Direct detection of photon spin angular momentum by a chiral graphene mid-infrared photodetector

We demonstrate the direct detection of photon spin angular momentum in the mid-infrared region by a single surface-plasmon-enhanced graphene photodetector. We utilize chiral surface plasmon nanostructures as photodetector electrodes to generate photocurrents of equal and opposite sign according to incident photon spin. Our detector possesses a current circular dichroism of 60% and a responsivity of 0.80 μA/W at a resonant wavelength of 3.8 μm. The photocurrent dichroism is attributed to the presence of opposite handedness circularly polarized surface plasmon resonances on adjacent electrodes, of which each enhances graphene absorption by a factor of 17. The direct detection of spin angular momentum will be useful in the next wave of multiplexed sensing and communications systems utilizing the optical angular momentum states of light to enhance bandwidth and information collection.