James H. Andrews

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.

Pulse compression in a synchronously pumped optical parametric oscillator from group-velocity mismatch

We report on experimental intracavity compression of generated pulses (down to one quarter of the pumppulse duration) in a widely tunable synchronously pumped picosecond optical parametric oscillator. This pulse compression takes place when the optical parametric oscillator is well above threshold and is due to the pronounced group-velocity mismatch of the pump and oscillating waves in the nonlinear crystal.

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.

Phase-Shifted Laser Feedback Interferometry

We have introduced the techniques of phase-shifting interferometry into a laser feedback interference microscope based on a helium-neon laser. With moderate feedback, multiple reflections between the sample and the laser are shown to be negligible, and the interferometer responds sinusoidally with a well-characterized fringe modulation. One can obtain higher signal-to-noise ratios by determining the number of additional terms required for modeling the effect of multiple reflections on the phase and visibility measurements in the high-feedback regime. Changes in optical path length are determined with nanometer precision without phase averaging or lock-in detection.

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.

Cutting a drop of water pinned by wire loops using a superhydrophobic surface and knife.

A water drop on a superhydrophobic surface that is pinned by wire loops can be reproducibly cut without formation of satellite droplets. Drops placed on low-density polyethylene surfaces and Teflon-coated glass slides were cut with superhydrophobic knives of low-density polyethylene and treated copper or zinc sheets, respectively. Distortion of drop shape by the superhydrophobic knife enables a clean break. The driving force for droplet formation arises from the lower surface free energy for two separate drops, and it is modeled as a 2-D system. An estimate of the free energy change serves to guide when droplets will form based on the variation of drop volume, loop spacing and knife depth. Combining the cutting process with an electrofocusing driving force could enable a reproducible biomolecular separation without troubling satellite drop formation.

20‐fold pulse compression in a synchronously pumped optical parametric oscillator

We have developed a model of pulses interacting in a synchronously pumped optical parametric oscillator to predict the pump intensity dependence of generated pulse compression due to pronounced group velocity walk off between the pump, signal, and idler pulses in the nonlinear crystal. We have experimentally demonstrated up to 20‐fold compression of the generated pulse relative to an 11 ps pump, and measured transform‐limited spectral broadening of the compressed pulse.

Contribution of the 2 1 Ag state to the third-order optical nonlinearity in a squaraine dye

We have measured the third-harmonic response, gamma, of a centrosymmetric squaraine dye (ISQ) in chloroform at a range of frequencies for which the third harmonic is above the strong, narrow peak in the dyes linear absorption spectrum but below the UV absorption band. By fitting the experimental dispersion of gamma using a four-level model, we determine the strength, location, and width of the lowest-lying two-photon transition. We find that the 2(1)Ag state appears just above the 1(1)Bu state in energy and that the 1(1)Bu-2(1)Ag transition moment is somewhat smaller than the transition moment between the ground state and the 1(1)Bu state but much larger than previously predicted for comparable squaraine dyes.

Coherent Perfect Rotation

Two classes of conservative, linear, optical rotary effects (optical activity and Faraday rotation) are distinguished by their behavior under time reversal. In analogy with coherent perfect absorption, where counterpropagating light fields are controllably converted into other degrees of freedom, we show that only time-odd (Faraday) rotation is capable of coherent perfect rotation in a linear and conservative medium, by which we mean the complete transfer of counterpropagating coherent light fields into their orthogonal polarization. This highlights the necessity of time reversal odd processes (not just absorption) and coherence in perfect mode conversion and may inform device design.

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.