Carl W. Dirk

Guest‐host polymer fibers for nonlinear optics

We report on the fabrication of poly(methyl methacrylate) (PMMA) nonlinear optical fibers with dye‐doped cores. The dye‐doped cores have an elevated refractive index that defines a waveguiding region with a large third‐order susceptibility and with single‐mode dimensions. The measured third‐order susceptibility of a squarylium‐doped PMMA film material and the measured optical loss of the dye‐doped fiber core results in a figure of merit that is suitable for all‐optical device applications at λ=1.3 μm. The impact of further improvements in PMMA loss and chromophore nonlinearity are also discussed.

Single-mode nonlinear-optical polymer fibers

We report on the successful demonstration of a single-mode polymer-optical fiber with an 8-µm-diameter nonlinear-optical core composed of a dye-chromophore-doped polymer. Both solid-solution cores and copolymer cores were successfully fabricated. Using an imaging system, we show that the far-field transverse light pattern is that of a single-mode guide. We find that the loss at 1064 nm for the single-mode fiber is approximately 0.2 dB/cm and that it preserves polarization to better than 99.8%/cm.

Measurement of ultrafast optical nonlinearities using a modified Sagnac interferometer

A method for the measurement of fast, intensity-dependent refractive-index changes with the use of a modified Sagnac ring interferometer is presented. The measurement is not degraded by slowly responding background index changes. Nonlinear refractive-index changes in an undoped silicon wafer, and in poly-bis toluene sulfonate polydiacetylene and dye-doped polymethyl methacrylate waveguides, were measured with the use of a cw mode-locked Nd:YAG laser.

Mechanisms of the nonlinear optical properties of squaraine dyes in poly(methyl methacrylate) polymer

We report on the nonlinear optical characterization experiments of squaraine dyes in solid poly(methyl methacrylate) solutions. We find that a quantum three-level model of a squaraine best agrees with the quadratic electroabsorption measurements when the isomer composition is taken into account. We compare the nonlinear response of several dyes with the three- and four-level models to help determine which models are most suitable for predicting nonlinear behavior. Four-level models that include either two one-photon states or two two-photon states are inconsistent with quadratic electroabsorption spectroscopy. Furthermore, we find that the effect of isomers is more significant than molecular reorientation at room temperature and that the isomer model best agrees with the experimental results.

Sagnac interferometric intensity-dependent refractive-index measurements of polymer optical fiber

We have applied a new modified Sagnac interferometric technique to measure the real part of the intensitydependent refractive index of a single-mode polymer optical fiber. For a 0.1% by weight squaraine dye in a poly(methyl methacrylate) core, Re([chi((3))1111]) is 12(+/-7) x 10(-13) cm(3)/erg at lambda = 1064 nm. We discuss the effect of these measurements on all-optical devices.

Electric‐field‐induced polarization current studies in guest–host polymers

We apply polarization current measurements to study dipole reorientation of nonlinear chromophores that are embedded in a polymer matrix. In particular, we apply a step voltage to a thin film of poly(methyl methacrylate) that is doped with a well‐known dipolar chromophore, Disperse Red 1, or an apparently centrosymmetric squarylium chromophore, and measure the isothermal temporal behavior of the current at several temperatures. By correlating the current with electric‐field‐induced second‐harmonic generation, we find that at very early times, polarization current dominates, while at later times, charge hopping mechanisms probably dominate. For the centrosymmetric squarylium chromophore dopant, we find an early‐time current suggesting polarization decay with no corresponding second‐harmonic signal. This suggests that the squarylium molecule has a dipole moment that is perpendicular to the dominant axis of the polarizability tensor and is consistent with the presence of the cis isomer. A time–temperature–vo...

A Prism-Grating-Prism Spectral Imaging Approach

Abstract This paper describes the development and application of a spectral imaging system based on the prism.grating.prism (PGP) line-scan concept. The system is validated against an integrating sphere spectrometer using 22 pigments. Measurements were made on several Old Master drawings and one nineteenth-century watercolour. Results are reported for a watercolour by Eugene Louis Lami to illustrate application of the system. For a square object, 23970 individual reflection spectra are captured in a single scan resulting in file sizes of typically 115 MB. Software has been created that permits direct access, by on-screen selection, to the reflection spectrum of any given pixel in an image, reconstructed from the reflection data. PGP-type spectral scanning systems offer the advantage of potentially very high spectral resolutions of <0.5 nm, though the implementation here is for 5 nm resolution. Digital resolution across a square object is 512 × ∼50 for a PGP slit width of 80 μm. Details of the basic device construction, calibration and operation are provided. The system described here was designed for relatively low spatial resolution, although higher resolution can be achieved with modifications suggested in this publication.

Efficient Photochemical Synthesis of Peptide-α-Phenylthioesters

Low yields and substantial epimerization of peptide‐α‐thioesters often compromise the overall efficiency of native chemical ligation (NCL). Peptide arylthioesters are more reactive than peptide alkylthioesters in NCL, but are also more difficult to handle due to their propensity to hydrolyze, and are therefore often generated in situ. However, pre‐prepared peptide arylthioesters are required for some NCL applications. Here we present a 7‐nitroindoline‐based photochemical method that generates protected peptide phenylthioesters under neutral reaction conditions via their activated esters from photoreactive peptide precursors in high isolated yields, and with low levels of epimerization. This method is fully compatible with Fmoc‐strategy solid‐phase peptide synthesis. Global deprotection with trifluoroacetic acid furnishes peptide phenylthioesters for NCL. Photoreactive peptide precursors can also be converted into their hydrazides in two steps by this method.

Characterization of Fluorescence Induced by Side Illumination of Rhodamine B Doped Plastic Optical Fibers

Acrylic plastic optical fibers containing the laser dye Rhodamine B chloride have been drawn from unclad doped acrylic preforms. Different samples of these doped plastic optical fibers were illuminated laterally with a He-Ne laser centered at 543.4nm and the fluorescence was induced. Experimental results show evidence that the fluorescence peak position and absorption coefficient are determined by the fiber length and dye concentration. A rate of (5.55 ± 0.32) nm/cm of the fluorescence red shift and (0.41 ± 0.02)/cm at λ λ=600 nm of the absorption coefficient was obtained by one sample doped at 0.0001 mol%.Acrylic plastic optical Wbers containing the laser dye Rhodamine B chloride have been drawn from unclad doped acrylic preforms. DiVerent samples of these doped plastic optical Wbers were illuminated laterally with a He-Ne laser centered at 543.4 nm and the Xuorescence was induced. Experimental results show evidence that the Xuorescence peak position and absorption coeYcient are determined by the Wber length and dye concentration. A rate of (5.55 § 0.32) nm/cm of the Xuorescence red shift and (0.41 § 0.02)/cm at ¶ ˆ 600 nm of the absorption coeYcient was obtained by one sample doped at 0.0001 mol% .

Characterization of the switching properties of a singlemode polymer optical fiber

We report on the all-optical switching characteristics of a single-mode polymer optical fiber with a squaraine dye-doped poly(methyl)methacrylate core using a Sagnac interferometer.