Incoherent radiation of amplifying random media

Incoherent radiation of amplifying random media is investigated using Monte Carlo simulation and the characteristics of random laser are observed. The entire emission spectra of random media become narrow abruptly when pumping energies exceed certain threshold values, and when the pumping energies further increase, distinct sharp peaks emerge on spectrum background. The intensity of a certain spatial point within the scattering media is contributed from emissions of many frequencies. And the intensity of single frequency of the entire spectrum is contributed from emissions with wide spatial and angle ranges. It has been pointed out that the incoherent radiation of amplifying random media is essentially different from amplified spontaneous emission without any feedback as well as conventional resonant-feedback laser. The explanation on distinct sharp peaks is that rare photons have experienced rather more scattering events and longer paths in amplifying random media, and these photons accumulate more gain.