Graphene: electrons en masse.

575 news & views the tunnel current while slowly varying the time delay between pump and probe. As with many experiments using near-field probes, the key to success lies in suppressing spurious effects while boosting the desired signal above the noise floor. Shigekawa and colleagues demonstrate a clever lock-in detection scheme akin to heterodyne detection in a radio receiver to boost their sensitivity and isolate the spin-dependent signal while keeping the average laser power impinging on the tip constant. Maintaining a constant power that is absorbed by the tip is an essential step to avoid spurious signals related to time-dependent heating, which have plagued laser-combined STM in the past 7. An important aspect of the experiment is the non-local nature of the optical excitation. The micrometre-sized laser beam presents a global excitation compared with the tunnel current detection, which is localized on the ångström scale. It is possible that many photoexcited electrons, not just those generated immediately under the scanning tunnelling microscope tip, are detected at each pump–probe cycle. Consequently, this measurement scheme may also be applicable to the study of non-local effects, such as spin diffusion in real space. It also opens the door to take other established methods in ultrafast optics to the nanoscale, such as the resonant excitation of phonons 8. Reliance on a global excitation, however, also creates a challenge for other sample systems, such as metals, where vanishing optical penetration depth significantly reduces the excitation volume and may preclude the application of laser-combined STM. Surmounting this challenge will be important for expanding the scope of this technique. Local enhancement of the light intensity under the tip, as achieved, for example, with plasmonic waveguides 9 , could overcome limitations in excitation efficiency and reach the same highly localized control as approaches employing fast gating of the tunnel junction voltage 5,6. Perhaps the most important finding of the work by Shigekawa and colleagues is the measurement of spin dynamics beyond population decay, showing that the scanning tunnelling microscope can detect coherent spin precession with high fidelity. Hallmark features are the detection of the quantum beat of the spin polarization in a magnetic field 10 and its resonant amplification with matched laser-pulse repetition 11. Electron spins in semiconductors can maintain quantum–mechanical coherence only for short periods of time. In this time, they can, in principle, be used for spintronic applications that go further than static magnetoresistance. The method …