Mid-infrared (MIR) lasers operating within a spectral region of 2.7-3.0 μm have a significant importance in applications such as laser surgery and remote sensing. Trivalent erbium (Er³⁺) ions are among the most efficient activators for MIR lasers, leveraging the ⁴I_11/2→⁴I_13/2 transition. Nevertheless, the realization of high-power and efficient laser output is often hindered due to the self-terminating effect and substantial thermal loading in Er³⁺-doped gain medium. This study was to investigate the high-performance Er³⁺-doped Gd₃Ga₅O₁₂ (GGG) and Lu₃Ga₅O₁₂ (LuGG) laser crystals with the low phonon energy and high thermal conductivity.

High-quality single crystals of Er: GGG, Er/Pr: GGG, and Er: LuGG were grown by the Czochralski method in an optimized atmosphere (i.e., 98% N₂ + 2% O₂) with iridium crucibles and (111)-oriented seeds. Raw materials underwent multi-step sintering to guarantee the phase purity. Comprehensive spectroscopic analysis was conducted on the polished samples. The room-temperature absorption spectra were determined by a model Lambda 900 spectrophotometer. The fluorescence spectra at the mid-infrared (MIR: 2500-3000 nm), near-infrared (NIR: 950-1750 nm) and upconversion (UC: 500-700 nm) bands were measured under 965/967 nm excitation by an optical parametric oscillator (OPO) or a xenon lamp using models FLS920 and FSP920 fluorescence spectrometers. The fluorescence decay curves for the ⁴I_11/2 and ⁴I_13/2 energy levels were recorded to determine the fluorescence lifetime. The laser performance was assessed under two pumping schemes (i.e., a xenon lamp and a 965 nm fiber-coupled laser diode (LD)). The crystals were fabricated into laser gain media of various sizes and doping profiles, including a bonded configuration (undoped GGG/56% Er: GGG) to alleviate thermal lensing. The continuous-wave (CW) performance was evaluated via input-output power curves, slope efficiency, laser threshold, and beam quality factor (M²). The passive Q-switching was implemented with two-dimensional materials (i.e., graphene and a Bi₂Te₃/graphene heterostructure) as saturable absorbers. The main pulsed laser metrics (i.e., pulse train profile, average output power, pulse width, repetition rate, and peak power) were systematically determined.

The as-grown crystals display a high optical quality with intense and broad absorption bands centered at 965 nm. The 30% Er, 0.5% Pr: GGG composition has a notably high absorption cross-section of 5.13×10^-21 cm² and a full width at half maximum (FWHM) of 15 nm, indicating a superior spectral overlap with commercial 980 nm InGaAs laser diodes and enabling efficient pump coupling. The incorporation of Pr³⁺ ions can enhance the MIR fluorescence emission based on a cross-relaxation energy transfer mechanism, i.e., ⁴I_13/2 (Er³⁺) + ³H₄ (Pr³⁺) → ⁴I_15/2 (Er³⁺) + ³F₄ (Pr³⁺). This process effectively depopulates the lower laser level (⁴I_13/2), leading to a reversal of the intrinsic lifetime ratio between metastable states. Specifically, a ratio of τ(⁴I_11/2)/τ(⁴I_13/2) of 2.348 appears in the sample 30% Er, 0.5% Pr:GGG, which is critical for overcoming the self-termination barrier in Er³⁺ -based~3 μm lasers.

Under 965 nm LD pumping, a CW laser emission at 2.75 μm is attained. The 56% Er: GGG crystal has a maximum output power of 325 mW with a slope efficiency of 16.15%. The bonded GGG/56% Er: GGG structure improves the thermal management, increasing the maximum power to 453 mW and the slope efficiency to 17.1%, while also improving beam quality (i.e., M² reduces from 1.68 to 1.48). The Er/Pr: GGG crystal (11% Er, 0.4% Pr) shows a superior thermal performance, reaching 372 mW output and a higher damage threshold, due to the lower Er³⁺ concentration and efficient energy transfer.

The passive Q-switching with graphene and Bi₂Te₃/graphene absorbers produce impressive pulsed outcomes. The graphene-based Q-switched bonded GGG/Er, Pr: GGG/GGG crystal has a maximum average power of 186 mW, a pulse width of 360 ns, and a repetition rate of 120.5 kHz, corresponding to a pulse energy of 1.54 μJ and a peak power exceeding 4.28 W. The Bi₂Te₃/graphene Q-switched Er: LuGG laser can deliver a higher average power of 274 mW with a shorter pulse width of 340 ns at 135 kHz, yielding a pulse energy of 2.03 μJ and a peak power of > 5.97 W. These findings affirm a potential of these materials in high-performance pulsed MIR lasers.

This work established Er³⁺-doped GGG and LuGG crystals as highly promising gain media for efficient ~2.75 μm mid-infrared lasers. The strategies of Pr³⁺ co-doping and crystal bonding proved effective in mitigating the self-terminating effect and thermal load, leading to the improvement of substantial performance. The effective operation in both continuous-wave and passively Q-switched regimes could underscore the applicability of these crystals in practical high-power MIR laser systems.