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Random Lasing from Weakly Scattering Media: Universality in the Emission Spectra from Pi-Conjugated Polymer Films

R. C. Polson·M. E. Raikh·Z. V. Vardeny
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Abstract

When films of Pi-conjugated polymers are optically excited above a certain threshold intensity, then the emission spectrum acquires a multimode finely structured shape, which depends on the position of the excitation spot. We demonstrate that the power Fourier transform (PFT) of the emission spectrum exhibits a certain peak-like structure, which also depends on the excitation spot. Our intriguing observation is that averaging the individual PFTs does not lead to a structureless curve, but rather yields a series of distinct transform peaks. This suggests universality, namely that the underlying random resonators that are responsible for the laser emission from the pi-conjugated polymer film are almost identical. We argue that the reason for such an universality is the large size of a typical resonator, which we determined from the PFT, as compared to the emission wavelength, lambda. This fact is, in turn, a consequence of the large light mean free path, l* approximately 10 lambda in the polymer film. This contrasts previous observations of random lasing in powders, where l* approx. lambda. We develop a simple theory that explains the presence of peaks in the average PFT and predicts their shape. The results of the theory agree quantitatively with the data.

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R. C. Polson, M. E. Raikh, Z. V. Vardeny. 2001-05-18. Random Lasing from Weakly Scattering Media: Universality in the Emission Spectra from Pi-Conjugated Polymer Films. https://arxiv.org/abs/cond-mat/0105360

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