Good Quantum Defects Make Good Lasers

Breck Hitz

The quantum defect -- the energy difference between a pump photon and a laser photon -- is one of the most significant parameters in selecting a potential laser material. Another one for a solid-state laser is the material's thermal ruggedness, or the amount of power it can absorb without physically rupturing. But there is a trade-off between the two: The larger a laser material's quantum defect, the more waste heat it must absorb at a given pump level, and, hence, the greater its thermal ruggedness must be.

Now researchers at Ecole Nationale Supérieure de Chimie in Paris have suggested a figure of merit that takes both factors into account. They have applied it to several laser materials and have explored a new material, Yb:CaGdAlO₄, with a very low (~3.5 percent) quantum defect.

Their figure of merit -- which they call the laser power resistance parameter, or R_P -- is defined as R_P∝^κ2/∝η, where κ is the thermal conductivity, ∝ is the expansion coefficient and η is the quantum defect (η = 1 [λ_pump/λ_laser]). Applying this to several common laser materials, they have concluded that the new host material, CaGdAlO₄, is a good competitor for YAG for use in high-power applications.

Figure 1. Yb:CaGdAlO₄ is a relatively easy crystal to grow by the Czochralski technique. The Yb dopant substitutes for either the Gd³⁺ or the Ca²⁺ in the crystal lattice. Images ©OSA.

In experiments with CaGdAlO₄, the researchers grew a high-quality boule of 2-percent-atomic Yb:CaGdAlO₄ by the Czochralski technique (Figure 1), from which they cut several laser-quality crystals. Pumping one of the crystals with a Ti:sapphire laser at 979 nm, they observed what they believe is the first laser emission from this crystal. The output was centered at 1016 nm and exceeded 0.5 W at 2 W of pump power (Figure 2). For the wavelengths in question, the quantum defect of the laser is 3.5 percent, significantly lower than in many other lasers.

Figure 2. Pumped with 979-nm radiation from a Ti:sapphire laser, the Yb:CaGdAlO₄ laser generated more than 0.5 W at 1016 nm. The quantum defect in this case was 3.5 percent.

The researchers measured a broad emission peak for Yb:CaGdAlO₄, indicating that laser action at shorter wavelengths -- and, hence, with even smaller quantum defects -- might be possible. Very recently, they obtained laser action at 994 nm, corresponding to a quantum defect of only 1.4 percent. They are enhancing that performance in the laboratory and attempting to use the broad bandwidth to generate ultrafast pulses.