Determination of excited state absorption cross sections for spectrally overlapping transitions via phase sensitive detection

In this paper we show that in an excited state absorption (ESA) setup the phase sensitive detection features of the lock-in amplifier can be exploited for a complete discrimination of even spectrally overlapping transitions. In connection with Judd Ofelt calculations it allows the determination of the excited state absorption cross sections. The method of phase sensitive detection in the double modulation technique was described in detail for the example of excited state absorption measurements in Ho³⁺,Yb³⁺:YLiF₄ in the 720 nm - 860 nm spectral range.