The smallest quantum defect for an optically pumped laser crystal could be demonstrated at room temperature, using a 125 µm-thin platelet of KYb(WO 4 ) 2 , a stoichiometric Yb laser material. While pumping at 1025 nm, lasing occurred at 1042 nm which corresponds to a quantum defect of only 1.6%.
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Klopp et al. (2003) studied this question.