  Reconfigurable photonic systems featuring minimal power consumption are
crucial for integrated optical devices in real-world technology. Current active
devices available in foundries, however, use volatile methods to modulate
light, requiring a constant supply of power and significant form factors.
Essential aspects to overcoming these issues are the development of nonvolatile
optical reconfiguration techniques which are compatible with on-chip
integration with different photonic platforms and do not disrupt their optical
performances. In this paper, a solution is demonstrated using an optoelectronic
framework for nonvolatile tunable photonics that employs undoped-graphene
microheaters to thermally and reversibly switch the optical phase-change
material Ge$_2$Sb$_2$Se$_4$Te$_1$ (GSST). An in-situ Raman spectroscopy method
is utilized to demonstrate, in real-time, reversible switching between four
different levels of crystallinity. Moreover, a 3D computational model is
developed to precisely interpret the switching characteristics, and to quantify
the impact of current saturation on power dissipation, thermal diffusion, and
switching speed. This model is used to inform the design of nonvolatile active
photonic devices; namely, broadband Si$_3$N$_4$ integrated photonic circuits
with small form-factor modulators and reconfigurable metasurfaces displaying
2$\pi$ phase coverage through neural-network-designed GSST meta-atoms. This
framework will enable scalable, low-loss nonvolatile applications across a
diverse range of photonics platforms.
