of these modes, however, remains challenging. Conventional approaches rely on rotating the meta‑atoms or varying their vertical geometry, which complicates fabrication and limits design flexibility. Here, we introduce an alternative strategy: embedding rectangular slotss directly within the dielectric meta‑atoms. This intra‑unit perturbation locally modifies the permittivity distribution, thereby altering the optical response without requiring complex three‑dimensional reconfiguration.
The physical mechanism is analyzed using coupled‑mode theory and multipole expansion. The intrinsic polarization direction is shown to be governed by the superposition of electric dipole moments in the dimer unit cell. Introducing slots into a nanorod reduces its effective polarizability, selectively modifying specific components of the dipole moment. As a result, the overall polarization axis can be rotated in a controlled manner. We experimentally realized this concept using amorphous‑silicon nanorod dimers fabricated on quartz. The base q‑BIC resonance was excited by rotating rods by 12.5° to break in‑plane symmetry. Through electron‑beam lithography and dry etching, we introduced 0 to 3 slots per dimer in various arrangements. Polarization‑resolved transmission spectroscopy was performed using a motorized rotating polarizer and a microscope‑coupled spectrometer.
Measurements reveal that increasing the number of slots in one rod from 0 to 2 progressively rotates the intrinsic polarization clockwise from 0°, accompanied by a blue shift of the resonance. For instance, samples with 0, 1, and 2 slots exhibited polarization angles of 0°, 162°, and 146°, respectively. When slots are added to both rods to restore approximate intra‑unit symmetry (e.g., 3‑slot designs), the polarization realigns closer to the unperturbed orientation. These results confirm that slot‑based perturbations offer a reliable and lithographically accessible means to independently tailor both the polarization state and the spectral position of q‑BIC resonances.
This work demonstrates a straightforward and effective method for polarization engineering in high‑Q metasurfaces, opening avenues for designing versatile polarization‑sensitive devices such as tunable waveplates, polarimetric sensors, and compact polarization encoders in integrated photonic systems.