Two dimensional materials for on-chip photonics

Optoelectronic applications such as photodetectors and emitters rely on the ability of its active material to interact strongly with light and one could not be blamed for questioning the ability of two dimensional (2D) materials here. After all, its cross-section is made up of at most few atoms which means there is hardly enough matter to interact with any amount of light. However, even a customary google search would yield an astounding number of papers and patents on graphene and 2D material photonics. So what makes these materials so attractive?

1. Superfast and free electrons floating on the graphene lattice

Graphene’s atomic structure where each carbon atom is bonded to 3 others has a very profound effect on its properties. This is because such an arrangement creates a cloud of free electrons which move at extremely high speeds which is the source of its unprecedented electron mobilities. Therefore absorption of even a small amount of light can be used quite effectively in detecting changes at high frequencies. (If you are wondering why that is important read blog by Dr Mark Dineen on InP Light). Using this property of graphene and some clever engineering there have been several demonstrations of graphene based photodetectors with extremely high responsivities both in the visible and NIR. However, the really exciting developments are happening in the telcom band around 1550nm where graphene photodetectors have been shown to operate at speeds greater than tens of gigahertz!

2. Passive surface and absence of lattice mismatch issues

2D materials interact with surfaces purely on the basis of Van der Waal’s forces (these are the weak forces that keep the layers of graphite together!) so they are not subject to strain which arises when depositing conventional materials on silicon for example. Their surfaces are also naturally passivated i.e., there are no dangling bonds which not only minimizes losses but also alleviates difficulties during integration with photonic waveguides. These properties have enabled researchers globally to extract light out of semiconducting 2D materials with extremely high quantum yields (near unity yield has been demonstrated using a perfect crystal) on not only conventional hard substrates but also on flexible and transparent substrates.

One does not need a crystal ball to predict a surge in research and development efforts on light emitters, modulators and photodetectors in the next few years. We are already seeing a great deal of technology readiness leaps in the integration of graphene based detectors with Silicon technology but there are great opportunities for researchers in bringing this fascinating class of materials towards on chip photonic integrated circuits!

We at Oxford Instruments Plasma Technology are working alongside you to take this technology forward by developing device fabrication solutions through continuous improvements in our processes and systems. We would be delighted to talk further regarding any of these solutions so please get in touch with us at plasma-experts@oxinst.com 

Author: Dr Ravi Sundaram