Ultrafast graphene-based terahertz detector

The gapless band structure, universal interband absorption and considerably strong intraband absorption make graphene attractive for applications in the terahertz frequency range [1]. Furthermore the high mobility and fast carrier relaxation [2] enable a fast response when graphene is used a material for terahertz detection. Graphene-based fast bolometers [3] and rectifying transistors [4] have been reported recently. We demonstrate an ultrafast broadband detector operated at room temperature. Response times of 50 ps are observed in the wavelength range from 30 µm to 220 µm.
The detector is based on an exfoliated graphene flake on Si/SiO2, which is coupled to a logarithmic-periodic planar antenna. In the center of the antenna interdigitated electrodes provide the electrical contacts between the antenna arms and the graphene flake (cf. Fig. 1). The response of the detector to picosecond terahertz radiation pulses from a free-electron laser was studied. A responsivity of ~5 nA/W and a signal rise time of 50 ps was found for the spectral range from 30 µm to 220 µm. The linear detection range is followed by region of saturation for high pulse powers. The characteristic pulse energy E_sat for the onset of saturation is ~10 nJ for lamda = 220 µm. With decreasing wavelength the saturation regime shifts towards higher pulse energies (E_sat ≈ 150 nJ for lamda = 30 µm). Comparing detectors fabricated on either intrinsic or doped silicon we find that a high substrate resistivity is crucial for the short response time. We attribute this to large RC time constants occurring for the conductive substrates.
These easy-to-use detectors are well suited for monitoring the timing of terahertz pulses. We will discuss several physical mechanisms, which may be involved in the detection process. As an outlook we suggest that the detector principle allows one to produce detectors covering an even larger spectral range, namely from the UV to THz range. To this end appropriate non-absorbing substrates have to be used.

References:

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