All-optical modulation with single-photons using electron avalanche

The distinctive characteristics of light, such as high-speed and low-loss propagation, low cross-talk and low power consumption, along with photons unique quantum properties, make it most suitable for various applications in communication, high-resolution imaging, optical computing, and emerging quantum information technologies. One limiting factor, though, is the weak optical nonlinearity of conventional media that poses challenges for the control of light with ultra-low intensities. In this work, we demonstrate all-optical modulation enabled by electron avalanche process in silicon, using a control beam with single-photon light intensities. The observed process corresponds to a record-high nonlinear refractive index of $n_{2}$~$1.3*10^{-2} m^2/W$, which is several orders of magnitude higher than the best known nonlinear optical materials. Our approach opens the possibility of gigahertz-speed, and potentially even faster, optical switching at the single-photon level, which could enable a family of novel on-chip photonic and quantum devices operating at room temperature.