Key Takeaways & Executive Findings
- •• Large-sized Co2+-doped ZnGa2O4 single crystals (volume ~20 cm3) were successfully grown using the vertical gradient freeze (VGF) method, exhibiting high crystalline quality with a rocking curve FWHM of only 58 arcsec. • The Co2+ ions occupy tetrahedral Zn2+ sites in the spinel lattice, leading to strong absorption bands in the visible (550–670 nm) and near-infrared (1100–1700 nm) regions, attributed to 4A2(4F) → 4T1(4P) and 4A2(4F) → 4T1(4F) transitions, respectively. • The optical band gap of Co2+-doped ZnGa2O4 was measured to be 4.44 eV, confirming its ultra-wide bandgap semiconductor nature. • The ground state absorption cross section at the near-infrared wavelength was determined to be 3.07 × 10−19 cm2, which is comparatively large, indicating ZnGa2O4 as a promising saturable absorber for 1.5 µm passive Q-switched solid-state lasers.
Abstract
The transition of cobalt ions located at tetrahedral sites will produce strong absorption in the visible and near-infrared regions, and is expected to work in a passively Q-switched solid-state laser at the eye-safe wavelength of 1.5 µm. In this study, Co2+ ions were introduced into the wide bandgap semiconductor material ZnGa2O4, and large-sized and high-quality Co2+-doped ZnGa2O4 crystals with a volume of about 20 cm3 were grown using the vertical gradient freeze (VGF) method. Crystal structure and optical properties were analyzed using X-ray powder diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and absorption spectroscopy. XRD results show that the Co2+-doped ZnGa2O4 crystal has a pure spinel phase without impurity phases and the rocking curve full width at half maximum (FWHM) is only 58 arcsec. The concentration of Co2+ in Co2+-doped ZnGa2O4 crystals was determined to be 0.2 at.% by the energy dispersive X-ray spectroscopy. The optical band gap of Co2+-doped ZnGa2O4 crystals is 4.44 eV. The optical absorption spectrum for Co2+-doped ZnGa2O4 reveals a prominent visible absorption band within 550−670 nm and a wide absorption band spanning from 1100 to 1700 nm. This suggests that the Co2+ ions have substituted the Zn2+ ions, which are typically tetrahedrally coordinated, within the lattice structure of ZnGa2O4. The visible region's absorption peak and the near-infrared broad absorption band are ascribed to the 4A2(4F) → 4T1(4P) and 4A2(4F) →4T1(4F) transitions, respectively. The optimal ground state absorption cross section was determined to be 3.07 × 10−19 cm2 in ZnGa2O4, a value that is comparatively large within the context of similar materials. This finding suggests that ZnGa2O4 is a promising candidate for use in near-infrared passive Q-switched solid-state lasers.
1. Introduction
In recent years, the 1.5 µm pulse laser has gained significant interest for its eye safety, atmospheric transmission efficiency, and ability to be transmitted through optical fibers. The saturable absorber-based passive Q-switching technology is an efficient method for achieving 1.5 µm pulse lasers. Saturable absorbers (SA) are key optical materials used in laser cavities and can be used to develop compact, low-cost short-pulse lasers for various applications.
In recent years, many saturable absorbers have been developed, including organic dyes, two-dimensional materials, and transition metal ion-doped saturable absorbers. Among the many saturable absorbers, transition metal ion-doped crystals have attracted considerable attention due to their high thermal stability, broad absorption bands, and tunable nonlinear optical properties. In particular, Co2+ ions in tetrahedral coordination exhibit strong absorption in the visible and near-infrared regions, making them suitable for passive Q-switching at the eye-safe wavelength of 1.5 µm. However, the performance of Co2+-doped saturable absorbers is highly dependent on the host material. ZnGa2O4, a wide bandgap semiconductor with a spinel structure, offers a promising host lattice for Co2+ ions due to its tetrahedral Zn2+ sites that can be readily substituted by Co2+ ions. In this study, we report the growth of large-sized Co2+-doped ZnGa2O4 single crystals using the vertical gradient freeze method and investigate their structural and optical properties, demonstrating their potential for near-infrared passive Q-switched solid-state lasers.
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Zhengyuan Li, Jiaqi Wei, Yiyuan Liu, Huihui Li, Yang Li, Zhitai Jia, Xutang Tao, Wenxiang Mu (2025). Growth and optical properties of large-sized Co2+: ZnGa2O4 single crystal. SinoTechIntel Verified Research. https://doi.org/10.1088/1674-4926/25010017
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Frequently Asked Questions
What is the significance of Co2+-doped ZnGa2O4 single crystals?
Co2+-doped ZnGa2O4 single crystals exhibit strong absorption in the visible and near-infrared regions due to Co2+ ions at tetrahedral sites, making them promising saturable absorbers for passively Q-switched solid-state lasers operating at the eye-safe wavelength of 1.5 µm.
How were the Co2+-doped ZnGa2O4 crystals grown?
Large-sized Co2+-doped ZnGa2O4 crystals with a volume of about 20 cm3 were grown using the vertical gradient freeze (VGF) method, which allows for high-quality bulk crystal growth.
What are the key optical properties of Co2+-doped ZnGa2O4?
The crystals have an optical band gap of 4.44 eV and exhibit absorption bands at 550–670 nm (visible) and 1100–1700 nm (near-infrared), corresponding to 4A2(4F) → 4T1(4P) and 4A2(4F) → 4T1(4F) transitions of Co2+ ions. The ground state absorption cross section at near-infrared is 3.07 × 10−19 cm2.
Why is ZnGa2O4 considered a promising host for Co2+ ions?
ZnGa2O4 has a spinel structure with tetrahedrally coordinated Zn2+ sites that can be readily substituted by Co2+ ions, providing a suitable environment for strong absorption in the desired wavelength regions.
What applications could benefit from Co2+-doped ZnGa2O4 crystals?
These crystals are particularly suited for use as saturable absorbers in passively Q-switched solid-state lasers emitting at 1.5 µm, which are important for eye-safe laser applications in ranging, remote sensing, and optical communications.
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