Enhancing Spin Photocurrent in Scalable Monolayer MoS₂ Plasmonic Phototransistors Using Chiral Metasurfaces

The broken inversion symmetry and valley-selection rule due to the different spin orientations within the split valence band of each K(K’) valleys in monolayer MoS₂ (1L-MoS₂) justifies their potential applications for spintronic devices, valley-sensitive optoelectronic devices and quantum-based information processing. However, the ability to distinguish the degree of freedom between left/right circularly polarized (L/RCP) spin photocurrent is still a challenge due to the low photoresponsivity and lack of polarization sensitivity in 1L-MoS₂ at room temperature conditions. To counter these concerns, we propose a spin photocurrent sensitive plasmonic phototransistors with wafer-scaled 1L-MoS₂ integrated with gold chiral metasurfaces upon a hafnium nitride substrate. The geometry and handedness of the chiral metasurfaces were designed <i>via</i> finite-difference time-domain (FDTD) methods and the calculated absorption spectra signified that the left/right handed orientation of the gold chiral plasmonic metasurfaces dictates the selective absorption enhancement of L/RCP light (or vice versa) upon the 1L-MoS₂ surface. An additional plasmonic hafnium nitride material (deposited via RF-magnetron sputtering at 800°C) was introduced as a gate electrode with excellent metallic properties and strong local surface plasmon resonance attributes within the visible spectrum [1,2]. The inclusion of hafnium nitride plays a crucial role in amplifying the light-matter interaction upon the 1L-MoS₂layer, where a 4-fold enhancement factor was observed from the photocurrent measurements (in comparison to pristine <i>p</i>⁺-Si substrate). Spin photocurrent measurements at different wavelengths showed up to 30.02% change at 660 nm (on-resonance with MoS₂) and less than 10% change at 450 nm and 550 nm (off-resonance). Overall, these promising results reveal a clear distinguishable approach to manipulate and tailor the L/RCP spin photocurrent at room temperature conditions. Ultimately, we will discuss the potential applications of ultrathin plasmonic phototransistors with high spin-selective photoresponse.<br/><br/><b><u>Reference</u></b><br/>[1] Z-Y Chiao, Y-C Chen, J-W Chen, Y-C Chu, J-W Yang, T-Y Peng, W-R Syong, H W H. Lee, S-W Chu, and <b>Y-J Lu</b>*, Full-Color Generation Enabled by Refractory Plasmonic Crystals. <i>Nanophotonics </i>11, 2891-2899 (2022)<br/>[2] Y-H Hsieh, B-W Hsu, K-N Peng, K-W Lee, C W Chu, S-W Chang, H-W Lin*, T-J Yen*, and <b>Y-J Lu</b>*, Perovskite Quantum Dot Lasing in a Gap-Plasmon Nanocavity with Ultralow Threshold, <i>ACS Nano </i>14, 11670 (2020).