Mark D. Rahn

COMPARISON OF LASER PERFORMANCE OF DYE MOLECULES IN SOL-GEL, POLYCOM, ORMOSIL, AND POLY(METHYL METHACRYLATE) HOST MEDIA

Laser performance is described for Rhodamine 590, Pyrromethene 567, Perylene red, and Perylene orange in inorganic porous sol-gel glass, poly(methyl methacrylate)(PMMA), a composite of porous sol-gel glass with PMMA and organically modified silicate ormosil glass. Lasers were excited with a flash-lamp-pumped dye laser in the long-pulse-length regime (3 µs, 506 nm, 300 mJ) and a second-harmonic Nd:YAG laser in the short-pulse-length regime (6 or 15 ns, 532 nm, 60 mJ). The feasibility of long-pulse-length operation is demonstrated, detailed characteristics of short-pulse operation are described, and laser damage measurements are given. The nonpolar perylene dyes had better performance in partially organic hosts, and the ionic rhodamine and pyrromethene dyes performed best in the inorganic sol-gel glass host.

Photostability enhancement of Pyrromethene 567 and Perylene Orange in oxygen-free liquid and solid dye lasers

We investigated the effect of oxygen on the photostability of the laser dyes Pyrromethene 567, Perylene Orange, and Rhodamine 590 by determining their longevity of laser operation when pumped by the second harmonic of aQ -switched Nd:YAG laser. In solution, dissolved oxygen accelerated the photodegradation of Pyrromethene 567 and Perylene Orange but not Rhodamine 590. The photostability of Pyrromethene 567 was also found to be dependent on the solvent and on the lifetime of singlet oxygen. Deoxygenated Pyrromethene 567 doped polycom glass and modified poly(methyl methacrylate) (MPMMA) samples were tested for longevity of laser operation. A factor of 6 improvement in photostability was found for Pyrromethene 567 in MPMMA upon deoxygenation, and the total absorbed energy per mole of dye molecules to one-half output pulse energy was 36 GJ mol-1 .

Singlet oxygen and dye-triplet-state quenching in solid-state dye lasers consisting of pyrromethene 567-doped poly(methyl methacrylate).

Solid-state dye lasers based on poly(methyl methacrylate) (PMMA) doped with Pyrromethene 567 dye (P567) have been investigated. The preparation techniques employed provided high photostability and laser damage threshold for P567 in pure PMMA with 270,000 pulses emitted before the conversion efficiency fell to half its initial value for a pump fluence of 0.16 J cm(-2). When PMMA was modified with 1, 4-diazobicyclo [2, 2, 2] octane singlet oxygen quencher, the longevity increased to 550,000 pulses, corresponding to a normalized photostability of 270 GJ mol(-1). Modification of PMMA with a triplet quencher (perylene) yielded no improvement, but in ethanol solutions both additives enhanced photostability. It is possible that in PMMA, stabilization by means of triplet quenching that depends on dye diffusion is prevented but that stabilization by means of singlet oxygen quenching that depends on the faster oxygen diffusion rate will succeed.

A pulse radiolysis and pulsed laser study of the pyrromethene 567 triplet state

Abstract The triplet state of pyrromethene 567, a molecule with potential as a solid state laser dye, has been characterized in benzene by pulse radiolysis in terms of its absorption spectrum, lifetime, self-quenching, electronic excitation energy, triplet–triplet extinction coefficient and oxygen quenching rate constant. The use of laser flash photolysis has then allowed determination of the triplet quantum yield, efficiency of formation of singlet oxygen ( 1 Δ g ), and the rate constant for reaction of the latter species with the ground state. The affects of oxygen on the fluorescence and triplet yields demonstrate that oxygen-induced intersystem crossing is important, the sum of these parameters being unity within experimental error. The mechanism of reaction of P-567 with 1 Δ g in benzene is predominantly physical in character with only a small (∼6%) contribution from chemical reaction.

Photorefractive performance of a CdSe/ZnS core/shell nanoparticle-sensitized polymer

We report the photorefractive performance of a polymer composite sensitized by CdSe/ZnS core/shell nanoparticles, and also comprising poly(N-vinylcarbazole) and an electro-optic chromophore. The nanoparticles are characterized by absorption and photoluminescence spectroscopy, elemental analysis, transmission electron microscopy, and powder x-ray diffraction. The electro-optic response of the composite is measured independently of the photorefractive effect by transmission ellipsometry. An asymmetric two-beam coupling gain of 30.6+/-0.4 cm(-1) is obtained, confirming photorefractivity. Degenerate four-wave mixing is used to assess photorefractive performance and, at a poling field of 70 V microm(-1), yields a diffraction efficiency of 4.21%+/-0.03%, a holographic contrast of 3.05 x 10(-4)+/-1 x 10(-6), a space-charge rise time of 25+/-2 s, and a sensitivity of 4.7 x 10(-5)+/-4 x 10(-6) cm3 J(-1). These results constitute a significant improvement on the performance of previous nanoparticle-sensitized photorefractive polymer composites.

Photorefractive polymer composite trapping properties and a link with chromophore structure

The photorefractive properties and the phase stability of polymer composites are dependent on the detail of the alkyl chain substituent attached to the electro-optic dye within the composite. Photorefractive composites based on poly (N-vinylcarbazole) (PVK), sensitized with trinitrofluorenone (TNF) and mixed with a concentration of 47.5 wt. % of electro-optic dye have been tested for photorefractive performance. Two alternative azo dyes of identical molecular weight have been used to produce alternative composites; both dyes were modified to suppress spatial isomerism and incorporated an eight carbon alkyl chain at the electropositive end of the chromophore: either a straight octyl chain or a branched ethylhexyl chain was substituted. The reorientational enhancement of photorefractive performance is similar in the composites resulting from these dyes. The dye with a straight octyl chain led to a composite with improved holographic performance. The dye with a branched ethylhexyl chain led to a composite ex...

Photorefractive holographic contrast enhancement via increased birefringence in polymer composites containing electro-optic chromophores with different alkyl substituents

The birefringence of a photorefractive polymer composite in response to an applied electric field is used to predict the holographic index contrast attainable. Predictions have been obtained for three different polymer composites based on the charge transfer complex matrix poly(N-vinyl carbazole)/2,4,7-trinitro-9-fluorenone, doped with electro-optic azo chromophores differing only in their alkyl substitutions. A comparison with experimental data indicates that unexpected large variations in the holographic contrast obtainable from different chromophores are, in most cases, accounted for by a corresponding variation in electric field-induced birefringence. This variation may be due to a combination of the chromophore number density and the composite viscosity. Where the birefringence does not correlate with holographic index contrast, a prediction based on a trap density limited space-charge field fits the data well.

Photorefractive performance of polymer composite sensitized by CdSe nanoparticles passivated by 1-hexadecylamine

The performance of a photorefractive polymer composite sensitized by 1-hexadecylamine capped CdSe nanoparticles is reported. The polymer composite also comprises the charge transporting matrix poly(N-vinylcarbazole) and the electro-optic chromophore 1-(2-ethylhexyloxy)-2,5-dimethyl-4-(4-nitrophenylazo) benzene. At an applied field of 70 V μ m−1 two beam coupling gain of 13.2 cm−1 was observed, confirming the photorefractive nature of the induced grating. At the same field, a holographic contrast of 9.12×10−4±6×10−6, a photorefractive sensitivity of 5.1×10−4 ±0.2×10−4 cm3 J−1 and a space-charge field rise time of 13±1 s were obtained.

Characteristics of dye-doped ormosil lasers

New hybrid organic-inorganic materials synthesized at room temperature and doped with laser dyes have been tested for laser action. The new ormosil material has excellent optical properties and is an effective host for many organic molecules. Two members of the perylene family of laser dyes, which are characterized by increased degrees of photostability, have been successfully incorporated into this host. The spectroscopic properties of the perylene dyes are not significantly altered by incorporation into ormosil. Dye doped samples with polished faces have been studied by longitudinal excitation in a compact cavity by the second harmonic of a Nd:YAG laser emitting 27 mJ in 6 ns pulses at 532 nm. Aspects of laser performance including slope efficiency, photostability, and output wavelength are reported. Experiments were conducted to establish the normalized photostability, which is the accumulated pump energy absorbed by the system per mole of dye molecules in the gain region before the output pulse energy falls to one-half of its initial value. These measurements allow a comparison between different dyes and hosts.

Comparison of solid state dye laser performance in various host media

Over the past twenty years there has been much research into the laser operation of solid materials doped with laser dyes. Although some research has developed high performance systems for operation at, for example, unusual wavelengths or with narrow linewidth, the most pertinent performance parameters remain the efficiency and the photostability. By reviewing all the published data concerning these parameters it is concluded that the main factor determining performance is the choice of laser dye. The performance of rhodamine 6G in solid-state dye lasers has not been improved and the only progress that has been made in this field is due to the introduction of the pyrromethene dyes which can now be used for many applications. Although optimization of the host material can improve performance of pyrromethene solid-state dye lasers no clear advantage can be gained by choosing either a glass or a plastic host. Long pulse length flashlamp pumped operation remains challenging, at least in part due to triplet state losses.