Broadband Photodetection in Graphene

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Cai, Xinghan
Fuhrer, Michael S.
Graphene, a single-atom-thick plane of carbon, has unique optoelectronic properties that result in a variety of potential photonic applications, such as optical modulators, plasmonic devices and THz emitters. In this thesis, the light-matter interaction in monolayer graphene and the subsequent photoexcited charge carrier transport are studied, and it is found that graphene has unique advantages for hot-electron photothermoelectric detection. Particularly promising is detection of terahertz (THz) radiation, in which graphene devices may offer significant advantages over existing technology in terms of speed and sensitivity. By using a tilted angle shadow evaporation technique, bi-metal contacted graphene photodetectors are realized experimentally. Efficient photodetection via the hot-electron photothermoelectric effect is demonstrated at room temperature across a broad frequency range (THz to near infrared). For THz detection, the best device shows sensitivity exceeding 10 V/W (700 V/W) and noise equivalent power less than 1100 pW/Hz1/2 (20 pW/Hz1/2), referenced to the incident (absorbed) power, implying a performance competitive with the best room-temperature THz detectors for an optimally absorbing device, while time-resolved measurements indicate that the graphene detector is eight to nine orders of magnitude faster than those. To increase the absorption and quantum efficiency, large area epitaxial graphene micro-ribbon array photodetectors are designed for resonant plasmon excitation in the THz range. By tailoring the orientation of the graphene ribbons with respect to an array of sub-wavelength bimetallic electrodes, a condition is achieved in which the plasmonic mode can be efficiently excited by an incident wave polarized perpendicular to the electrode array. The sensitivity of the detector is enhanced when the plasmon resonance frequency, which is tunable by adjusting the gate voltage, matches with the frequency of the incident radiation.