Efficient spectral broadening of light in compact graphene-covered photonic chips has recently been successfully demonstrated in the frame of GRAPHENICS, a Future and Emerging Technologies (FET) ‘Young Explorers’ project. The early-career GRAPHENICS researchers unveiled unexpected features in graphene’s nonlinear optical behaviour, playing a crucial role in the generation of spectrally broadband light on a chip.

Broadband light sources are devices that emit high-intensity light over a wide range of colours or wavelengths and could strongly impact various aspects of people’s daily lives. For example, broadband sources that generate mid-infrared light would allow for non-invasive all-optical monitoring of the glucose level of diabetes patients, so that pricking one’s finger is no longer needed. If furthermore these light sources can be made sufficiently compact, one could envisage integrating them in a “smart watch” which would allow the diabetes patients to assess their glucose levels at all times.

To realize such a compact light source, the GRAPHENICS young explorers, affiliated to the Vrije Universiteit Brussel (Belgium), the Institute of Electronic Materials Technology (Poland), Vienna University of Technology (Austria) and University of Toronto (Canada), developed a graphene-covered photonic chip to generate the broadband light, and succeeded in constructing a small-sized mid-infrared pump laser for exciting the chip. The introduction of a 2D atomic layer of graphene - a material with unusual nonlinear optical properties - on silicon nitride waveguide chips contributed to creating the desired wavelengths. The GRAPHENICS research team also observed, however, that when placing the same material on silicon waveguide chips as shown here in the figure, the graphene counteracted the generation of new wavelengths rather than enhancing it. This unexpected dual behavior of graphene was found to be the result of the unusual negative sign of graphene’s nonlinear optical coefficient, in stark contrast with the positive sign one had so far assumed. These new insights have enabled the GRAPHENICS team to optimally exploit graphene’s unique nonlinear optical properties in their pursuit for optimal on-chip broadband light emission and currently opens up new horizons for the realization of other graphene-based nonlinear optical devices. Further information is available in the paper [N. Vermeulen, Phys. Rev. Appl. 6, 044006 (2016)] published in Physical Review Applied. In addition, other GRAPHENICS research findings on the practical realization of the graphene-covered photonic chips and the pump laser exciting the chips can be found on the project website