A compact all-silicon 1×2 power divider based on multimode interference (MMI) is designed and numerically evaluated for J-band operation. An air-clad MMI core ensures low-loss propagation and strong lateral confinement, while periph-eral waveguides use effective-medium (EM) cladding to provide mechanical robustness and packaging compatibility. The EM cladding, realized with subwavelength air-hole lattices, maintains a low effective-index contrast for efficient mode guidance. Full-wave simulations confirm broadband operation from 220 to 320 GHz under WR-3.4 waveguide excitation, with transmission coefficients around ?3.3 dB and return loss better than ?10 dB. These results establish the proposed device as a compact (9.6 mm × 5.5 mm), integrable solution for high-efficiency power splitting and combining in ad-vanced THz systems.