Gallium (Ga) has attracted intense interest due to its low melting point (29.8 °C), non-toxicity and excellent performance [[1], [2], [3], [4]]. Its liquid state, which is easily deformable and flexible, has become a promising new material used in emerging fields such as microfluidics, heat conduction, neural connection, flexible electronics, optoelectronics and photonics [[5], [6], [7]]. Since each Ga atom has three valence electrons, Ga nanostructures have surface plasmon (SP) that can reach vacuum ultraviolet region [[8], [9], [10]], which is significantly different from Au and Ag, comparable to Al [11]. Because solid-liquid phase transitions can easily occur by changing temperature or external optical excitation, Ga nanostructures stand out in nanophotonics, used as an “active plasmon” phase-change optical material [12,13].

Several researchers have studied the surface plasmon polariton (SPP) of Ga-based metasurfaces with periodicity [[14], [15], [16]]. For example, R. C. Vivekchand et al. [14] manipulated the SPP of 1D Ga gratings by reversible liquid-to-solid phase transitions in the visible region, and they found that liquid gratings have higher SPP coupling efficiency, narrower resonance and better reflectivity compared to solid gratings. J. Wang et al. [15] reported liquid metal (eutectic gallium indium) supports SPP at terahertz (THz) frequency. They observed THz transmission enhancement of periodic arrays with subwavelength apertures by injecting the metal into a polydimethylsiloxane mold manufactured based on soft lithography. R. F. Waters et al. [16] presented an optically switchable Ga-based metasurface (Au disk array/Si3N4/Ga film), in which solid-to-liquid transition occurs reversibly in the confined nanoscale Ga film, driving significant changes in near-infrared spectra and providing optical switching with high contrast. In addition, A. V. Krasavin et al. [13] utilized Otto configuration to achieve modulation of light with light by control of SPP wave through a glass/MgF2/Ga structure in the visible and near-infrared regions, and the mechanism relies on a light-induced phase transition of the gallium layer, with a transient switching time of tens of nanoseconds.

Although some observable SPP optical differences between liquid and solid Ga have been experimentally found in periodic metasurfaces or Otto configuration, the research on local surface plasmon (LSP) differences between the two phases of individual Ga nanoparticles is very rare, and no significant differences have been found yet. K. F. MacDonald et al. [17] deposited Ga nanoparticle films on the ends of optical fibers, and indicated that the spectrum change of the nanoparticles was below 7 % when Ga melted.

In this work, in order to elucidate and predict the LSP differences between solid and liquid phases for individual Ga nanostructures, two typical and representative shapes of spherical and cylindrical Ga nanostructures in the two phases are explored and compared by using the discrete dipole approximation method. We focus on the effects of the shape and size of two-phase metal, and also consider the influence of the medium environment and surface oxidation layer. The conditions for providing “active” and “inactive” plasmon (i.e., high and low liquid-solid optical contrast) during the solid-liquid transition of Ga are revealed.