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Double‐negative refractive index materials have attracted sustained experimental and theoretical interest because they can display a range of surprising optical phenomena, including negative Doppler shifts and perfect lensing. Double‐negative indexes have been achieved experimentally in engineered metamaterials; however, these materials become increasingly challenging to fabricate at shorter wavelengths, and at optical wavelengths only 2D negative index materials have been achieved. Here, it is shown that a double‐negative index can occur in a natural material, near narrow optical transitions of dielectric crystals stoichiometric in a rare‐earth ion. Optical measurements of two candidate materials, the magnetically‐ordered erbium crystals, ErCl3 ·6H2O and 7LiErF4, which have ultra‐narrow optical linewidths of 3 GHz and 250 MHz, respectively, in the telecom band are presented. It is shown that the spectral density of 7LiErF4 is sufficient to achieve a negative index at 1530 nm. This material can enable the exploration of negative refractive index effects at optical wavelengths in a truly 3D, natural medium. [ABSTRACT FROM AUTHOR]

We propose a protocol to preserve the spin state of rare-earth ions when they are optically cycled. This technique uses large magnetic fields to increase the probability of an optically excited ion returning to its initial spin state. This Zeeman enhanced cyclicity is shown to be applicable to non-Kramers ions in various crystals irrespective of the site symmetry. The specific example of Pr3+:Y2SiO5 is investigated to demonstrate that the protocol can create closed optical transitions even where the point group symmetry of the site is C1. In this example, the predicted cyclicity exceeds 104. This high level of cyclicity extends the usefulness of rare-earth ion crystals for applications in quantum and classical information processing. We explore the use of this technique to enable single-ion, spin-state optical readout and the creation of ensemble-based spectral features that are robust against optical cycling. [ABSTRACT FROM AUTHOR]