  High-performance optical beam splitters are of fundamental importance for the
development of advanced silicon photonics integrated circuits. However, due to
the high refractive index contrast of the silicon-on-insulator platform, state
of the art Si splitters are hampered by trade-offs in bandwidth, polarization
dependence and sensitivity to fabrication errors. Here, we present a new
strategy that exploits modal engineering in slotted waveguides to overcome
these limitations, enabling ultra-wideband polarization-insensitive optical
power splitters, with relaxed fabrication tolerances. The proposed splitter
relies on a single-mode slot waveguide which is transformed into two strip
waveguides by a symmetric taper, yielding equal power splitting. Based on this
concept, we experimentally demonstrate -3$\pm$0.5 dB polarization-independent
transmission in an unprecedented 390 nm bandwidth (1260 - 1650 nm), even in the
presence of waveguide width deviations as large as $\pm$25 nm.
