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We report on a laser system based on difference frequency generation (DFG) to produce tunable, narrow-linewidth (<30pm), and comparatively high-energy mid-IR radiation in the 6.8 µm region. The system exploits a lithium thioindate (LiInS_2) nonlinear crystal and nanosecond pulses generated by single-frequency Nd:YAG and Cr:forsterite lasers at 1064 and 1262 nm, respectively. Two experimental configurations are used: in the first one, single-pass, the mid-IR energy achieved is 205 µJ. Additional increments, up to 540 µJ, are obtained by performing double-pass through the nonlinear crystal. This laser has been developed for high-resolution photon-hungry spectroscopy in the mid-IR.

The United Nations Sustainable Development Goals outline a roadmap towards a more equitable future for humanity. Along with other scientists, physicists have long made valuable contributions to this endeavour.

We have developed a device to investigate finite-size scaling of the thermal expansion coefficient in liquid helium near the lambda transition. Motivated by the need for a range of well known, uniform, sub-micron gaps with constant surface conditions, we have built a variable-gap parallel-plate capacitor that can be adjusted in situ. We measure the gap at three points using laser interferometry. A careful choice of the material and thickness of the reflecting surfaces results in asymmetric fringes, for which both exceptional sensitivity and a high contrast ratio can be achieved simultaneously. Both the gap size and its uniformity are actively controlled using voice coil actuators. This design has a number of additional benefits: the gap can be kept closed until low temperature use, data can be collected for all gaps with no thermal cycling, and no spacers are needed to establish the gap.

Intrinsic nucleation of quantized vortices in Helium II can be studied by means of rotating freely suspended superfluid drops at angular velocities above some critical value. The motivation for doing so is described, as well as recent progress in the electrostatic levitation of Helium II drops charged with positive ions. To date, stable levitation has been achieved for drops of order 100–150 micrometers in diameter, with a surface charge density about a factor of ten smaller than Rayleigh limit, and a diameter a similar factor less than the maximum allowed in normal gravity. We discuss the possibility of rotating these drops via the surface charge density and discuss the advantages of a microgravity environment, including the attainment of significantly larger suspended drops. Recent efforts to find optical seed particles for angular velocity measurements are discussed.

Measurements have been made of the dielectric constant of liquid helium from 1.15 K to 4.9 K. The density and thermal expansion coefficient have been derived from these measurements using the Clausius-Mossotti equation. Comparison is made to modern measurements. Formulae are presented representing the absolute density and thermal expansion coefficient from 0 K to 4.9 K. Difficulties and uncertainties in these results are discussed.

Liquid Helium II drops, of diameter 1 mm or less, are charged with positive helium ions and subsequently levitated by static electric fields. Stable levitation was achieved for drops of order 100–150 micrometers in diameter. The suspended drops could be translated to arbitrary positions within the levitator using additional superimposed DC electric fields, and also could be made to oscillate stably about their average positions by means of an applied time-varying electric field. A weak corona discharge was used to produce the necessary ions for levitation. A novel superfluid film flow device, developed for the controlled deployment of large charged drops, is described. Also discussed is an adjustable electric fountain that requires only a field emission tip operating at modest potentials, and works in both Helium I and Helium II.

We present a progress report on our investigation of finite size effects on the superfluid transition. In our experiment, the helium is confined between the parallel plates of a carefully constructed capacitor. We measure the dielectric constant as a function of temperature, from which we can calculate the thermal expansivity. We discuss some of the following important features of our experiment: The capacitor, built in collaboration with the Chex experiment, gives us a simple, well defined planar geometry, extremely flat surfaces, and a sub-micron gap. A low temperature audio-frequency preamp improves the sensitivity of the capacitance measurement. A high resolution paramagnetic salt thermometer, also from Chex, enables us to work extremely close to the lambda point. We also present a novel design for a simple low temperature burst disc that is part of our apparatus.

Liquid Helium II drops, of diameter 1 mm or less, are charged with positive helium ions and subsequently levitated by static electric fields. Stable levitation was achieved for drops of order 100–150 micrometers in diameter. The suspended drops could be translated to arbitrary positions within the levitator using additional superimposed DC electric fields, and also could be made to oscillate stably about their average positions by means of an applied time-varying electric field. A weak corona discharge was used to produce the necessary ions for levitation. A novel superfluid film flow device, developed for the controlled deployment of large charged drops, is described. Also discussed is an adjustable electric fountain that requires only a field emission tip operating at modest potentials, and works in both Helium I and Helium II.