Metamaterial Shrinks Integrated-Photonics Devices
Author: Rajesh Menon
Affiliation: Department of Electrical & Computer Engineering, University of Utah, USA.
Integrated electronics is the driving force behind the information revolution of the last 6 decades. A similar revolution is happening in photonics, where devices that manipulate the flow of light (or photons) are being miniaturized and integrated. The main challenge for integrated photonics is that the wavelength of light is far larger than the equivalent wavelength of electrons. This is the main reason that devices fundamental to integrated electronics are significantly smaller than those used in integrated photonics. Furthermore, no one had come up with a way to design devices close to this limit for integrated photonics.
We recently solved this problem by first coming up with a new design algorithm and then experimentally verifying that our devices work as intended [1-3]. One crucial advantage of our method is that our fabrication process is completely compatible with the very mature processes already developed for silicon electronics. This means that we can exploit the vast existing manufacturing infrastructure to enable integrated photonics.
In our recent publication, we demonstrated the smallest polarization beam-splitter to date . This device (shown in the figure below) has 1 input and 2 outputs. The 2 outputs correspond to the 2 linear polarization states of light. The device is designed to take either polarization of light (or both) as the input and separate the 2 polarizations into the 2 outputs. We input light into our device one polarization at a time and measured the transmission efficiency into the correct output. This allowed us to verify that the device performs as designed. This is analogous to separating two channels of communication (for example, a video stream from PBS and another from Netflix). Previously such separation would have required time and power-consuming electronics or if photonics devices were used, they would have been much larger (so much harder to integrate onto a chip).
Figure 1: (a) Scanning-electron micrograph of fabricated polarization beamsplitter. Simulated intensity distribution at (b) TE polarization and (c) TM polarization showing the separation of the beams.
In the big picture, our research has the potential to maintain Moore's law for photonics. By enabling integrated photonics devices to be much smaller (in fact, close to their theoretical limits), we allow the integration of more devices in the same area (which increases functionality) and also enable the devices to communicate faster (since they are closer together; light has to travel shorter distances). Finally, by packing more devices into the same chip, one also exploits economies of scale to reduce the cost per chip (similar to what has happened in electronics). The practical impact for customers is that one can expect to drastically reduce power consumption and enable faster communications and computing. Data centers today consume over 2% of the total global electricity. Reducing power consumption in data centers and other electronics can go a long way to reduce our CO2 emissions and stem global climate change.
Our vision is to create a library of ultra-compact devices (including beamsplitters, but also other devices) that can then be all connected together in a variety of different ways to enable both optical computing and communications. The first devices were fabricated at a University. Next, we need to fabricate these in a standard process at a company, and then provide this library of devices to designers and hopefully, unleash their creativity. We believe that these devices will usher in unpredictable, but unbelievably exciting applications.
 Bing Shen, Peng Wang, Randy Polson, Rajesh Menon, “An integrated-nanophotonic polarization beamsplitter with 2.4 × 2.4 μm2 footprint”, Nature Photonics, 9, 378-382 (2015). Abstract.
 Bing Shen, Peng Wang, Randy Polson, Rajesh Menon, “Integrated metamaterials for efficient, compact free-space-to-waveguide coupling”, Optics Express, 22, 27175-27182 (2014). Abstract.
 Bing Shen, Randy Polson, Rajesh Menon, “Integrated digital metamaterials enables ultra-compact optical diodes”, Optics Express, 23, 10847-10855 (2015). Abstract.