Sub‐Picosecond Response Time of a Hybrid VO2:Silicon Waveguide at 1550 nm

Hybrid material systems are a promising approach for extending the capabilities of silicon photonics. Given the weak electro‐optic and thermo‐optic effects in silicon, there is intense interest in integrating an ultrafast‐switching phase‐change material with a large refractive index contrast into the waveguide, such as vanadium dioxide (VO2). It is well established that the phase transition in VO2 thin films can be triggered by ultrafast, 800 nm laser pulses, and that pump‐laser fluence is a critical determinant of the recovery time of thin films irradiated by femtosecond pulses. However, thin‐film experiments are not reliable guides to a VO2:Si system for all‐optical, on‐chip switching because of the differences in VO2 optical constants in the telecommunication band, and the complex sample geometry and alignment issues in a waveguide geometry. This paper reports the first demonstration that the reversible, ultrafast photoinduced phase transition in VO2 can achieve sub‐picosecond response when small VO2 volumes are integrated into a silicon waveguide as the active element. The result suggests that VO2 can be pursued as a strong candidate for waveguide switching with sub‐picosecond on‐off times. Sub‐picosecond all‐optical switching of ultrashort TE‐mode pulses at 1550 nm is achieved in a silicon waveguide containing an embedded section of vanadium dioxide, when pumped by synchronously generated femtosecond pulses generated in an optical parametric amplifier. The nonlinear response of the hybrid waveguide creates both design challenges and novel opportunities for optimizing switching performance.