Upconversion photoluminescence (PL) from graphene quantum dots (GQDs) excited by a xenon lamp is artificial. It is essentially excited by the second-order diffraction light of wavelength λ/2 co-existing in the red light. Real upconversion PL from GQDs is observed under excitation with a femtosecond pulsed laser, implying that coherent photons with high enough power density can be upconverted into blue light via GQDs.
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Gan et al. (2013) studied this question.
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