Negative Refractive Index in Dielectric Crystals Containing Stoichiometric Rare‐Earth Ions

Abstract

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.

Spectral Multiplexing of Rare‐Earth Emitters in a Co‐Doped Crystalline Membrane

“…[ 8–16 ] In a growth from the melt, this is hard to achieve – in particular for the best‐studied rare‐earth host crystals, such as yttrium orthosilicate (YSO) [ 8,9,11,20 ] and yttrium orthovanadate. [ 10 ] In these and other crystals containing rare‐earth elements, [ 21,22 ] trace impurities of the dopants typically exceed the desired concentration by orders of magnitude. Thus, previous experiments have worked in the far‐out tails of the inhomogeneous distribution, where the spectral density is low.…”