We propose a ${\sigma }_z$-symmetry-preserving strategy to realize Janus bound states in the continuum (Janus BICs) exhibiting asymmetric topological charges in upward and downward radiation channels. Unlike the prior approach that typically involve explicit structural perturbations to break vertical symmetry, our design employs bilayer photonic crystals (PCs) with independently tunable refractive index (RI), enabling optical asymmetry without altering the geometric symmetry. In the optically symmetric case, the system supports symmetry-protected BICs at $\mathrm{\Gamma }$ point with topological charge $q=-1$ and accidental BICs at off-$\mathrm{\Gamma }$ point with $q=+1$. Upon introducing RI detuning, the polarization vortex splits into two circularly polarized states (C points) with half-integer topological charge ($q=+1/2$), shifting oppositely in momentum space for upward and downward radiation, while the symmetry-protected BICs remain unaffected. Janus BICs emerge through the migration of upward-radiating C points toward the $\mathrm{\Gamma }$ point, accumulating a total topological charge of $q=+1$, while the downward-radiating counterpart contributes $q=-1$, leading to a net topological charge reversal between the two radiation channels. We further investigate the robustness of Janus BICs against variations in RI contrast and structural perturbations, demonstrating their stable existence over a broad parameter range. The experimental feasibility of this configuration is also examined when considering substrates, proposing two practical implementation schemes and validating one of them through numerical simulations. This purely optical asymmetry mechanism enables the realization of Janus BICs without any structural deformation, offering a platform for directional light sources, chiral photonic interfaces, and integrated topological photonics.

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• Bound states in the continuum
• Topological effects in photonic systems
• Phononic crystals