A highly stable and fully tunable open microcavity platform at cryogenic temperatures

Open-access microcavities are a powerful tool to enhance light–matter interactions for solid-state quantum and nanosystems and are key to advance applications in quantum technologies. For this purpose, the cavities should simultaneously meet two conflicting requirements—full tunability to cope with spatial and spectral inhomogeneities of a material and highest stability under operation in a cryogenic environment to maintain resonance conditions. To tackle this challenge, we have developed a fully tunable, open-access, fiber-based Fabry–Pérot microcavity platform that can be operated under increased noise levels in a closed-cycle cryostat. It comprises custom-designed monolithic micro- and nanopositioning elements with up to mm-scale travel range that achieve a passive cavity length stability at low temperature of only 15 pm rms in a closed-cycle cryostat and 5 pm in a more quiet flow cryostat. This can be further improved by active stabilization, and even higher stability is obtained under direct mechanical contact between the cavity mirrors, yielding 0.8 pm rms during the quiet phase of the closed-cycle cryocooler. The platform provides the operation of cryogenic cavities with high finesse and small mode volume for strong enhancement of light–matter interactions, opening up novel possibilities for experiments with a great variety of quantum and nanomaterials

Modular Coils with Low Hydrogen Content Especially for MRI of Dry Solids.

Recent advances have enabled fast magnetic resonance imaging (MRI) of solid materials. This development has opened up new applications for MRI, but, at the same time, uncovered new challenges. Previously, MRI-invisible materials like the housing of MRI detection coils are now readily depicted and either cause artifacts or lead to a decreased image resolution. In this contribution, we present versatile, multi-nuclear single and dual-tune MRI coils that stand out by (1) a low hydrogen content for high-resolution MRI of dry solids without artifacts; (2) a modular approach with exchangeable inductors of variable volumes to optimally enclose the given object; (3) low cost and low manufacturing effort that is associated with the modular approach; (4) accurate sample placement in the coil outside of the bore, and (5) a wide, single- or dual-tune frequency range that covers several nuclei and enables multinuclear MRI without moving the sample.The inductors of the coils were constructed from self-supporting copper sheets to avoid all plastic materials within or around the resonator. The components that were mounted at a distance from the inductor, including the circuit board, coaxial cable and holder were manufactured from polytetrafluoroethylene.Residual hydrogen signal was sufficiently well suppressed to allow 1H-MRI of dry solids with a minimum field of view that was smaller than the sensitive volume of the coil. The SNR was found to be comparable but somewhat lower with respect to commercial, proton-rich quadrature coils, and higher with respect to a linearly-polarized commercial coil. The potential of the setup presented was exemplified by 1H/23Na high-resolution zero echo time (ZTE) MRI of a model solution and a dried human molar at 9.4 T. A full 3D image dataset of the tooth was obtained, rich in contrast and similar to the resolution of standard cone-beam computed tomography

Representative photograph of several loop-gap inductors that were constructed (LG_1–6). Note the ledges used for connection. Dimensions are provided in Table 2.

<p>Representative photograph of several loop-gap inductors that were constructed (LG_1–6). Note the ledges used for connection. Dimensions are provided in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0139763#pone.0139763.t002" target="_blank">Table 2</a>.</p

3D maximum-intensity-projection ZTE MRI and approximate coil positions (left) acquired at 9.4 T and photograph of coil assembled from CB₂ and LG₂ (right), either with a conventional BNC connector and coaxial cable (top), or a custom-made PTFE cable that was soldered to the circuit board (bottom).

<p>Note that the signal originating from the BNC connector was removed using the custom build cable. In turn, signal originating from the variable capacitors appeared close to the noise level (FOV = 20 cm).</p

¹H-MR signal height of a tooth acquired with unlocalized spectroscopy as a function of pulse power using a constant flip angle (α ≈ 6°) at 9.4 T with CB₂ and LG₁.

<p>Note that the MR signal decreased for powers exceeding 22 W.</p

¹H- and ²³Na-ZTE MRI of an aqueous model solution containing 100 g NaCl / l acquired with LG₄ and CB₃ at 9.4 T.

<p>¹H- and ²³Na-ZTE MRI of an aqueous model solution containing 100 g NaCl / l acquired with LG₄ and CB₃ at 9.4 T.</p

Signal, noise, signal-to-noise-ratio (SNR) and reference pulse power (<i>P</i>_W⁹⁰) of a 1 ms, 90°-pulse for the coils composed of a circuit board (CB) and loop-gap (LG) inductor, as well as for commercial coils that were polarized in quadrature (QR₁, QR₂) and linear (LR).

<p>Signal, noise, signal-to-noise-ratio (SNR) and reference pulse power (<i>P</i>_W⁹⁰) of a 1 ms, 90°-pulse for the coils composed of a circuit board (CB) and loop-gap (LG) inductor, as well as for commercial coils that were polarized in quadrature (QR₁, QR₂) and linear (LR).</p

MRI of solids.

<p>3D rendering of a skull found in a late roman settlement, estimated 300–400 A.D., and head coil acquired with an UTE sequence at 1.5 T (left). 3D maximum intensity projection of a ¹H-ZTE image of a quadrature mouse coil at 9.4 T (center, QR₂) and corresponding photograph (right).</p