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Modern technologies rely not only on traditional inorganic semiconductors, but also on organic materials. In particular, organic semiconductors based on conjugated oligomers and polymers have technological applications owing to their compatibility with lowtemperature processing, the relatively simple thin-film device fabrication, and the tunability of their electronic properties based on the richness of synthetic organic chemistry. Here, we are interested in molecular materials in their crystalline form, which play a role in the fields of both theoretical and experimental research. We will focus the attention on their UVvisible optical properties. Compared to amorphous films, crystalline materials exhibit well defined polarization of the electronic states and interesting effects related to the anisotropic propagation of light. First of all, in order to maximize coupling to the crystal electronic transition moment, the pumping photon beams must be precisely aligned (Fichou et al. 1997). Emission is then observed with defined directions of propagation and of polarization. Optical gain can also result in amplified spontaneous emission (ASE), which is characterized by a spectrally narrowed emission upon increasing the excitation fluence above a threshold. Gainnarrowing has been demonstrated in a number of molecular crystalline materials, such as oligothiophenes, oligo(p-phenylene), oligo(p-phenylene vinylene), trans-1,4-distyrylbenzene, cyano derivatives, fluorene/phenylene co-oligomers, hydroxy-substituted tetraphenylimidazole, and 1,1,4,4-tetraphenyl-1,3-butadiene (Fichou et al. 1997, 1999; Hibino et al. 2002; Horowitz et al. 1999; Ichikawa et al. 2003, 2005; Losio et al. 2007; Nagawa et al. 2002; Park et al. 2005; Polo et al. 2008; Tavazzi et al. 2006c, 2007, 2008, 2010b; Xie et al. 2007; Zhu et al. 2003). Relatively low thresholds have been recently reported for p-sexiphenyl samples. Selfwaveguided emission has been observed in needle-shaped crystals of this molecular species grown on potassium chloride (001) substrates (Cordella et al. 2007; Quochi et al. 2005; Yanagi et al. 2001; Yanagi & Morikawa 1999) with threshold down to 0.5 J/cm2. Random lasing from isolated nanofibres of various widths has also been reported and discussed by Quochi et al. (Quochi et al. 2005). For the same molecular species in the bulk crystalline form, ASE was reported (Losio et al. 2007) with different thresholds depending on the

Optical Properties of Molecular Crystals: The Effect of Molecular Packing and Polymorphism