Optically pumped stimulated emission behavior was demonstrated in a slab-waveguide structure formed by substrate/organic film/air. The organic film was deposited by high vacuum (1×10

-3

Pa) co-evaporating of 8-hydroxyquinolinato aluminum(Alq) and laser dye DCM. A wafer of quartz was used as the substrate. The thickness of the organic film was about 300nm

and the percentage of DCM was about 3%. The third harmonic output(355nm) of a Nd:YAG laser was used as the excitation source. The pulse width was about 300ps. The repetition rate was 2Hz. The pump beam was focused onto the sample with a cylindrical lens (

f

=10cm). The excitation area was a typical 10mm×0.2mm stripe. Detected from the side or the surface of the organic film

the fluorescence spectrum with the FWHM (the full width at half maximum) of about 110nm was achieved at excitation pulses energy of 0.1μJ. Increasing the excitation pulse energy to 4μJ

the FWHM of the spectrum detected from the edge of the sample reduced to 13nm with the peak at 620nm and the spectrum from the surface was still a broad band. The change of the emission spectra detected from the edge of the sample showed a clear threshold action and gain narrowing phenomenon when increasing the excitation intensity. The spectrally gain narrowing results from the amplified spontaneous emission (ASE) of the waveguide along the excitation stripe in the film.