Thermosensitive Plasmonic Color Enabled by Sodium Metasurface

Abstract

Active plasmonic nanostructures have attracted tremendous interest in nanophotonics and metamaterials owing to the dynamically switchable capabilities of plasmonic resonances. In this study, tunable hybrid plasmon resonances (HPR) of sodium metasurfaces through heat‐initiated structural transformation is experimentally demonstrated. A HPR is formed by coupling surface plasmon polaritons (SPP) and gap plasmon resonances (GPR), whose resonant wavelengths are highly sensitive to gaseous nanogaps. By carefully manipulating the thermo‐assisted spin‐coating process and post‐thermal treatment, tuning of the HPR is achieved because of the phase transition between the antidome and nanodome structural profiles of liquid sodium inside the patterned fused silica substrates. Furthermore, the figure of merit of the heat initiated variable structure‐spectrum is demonstrated that is highly dependent on the size of the substrate patterns, based on which temperature‐sensitive plasmonic color and “invisible ink” of sodium metasurfaces are demonstrated. These findings can lead to new solutions for manipulating low‐cost and high‐performance active plasmonic devices.

Low-Loss Plasmonics with Nanostructured Potassium and Sodium–Potassium Liquid Alloys

“…Immediately after the contact of the molten K droplet with the PU template, the diffractive color of the PU template from the periodical nanoscale features was enhanced due to the filling of molten plasmonic K into the nanoscale trenches, forming K “nanopit” arrays (Figure b, right). Recent reports on Na found that thermo-assisted spin-coating can lead to a concave profile of Na on top of the nanoholes, and post-thermal treatment can facilitate the filling of Na into the nanoholes . We believe our TANE process has a similar effect to the post-thermal treatment to get K into nanopatterns.…”

“… However, alkali metals are highly reactive with air and water, which poses significant challenges in fabricating and investigating alkali metal nanostructures, excluding conventional lithography–deposition–lift-off methods. Recently, Prof. Jia Zhu’s group pioneered a thermo-assisted spin-coating technique to fabricate high-quality Na films. − This technique is innovative and successful because it has a simple procedure that can be implemented in an inert environment: it directly processes molten Na into Na films without exposure to the atmosphere or solvents. We also developed a large-scale fabrication method for Na nanostructures by combining phase-shift photolithography and the thermo-assisted spin-coating process. , Na surface plasmon polariton (SPP) resonances with a narrow line width of 9.3 nm have been achieved in our previous report …”

Recent progress on structural coloration

“…Over 1700 years ago, Roman artisans created the famous Lycurgus Cup [84] by mixing nano-sized metal particles into glass. Due to the strong modulation effect of SPR on electromagnetic waves, there have been numerous reports of high spatial resolution color filters based on SPR [49,85,86] . In addition to classical plasmonic metasurfaces, researchers have also designed new nanostructures such as gap plasmons [87] and Fabry-Pérot (FP) cavities [88][89][90] , and this section will briefly discuss them.…”

“…The dynamic and reversible control achievable by externally modulating the nanostructures responsible for structural colors makes them sensitive to external stimuli. This characteristic has led to the application of structural colors in sensing, with extensive research focusing on applications such as motion sensing [270] , tensile and shear stress sensing [271,272] , passive humidity sensing [273][274][275] , gas sensing [196,276] , refractive index sensing [277] , PH sensing [278] , spectrometers [279,280] , and temperature sensing [85,181,212] . In contrast to physical and chemical sensing, biological sensing often involves complex processes, lengthy detection times, and challenges in achieving real-time monitoring.…”