This article covers the historical background to the study of the helium atom by optical methods, and some essential results from the classical and quantum mechanical theory of optical dispersion. A simple undergraduate-level experiment, designed for refractivity measurements by cw laser interferometry, is described, and the ingredients of the theoretical calculation of the refractivity are discussed. It is shown that careful experimental work in this area can yield information that provides a valuable test of the accuracy and completeness of the available oscillator strength data.

Liquid microdroplet resonators provide an excellent tool for optical studies due to their innate smoothness and high quality factors, but precise control over their geometries can be difficult. In contrast, three dimensional (3D) printed components are highly customizable but suffer from roughness and pixelation. We present 3D printed structures which leverage the versatility of 3D printing with the smoothness of microdroplets. Our devices enable the reliable creation of microdroplet resonators of varying shapes and sizes in an ambient environment, and our coupling scheme allows for high control over droplet position.

Thin liquid films (TLF) have fundamental and technological importance ranging from the thermodynamics of cell membranes to the safety of light‐water cooled nuclear reactors. The creation of stable water TLFs, however, is very difficult. In this paper, the realization of thin liquid films of water with custom 3D geometries that persist indefinitely in ambient environments is reported. The wetting films are generated using microscale “mounts” fed by microfluidic channels with small feature sizes and large aspect ratios. These devices are fabricated with a custom 3D printer and resin, which are developed to print high resolution microfluidic geometries. By modifying the 3D‐printed polymer to be hydrophilic and taking advantage of well‐known wetting principles and capillary effects, self‐sustaining microscale “water fountains” are constructed that continuously replenish water lost to evaporation while relying on surface tension to stabilize their shape. To the authors' knowledge, this is the first demonstration of stable sub‐micron thin liquid films (TLFs) of pure water on curved 3D geometries.

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