Polymeric and nanoparticle - based photonic crystals for dfb lasers

The development of new smart structures to provide optical feedback mechanisms paves the way to the realization of novel laser emitters. An approach that has been attracting the interest of many researchers is the distributed feedback (DFB), provided by a periodic dielectric modulation along the propagation direction of the light. Such structures, namely photonic crystals, are the optical analogues of electronic semiconductors. In these systems a periodicity (comparable with the optical wavelengths) in the dielectric constant along one, two or three spatial dimensions generates stopgaps, photonic band gaps and slow photons. On the other side, point, line, bend and planar defects give rise to intra-gap states. To obtain laser emission, both an active material that exhibits strong stimulated emission and an optical feedback are necessary. For the first requirement, a variety of state-of-the-art conjugated molecules and polymers are available combining easy manufacturing with high gain properties. For the second one, the optical feedback, several photonic structures are reported in literature. Focusing on DFB provided by one dimensional photonic structures, two model systems are reported, namely corrugated substrates (1D gratings) and multilayer structures respectively. 1D DFB gratings are extensively studied: several works report structures with very high performance in term of lasing threshold and line narrowing. Nevertheless, expensive and complicated lithographic or UV embossing procedures are required to make such gratings. Although performances of common multilayer systems are still not comparable with the gratings, multilayer devices are really promising for low-cost technological applications, since their manufacturing employs standard and basic techniques. With this perspective, we have fabricated different types of multilayers. Polymeric 1D PCs have been grown on rigid and flexible substrates and have been doped with the laser dye Rhodamine 6G. Moreover, we have fabricated nanoparticle-based 1D PCs, which we have infiltrated with Rhodamine 6G or with the polymer emitter poly (phenylene vinylene). These systems behave as a DFB laser and stimulated emission has been observed. We have studied the laser characteristics and the observed thresholds are lower than the ones reported in literature for other types of organic lasers. In conclusion, we have employed various materials to manufacture multilayer DFB lasers. These materials give different interesting properties, as for example flexibility and porosity. Porous PCs shift the position of the photonic band gap as a function of the concentrations of several vapours and liquids: this gives the possibility to make “tunable” DFB laser by increasing/decreasing the concentration of a specific vapour or liquid, for laser switches and sensors. The morphological and optical characterization provided a comprehensive analysis of the fabricated lasers and give detailed guidelines for the improvement of such systems for application in real devices.