In recent years, a class of solid-state materials, called three-dimensional topological insulators, has emerged. In the bulk, a topological insulator behaves like an ordinary insulator with a band gap. At the surface, conducting gapless states exist showing remarkable properties such as helical Dirac dispersion and suppression of backscattering of spin-polarized charge carriers. The characterization and control of the surface states via transport experiments is often hindered by residual bulk contributions. Here we show that surface currents in Bi2Se3 can be controlled by circularly polarized light on a picosecond timescale with a fidelity near unity even at room temperature. We reveal the temporal separation of such ultrafast helicity-dependent surface currents from photo-induced thermoelectric and drift currents in the bulk. Our results uncover the functionality of ultrafast optoelectronic devices based on surface currents in topological insulators.

Non-radiative transfer processes are often regarded as loss channels for an optical emitter 1 because they are inherently difficult to access experimentally. Recently, it has been shown that emitters, such as fluorophores and nitrogen-vacancy centres in diamond, can exhibit a strong non-radiative energy transfer to graphene [2][3][4][5][6] . So far, the energy of the transferred electronic excitations has been considered to be lost within the electron bath of the graphene. Here we demonstrate that the transferred excitations can be read out by detecting corresponding currents with a picosecond time resolution 7,8 . We detect electronically the spin of nitrogen-vacancy centres in diamond and control the non-radiative transfer to graphene by electron spin resonance. Our results open the avenue for incorporating nitrogen-vacancy centres into ultrafast electronic circuits and for harvesting non-radiative transfer processes electronically.With the advancement of nanoscale photonics research it has become increasingly desirable to combine optical systems with electric circuits to create optoelectronic devices that can be miniaturized and integrated into chips. To this end, we can take advantage of the excellent optical and electronic properties of graphene 9 , which include good photodetection capabilities 8,10-15 , efficient energy absorption 3 and strong light-matter interactions at the nanoscale 16,17 . In particular, it has been reported recently that, because of graphene's specific properties, the near-field interaction between light emitters and graphene is greatly enhanced as compared to that of conventional metals 2-6 . This interaction manifests itself, for example, in a 100-fold enhancement of the excited-state decay rate of emitters placed 5 nm away from graphene as compared to the spontaneous emission of the emitter. The physical mechanism behind the interaction is the creation of an electron-hole pair in graphene through non-radiative energy transfer (NRET) from the emitter dipole 18 . The NRET process to graphene has been demonstrated to have an efficiency of nearly 100% when the emitter is less than 10 nm away from the graphene sheet 3 , which makes graphene an ideal material to detect electronically the optical properties of nearby emitters 6 . NRET has been studied extensively for fundamental as well as for biosensing applications. However, a fast energy transfer has not yet been observed because of quenching of the optical signal for short graphene-emitter distances. In contrast, an electronic readout of the NRET enables studies on fast energy processes. Moreover, if the transferred energy can be collected, as we show in this work, new ways for energy harvesting and biosensing can be implemented.We take advantage of the highly efficient NRET process to read out electronically, for the first time, the optical excitation of nitrogen-vacancy (NV) centres in diamond nanocrystals. To this end, we used graphene for the extraction of the excited-state energy of NV centres and converted it into a measurable ...

Ferrimagnetic rare earth -transition metal Tb-Fe alloy thin films exhibit a variety of different magnetic properties, which depend strongly on composition and temperature. In this study, first the influence of the film thickness (5-85 nm) on the sample magnetic properties was investigated in a wide composition range between 15 and 38 at.% of Tb. From our results, we find that the compensation point, remanent magnetization, and magnetic anisotropy of the Tb-Fe films depend not only on the composition but also on the thickness of the magnetic film up to a critical thickness of about 20-30 nm. Beyond this critical thickness, only slight changes in magnetic properties are observed. This behavior can be attributed to a growth-induced modification of the microstructure of the amorphous films, which affects the short range order. As a result, a more collinear alignment of the distributed magnetic moments of Tb along the out-of-plane direction with film thickness is obtained. This increasing contribution of the Tb sublattice magnetization to the total sample magnetization is equivalent to a sample becoming richer in Tb and can be referred to as an "effective" composition. Furthermore, the possibility of all-optical switching, where the magnetization orientation of Tb-Fe can be reversed solely by circularly polarized laser pulses, was analyzed for a broad range of compositions and film thicknesses and correlated to the underlying magnetic properties.

In order to clarify the temporal interplay of the different photocurrent mechanisms occurring in single GaAs nanowire based circuits, we introduce an on-chip photocurrent pump-probe spectroscopy with a picosecond time resolution. We identify photoinduced thermoelectric, displacement, and carrier lifetime limited currents as well as the transport of photogenerated holes to the electrodes. Moreover, we show that the time-resolved photocurrent spectroscopy can be used to investigate the drift velocity of photogenerated carriers in semiconducting nanowires. Hereby, our results are relevant for nanowire-based optoelectronic and photovoltaic applications.

To clarify the ultrafast temporal interplay of the different photocurrent mechanisms occurring in single InAs-nanowire-based circuits, an on-chip photocurrent pumpprobe spectroscopy based on coplanar striplines was utilized. The data are interpreted in terms of a photo-thermoelectric current and the transport of photogenerated holes to the electrodes as the dominating ultrafast photocurrent contributions. Moreover, it is shown that THz radiation is generated in the optically excited InAs-nanowires, which is interpreted in terms of a dominating photo-Dember effect. The results are relevant for nanowire-based optoelectronic and photovoltaic applications as well as for the design of nanowire-based THz sources.