ABSTRACT
LixAg1-xGaSe2 is a new series of solid solution crystals that has a large nonlinear optical (NLO) coefficient and laser-induced damage threshold (LIDT). It has great application prospects in mid-infrared laser frequency conversion. In this work, LixAg1-xGaSe2 (x = 0, 0.2, 0.4, 0.5, 0.6, 0.8, and 1) crystals (Φ 16 mm × 40 mm) were grown by the improved Bridgeman method in a four-zone furnace. It is found that the LixAg1-xGaSe2 (x = 0.2-0.8) crystals keep the same tetragonal symmetry with AgGaSe2 and the melting and solidification temperature increase with the Li content. Because the as-grown crystals are almost opaque in the visible-NIR range, an annealing experiment is necessary. After annealing, the transmittance is improved significantly, which can meet the application requirements. The band gap is changed by annealing atmosphere; for instance, the band gap of Li0.6Ag0.4GaSe2 annealed in a LiGaSe2 powder atmosphere increases from 2.35 to 2.56 eV, while the band gap of LiGaSe2 annealed in vacuum decreases from 3.39 to 3.01 eV. Finally, the LixAg1-xGaSe2 shows an extreme SHG response, especially Li0.8Ag0.2GaSe2, which has about five times that of LiGaSe2, proving the promising NLO properties.
ABSTRACT
The new quaternary single crystals AgGaGenSe2(n+1) (n = 1.5, 1.75, 2, 3, 4, 5, and 9) have high nonlinear optical property and can be used for mid-IR laser applications in high power. However, only AgGaGe3Se8 and AgGaGe5Se12 have been grown on a large scale and studied in detail. In this work, the AgGaGenSe2(n+1) (n = 1.5, 1.75, 2, 3, 4, 5, and 9) crystals (Φ 20 mm × 40 mm and Φ 40 mm × 100 mm) were grown by the modified Bridgman method. The crystal structure was studied by X-ray diffraction and the Rietveld refinement method. The composition and morphology were characterized by scanning electron microscopy and metallurgical microscopy. The chemical state and vibration modes of surface elements were characterized by X-ray photoelectron spectroscopy and Raman spectra, and the electrical property was investigated by the Hall effect measurement, which indicates that all the single AgGaGenSe2(n+1) crystals are n-type semiconductors. The transmittance of all as-grown AgGaGenSe2(n+1) crystal wafers exceed 65% in the transparent range, and the band gap increases from 2.05 eV for AgGaGe1.5Se5 to 2.14 eV for AgGaGe9Se20. Besides, after being annealed in two different conditions, the wafers show different changes. We discovered a special decomposition phenomenon during the annealing process and found the more appropriate annealing method at last. In addition, the absorption peaks at 4.2, 10, and 14.9 µm of wafers have been nearly eliminated, and the quality of most crystals has been improved.
