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Mid‐infrared (MIR) lasers generated by nonlinear optical (NLO) crystals have aroused extensive attention in various military and civilian applications. Among the MIR NLO crystal family, LiInSe2 (LISe) is a promising candidate in the generation of MIR lasers due to its wide transmission range, large bandgap, and high laser‐induced damage threshold. However, the intrinsic defects in LISe induce optical losses, which will induce laser damage and hinder the performance of MIR laser generation. In this work, the mechanism of optical absorption induced by defects is revealed in LISe through transient absorption (TA) spectroscopy. It is verified that the deep‐level defects in LISe give rise to the extra optical absorption, which is owing to the excited state absorption (ESA) and photo‐induced absorption (PIA). Accordingly, for improving the optical properties of LISe crystals, Li2Se vapor‐mediated annealing to reduce defects concentration is rationally designed. The results confirmed that the optical properties of annealed LISe are improved due to the decrease of defects concentration. Ultimately, the performance of MIR laser generation is successfully enhanced by using the annealed LISe crystal. This work provides deep insights into the defect‐induced absorption losses in LISe crystals and promotes the application of LISe crystals in MIR laser generation. [ABSTRACT FROM AUTHOR]

Abstract LiInSe2 crystals are promising semiconductor materials for neutron detectors due to the large neutron capture cross-sectional area of the specific isotopes (6Li) and high charge transport properties. However, the optoelectronic performance fails to reach the expected level due to the difficulty of controlling the crystal defects. Herein, we modulate the stoichiometric ratio to control the type of defects in single LiInSe2 crystals grown by the vertical Bridgman method. The UV‒vis–NIR transmission results indicate that the band gap of the yellow colored Li1.01In1Se2 sample is close to ~ 2.83 eV at room temperature, and this value is consistent with the theoretical band gap of LiInSe2 (~ 2.86 eV). Photoluminescence (PL) spectroscopy was used to analyze the defect concentration. The results indicate that the defect types in the yellow Li1.01In1Se2 single crystal are VSe + and LiIn 2−; these result from the introduction of excess Li and the suppression of the adverse defects in InLi 2+ and VLi −. These results demonstrate a feasible route for obtaining high-quality yellow 6LiInSe2 crystals and promote the application of 6LiInSe2 neutron detectors.

Optical quality cm-sized LiInSe2 crystals were grown using the Bridgman–Stockbarger method, starting from pure element reagents, under the conditions of a low temperature gradient of 5–6 degrees/cm and a slight melt overheating. The phase purity of the grown crystal was verified by the powder XRD analysis. The thermophysical characteristics of LiInSe2 were determined by the XRD measurements in the temperature range of 303–703 K and strong anisotropy of the thermal expansion coefficients was established. The following values of thermal expansion coefficients were determined in LiInSe2: αa = 8.1 (1), αb = 16.1 (2) and αc = 5.64 (6) MK−1. The electronic structure of LiInSe2 was measured by X-ray photoelectron spectroscopy. The band structure of LiInSe2 was calculated by ab initio methods. [ABSTRACT FROM AUTHOR]

Optical quality cm-sized LiInSe2 crystals were grown using the Bridgman–Stockbarger method, starting from pure element reagents, under the conditions of a low temperature gradient of 5–6 degrees/cm and a slight melt overheating. The phase purity of the grown crystal was verified by the powder XRD analysis. The thermophysical characteristics of LiInSe2 were determined by the XRD measurements in the temperature range of 303–703 K and strong anisotropy of the thermal expansion coefficients was established. The following values of thermal expansion coefficients were determined in LiInSe2: αa = 8.1 (1), αb = 16.1 (2) and αc = 5.64 (6) MK−1. The electronic structure of LiInSe2 was measured by X-ray photoelectron spectroscopy. The band structure of LiInSe2 was calculated by ab initio methods.

• High-quality LiInSe 2 crystals (φ26 mm) were obtained by optimized Bridgman method. • Defects of In Li , V Li and V Se in LiInSe 2 are verified by spectroscopy. • The defect concentration was reduced greatly by annealing under Li 2 Se vapor. Mid-infrared (MIR) lasers based on nonlinear frequency conversion by nonlinear optical (NLO) crystals are urgently desired for various military and civilian applications. LiInSe 2 (LISe) crystal has aroused intense attention due to its remarkable optical properties. However, the presence of point defects in LISe increases absorption loss and degrades the laser damage threshold (LDT), which limits the practical application in high-power MIR lasers. Therefore, it is essential to reveal the defect types and their formation mechanism in LISe, which is beneficial to design a rational strategy to improve the crystal quality. Here, large-sized LISe crystals with a diameter of 26 mm were successfully grown by the optimized Bridgman method using oriented seed crystal. Furthermore, three types of defects, V Se , V Li , and In Li in the as-grown LISe crystals were verified by photoluminescence (PL) spectra. Therefore, we designed the thermal annealing process under Li 2 Se vapor to compensate for the losses of Li and Se elements and reduce the defect concentration in LISe. Moreover, the evolution of defects in LISe crystals was determined by X-ray photoelectron spectroscopy (XPS), PL and ultrafast transient absorption spectra (TAS). All the experimental results confirm that the defect concentration in LISe greatly decreases and the crystal quality was obviously improved after annealing under Li 2 Se vapor. More importantly, this work provides a general strategy to reveal the defects in crystals and to improve the crystal quality of MIR chalcogenides for practical applications. [ABSTRACT FROM AUTHOR]