Modes interaction and light transport in bidimensional organic random lasers in the weak scattering limit M. Anni,* S. Lattante, ² T. Stomeo, R. Cingolani, and G. Gigli National Nanotechnology Laboratory (NNL) of INFM, Dipartimento di Ingegneria dell’Innovazione, Università degli Studi di Lecce, Via per Arnesano 73100 Lecce, Italy G. Barbarella and L. Favaretto ISOF, Area della Ricerca CNR, Via Gobetti 101, I-40129 Bologna, Italy (Received 1 October 2003; revised manuscript received 5 August 2004; published 24 November 2004) We report on the modes interaction and light transport in weakly scattering neat films of small molecular weight organic molecules, showing coherent random lasing. The lasing modes interaction exhibits peculiar properties, with spatially overlapped modes at low excitation density and modes competition at high excitation density. This results in a progressive decrease and in a saturation of the number of lasing modes with increasing the excitation density. The weak scattering results in diffusive light transport with a transport mean free path in the range 14.6–125 m, which is much higher that the lasing wavelength   620 nm. The average transport properties are correlated with the single scattering event cross section. We demonstrate that the light propaga- tion is mainly affected by single scattering properties rather than collective effects. DOI: 10.1103/PhysRevB.70.195216 PACS number(s): 71.55.Jv, 42.55.Zz, 42.55.Px I. INTRODUCTION The study of electron and light transport in disordered systems has become a prominent part of condensed matter physics. In particular the formation of bands of spatially lo- calized electronic states, called Anderson localization, has been predicted to occur in sufficiently disordered solids, 1,2 as a consequence of the interference of electron wave functions scattered by randomly distributed defects. Regarding light transport in disordered materials the in- terplay between light amplification and multiple scattering results in the fascinating phenomenon of random lasing. Such a process, after the theoretical prediction of Letokhov, 3 has recently received considerable attention, both from the fundamental point of view and for the application to micro- sized active elements in photonic devices. 4 Adding optical gain to a random system indeed provides a unique tool to study light transport and localization, as the behavior of las- ing modes reflects the properties of the eigenstates of the disordered system. 5 Random lasing has been demonstrated in various materi- als including strongly scattering tridimensional (3D) sys- tems, like ZnO and GaN powders, 6,7 blends of TiO 2 and rhodamine dyes in solution and in inert matrix, 8,9 and weakly scattering bidimensional (2D) polymeric films. 10 Light local- ization has been theoretically predicted 11–17 and recently cor- related to random lasing 18 for ZnO strongly scattering nano- particles films, i.e., with the scattering mean free path length comparable to the lasing wavelength. In addition the mode interaction of the lasing modes has been investigated in strongly scattering systems, showing the occurrence of spec- tral and spatial modes repulsion, 9,12 as well as modes coupling. 19 In this frame, the analysis of the emission prop- erties of random lasers in the opposite limit of weak scatter- ing is still lacking. Some preliminary results on polymer neat films indicate a weaker scattering with respect to inorganic powders, 10,20 whereas the effects on the modes interaction of a reduced scattering efficiency, the light transport properties as well as the origin of the feedback for lasing have not been studied yet. Moreover despite the theoretical prediction of enhance- ment of the localization effects as the system dimensionality is reduced, 21 most of the recent experimental works have been conducted in a strongly scattering 3D system and a detailed analysis of the emission properties of the 2D system is still missing. In this paper we report on coherent random lasing in bi- dimensional (2D) neat films of low molar weight organic molecules (T5oCx, see inset of Fig. 1). We investigate the modes interaction and the light transport in asymmetric glass-T5oCx-air waveguides, which are characterized by a FIG. 1. Emission spectra of the S1 sample as a function of the excitation density for a stripe length of 6.5 mm. The gray lines are fit of the spectra without the contribution of random lasing peaks. The dotted vertical lines and the numbers indicate three modes with different intensity dependence on excitation density. Inset: Chemical structure of the T5oCx molecule (Hex=Hexyl chain, Cx = Ciclohexyl group). PHYSICAL REVIEW B 70, 195216 (2004) 1098-0121/2004/70(19)/195216(7)/$22.50 ©2004 The American Physical Society 70 195216-1