Justin R. Lawrence

Nonlocal-response diffusion model of holographic recording in photopolymer

The standard one-dimensional diffusion equation is extended to include nonlocal temporal and spatial medium responses. How such nonlocal effects arise in a photopolymer is discussed. It is argued that assuming rapid polymer chain growth, any nonlocal temporal response can be dealt with so that the response can be completely understood in terms of a steady-state nonlocal spatial response. The resulting nonlocal diffusion equation is then solved numerically, in low-harmonic approximation, to describe grating formation. The effects of the diffusion rate, the rate of polymerization, and a new parameter, the nonlocal response length, are examined by using the predictions of the model. By applying the two-wave coupled-wave model, assuming a linear relationship between polymerized concentration and index modulation, the resulting variation of the grating diffraction efficiency is examined.

Photopolymer holographic recording material

Summary Photopolymers are promising materials for use in holography. They have many advantages, such as ease of preparation, and are capable of efficiencies of up to 100%. A disadvantage of these materials is their inability to record high spatial frequency gratings when compared to other materials such as dichromated gelatin and silver halide photographic emulsion. Until recently, the drop off at high spatial frequencies of the material response was not predicted by any of the diffusion based models available. It has recently been proposed that this effect is due to polymer chains growing away from their initiation point and causing a smeared profile to be recorded. This is termed a non-local material response. Simple analytic expressions have been derived using this model and fits to experimental data have allowed values to be estimated for material parameters such as the diffusion coefficient of monomer, the ratio of polymerisation rate to diffusion rate and the distance that the polymer chains spread during holographic recording. The model predicts that the spatial frequency response might be improved by decreasing the mean polymer chain lengths and/or by increasing the mobility of the molecules used in the material. The experimental work carried out to investigate these predictions is reported here. This work involved (a) the changing of the molecular weights of chemical components within the material (dyes and binders) and (b) the addition of a chemical retarder in order to shorten the polymer chains, thereby decreasing the extent of the non-local effect. Although no significant improvement in spatial frequency response was observed the model appears to offer an improved understanding of the operation of the material.

Polymer laser fabricated by a simple micromolding process

We report polymer distributed feedback lasers fabricated using solvent-assisted microcontact molding. The poly[2-methoxy-5-(3,7-dimethyloctyloxy) paraphenylenevinylene] film is patterned by placing it in conformal contact with an elastomeric mould inked with a suitable solvent. When the resulting microstructured film is pumped with the 532 nm pulsed output of a microchip laser, we observe lasing above a threshold pump energy of 225 nJ. Above threshold the emission narrows to a linewidth of less than 0.6 nm at a wavelength of 638 nm. This micromolding technique may find application to a wide range of wavelength-scale microstructured organic photonic devices.

Comparison of holographic photopolymer materials by use of analytic nonlocal diffusion models

The one-dimensional diffusion equation governing holographic grating formation in photopolymers, which includes both nonlocal material response and generalized dependence of the rate of polymerization on the illuminating intensity, has been previously solved under the two-harmonic expansion assumption. The resulting analytic expressions for the monomer and polymer concentrations have been derived and their ranges of validity tested in comparison with the more accurate numerical four-harmonic case. We used these analytic expressions to carry out a study of experimental results presented in the literature over a 30-year period. Automatic fitting of the data with these formulas allows material parameters, including the nonlocal chain-length variance sigma, to be estimated. In this way, (i) a quantitative comparison of different materials can be made, and (ii) a standard form of experimental result presentation is proposed to facilitate such a procedure.

Broadband optical amplifier based on a conjugated polymer

We demonstrate a compact, broadband optical amplifier using the conjugated polymer poly[2-methoxy-5-(2′,6′-dimethyloctyloxy)-paraphenylenevinylene] (OC1C10–PPV) in dilute solution. Gains of 30–40 dB in a wavelength range of 575–640 nm, corresponding to a 50 THz bandwidth, are observed due to the broad luminescence spectrum and large cross section for stimulated emission of the polymer. The variation in gain as a function of solution concentration and probe intensity is examined. For a 1 cm path length we observe a small signal gain of 44±1 dB, and deduce a stimulated emission cross-section for OC1C10–PPV of (5.3±0.6)×10−17 cm2.

Thickness variation of self-processing acrylamide-based photopolymer and reflection holography

There are many types of holographic recording material. The acrylamide-based recording material examined here has one significant advantage: it is self-processing. This simplifies the recording process and enables holographic interferometry to be carried out without the need for complex realignment procedures. However, the effect that the polymerization process has on the grating thickness must be examined. This question is fundamental to the materials use in holographic optical elements, as thickness variations affect the replay conditions of the pro- duced elements. This paper presents a study of this thickness variation and reports for the first time the production of reflection holographic grat- ings in this material.

Optical properties of a light-emitting polymer directly patterned by soft lithography

We present the optical properties of a directly patterned light-emitting polymer. The patterned poly~2-methoxy-5-~38,78-dimethyloctyloxy!-paraphenylenevinylene film is fabricated using hot embossing lithography. The effect of the embossed microstructure on the light emitted from the polymer is examined by measuring the angle-dependent photoluminescence and its photonic band structure. The imposed grating modifies the emitted light by Bragg scattering into free space light that would otherwise be trapped as waveguide modes. This simple patterning technique may find application in improving the performance of light-emitting polymer devices.

Photopolymer holographic recording material parameter estimation using a nonlocal diffusion based model

It has recently been shown [Sheridan and Lawrence, J. Opt. Soc. Am. A 17, 1108 (2000)] that by introducing a nonlocal response function into the one-dimensional diffusion equation governing holographic grating formation in photopolymers, both high frequency and low frequency cutoffs in the spatial frequency response of photopolymer materials can be deduced. Starting with this result, analytic solutions to this equation are derived assuming a two harmonic expansion of the monomer concentration. These expressions are first compared to the more rigorous four harmonic numerical results, indicating the range of physical parameters for which they are accurate. They are then compared to the experimental diffraction intensity growth curves produced during the formation of sinusoidal gratings recorded with different spatial frequencies. Fitting these curves using the analytic expressions, various material parameters, including the monomer diffusion constant, D, and the nonlocal variance, σ, can be estimated. Trend...

Electroluminescent colloidal inks for flexographic roll-to-roll printing

The academic and commercial interest in organic light-emitting devices is motivated in part by the potential of building devices utilizing simple and inexpensive fabrication routes, for example, commercial printing techniques. The focus on synthetically challenging small molecules and π-conjugated polymers for these devices is countered by the alternative of developing emissive materials that utilize an electroluminescent dye embedded in a hole and electron transporting host. In this effort, we exploit readily obtainable materials and simple fabrication routes to produce light-emitting colloidal particles, which in turn allows us to invoke the concept of a “particle device”. Specifically, we present colloidally based organic light emitting devices that can be designed to produce a range of colors by mixing together various ratios of red-, green-, and blue-emitting particles. These aqueous-based colloids are adaptable to form printing inks and may be utilized in fabricating devices through high-throughput commercial printing technologies.

Optical amplification in a first-generation dendritic organic semiconductor

We report a study of a new class of organic semiconductor as an optical gain medium. We demonstrate amplification of violet light by use of stimulated emission in a solution of a first-generation bis-fluorene-cored semiconducting dendrimer. Amplification is also observed in the solid state by means of amplified spontaneous emission in an optically pumped dendrimer planar waveguide. Gains of 36 dB cm(-1) at 420 nm and 26 dB cm(-1) at 390 nm in solution and 350 dB cm(-1) in the solid state are obtained. These results show that semiconducting dendrimers have potential as visible laser and amplifier materials.