Graphene-Based Light Detector

Schematic representation of the ultrafast optical pump-terahertz probe experiment, where the optical pump induces electron heating and the terahertz pulse is sensitive to the conductivity of graphen. (Source: F. Vialla, ICFO)

Light detection and control lies at the heart of many modern device appli­cations, such as the camera you have in your phone. Using graphene as a light-sensi­tive material for light detectors can offer signi­ficant improve­ments with respect to materials being used nowadays. For example, graphene can detect light of almost any colour, and it gives an extremely fast elec­tronic response within one millionth of a millionth of a second. Thus, in order to properly design graphene-based light detectors it is crucial to under­stand the processes that take place inside the graphene after it absorbs light.

A team of European scientists including ICFO from Barce­lona, IIT from Genova, the Univer­sity of Exeter from Exeter and Johannes Guten­berg Uni­versity from Mainz, have now succeeded in under­standing these processes. Their work gives a thorough expla­nation of why, in some cases, the graphene conduc­tivity increases after light absorp­tion and in other cases, it decreases. The researchers show that this behaviour correlates with the way in which energy from absorbed light flows to the graphene electrons: After light is absorbed by the graphene, the processes through which graphene electrons heat up happen extremely fast and with a very high effi­ciency.

For highly doped graphene with many free electrons, ultra­fast electron heating leads to carriers with elevated energy which, in turn, leads to a decrease in conduc­tivity. Interes­tingly enough, for weakly doped graphene with less free electrons, electron heating leads to the creation of addi­tional free electrons, and there­fore an increase in conduc­tivity. These addi­tional carriers are the direct result of the gapless nature of graphene. In gapped materials, electron heating does not lead to addi­tional free carriers.

This simple scenario of light-induced electron heating in graphene can explain many observed effects. Aside from descri­bing the conduc­tive proper­ties of the material after light absorption, it can explain carrier multi­plication, where under specific conditions one absorbed photon can indi­rectly generate more than one addi­tional free electron, and thus create an effi­cient photo­response within a device. In parti­cular, under­standing electron heating processes accurately, will defi­nitely mean a great boost in the design and develop­ment of graphene-based light detec­tion tech­nology. (Source: ICFO)

Reference: A. Tomadin et al.: The ultrafast dynamics and conductivity of photoexcited graphene at different Fermi energies, Sci. Adv. 4, eaar5313 (2018); DOI: 10.1126/sciadv.aar5313

Link: Quantum Nano-Optoelectronics (F. Koppens), The Institute of Photonic Sciences ICFO, Castelldefels, Spain

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