Joseph Lott

Influence of Temperature on Low-Power Upconversion in Rubbery Polymer Blends

The upconverting properties of a dye cocktail composed of palladium(II) octaethylporphyrin (PdOEP, triplet sensitizer) and 9,10-diphenylanthracene (DPA, triplet acceptor/annihilator) were investigated as a function of temperature in several low glass transition temperature (T(g)) polymer hosts including an ethyleneoxide-epichlorohydrin copolymer (EO-EPI) and the polyurethanes Texin 270, Texin 285, and Tecoflex EG-80A. Selective excitation of PdOEP at 544 nm in the presence of DPA in these materials resulted in anti-Stokes blue emission from DPA, a consequence of sensitized triplet-triplet annihilation (TTA) photochemistry, confirmed by the quadratic dependence of the upconverted fluorescence intensity with respect to incident light power. The upconversion process was completely suppressed by cooling a PdOEP/DPA blend film to below the T(g) of the respective polymer. However, the blue emission was clearly visible by the naked eye upon heating these films to room temperature (290 K). Subsequently, the upconverted emission intensity increased with increasing temperature and was found to be completely reversible upon several heating and cooling cycles provided the temperature remained below 400 K. Heating samples above this temperature resulted in unrecoverable failure of the material to produce upconverted photons. The phosphorescence intensity decay of PdOEP in the polymer host, Tecoflex EG-80A, adequately fits to a sum of two exponential functions as well as the Kohlrausch-Williams-Watts (KWW) stretched exponential model. Increasing the temperature of the film increases the complexity and heterogeneity of the system as evidenced by the lower beta values obtained from the KWW model as the temperature increases.

Two-photon 3D optical data storage via aggregate switching of excimer-forming dyes.

N S Current optical data storage (ODS) technologies use onephoton-absorption processes to write data by locally changing the optical properties of the medium. [ 1 , 2 ] Since the lateral dimensions of spots that can be written are near the diffraction limit, signifi cant capacity increases require new approaches such as storage in three dimensions. DVDs, which comprise up to four individually addressable storage layers, exemplify the potential of this concept, but the complexity of producing and using multilayer systems increases with the number of layers. In bulk materials, changes can be confi ned in the third dimension via nonlinear optical processes, such as two-photon absorption (TPA). [ 3– 6 ] We have developed a novel ODS system that relies on the optically-induced switching of the aggregation state and fl uorescence of a TPA dye in a polymer matrix. Welldefi ned, ∼ 3 × 3 × 6 μ m-large voxels were written with single focused laser pulses and read by confocal laser scanning microscopy. Such ODS systems are easily produced and promise a storage capacity of up to several Tbytes on a DVD-size disk, which is ∼ 100× higher than that of current commercial ODS technologies. [ 6 , 7 ]

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.

Roll‐to‐Roll Fabrication of Multilayer Films for High Capacity Optical Data Storage

Optical data storage (ODS) has led to transformative advances in information storage and distribution technology. Conventional two-dimensional ODS media have allowed storage capacities necessary for high-defi nition video. The capacity is limited, however, by the size of the disk and the number of layers that can be addressed using highly scattering phase-change materials employed in these media. [ 1 ] Here we report on a co-extrusion process for fabricating roll-to-roll multilayer (ML) fi lms for high-density ODS. This process can easily produce a continuous, complete storage medium hundreds of meters in length and meters in width, ready for fabrication into the standard 120 mm diameter disk or a variety of other potential formats, with total writable areas suffi cient for terabyte (TB) to petabyte (PB)-scale capacity. The co-extrusion process is also low-cost and far simpler than current manufacturing approaches, such as spin-coating [ 2 , 3 ] and lamination. [ 4–6 ] We demonstrate data storage in 23 layers of a 78 μ m thick ML fi lm using a continuous-wave Blu-Ray (BR) laser by fl uorescence (FL) quenching of an organic dye. The areal density is found to be similar to that of commercial disks, and the small layer spacing allowed by a FL-based scheme leads to a bit density of 1.2 × 10 12 cm − 3 . Given the mechanism and high axial density, the cross-talk during writing is also examined. The approach is generic so that materials already developed for high-density ODS can be exploited for innovations including “cloud”-scale data storage. Commercial ODS disks are made by fi rst injection molding of the thick, plastic substrate. The refl ective and active layers are added by a combination of sputtering and spin-coating. [ 1 ] For simple read/write systems based on a bit-wise one-photon write scheme, a multilayer architecture would require a multiplicity of steps with the current manufacturing processes. Several organic dye/polymer schemes suitable for three-dimensional

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.

Melt-processed polymer glasses for low-power upconversion via sensitized triplet–triplet annihilation

The process of low-power light upconversion by triplet–triplet annihilation is well established in solutions of appropriate sensitizer–emitter pairs, but has only recently been reduced to practice in polymeric materials. Here, the fabrication of upconverting glasses based on poly(methyl methacrylate) (PMMA), palladium octaethylporphyrin (PdOEP, sensitizer, 0.005–0.5% w/w relative to the polymer) and large amounts of diphenylanthracene (DPA, emitter, 25% w/w relative to the polymer) is reported. These materials were produced by compression-molding pre-mixed blends and subsequently quenching the samples in a molecularly mixed state. The resulting films upconvert green incident light (543 nm) of low incident power density (34 mW cm−2) into blue light (440 nm). The dependence of the upconversion intensity on the sensitizer concentration was studied and the results suggest that an optimal composition range exists, where the upconversion efficiency is maximal.

Functionalized cyano-OPVs as melt-processable two-photon absorbers

Several cyano-functionalized oligo(phenylenevinylene) (cyano-OPV) dyes were modified with different alkyl substituents with the objective to develop melt-processable, two-photon absorbing materials. The new dyes exhibit two-photon absorption cross-sections in the range of 400–850 GM, can be melt-processed at temperatures as low as ∼100 °C, and offer high thermal stability. The propensity of these molecules to form excimers also makes them interesting for sensor and data storage applications.

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.

Co-extruded multilayer films for high capacity optical data storage

New approaches for optical data storage (ODS) applications are needed to meet the future requirements of applications in multimedia, archiving, security, and many others. Commercial data storage technologies are moving to threedimensional (3D) materials, but the capacity is limited by the fabrication cost and the number of layers that can be addressed using the reflection-based storage mechanism. We demonstrate here storage systems based on co-extrusion of multilayer (ML) films that can overcome these problems. The organic roll-to-roll films produced can easily be produced hundreds of meters in length, in a far simpler and cheaper manner than current manufacturing methods such as spin coating and lamination. The medium consists of alternating active and buffer layers, and data storage is demonstrated by writing images in 23 layers of 78 μm thick films by fluorescence (FL) quenching of an organic dye. The areal data resolution is at the diffraction limit of the CW Blu-ray (BR) laser employed, and the co-extrusion technique allows for small layer spacings, leading to a total bit density 1.2 Tb/cm3. We anticipate materials already demonstrated successful for 3D ODS will be adapted to this technique, as well as new systems developed, to take full advantage of this medium.

Time-resolved optical writing on a photosensitive and fluorescent polymer film

Recently a melt-processed blend of 1,4-bis(α-cyano-4-octadecyloxystyryl)-2,5-dimethoxybenzene (C18-RG) dye and polyethylene terephthalate glycol (PETG) has been demonstrated as a promising 3-dimentional optical data storage (ODS) medium 1. ODS in this novel system relies on the laser-induced switching of the aggregation state of the excimerforming fluorescent dye in the inert host polymer. Here we investigate the mechanism and the time scales involved in the writing process. The optical writing was realized by the laser-induced localized excimer to monomer conversion and was characterized by the emergence of the monomer fluorescence. We obtained the dependence of the excimer to monomer conversion on the writing time. Our result indicates that the effective optical writing time is controlled by heating and cooling time of the host polymer and the excimer-to-monomer conversion time. The effective laser writing time, under the specific writing conditions employed in our experiments, is on the order of 10 ms.