Scientists at Stanford University say they have developed a miniature optical isolator that can send light in one direction at a time. The device uses nano-scale photonic structures made of silicon waveguides and ring resonators. This development is expected to impact on-chip optical circuitry in the future.
Achieving on-chip optical signal isolation is a fundamental difficulty in integrated photonics, the scientists say. And yet the need to overcome this difficulty is becoming increasingly urgent, especially with the emergence of silicon nano-photonics which promises on-chip optical systems at an unprecedented scale of integration. Backward light propagation is common and frequently causes severe performance degradation in photonic devices and hence optical communication systems. Current optical isolators use magneto-optical materials that are incompatible with integrated circuit manufacturing techniques, in particular silicon complementary metal-oxide-semiconductor (CMOS) technology.
Now, in a paper published in Nature Photonics, Stanford researchers Shanhui Fan and Zongfu Yu have described an ultra-compact complete optical isolator that can be built using CMOS-compatible materials systems. By modulating the refractive index of the nanostructures in time and space, the device imparts energy and momentum to the forward-propagating, (but not to the backward-propagating) light, allowing the two to be separated. The researchers further show that a non-reciprocal effect can be accomplished in dynamically modulated micrometre-scale ring-resonator structures. This work demonstrates that on-chip isolation can be accomplished with dynamic photonic structures in standard material systems that are widely used for integrated optoelectronic applications.
For more information, see the paper Complete optical isolation created by indirect interband photonic transitions in Nature Photonics. See also the web page of the Fan Group, whose work is focused on theoretical and computational research of photonic crystals, micro-photonic and nano-photonic structures, as well as solid state devices.