Optical Characteristics of Superconducting Plasmonic Transition Metal Nitrides

Transition metal nitrides (TMNs) exhibit exceptional promise due to their elevated melting points, metallic optical characteristics, and superconductivity at cryogenic temperatures. Recent attention has turned toward TMNs as potential substitutes for conventional plasmonic materials like gold and silver due to their unique plasmonic properties. However, the optical properties of plasmonic TMN films in the superconducting state have remained uncharted territory. In the past, we have demonstrated the utility of plasmonic TMN films (including TiN, ZrN, HfN, and NbN) as alternative plasmonic materials for various applications [1-3]. Here, we sputtered a thin film of NbN onto a MgO (100) substrate at 800°C, revealing a measured critical temperature (<i>T</i>_C) of 10.5 K. Employing temperature-dependent spectroscopic ellipsometry, we systematically characterized the optical behavior of NbN films from 4 K to 300 K within a wavelength range spanning 300 nm to 2 μm. As the temperature decreased, we observed a remarkable reduction in the magnitude of the imaginary part of the permittivity, the magnitude dropped by 13.8 times at 1550 nm. Subsequently, we applied the Drude-Lorentz model for parameter fitting, elucidating that the substantial reduction in the imaginary component primarily resulted from a sharp increase in the relaxation time. These findings underscore NbN's potential as a highly promising material for the development of low-loss superconducting plasmonic devices.<br/><b>Reference</b><br/>[1]. M-J Yu, C-L Chang, H-Y Lan, Z-Y Chiao, Y-C Chen, H W H. Lee, Y-C Chang, S-W Chang, T. Tanaka, V. Tung, H-H Chou*, and <b><u>Y-J Lu</u></b>*, Plasmon-Enhanced Solar-Driven Hydrogen Evolution Using Titanium Nitride Metasurface Broadband Absorbers. <i>ACS Photonics</i> <b>8</b>, 3125–3132 (2021).<br/>[2] 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><u>Y-J Lu</u></b>*, Full-Color Generation Enabled by Refractory Plasmonic Crystals. Nanophotonics 11, 2891-2899 (2022)<br/>[3] Sheng-Zong Chen, Jing-Wei Yang, Tzu-Yu Peng, Yu-Cheng Chu, Ching-Chen Yeh, I-Fan Hu, Swapnil Mhatre, Yu-Jung Lu*, and Chi-Te Liang*, Disorder-Induced 2D Superconductivity in a NbTiN Film Grown on Si by Ultrahigh-Vacuum Magneton Sputtering. Superconductor Science and Technology 35, 064003 (2022)