Fig. 1: Spin accumulation induced photoresistance and beam shift induce photoresistance.

Illustration of spin accumulation and the corresponding helicity dependent photoresistance detected for a a straight current path and b a ring shaped current path. The up/down spin accumulation appears at the edges of the device on the left/right side of the electric current. Thus, when illuminated by a circular polarized light, the absorption of one particular circularly-polarized light is different for spin-up and spin-down. The longitudinal resistance of the illuminated area on one edge will exhibit a higher value (blue) when illuminated with one particular helicity light, while on the other edge will be lower (red). c Cartoon illustration of the laser spot barycenter displacement associated to the polarization change from circular left (T1) to circular right (T3). The beam shift effect is illustrated by the two dashed circles the right y-axis, indicating the beam center positions are different at T1 and T3. An example of two laser spot profiles simulated with a 2D skewed Gaussian distribution are plotted in the insets. The laser spot with a different shape inherently will have a different effective position, which creates a different thermal gradient. To a first order, this effect can be approximated by a beam-shift. d The thermal gradients produced by the oscillating beam induce opposite longitudinal resistance variations at the two edges (blue and red). This heat gradient induced photoresistance is very similar to the helicity dependent photoresistance expected in the straight device. However, for the ring shaped device in e, these two effects have different symmetries: the contrast on the edges of the device changes sign along the circle. f Measured photoresistance mapping for 3 nm thick Pt ring shaped device. The angle difference Δθ between the zero photoresistance position (dash arrow line) and the diameter across the other side of the zero photoresistance position (solid line) indicates the mixture of both spin accumulation effect (Δθ = 180°) and beam shift effect (Δθ = 0°).