Dynamic Tuning of Organic Lasers with Colloidal Crystals

The prospect of an inexpensive organic laser which can dynamically (i.e., in real time) alter its lasing wavelength is desirable for a number of display and communication technologies. Extensive efforts have focused on developing liquid-crystalbased lasers which can exhibit emission tuning through the application of an external stimuli, such as a mechanical stress, a temperature variation, or a radiation exposure. Polymeric-based lasers are especially interesting organic systems due to their low cost, stability, and ease of fabrication relative to liquid-crystal-based systems. To this end, tunable distributed-feedback (DFB) lasers, which rely on the dynamic variation of a grating in the gain medium, are of particular interest. Mechanically tunable DFB lasers have been fabricated by using an elastomeric substrate with an overlaid grating, where the straining of the substrate changes the period of the grating and the resulting lasing wavelength. In addition, a phototunable organic DFB laser has recently been presented which utilizes an azopolymer that allows for the phototemplating of variable-period gratings. Although these systems are promising, they often require an extensive synthetic effort to produce the gain medium or complicated interference lithography to form the grating. Lasers require two main elements for operation, an active material to provide optical gain and a resonator structure to provide optical feedback. One strategy to simplify the development of tunable organic lasers is to choose a laser design that decouples the resonator from the selection of the gain medium. In a conventional “external” resonator cavity design, an organic gain material is deposited onto a mirror and capped with a second mirror to form a cavity. The light generated in the active material is reflected between the two mirrors, and, if the reflectivity of the mirrors is sufficiently high and the active material has adequate optical gain to offset the sources of loss, the structure will begin to lase. The current effort focuses on the utilization of colloidal crystals to provide the required reflectivity in an external resonator cavity design. Previous efforts to fabricate lasers utilizing colloidal crystals have focused on the assembly of the particles into close-packed arrays through sedimentation, relying on non-specific particle–particle “hard sphere” packing to induce order. These close-packed crystals have typically been infiltrated with a laser dye and exhibited either random or DFB lasing. The current approach utilizes charged polymeric colloidal particles that self-assemble through long-range electrostatic repulsive interactions to procure order. These ordered systems have been employed in the fabrication of opalescent films that exhibit a mechanochromic response (i.e., photonic crystals that exhibit a strain-induced variation of their rejection wavelength), where the response has been attributed to an affine deformation of the lattice. This response is intimately tied to the large interparticle spacings afforded to colloidal arrays based on electrostatically self-assembled particles. In this communication, we report on the exploitation of mechanochromic photonic bandgap composites to dynamically modify the emission spectra of a thin-film organic laser. The thin-film laser is presented schematically in Figure 1a. In this configuration, a commercial dielectric mirror (mirror 1, cf. Fig. 1a) is spin-coated with a thin layer of poly(methyl methacrylate) (PMMA) which has been doped with the photoluminescent dye rhodamine B (gain medium, cf. Fig. 1a). The uniform PMMA/rhodamine B layer acts as the gain medium and is relatively thin (ca. 1 lm thick) in an effort to minimize the number of resonator modes, while mirror 1 exhibits a broadband reflectivity of greater than 95 % across the entire emission spectrum of the dye. A photonic bandgap (PBG) composite (mirror 2, cf. Fig. 1a) is laminated on the top of the gain medium and acts as the second reflector of the cavity. The preparation of the PBG composite films has recently been reported. Briefly, the film fabrication involves the following steps: Monodisperse crosslinked polystyrene particles dispersed in water and with a particle diameter of 150 nm were allowed to electrostatically self-assemble and then encapsulated in a photoinitiated free-radical-polymerized methacrylate functionalized poly(ethylene glycol) (PEG) hydrogel. The water from the hydrogel was removed, and the stabilized arrays were reswollen in 2-methoxyethyl acrylate (MOEA). The incorporation of a small quantity of ethylene glycol dimethacrylate and the photoinitiator 2,2-diethoxyacetophenone (DEAP) into the reswollen system, with subsequent photopolymerization, resulted in a robust composite film. In the case of colloidal crystals in which the lattice adopts a face-centered cubic (fcc) symmetry, the most densely packed planes (i.e., (111) planes) pack parallel to the walls of the container. At a normal incidence, the approximate diffraction characteristics of these systems can be predicated with the Bragg diffraction equation, k0 = 2 nc d111, where k0 is the waveC M M U N IC A TI O N S