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

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: F-3(4)-->F-3(2) (from 1040 nm to 1096 nm), F-3(4)-->F-3(3) (from 1111 nm to 1169 nm), and H-3(4)-->(1)G(4) (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 F-3(4) level is much longer (15.6 ms) than the lifetime of the H-3(4) level (1.56 ms). In order to observe the H-3(4)-->(1)G(4) ESA band, pump modulation dependent measurements were performed, where the modulation frequency of the pump selects the ESA signal to be measured.