TiN-GST-TiN All-Optical Reflection Modulator for 2 $\mu$m Waveband Reaching 85% Efficiency

In this study, we present an all-optical reflection modulator for 2$\mu$m communication band exploiting a nano-gear-array metasurface and a phase-change-material Ge$_2$Sb$_2$Te$_5$ (GST). The reflectance of the structure can be manipulated by altering the phase of GST by employing optical stimuli. The paper shows details on the optical and opto-thermal modeling techniques of GST. Numerical investigation reveals that the metastructure exhibits a conspicuous changeover from $\sim$ 99% absorption to very poor interaction with the operating light depending on the switching states of the GST, ending up with 85\% modulation depth and only 0.58 dB insertion loss. Due to noticeable differences in optical responses, we can demonstrate a high extinction ratio of 28 dB and a commendable FOM of 49, so far the best modulation performance in this wavelength window. In addition, real-time tracking of the reflectance during phase transition manifests high-speed switching expending low energy per cycle, on the order of sub-nJ. Hence, given its overall performance, the device will be a paradigm for the optical modulators for upcoming 2 $\mu$m communication technology.