Although the detection of light at terahertz (THz) frequencies is important for a large range of applications, current detectors typically have several disadvantages in terms of sensitivity, speed, operating temperature, and spectral range. Here, we use graphene as photoactive material to overcome all of these limitations in one device. We introduce a novel detector for terahertz radiation that exploits the photo-thermoelectric effect, based on a design that employs a dual-gated, dipolar antenna with a gap of ∼100 nm. This narrow-gap antenna simultaneously creates a pn-junction in a graphene channel located above the antenna, and strongly concentrates the incoming radiation at this pn-junction, where the photoresponse is created. We demonstrate that this novel detector has excellent sensitivity, with a noise-equivalent power of 80 pW/ √ Hz at room temperature, a response time below 30 ns (setup-limited), a high dynamic range (linear power dependence over more than 3 orders of magnitude) and broadband operation (measured range 1.8 -4.2 THz, antenna-limited), which fulfills a combination that is currently missing in the state of the art. Importantly, based on the agreement we obtain between experiment, analytical model, and numerical simulations, we have reached a solid understanding of how the PTE effect gives rise to a THz-induced photoresponse, which is very valuable for further detector optimization.

We present a new fabrication method of graphene spin-valve devices which yields enhanced spin and charge transport properties by improving both the electrode-to-graphene and graphene-to-substrate interface. First, we prepare Co/MgO spin injection electrodes onto Si ++ /SiO 2 . Thereafter, we mechanically transfer a graphene-hBN heterostructure onto the prepatterned electrodes. We show that room temperature spin transport in single-, bi-and trilayer graphene devices exhibit nanosecond spin lifetimes with spin diffusion lengths reaching 10 µm combined with carrier mobilities exceeding 20, 000 cm 2 /Vs.

Quantum point contacts are cornerstones of mesoscopic physics and central building blocks for quantum electronics. Although the Fermi wavelength in high-quality bulk graphene can be tuned up to hundreds of nanometres, the observation of quantum confinement of Dirac electrons in nanostructured graphene has proven surprisingly challenging. Here we show ballistic transport and quantized conductance of size-confined Dirac fermions in lithographically defined graphene constrictions. At high carrier densities, the observed conductance agrees excellently with the Landauer theory of ballistic transport without any adjustable parameter. Experimental data and simulations for the evolution of the conductance with magnetic field unambiguously confirm the identification of size quantization in the constriction. Close to the charge neutrality point, bias voltage spectroscopy reveals a renormalized Fermi velocity of ∼1.5 × 106 m s−1 in our constrictions. Moreover, at low carrier density transport measurements allow probing the density of localized states at edges, thus offering a unique handle on edge physics in graphene devices.