The generation and manipulation of spin currents in semiconductors lie at the cutting edge of Spintronics[1]. To this purpose, group-IV semiconductors represent excellent candidates for spintronic applications, due to their large spin-orbit interaction and electron spin lifetimes. Spin currents in semiconductors can be photo-induced via optical orientation, that is the generation of a spin-oriented electron population in the conduction band of the semiconductor via circularly-polarized light absorption [2]. Although optical orientation allows overcoming the drawbacks of the electrical spin-injection schemes, this approach generates electron spin-polarization vectors always parallel (or antiparallel) to the k-vector of light, preventing the generation of relevant in-plane spin-polarization projections on the sample. This feature is certainly detrimental in view of future potential applications.

Here we show that, by properly engineering the wave-front of the incoming light, using a normal incidence geometry complementary spin densities (i.e. two spin populations with opposite spin-polarization projections) can be simultaneously injected in the plane of the device, thus obtaining the spintronic equivalent of a photovoltaic cell. While a photovoltaic generator spatially separates photoexcited electrons and holes, our device exploits circularly polarized light to produce two spatially well-defined electron populations with opposite in-plane spin projections. This is achieved by modulating the phase and amplitude of the light wavefronts entering a semiconductor (germanium, Ge) through micro and nanopatterned metal (platinum, Pt) overlayers [3]. The resulting spin distribution inside the semiconductor can be then detected through inverse spin-Hall effect (ISHE) technique, where the spin current, flowing from Ge towards the Pt layer, is converted into a transverse electromotive field via spin-dependent scattering with the Pt nuclei. In presence of complementary spin populations, the driving force for the spin-oriented electron motion is only given by the spin density gradient. This allows to study spin transport and dynamics in semiconductors, totally decoupled from the electron diffusion, hence exploiting the non-dissipative features provided by pure spin transport.

We will also show that the simultaneous generation of complementary spin populations can be achieved with high spatial confinement [3], i.e. well below the spin diffusion length. By properly combining optically-induced confined spin injection to common readout blocks, such as magnetic tunnel junctions, it will be possible to achieve the mandatory versatility for the engineering of future nanoscale spintronic devices.

1. I. Zuti?, J. Fabian & S. Das Sarma, Rev. Mod. Phys. 76, 323-410 (2004).

2. T. Jungwirth, J. Wunderlich & K. Olejn-k, Nat. Mater. 11, 382-390 (2012).

3. F. Bottegoni, M. Celebrano, M. Bollani, P. Biagioni, G. Isella, F. Ciccacci & M. Finazzi, accepted in Nat. Mater.

Michele Celebrano

Politecnico Milano

Appearances