Quasi-bound states in the continuum (q-BIC) supported by metasurfaces 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 slots 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 the 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 is 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.