On Fluidic Plasmonics: Harnessing Liquid Metals for Tunable Nanophotonics

Abstract

Low melting point metals, in particular, Ga and its alloys, have been used in electronics applications extensively, while their photonic applications in the visible region (2-3eV photon energy) of the electromagnetic spectrum have been limited owing to the difficulties in obtaining a homogenously dispersed nanoparticle [1]. Ga has a bulk plasma frequency of 13.7eV [2], whereas the Frolich condition [3] for nanoparticles determines the Localised Surface Plasmon Resonance (LSPR) to be at 9eV in an air medium. However, with high refractive index material as a medium enclosing Ga nanodroplets, the LSPR resonance can be reduced further. For example, Ga embedded in a transparent medium of refractive index 1.4 has an LSPR at 7eV, which is still deep in the ultraviolet region, unsuitable for application in visible frequencies [4]. To overcome this problem, plasmon hybridization [5] can be employed, which predicts the splitting of the single particle plasmon mode energies as two nanoparticles come close sufficiently enough for coulombic interactions between the charge densities excited to localized surface plasmons. The lower energy modes, also known as the bonding mode, can have the plasmon frequencies in the visible range. To obtain the hybridized modes, a non-coalescent assembly of the nanoparticles with static relative positions with respect to each other is required. However, most of the metal nanoparticles are obtained in colloidal media where the surrounding medium is a liquid at non-zero temperature, resulting in random thermal motion of the nanoparticles and thus inhibiting the hybridization of LSPR modes. Furthermore, owing to the surface tension of Ga, it is energetically difficult to obtain homogenously dispersed Ga nanoparticles [6]. In this work, we overcome the above challenges by employing single-step thermal evaporation of Ga with a smart choice of substrate. Polydimethylsiloxane (PDMS) is an optically transparent elastomer with a refractive index of 1.4. PDMS is thermally cured after mixing its base with a cross-linking agent. However, even after thermal curing, PDMS retains uncured oligomers, which are in a liquid state. As soon as Ga nanodroplets start forming on the substrate, the oligomers engulf them to reduce the system's overall surface energies. Owing to the liquid state and high surface tension of liquid Ga, the nanoparticle remains spherical in shape. Furthermore, this phenomenon of engulfing keeps Ga nanodroplets from coalescing into bigger particles. The assembly of Ga nanodroplets, engulfed with the oligomers, thus remains on the substrate with their relative positions static to each other. Macroscopically, the optical images of the sample exhibit vibrant, visible colors, owing to the nanostructure characterized by the distribution of Ga nanosphere sizes and the gaps between them, which depend on the amount of oligomers and the cross-linking of the PDMS substrate. These structural colors can find applications in reflective displays, as they are shown to possess the properties of non-fading, longevity, and robustness. By varying the proportion of the cross-linking agent of PDMS, the amount of oligomers in the substrate can be tuned. Thus, with a single physical vapor deposition process, multiple structural colors can be fabricated. Moreover, since the substrate is an elastomer, the inter droplet gaps can be tuned by mechanical deformations like linear strain, twists, and bending of the sample, enabling a controlled manipulation of plasmon hybridization. On applying a uniaxial strain, the sample exhibits a blue shift of the optical spectra, resulting in a color change. The strain causes the inter-droplet gap to increase, resulting in a lowering of the viii strength of plasmon hybridization, leading to an increase in the energy of bonding mode energies and, hence, a blue shift. This property of mechanoresponsiveness of the samples is shown to be repeatable and reversible, and hence, it is robust and reliable for thousands of strain cycles. They can have potential applications in the fabrication of soft-robotics sensors, strain gauges with visual calibration scales, and prosthetic components. The fabricated samples exhibit temperature-dependent optical spectra, due to the PDMS embedded liquid metal morphology featuring sub-10 nm spatial proximity of nanodroplets. With an increase in temperature, the samples exhibit a blue shift in their optical spectra, owing to a decrease in the refractive index of PDMS [8]. In-situ thermal Electron Energy Loss Spectroscopy (EELS) allows for probing the plasmonic modes responsible for this spectral blue shift due to a rise in temperature. This phenomenon is exclusively exhibited in morphologies characterized by metallic nanoparticles surrounded by PDMS. Single-step realization of such a morphology by physical vapor deposition, thermal evaporation in particular, is exclusive to liquid metal on PDMS substrates.