Realizing Tunable Evolution of Bound States in the Continuum and Circularly Polarized Points by Symmetry Breaking

Bound states in the continuum (BICs) and circularly polarized points (C points), being well-known momentum-space polarization singularities in photonic crystal (PhC) slabs, have attracted much attention due to their novel properties. The further investigations on the generation and evolution of BICs and C points in momentum space have provided more perspectives to modulate these polarization singularities by tuning parameters. Recently, it was theoretically proposed that starting from a high-order BIC, various tunable evolutions of BICs and C points could be realized by symmetry breaking, offering an effective method to create and modulate polarization singularities in momentum space. To date, there is still no experimental realization of tunable evolution of polarization singularities from a high-order BIC. Here, we experimentally realized tunable evolution of BICs and C points in momentum space by symmetry breaking on purpose. The studied high-order BIC of -2 charge exists in a PhC slab of C-6 symmetry. The off-Gamma BICs of -1 charge were observed by breaking the C-6 symmetry to the C-2 symmetry. The at-Gamma BIC of +1 charge and off-Gamma C points of -1/2 charge were observed by breaking the C-6 symmetry to the C-3 symmetry. The symmetry breaking factors and unit-cell configurations were further applied to continuously modulate the movement of polarization singularities in momentum space. Our results can promote the understanding of polarization singularities' evolution and provide effective approaches of symmetry breaking to on-purpose design BICs and C points in momentum space.