We have prepared random lasers with conjugated polymer(MEH-PPV) coated on TiO2
nanoparticle films. The threshold of the lasing based on random TiO2
nanoparticle film is significantly reduced by a factor of 9 in comparison with the amplified spontaneous emission(ASE) threshold of the flat MEH-PPV film, which is caused by multiple scattering induced by TiO2
nanoparticles. Further investigation by femtosecond fluorescence up-conversion experiment reveals that the dwell time of light inside the gain medium is increased in the random laser device, which directly confirms that light travels longer path due to the multiple scattering in the random structure. So it stimulates the emission of more light and the threshold of the random laser is lowered.