  We propose a multifunctional integrated plasmonic-photonic polarization
converter for polarization demultiplexing in an indium-phosphide membrane on
silicon platform. Using a compact 1$\times$4 multimode interference coupler,
this device can provide simultaneous half-wave plate and quarter-wave plate
(HWP and QWP) functionalities, where the latter generates two quasi-circular
polarized beams with opposite spins and topological charges of $l$ = $\pm$1.
Our device employs a two-section HWP to obtain a very large conversion
efficiency of $\geq$ 91% over the entire C to U telecom bands, while it offers
a conversion efficiency of $\geq$ 95% over $\sim$ 86% of the C to U bands. Our
device also illustrates QWP functionality, where the transmission contrast
between the transverse electric and transverse magnetic modes is $\approx$ 0 dB
over the whole C band and 55% of the C to U bands. We expect this device can be
a promising building block for the realization of ultracompact on-chip
polarization demultiplexing and lab-on-a-chip biosensing platforms. Finally,
our proposed device allows the use of the polarization and angular momentum
degrees of freedom, which makes it attractive for quantum information
processing.
