Spontaneous and stimulated emission from a ladder-type conjugated polymer

Fluorescence from films as well as from a solid solution of a ladder-type poly(para-phenylene) has been studied as a function of pump energy upon excitation at variable spectral positions within the inhomogeneously broadened absorption band. Photoinduced transmission changes following excitation with 200 fs laser pulses at $25000\hspace{0.5em}{\mathrm{cm}}^{-1}$ have been recorded at variable delay time. The latter testify on the occurrence of optical gain in the spectral range of spontaneous emission and of absorption bleaching. The initial decay time of the gain is on the order of 1 ps. Above a certain pump power a $180\hspace{0.5em}{\mathrm{cm}}^{-1}$ broad emission feature emerges from the inhomogeneously broadened fluorescence spectrum of the film. Upon exciting within the low energy tail of the $S_1\leftarrow S_0\hspace{0.5em}0-0$ absorption band sharp $(7\mathrm{}–9\hspace{0.5em}{\mathrm{cm}}^{-1})$ emission features show up, off-set from the laser line by the energies of the dominant vibrational modes of the polymer. The phenomenon is identified as amplified spontaneous emission induced by spontaneous emission from polymer segments prior to the depopulation by energy transfer towards lower energy sites.