Gigahertz speed operation of compact, non-resonant epsilon-near-zero silicon photonic modulators

We describe the design, fabrication, and performance of a fast, compact electroabsorption modulator based on transparent conducting oxides. The modulator works by using bias voltage to increase carrier density in the conducting oxide which changes the permittivity and, hence, optical attenuation by almost 10 dB. Under bias, light is tightly confined to the conducting oxide layer through non-resonant epsilon-near-zero (ENZ) effects which enable modulation over a broad range of wavelengths in the telecommunications band. Our approach features simple integration with passive silicon waveguides, the use of stable inorganic materials, and the ability to modulate both transverse electric (TE) and magnetic (TM) polarizations with the same device design. Using a 4 micrometer long modulator and a drive voltage of 2 Vpp, we demonstrate digital modulation at rates of 2.5 Gb/s. This work verifies that high-speed ENZ devices can be created using conducting oxide materials and paves the way for additional technology development that could have broad impact on future optical communications systems.