Graphene based broadband THz detector working at room temperature

Graphene can serve as an attractive detector material for the whole visible and infrared spectral range. Due to the unique band structure of graphene, with a linear dispersion next to the Dirac point where valence and conduction band touch, the absorption is constant for nearly all photon energies. Furthermore the fast carrier relaxation in graphene allows one to build up fast detectors with an electrical response in the GHz range. We demonstrate a detector based on a graphene flake combined with a logarithmic-periodic antenna structure with an outer diameter of 1mm, which connects the flake via an interdigitated structure. The graphene flake was produced by the scotch-tape method on SiO2/Si. To maximize the photocurrent the two halves of the antenna were made of different metals. The metallization was patterned by electron beam lithography. One arm of the antenna consists of a 60nm thick layer of palladium, the other one of 20nm titanium combined with 40nm of gold. With the free-electron laser FELBE at Dresden-Rossendorf we proved room temperature operation in a wide spectral range (wavelength: 8µm-220µm), the coupling of the antenna was verified by measurements of the polarization dependence of the detector for different wavelengths. Additionally we measured FEL-pulses with pulse durations around 20 ps in the wavelength range of 30µm to 220µm. The rise time of the measured signal was approximately 50 ps, the pulse length was in the range of 200 ps. Even though the responsivity was fairly low (~ 1nA/W), this detector could be very useful for timing purposes in two-color experiments.