All-optical modulation with single photons using an electron avalanche

The distinctive characteristics of light, such as high-speed and low-loss propagation, low crosstalk and low power consumption, along with the unique quantum properties of photons, make it most suitable for various applications in communications, high-resolution imaging, optical computing and quantum information technologies. One limiting factor, however, is the weak optical nonlinearity of conventional media, which poses challenges for controlling light at ultralow intensities. Here we demonstrate all-optical modulation of the refractive index enabled by the electron avalanche process in silicon using a control beam with single-photon light intensities. The observed process corresponds to an extremely high nonlinear refractive index of $n_2\approx 1.3\times {10}^{-2}{m}^2{W}^{-1}$ , which is several orders of magnitude higher than those of the best-known nonlinear optical materials. Using single photons for light modulation opens the possibility of gigahertz-frequency-and potentially even faster-optical switching for on-chip photonic and quantum devices operating at room temperature.