Abstract: The demand for information processing at ultrahigh speed with large data transmission capacity is continuously rising. Necessary building blocks for on-chip photonic integrated circuits (PICs) are reconfigurable integrated low-loss high-speed modulators and switches. Phase change materials (PCMs) provide unique opportunities for integration into PICs. Here, the investigation of layered gallium monosulfide (GaS) as a novel low-loss PCM from infrared to optical frequencies is pioneered, with high index contrast (?n ?0.5) at the optical telecommunication band. The GaS bandgap switches from 1.5 ± 0.2 eV for the amorphous state to 2.1 ± 0.1 eV for the crystalline state. It is demonstrated that the reversible GaS amorphous-to-crystalline phase transition can be operated thermally and by picosecond green (532 nm) laser irradiation. The design of a reconfigurable integrated optical modulator on-chip based on Mach-Zehnder Interferometers (MZI) with the GaS PCM cell deposited on one of the arms for application is presented at the telecommunication wavelength of ? = 1310 nm, where the standard single mode optical fiber exhibits zero chromatic dispersion, and at ? = 1550 nm, where a minimum optical loss of 0.22 dB km?1 is obtained. This opens the route to applications such as reconfigurable modulators, beam steering using phase modulation, and photonic neural networks.

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