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We measured fluorescence spectra of visible and infrared electronic transitions at 530 (2H 11/2 → 4I 15/2), 560 (4S 3/2 → 4I 15/2), 670 (4F 9/2 → 4I 15/2), 980 (4I 11/2 → 4I 15/2), 1530 (4I 13/2 → 4I 15/2) and 2700 nm (4I 11/2 → 4I 13/2) of Er3+ in Gd 3 (Al 0.4 Ga 0.6) 5 O 12 garnet single-crystal excited at the wavelength 520 nm (for the 980 nm emission) or 980 nm (for other emissions). From measured fluorescence spectrum, Er3+ emission cross-section spectrum was calibrated for each transition. Absorption cross-section spectra were further derived on the basis of McCumber equation, and compared with those spectra obtained from measured absorption spectrum on the basis of the Beer-Lambert law. The comparison showed that the results given by two methods can be regarded as same within the error. The transient features of visible and infrared emissions were also studied under pulse excitation. The results showed that, while the three IR emissions at 980, 1530 and 2700 nm all follow a mono-exponential function, both green and red 980-nm-upconverted emissions at 560 and 670 nm exhibit non-mono-exponentially decaying feature, and each decaying trace can be fitted by a sum of two exponential functions. The non-mono-exponential feature is related to cross relaxation caused by non-radiative short-distance energy transfer between clustered Er3+ sites. • Cross-section spectra of Er3+ ions in Gd 3 (Al,Ga) 5 O 12 garnet crystal were studied. • Emission cross-section spectra are computed from measured fluorescence spectra. • Absorption cross-section spectra are derived by two ways and results are same. • 980-nm-upconverted visible emissions show non-mono-exponential transient feature. • It is caused by non-radiative energy transfer between clustered Er3+ sites. [ABSTRACT FROM AUTHOR]