Pump modulation frequency resolved excited state absorption spectra in Tm³⁺ doped YLF

Abstract

Excited state absorption (ESA) in a 1% doped Tm:YLF crystal in the range from 1000 nm to 1300 nm is reported. In this spectral range, three ESA bands are expected: ³F₄→³F₂ (from 1040 nm to 1096 nm), ³F₄→³F₃ (from 1111 nm to 1169 nm), and ³H₄→¹G₄ (from 1118 nm to 1240 nm). In a conventional continuous wave ESA setup only the first two bands are observed because the lifetime of the ³F₄ level is much longer (15.6 ms) than the lifetime of the ³H₄ level (1.56 ms). In order to observe the ³H₄→¹G₄ ESA band, pump modulation dependent measurements were performed, where the modulation frequency of the pump selects the ESA signal to be measured.