Yeheng Wu

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.

Third harmonic generation in self-focused filaments in liquids.

Intense filaments of third harmonic generation have been observed in a variety of liquids when pumped by 1300 nm wavelength femtosecond light. The onset of third harmonic generation coincides with the threshold for self-focusing as deduced from n(2) measurements performed on the liquids using the z-scan technique. We have found self-phase modulation above the threshold leading to continuum generation at about 3 times threshold. In the third-harmonic regime, the third harmonic output power is independent of the input power. Our observations are consistent with those for similar processes in gases as described by a phase-locking model. The measured conversion efficiency was approximately 10(6).

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.

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.

Control of multiphoton excited emission and phase retardation in Kleinman-disallowed hyper-Rayleigh scattering measurements

We describe improvements to the Kleinman-disallowed hyper-Rayleigh scattering (HRS) measurement technique, which can be used to measure all of the rotationally invariant figures of merit for the first molecular nonlinear optical hyperpolarizability tensor βijk. As multiphoton excited fluorescence continues to be an accuracy-limiting factor in any HRS experiment, we have implemented time-resolved single photon counting in HRS using a Ti:sapphire laser. We show, however, that spectral analysis is necessary to ensure that the fluorescence and second harmonic are emitted on different time scales in order to allow temporal separation and thus improved accuracy. We also demonstrate how an electrically controlled liquid-crystal phase retarder can be used to vary the polarization state of the probe light in order to determine the second-order nonlinear response tensor. It was found that results agree with those obtained using a rotating quarter-wave plate to control the polarization state and demonstrate improved precision and reproducibility.

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.

Surface-Emitting Distributed Feedback Lasing Based on Multilayer Polymer Films

We report on the fabrication of a multilayer all-polymer surface emitting distributed feedback dye laser. Output power, emission spectrum, and threshold for various designs are described.