  We propose an all-photonic, non-volatile memory and processing element based
on phase-change thin-films deposited onto nanophotonic waveguides. Using
photonic microring resonators partially covered with Ge2Sb2Te5 (GST)
multi-level memory operation in integrated photonic circuits can be achieved.
GST provides a dramatic change in refractive index upon transition from the
amorphous to crystalline state, which is exploited to reversibly control both
the extinction ratio and resonance wavelength of the microcavity with an
additional gating port in analogy to optical transistors. Our analysis shows
excellent sensitivity to the degree of crystallization inside the GST, thus
providing the basis for non-von Neuman neuromorphic computing.
