Graphene can be used to create fast detectors for photons in the infrared and terahertz parts of the electromagnetic spectrum. Now, an interdisciplinary team of researchers at the Universities of Manchester, Cambridge and Ioannina (Greece) have improved the photoresponsivity of these detectors by 400%, and greatly increased their photoactive length, by using the incoming light to excite the 'surface plasmon' in a metal grating coupled to the photodetector.
When light hits the surface of a conductor such as a metal under the right conditions, it is able to interact with the sea of electrons in it. This can lead to the generation of waves of electrons, called plasmons, which travel along the surface of the metal. In this work, the researchers integrate a diffraction grating into the metal to ensure the incident light excites the plasmons in the contacts of the device. These plasmons then travel along the contact until they reach the graphene channel where they are converted into an electrical signal - so the plasmons are effectively used to deliver the collected photons to the detector. By appropriately designing the contacts it is also possible to utilise interference effects between the light and the plasmons to tune the spectral response of the photodetectors.
One potential application is in biosensing - in detecting the biomarkers of specific diseases. Because the sensitivity of the detector is improved so much, it is possible to detect the attachment of analytes onto the sensor surface with high sensitivity. As opposed to conventional surface plasmon resonance sensors which rely on an optical readout and bulky optics, it is now possible to directly read-out the signal from the sensor electrically. This allows plasmonic sensing systems to be miniaturised further, opening the route towards point-of-care testing.