This paper introduces ZnO semiconductor optical amplifiers. In this paper, stimulated emission from zinc oxide nanoparticles are discussed in low and high powder density samples. 4 nm linewidth emission peak are observed in ZnO nanoparticles pump by Nd:YAG laser with 8 ns pulse width in high powder density sample. Scattering is strong when the nanoparticle density is high. And strong scattering is the reason of narrow peak in the nanoparticle. We also investigate the influence of the excitation spot area and the scatter concentration on the laser threshold in the TiO<sub>2</sub>/Rhodamine B gain media, with frequency-doubled output of a mode-locked Nd:YAG laser (20 Hz repetition rate, 6 ns pulse width). Monte Carlo methods with ring laser were employed to calculate the threshold gain required for modeling the input-output characteristics in scattering gain media. We performed a Monte Carlo simulation of random walks in the geometry. With the increased pump area, the laser threshold is decreased. If we add more scatterers in the Rhodamine B, the laser threshold will reduce.