Upconversion threshold measurement: Upconversion threshold was measured using the photoluminescence set up described above. The laser was set to the highest power required for the measurement and the power was attenuated using ND filters. The UC PL for the samples was measured across orders of magnitude of input power to span the linear and quadratic regimes in the log-log input-output power plots. The integration times were increased to achieve high signal at low input powers.