Optical non-linearity is used for wavelength multiplication and pulse shaping, both of which can improve optical communications.

For example if you input two colors of light, instead of the original two colors coming out, a third color also comes out containing the same modulated information as the original signals. The third signal is created by the non-linear interaction. Using nonlinearity we can perform the same functions as an electronic mixer in a radio receiver.

The other possible use is to generate very short optical pulses, which enables us to increase the data-rate in the communication network. In 20 years we could be using optical links operating faster than 100 Gbits/second.

Optical nonlinearity is the only way to change light on the optical time scale – think the time it takes for a photon to oscillate one cycle. Its discovery yielded the Nobel prize in 1981.

Yes, graphene serves as the top cladding layer for enhancing the optical nonlinearity (Two photon absorption and Kerr nonlinearity). Graphene itself has the third order nonlinearity 5 orders of the silicon, and thus the single layer carbon sitting on the 250nm Silicon ‘bulk’ can enhance Chi(3) of the effective media 20 times – compared to the monolithic silicon device.
After transferring graphene, We measured the linear propagation loss increases less than 1 dB over the 200 micrometer long photonic crystal waveguide, compared to the control without graphene.

One more point about the propagation loss in waveguides – the loss is mostly due to edge roughness from the fabrication process. Over the years this has steadily improved and now we can expect 1-2 dB per centimeter of loss on chip.
The manufacturing process at the Institute of Microelectronics in Singapore, which was used to manufacture some of the devices, is very good and reproducible.

Hi Prof. Noginova,
Yes. Graphene is a zero-bandgap material, and enable optical devices responsing from terahertz to visible light. Its high mobility, high nonlinearity allows high speed, low power operation.
Plasmonics offers tight confinement in small scale devices, but its realization with conventional metal always face the problem of optical damping/loss. The low linear absorption (2.3%) of graphene, two dimensional electron gas, maybe a key to solve the problem.
- Tingyi & Xiaoliang

I remember seeing a very exciting application of plasmonics as a high-speed absorption based modulator at CLEO 2011 (post deadline) from Prof. Xiang Zhang’s lab at Berkeley. They were able to perform optical modulation using a plasmonic absorption effect in a wavelength size device. So there are definitely applications for plasmonics in next generation optical interconnects.
http://www.opticsinfobase.org/abstract.cfm?URI=CLEO_AT-2012-CTh5D.1

Yes, we use cavity for enhancing light-matter interaction by circulating photon in a small area for longer time.

Graphene has delocalized bond electrons for high optical nonlinear response at all photon energies. Its nonlinear coefficient is much higher than the conventional semiconductors and metals, e.g. five order higher than silicon, fourth order of GaAs. Its single layer structure leads to easy integration.

Tingyi & Xiaoliang

Hi Prof. Kong,

We use CVD to grow single layer graphene on copper, etched away the backside copper by acid, and then transfer the graphene on to the silicon membrane. The monolayer graphene is confirmed by the Raman spectrum. The narrow 2D band indicates the single layer feature. For more details, please refer to the paper:
http://www.nature.com/nphoton/journal/v6/n8/ful...

Thanks,
Tingyi & Xiaoliang

Great question. I’m an Electrical Engineer and Tingyi is in the Mechanical Engineering department with a background in physics. I’m interested in the performance of the devices as modulators and switches, and the implications of such performance on a systems and applications level. Tingyi is the expert in the material properties and fabrication requirements of these devices.
It may seem that these areas are completely different, but once you start working in lab it is easy to learn how different methodologies work. The biggest learning curve for me was understanding the properties of graphene and step into quantum mechanics.