A strong linear-in-$k$ Rashba spin-orbit coupling (SOC) has been discovered theoretically for holes in semiconductor nanowires originating from direct dipolar coupling to the external electric field. Such direct Rashba SOC effect not only overcomes the major drawback of the hole Rashba SOC for its leading order being cubic-in-$k$ but also renders its strength orders of magnitude stronger than the corresponding electron counterpart. Here, the direct hole Rashba SOC effect has been experimentally observed through the circular photogalvanic effect (CPGE) by conducting the experimental measurement of a helicity-dependent photocurrent in the $p$-type $\mathrm{Ga}{\mathrm{As}}_{1-x}{\mathrm{Sb}}_x$ semiconducting nanowires upon application of an external gate voltage at room temperature. We have observed that the CPGE current undergoes a transition from rapid increase to saturation as the gate voltage varies. This finding is consistent with the predicted common feature of the direct Rashba SOC of holes in semiconductor nanowires, providing clear evidence for the theoretical concept.

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• Photocurrent
• Spin optoelectronics
• Spin-orbit coupling
• Nanowires
• Narrow band gap systems
• Photoexcitation