A recent demonstration by a team at Huazhong University of Science and Technology (HUST) spotlights the possibility of stable multiband lasing by rare-earth elements. In the work, the research team used polymer-assisted thermal doping to fabricate rare-earth-doped microcavities with ultrahigh intrinsic Q factors exceeding 108. The doping process did not introduce any obvious ion clustering or scattering loss. The ultrahigh intrinsic Q factor makes the process a natural platform for achieving lasing and further nonlinear phenomena that require low power.

Aside from the advantages for laser applications, the ultrahigh-Q doped microcavity may also offer a platform for ultrahigh-precision sensing, optical memories, and investigation of cavity-matter-light interactions.

Microlasers with multiple lasing bands are crucial components in various applications, such as full-color display, optical communications, and computing. Rare-earth elements offer abundant long-lived intermediate energy levels and intraconfigurational transitions necessary for emissions over a wide range of lightwave bands.

It is possible to generate deep-ultraviolet (UV) to mid-infrared light by pumping photons through downshifting, to lower frequency — and upconversion, to increase energy. Though upconversion offers advantages that include better penetration depth and less ionization damage, it is generally more difficult than downshifting. Combining downshifting with upconversion can expand the emission wavelength range for the greatest potential.

Since using rare-earth elements for upconversion eliminates the need for rigorous phase-matching conditions or high pump density, researchers have asked if it could be possible to construct a multiband laser by doping rare-earth elements into an ultrahigh-Q microcavity without degrading its intrinsic Q factor.

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HUST researchers demonstrated simultaneous ultraviolet, visible, and near-infrared continuous-wave lasing at room temperature. The work supports ultrahigh-precision sensing, optical memories, and investigation of cavity-matter-light interactions. Courtesy of B. Jiang et al.

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Research for high-order upconversion lasers typically uses a pulsed laser pump in a cryogenic environment, which aims to reduce thermal damage for gain materials and resonant cavities.

In the recent demonstration, the HUST team achieved UV and violet continuous-wave (CW) upconversion lasing from rare-earth elements at room temperature.

The team doped a microcavity with erbium and ytterbium and pumped it with a CW 975-nm laser. The resulting laser spanned a wavelength range of about 1170 nm, covering the UV, visible, and near-infrared (NIR) bands. The team estimated that all the lasing thresholds were at the submilliwatt level. The microlasers exhibited good intensity stability over 190 min, which makes them suitable for practical applications.

Additionally, other rare-earth elements — such as thulium, holmium, and neodymium — may allow for flexible pump schemes and abundant lasing wavelengths.

The research was published in Advanced Photonics (www.doi.org/10.1117/1.AP.4.4.046003).