Resonance in Er³⁺ upconversion excitation

Upconversion (UC) processes are discussed on the examples of excited state absorption (ESA) in LaCl₃: 1% Er³⁺ for 980 nm excitation and energy transfer upconversion (ETU) in β-NaYF₄: 20% Er³⁺ for 1523 nm excitation. The crucial importance of resonance is demonstrated for one- and two-color excitations in LaCl₃:Er. For ESA the excitation spectra are convolutions of the individual excitation steps and the UC luminescence intensity shows a linear dependence on the excitation power for each step. For one-color excited ESA and ETU in the low power limit, the exponent of the power dependence is equal to the number of excitation steps. The exponent is reduced due to saturation effects for high power excitation. The ETU excitation spectra resemble the ground state absorption as shown for β-NaYF₄:Er. Upon broad-band excitation by sub-Si-band-gap infrared light β-NaYF₄:Er shows UC luminescence mainly from the ⁴I_11/2 state around 980 nm which efficiently excites a Si solar cell. This Er UC luminescence is two orders of magnitude more intense than that of other rare earth ions under the same experimental conditions. - Highlights: • Excitation spectra and power dependences of ESA and ETU UC processes are discussed. • Resonance is crucial for one- and two-color laser-excited ESA upconversion spectra. • Broad-band ⁴I_15/2→⁴I_13/2 β -NaYF₄: 20% Er excitation results in very efficient ETU UC.