One of the most fundamental components in electronics is the diode. It regulates electric current by letting electrons pass in one direction but not the other, and forms a critical part of almost all circuits. A similar device that works for light on a computer chip would be equally important for developing integrated optical devices. Due to fundamental differences between light and electrons, however, such devices have proved very difficult to create. Yan-Feng Chen and researchers from Nanjing University in China in collaboration with colleagues from Caltech and the University of California, San Diego in the USA have now demonstrated the first silicon chip-based device that provides diode-like one-way transmission functionality for light.1

The properties of light mean that it is very difficult to break the back-and-forth current flow symmetry that is achieved so easily in electronics, and most known methods for producing a ‘light diode’ effect are unsuitable for implementation on a small silicon chip. The device developed by Chen and his colleagues were able to break this symmetry by devising a specially structured waveguide on a silicon chip. “Our device could lead to chip-scale asymmetric photonic components that complement conventional symmetric devices,” says Chen.

Fig. 1: One-way light propagation in a metallic–silicon waveguides

Their design is based on a silicon waveguide that is patterned with different sections of silicon and germanium–chrome alloy (see image). These sections, which are about the same size as the wavelength of the light, alter the refractive index of the medium in a way that leads to an asymmetry in the propagation of light — in one direction light passes straight through the waveguide but is partially absorbed, while in the other direction, the patterned refractive index modulations cause a relative enhancement in transmitted light power.

Although the operation of the device has not reached the perfect one-way current-blocking effect offered by electrical diodes, the researchers are optimistic that the difference in power transmission between the two directions can be enhanced by introducing other complementary processes. “We believe complete on-chip one-way operation is possible by combining this asymmetric light propagation with other phenomena such as nonlinear optical effects, and that is what we are working on now,” says Chen.