Cooperative emission has been observed in sol-gel derived nano-powders doped with yitterbium ions when excited with the 950 nm emission of a laser diode. Measurements of the sample emission intensity dependence on the pumping power under atmospheric pressure condition were performed to provide possibly an explanation for the cooperative emission mechanism. The emission intensities of each nano-powder increased when the pressure was increased from 0.01 mbar to 1 atm with the observation of the bright blue emission. When the powders were subject to very low pressure at 0.01 mbar, the blue emission intensities decreased and green emission intensities increased. The CIE coordinates (x, y) for YSY1 were found to be (0.187, 0.196) under 1 atm. with a color rendering index (CRI) of 67 under the 950 nm laser diode excitation. (C) 2015 Elsevier Ltd and Techna Group S.r.l. All rights reserved.