Photon-localization induced random lasing from an amplifying periodic-on-average random system

Summary form only given. Lasing from random media continues to generate significant interest[1]. A particular random system of interest is the periodic-on-average random system (PARS, in short), which comprises a random structure that has an underlying periodicity[2]. In this case, the passband and stopband characteristics of the underlying structure influence the modes of the random system. Despite the interesting transport properties of such systems[3,4], a PARS-based amplifying system has so far not been studied. In this paper, we present a practical implementation of an amplifying PARS system (aPARS), and analyze the random lasing properties thereof. Our experimental observations are in excellent agreement with transfer matrix calculations.We created an aPARS system by generating an array of monodisperse microresonators by using a vibrating orifice aerosol generator. The microresonators were in the form of liquid droplets of Rhodamine 6G in methanol. The aerosol generator could create arrays with varying degrees of monodispersity, as shown in Fig1[A]. The longitudinally emitted radiation upon optical excitation exhibited frequency-sensitive properties. As shown in Fig1[B], under polydisperse conditions (blue lines), lasing modes at arbitrary wavelengths were generated. Consequently, the histogram shown in bottom panel, taken over 200 spectra, shows a continuous distribution of the lasing wavelengths. Under monodisperse conditions (red lines), the lasing modes occur at restricted wavelength intervals, as seen from the bunches in the histograms. We analyzed the spectra features by using transfer matrix calculations on a one-dimensional system comprising a multilayer with refractive index profiles similar to those seen in Fig1[A]. The results are shown in the Fig1[C]. Under monodisperse conditions (red lines), sharp lasing peaks occurred at specific wavelengths, as against the spectra from polydisperse system shown in blue. The corresponding histograms are shown u- derneath, which exactly reproduced the experimental observations. Further transfer matrix analysis shows that the frequency sensitivity arises from the bandstructure effect of the underlying periodic lattice. The lasing modes are essentially the perturbed band-edge modes that migrate to the stopband, and are restricted to a small frequency neighbourhood near the edge. The modes here have a high quality factor, and also a small localization length, which leads to strong random lasing. The strongly polydisperse system washes out the photonic band structure, and hence does not qualify as an aPARS system.