Stimulated emission and lasing have been studied in various scattering gain media, including luminescent B-conjugated polymer films and opal micro-crystals infiltrated with polymer or dye solutions. In addition to amplified spontaneous emission that is usually observed in high gain media upon intense photoexcitation, a lasing process was revealed using stripe excitation geometry. The new emission regime is characterized by a finely structured spectrum with narrow lines, of the order of 0.1nm. We interpret this laser like emission as due to random lasing where the optical feedback is provided by weak light scattering inside the gain medium. Coherent back-scattering measurements of the same media systems were found to be in agreement with this interpretation.
In addition we demonstrate a three-dimensional photonic crystal laser based on infiltrated dyes in a single crystal opal, operating simultaneously in the blue (435nm) and red (670-720nm) spectral regions. The feedback mechanism underlying this type of laser is the Bragg scattering off different sets of crystalline (hkl) silica planes, via a distributed feedback process. The laser emission wavelength was selected by proper orientation of the lasing axis with respect to the [111] and [220] directions for the red and blue lasing, respectively. Defect modes caused by stacking faults in the opal crystal were identified in the laser spectrum as sharp lines, which were found to depend on the specific illuminated area on the opal surface.
The advantages of spectral gaps formed by local resonances are: (1) The gaps are neither related to nor pinned by the periodicity of the structure. Thus spectral gaps are possible for any periodic structure, provided the filling fraction exceeds a certain threshold. (2) The gaps are tunable by adjusting the locally resonant unit. (3) Spectral gaps can be formed with lattice constant, or the basic length scale of the structure, orders of magnitude smaller than the relevant wavelength in the bandgap region. Thus very compact spectral gap crystals can be fabricated.
We substantiate our notion through both experimentation and rigorous calculations. Examples for both the photonic crystal and the elastic bandgap crystal fabricated from locally resonant structures will be shown and discussed.