Near-room-temperature liquid metals offer unique and crucial advantages over solid metals for a broad range of applications which require soft, stretchable and/or reconfigurable structures and devices. In particular, gallium-based liquid metals are the most suitable for a wide range of applications, not only owing to their low melting points, but also thanks to their low toxicity and negligible vapor pressure. In addition, gallium-based liquid metals exhibit attractive optical properties which make them highly suitable for a variety of photonics applications. This review summarizes the material properties of gallium-based liquid metals, highlights several effective techniques for fabricating liquid-metal-based structures and devices, and then focuses on the various photonics applications of these liquid metals in different spectral regions, following with a discussion on the challenges and opportunities for future research in this relatively nascent field.

We characterize three commercial resins suitable for three-dimensional two-photon printing of mm volume micro-optical components for visible light - IP-S, IP-n162, and IP-Visio - under different print modes and post-processing conditions. Due to the combination of cured resin absorption and bulk scattering, we find a maximum total printed thickness of 4 mm (or greater) for at least 50% transmittance of red light, up to 2 mm for green light, and large maximum thickness variation for blue light (0.1 to 1 mm).

We report the optical properties of SU-8 in the mid-infrared (mid-IR) region before and after UV treatment. Samples consisted of SU-8 films of thickness ranging from 10 um to 157 um deposited on gold coated silicon substrates and were prepared using spin coating. Mid-IR diffuse reflectance measurements were conducted using Fourier transform infrared spectroscopy. Spectra measurements imply a change in optical properties of SU-8 upon exposure to UV and heat treatment. A gradual change in optical properties is seen after each step of UV treatment and the baking process. Reflectance spectra of thin-films were also observed to be thickness dependent. We calculate the dielectric function of SU-8 in the range 2 um to 15 um using the reflectance spectra of the samples.

A hybrid optical nanostructure of plasmon-coupled SQDs was developed for photonic applications. The coupling distances between the mono-layers of Au nanoparticles with a surface concentration of ~9.18 × 10 nm and CdSe/ZnS SQDs with that of ~3.7 × 10 nm were controlled by PMMA plasma etching. Time-resolved spectroscopy of plasmon-coupled SQDs revealed a strong shortening of the longest lifetime and ~9-fold PL enhancement. Polarization-resolved PL spectroscopy displayed linear polarization and depolarization at near- and far-field plasmon-coupling, respectively. The physical origin of PL enhancement could be attributable to both the large local field enhancement and the fast resonant energy transfer.

We determine the linear birefringence magnitude, i.e. the difference between refractive indexes along the extraordinary and ordinary axes, of artificial uniaxial DNA crystals assembled with the so-called DNA tile approach. Based on the ellipsometric measurements, the birefringence magnitude is between 0.001 and 0.0018 in the visible and near infrared range. Besides being of fundamental interest, the optical properties of DNA crystals are crucial in the design of novel photonic nanostuctures.

We introduce a manufacturable and scalable method for creating tunable wrinkled ferromagnetic-metallic structures to enhance fluorescence signals. Thin layers of nickel (Ni) and gold (Au) were deposited onto a pre-stressed thermoplastic (shrink wrap film) polymer. Heating briefly forced the metal films to buckle when the thermoplastic retracted, resulting in multi-scale composite 'wrinkles'. This is the first demonstration of leveraging the plasmons in such hybrid nanostructures by metal enhanced fluorescence (MEF) in the near-infrared wavelengths. We observed more than three orders of magnitude enhancement in the fluorescence signal of a single molecule of goat anti-mouse immunoglobulin G (IgG) antibody conjugated to fluorescein isothiocyanate, FITC, (FITC-IgG) by two-photon excitation with these structures. These large enhancements in the fluorescence signal at the nanoscale gaps between the composite wrinkles corresponded to shortened lifetimes due to localized surface plasmons. To characterize these structures, we combined fluctuation correlation spectroscopy (FCS), fluorescence lifetime imaging microscopy (FLIM), and two-photon microscopy to spatially and temporally map the hot spots with high resolution.