The richness of optical and electronic properties of graphene attracts enormous interest. So far, the main focus has been on fundamental physics and electronic devices. However, it has also great potential in photonics and optoelectronics, where the combination of its unique optical and electronic properties can be fully exploited, the absence of a bandgap can be beneficial, and the linear dispersion of the Dirac electrons enables ultra-wide-band tunability[1]. The rise of graphene in photonics and optoelectronics is shown by several recent results, ranging from solar cells and light emitting devices, to touch screens, photodetectors and ultrafast lasers. Despite being a single atom thick, graphene can be optically visualized. Its transmittance can be expressed in terms of the fine structure constant. The linear dispersion of the Dirac electrons enables broadband applications. Saturable absorption is observed as a consequence of Pauli blocking. Chemical and physical treatments enable luminescence. Graphene-polymer composites prepared using wet chemistry can be integrated in a fiber laser cavity, to generate ultrafast pulses, down to 200fs, and enable broadband tunability .

In this talk I will review and present results on the application of graphene for photodetection. Graphene’s suitability for high-speed photodetection was demonstrated in an optical communication link operating at 10 Gbit s^-1. However, the underlying physical mechanism is still under debate as both photo-thermoelectric and photoelectric effects are being suggested. We carry out wavelength and polarization dependent photovoltage mapping of metal-graphene-metal junctions, demonstrating that both effects simultaneously contribute to the photoresponse. These measurements allow us to quantify the wavelength dependent ratio of thermo- vs. photoelectric effects from the visible to the near-infrared.

Overall, the electrical signal produced when shining light on graphene-based photodetectors is small compared to traditional semiconductor based detectors, due to the small 2.3% absorption of graphene. In order to increase the effective light absorption, we integrate plasmonic nanostructures into the devices, so to collect light over a larger area and concentrate the energy into the near-field region where the pn-junction is located. Metal nanogratings and nanodots are fabricated on graphene by e-beam lithography. Photovoltage mapping is then carried out at different gate voltages, laser powers, polarizations and wavelengths. Raman spectroscopy is used to confirm monolayer thickness, probe doping levels and absorption enhancement. We detect up to 20 times photovoltage enhancement in the device with metal nanostructures. Also, we find a wavelength dependent response, tuneable by the geometry of the applied metal nanostructures. Further, the polarization dependence of the incident light with respect to the nanostructures orientation strongly influences the magnitude of the generated photovoltage, being maximum for polarization perpendicular to the axis of our nanogratings.


Seminar, April 2, 2012, 12:00. Seminar Room

Hosted by Prof. Frank Koppens