  Inducing a large refractive-index change is the holy grail of reconfigurable
photonic structures, a goal that has long been the driving force behind the
discovery of new optical material platforms. Recently, the unprecedentedly
large refractive-index contrast between the amorphous and crystalline states of
Ge-Sb-Te (GST)-based phase-change materials (PCMs) has attracted tremendous
attention for reconfigurable integrated nanophotonics. Here, we introduce a
microheater platform that employs optically transparent and electrically
conductive indium-tin-oxide (ITO) bridges for the fast and reversible
electrical switching of the GST phase between crystalline and amorphous states.
By the proper assignment of electrical pulses applied to the ITO microheater,
we show that our platform allows for the registration of virtually any
intermediate crystalline state into the GST film integrated on the top of the
designed microheaters. More importantly, we demonstrate the full reversibility
of the GST phase between amorphous and crystalline states. To show the
feasibility of using this hybrid GST/ITO platform for miniaturized integrated
nanophotonic structures, we integrate our designed microheaters into the arms
of a Mach-Zehnder interferometer to realize electrically reconfigurable optical
phase shifters with orders of magnitude smaller footprints compared to existing
integrated photonic architectures. We show that the phase of optical signals
can be gradually shifted in multiple intermediate states using a structure that
can potentially be smaller than a single wavelength. We believe that our study
showcases the possibility of forming a whole new class of miniaturized
reconfigurable integrated nanophotonics using beyond-binary reconfiguration of
optical functionalities in hybrid PCM-photonic devices.
