Novel thulium-doped crystal shows potential for ultrashort-pulse operation

A large group of Asian and European researchers has developed and experimented with a novel laser gain material, saying that it has potential for broadband tunable lasing. The new laser crystal, trigonal calcium vanadate, or Tm³⁺:Ca₉La(VO₄)₇, has a laser emission in the 2 μm spectral region. The researchers hail from Chinese Academy of Sciences (Fuzhou, China), the Graduate School of Fujian Normal University (Fuzhou, China), ITMO University (St. Petersburg, Russia), Universitat Rovira i Virgili (URV; Tarragona, Spain), Max Born Institute for Nonlinear Optics and Short Pulse Spectroscopy (Berlin, Germany), Belarusian National Technical University (Minsk, Belarus), and State Key Laboratory of Structural Chemistry (Fuzhou, China).

The group grew the crystals using the Czochralski method (a standard method for growing crystals that results in an end product called a boule—a large, rough cylindrical shape from which crystals can be cut), modeled them within the Judd-Ofelt (J-O) theory, and then measured their absorption, emission, and Raman spectra, luminescence decay, and transition cross-sections. The thulium dopant ions are interspersed in a disordered way throughout the crystal, resulting in structureless absorption and emission bands while still preserving polarization anisotropy. The gain spectra in both polarizations show that broadband tunable laser emission between 1.8 and 2.05 μm can be achieved, and thus ultrashort-pulse operation. Energy is stored in the upper laser level for a relatively long 1.2 ms time, which allows passive Q-switching. Improving fabrication via annealing and purification of the raw materials should boost the luminescence quantum yield. Reference: Z. Zhang et al., Opt. Mater. Express, 484 (Feb. 2017); http://dx.doi.org/10.1364/ome.7.000484.