  One of the fundamental difficulties in implementing organic dyes in random
lasers is irreversible photodegradation of the dye molecules, leading to loss
of performance and the need to replace the dye. We report the observation of
self-healing after photodegradation in a Rhodamine 6G dye and nanoparticle
doped polyurethane random laser. During irradiation we observe two distinct
temporal regions in which the random lasing (RL) emission first increases in
intensity and redshifts, followed by further redshifting, spectral broadening,
and decay in the emission intensity. After irradiation the emission intensity
is found to recover back to its peak value, while still being broadened and
redshifted, which leads to the result of an enhancement of the spectrally
integrated intensity. We also perform IR-VIS absorbance measurements and find
that the results suggest that during irradiation some of the dye molecules form
dimers and trimers and that the polymer host is irreversibly damaged by
photooxidation and Norrish type I photocleavage.
