Recent Advances in Liquid Metal Photonics: Technologies and Applications

Mechanically reconfigurable cavity filters using liquid metals in the W-band

The high adaptability and flexibility of reconfigurable cavity filters (RCFs) provide a compelling solution for signal modulation in communication systems. However, current reports on these filters are predominantly concentrated within the frequency range of a few GHz. As communication frequencies continue to escalate, there is a pressing need for filters that can operate at higher frequencies. This study presents a mechanical RCF operating in the W-band (75-110 GHz) that address the need for high-frequency cavity filters. The reconfigurability of the filter is enhanced by adjusting the height of the liquid metal posts and interchanging different liquid metal (LM) media, thereby augmenting its tuning flexibility. During the experiment, we observed the potential dispersion effects of the LMs near 100 GHz, which were hypothetically validated. Finally, we tested the performance of the RCF with single or double LM tubes using a vector network analyzer to achieve consistency between the experimental and simulated results. This study offers a unique solution for sub-terahertz communication systems with critical requirements for dynamic filtering capabilities.

Deterministic Fabrication of Liquid Metal Nanopatterns for Nanophotonics Applications

Gallium-based liquid metals (LMs) are widely used for stretchable and reconfigurable electronics thanks to their fluidic nature and excellent conductivity. These LMs possess attractive optical properties for photonics applications as well. However, due to the high surface tension of the LMs, it is challenging to form LM nanostructures with arbitrary shapes using conventional nanofabrication techniques. As a result, LM-based nanophotonics has not been extensively explored. Here, a simple yet effective technique is demonstrated to deterministically fabricate LM nanopatterns with high yield over a large area. This technique demonstrates for the first time the capability to fabricate LM nanophotonic structures of various precisely defined shapes and sizes using two different LMs, that is, liquid gallium and liquid eutectic gallium-indium alloy. High-density arrays of LM nanopatterns with critical feature sizes down to ≈100 nm and inter-pattern spacings down to ≈100 nm are achieved, corresponding to the highest resolution of any LM fabrication technique developed to date. Additionally, the LM nanopatterns demonstrate excellent long-term stability under ambient conditions. This work paves the way toward further development of a wide range of LM nanophotonics technologies and applications.

Surface-Enhanced Raman Scattering Sensors Employing a Nanoparticle-On-Liquid-Mirror (NPoLM) Architecture

Surface-enhanced Raman scattering (SERS) sensors typically employ nanophotonic structures that support high-field confinement and enhancement in hotspots to increase the Raman scattering from target molecules by orders of magnitude. In general, high field and SERS enhancement can be achieved by reducing the critical dimensions and mode volumes of the hotspots to nanoscale. To this end, a multitude of SERS sensors employing photonic structures with nanometric hotspots have been demonstrated. However, delivering analyte molecules into nanometric hotspots is challenging, and the trade-off between field confinement/enhancement and analyte delivery efficiency is a critical limiting factor for the performance of many nanophotonic SERS sensors. Here, a new type of SERS sensor employing solid-metal nanoparticles and bulk liquid metal is demonstrated to form nanophotonic resonators with a nanoparticle-on-liquid-mirror (NPoLM) architecture, which effectively resolves this trade-off. In particular, this unconventional sensor architecture allows for the convenient formation of nanometric hotspots by introducing liquid metal after analyte molecules are efficiently delivered to the surface of gold nanoparticles. In addition, a cost-effective and reliable process is developed to produce gold nanoparticles on a substrate suitable for forming NPoLM structures. These NPoLM structures achieve two orders of magnitude higher SERS signals than the gold nanoparticles alone.

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

Alkali metals have low optical losses in the visible to near-infrared (NIR) compared with noble metals. However, their high reactivity prohibits the exploration of their optical properties. Recently sodium (Na) has been experimentally demonstrated as a low-loss plasmonic material. Here we report on a thermo-assisted nanoscale embossing (TANE) technique for fabricating plasmonic nanostructures from pure potassium (K) and NaK liquid alloys. We show high-quality-factor resonances from K as narrow as 15 nm in the NIR, which we attribute to the high material quality and low optical loss. We further demonstrate liquid Na-K plasmonics by exploiting the Na-K eutectic phase diagram. Our study expands the material library for alkali metal plasmonics and liquid plasmonics, potentially enabling a range of new material platforms for active metamaterials and photonic devices.