Hyunmin Song

Melt-processed all-polymer distributed Bragg reflector laser

We have assembled and studied melt-processed all-polymer lasers comprising distributed Bragg reflectors that were fabricated in large sheets using a co-extrusion process and define the cavities for dye-doped compression-molded polymer gain core sheets. Distributed Bragg reflector (DBR) resonators consisting of 128 alternating poly(styrene) (PS) and poly(methyl methacrylate) (PMMA) layers were produced by multilayer co-extrusion. Gain media were fabricated by compression-molding thermoplastic host poly notmers doped with organic laser dyes. Both processing methods can be used in high-throughput roll-to-roll manufacturing. Optically pumped DBR lasers assembled from these components display single and multimode lasing in the reflection band of the resonators, with a slope efficiency of nearly 19% and lasing thresholds as low as 90microJ/cm(2). The lasing wavelength can be controlled via the layer thickness of the DBR resonator films, and variation of the laser dye. Studies of threshold and efficiency are in agreement with models for end-pumped lasers.

Continuous melt processing of all-polymer distributed feedback lasers

Novel processing techniques for low-cost production of photonic devices could open up new applications for functional polymer systems. To this end, we have used multilayer coextrusion in a continuous melt process to fabricate large-area polymeric nanolayer films for optically-pumped all-polymer distributed feedback (DFB) surface-emitting lasers. Each laser film consists of hundreds of alternating layers of two transparent polymers with different refractive indices, of which one contains a laser dye. The resulting DFB lasers emit at defect states and show efficiencies as high as 8% and threshold fluences as low as 100 µJ/cm2.

Co-extruded mechanically tunable multilayer elastomer laser

We have fabricated and studied mechanically tunable elastomer dye lasers constructed in large area sheets by a single-step layer-multiplying co-extrusion process. The laser films consist of a central dye-doped (Rhodamine-6G) elastomer layer between two 128-layer distributed Bragg reflector (DBR) films comprised of alternating elastomer layers with different refractive indices. The central gain layer is formed by folding the coextruded DBR film to enclose a dye-doped skin layer. By mechanically stretching the elastomer laser film from 0% to 19%, a tunable miniature laser source was obtained with ~50 nm continuous tunability from red to green. Optically pumped by a frequency-doubled Nd:YAG laser, the elastomer laser showed a lasing threshold of 0.9 mJ/cm2 at 600 nm.

Layered polymeric optical systems using continuous coextrusion

Polymers are receiving considerable attention as components in novel optical systems because of the tailored functionality, ease of manufacturing, and relatively low cost. The processing of layered polymeric systems by coextrusion is a method to produce films comprising hundreds to thousands of alternating layers in a single, one-step roll-to-roll process. Several layered polymer optical systems have been fabricated by coextrusion, including gradient refractive index lenses, tunable refractive index elastomers, photonic crystals, and mechanically tunable photonic crystals. Layered polymeric optical systems made by coextrusion can also incorporate active components such as photoreactive additives for multilayered patterning and laser dyes for all-polymer laser systems. Coextrusion is a process which allows for the flexible design of polymeric optical systems using layers with thickness spanning the nanoscale to the microscale.

Mode delocalization in 1D photonic crystal lasers

We have investigated the formation of in-bandgap delocalized modes due to random lattice disorder as determined from the longitudinal mode spacing in a distributed Bragg laser. We were able to measure the penetration depth, and from transfer matrix simulations, determine how the localization length is altered for disordered lattices. Transfer matrix simulations and studies of the ensemble average were able to connect the gap delocalized modes to localized modes outside of the gap as expected from consideration of Anderson localization, as well as identify the controlling parameters.

Thermo-spectral study of all-polymer multilayer lasers

We investigate the temperature dependence of the emission wavelength and reflection band of polymer Distributed Bragg Reflector (DBR) and defect Distributed FeedBack (DFB) lasers fabricated using a coextrusion melt-process. We show the measured spectral shifts are a direct consequence of the optical path modifications associated with layer expansion and thermo-optic coefficients. By varying the choice of polymer bilayers and sandwiching the DBR laser films between glass coverslips, we fabricated DBR lasers that are either readily tunable up to 0.36nm/°C or made thermally stable at 0.035nm/°C.

Toward Roll-to-Roll Production ofPolymer Microresonator Lasers

Lasers: We have created lasers with operating properties that we never would have dreamed of when the technology was invented 50 years ago.

Role of group velocity delay in Faraday rotation in a multilayer polymer lattice

We measure and model the spectral dependence of Faraday rotation in one-dimensional lattice structures composed of co-extruded alternating polymer layers of polymethylmethacrylate and polystyrene. We develop a theory that shows that the net Faraday rotation in a symmetric multilayer system is determined not by the total thickness of the constituent materials but by the time spent in each constituent material as measured by the overall group velocity delay of the structure and the relative energy distribution per material. We compare measured and computed Faraday rotation spectra for these films to theoretical predictions, taking into account ellipticity as well as layer thickness variations and finite spectral width detection. To measure rotations of these thin, non-magnetic, weak Faraday rotators, we constructed and optimized an apparatus capable of measuring broadband Faraday rotation spectra at 0.001° resolution for rotation angles as small as 0.002°.

Thermo-spectral properties of plastic lasers

We study the effects of temperature changes on the operating wavelength of all-polymer microresonator lasers, particularly on multilayered defect distributed feedback and distributed Bragg reflector lasers. The parameters that change the operating wavelength are discussed with comparisons between experiments and simulations.

Multilayer Polymer Films for Photonic Applications

Polymers are receiving considerable attention as components in novel optical systems because of the tailored functionality, easy manufacturing, and relatively low cost. The processing of layered polymeric systems by coextrusion is a method to produce films comprising hundreds to thousands of alternating layers with thickness spanning the nanoscale to microscale in a single, one-step roll-to-roll process. Several layered polymer optical systems have been fabricated by coextrusion, including tunable refractive index elastomers, photonic crystals, and mechanically tunable photonic crystals. Layered polymeric optical systems made by coextrusion can also incorporate active components such as laser dyes for all-polymer laser systems.