A novel ultracompact (<7 × 0.4 μm2) polarization mode converter for converting quasi-TE00 mode of a silicon strip waveguide (WG) to the fundamental quasi-TM mode of a hybrid-plasmonic WG (HPW) without lateral structures is described. The mode-evolution-based design consists of three-section asymmetric HPWs, each of which has a unique top silver structure. A newly defined asymmetry index whose minimum suggests the strongest decoupling of longitudinal field components in three-dimensional guiding structures is used to rapidly obtain a suboptimal design of top metallic structure without relying heavily on parameter sweeps using numerical computations. The optimized converter exhibits a mode conversion efficiency (MCE) of 88.79%, an insertion loss (IL) of 0.5139 dB, and a polarization conversion efficiency (PCE) of 99.82% at 1550 nm. The respective 1-dB optical bandwidths for the MCE and IL are about 104 and 108 nm, while the PCE is >92% over a 172 nm spectral range. The fabrication tolerances for top silver structures are also discussed.