Paras N. Prasad

High contrast switching of distributed-feedback lasing in dye-doped H-PDLC transmission grating structures

Electrically switched distributed-feedback (DFB) lasing action is presented in a Pyrromethene 580 lasing dye-doped holographic polymer dispersed liquid crystal (H-PDLC) transmission grating structure. This design, when compared with the previously utilized H-PDLC reflection grating structure, has the advantage of a greatly enlarged gain length (10 mm) and a low concentration of liquid crystal (20%) while maintaining sufficient refractive index modulation. The experimental results demonstrate that the emitted laser bandwidth (~5 nm) can be obtained with a pump energy threshold of ~0.3 mJ at three different wavelengths, 561 nm, 569 nm and 592 nm, corresponding to three different grating spacings. The near- and far-field measurements have shown a high directionality of the lasing output. The lasing can be electrically switched off by an applied field of 30V/mum. The temporal, spectral, and output/input properties of the laser output are also presented.

Novel two-photon-absorbing, 1,10-phenanthroline-containing π-conjugated chromophores and their nickel(II) chelated complexes with quenched emissions

Novel 1,10-phenanthroline-containing π-conjugated chromophores with varied electron donors were synthesized and characterized together with their corresponding nickel(II) chelated complexes. The linear absorption maxima for these chromophores could be tuned from 357 to 415 nm by using different electron donors. The 1,10-phenanthroline-containing π-conjugated chromophores exhibit strong fluorescence in solutions upon one- or two-photon excitation. The fluorescence of these chromophores can be quenched or partially quenched by some metal ions due to an excellent metal ion chelating ability of the 1,10-phenanthroline backbone. The nickel(II) chelating complexes show quenched emission, both in dilute and concentrated solutions, compared to their metal ion-free chromophores. Their two-photon absorption (TPA) spectra are obtained using a newly developed multi-photon spectroscopic technique which utilizes a single strong femtosecond white-light continuum generation beam. The two-photon absorption spectra show that the TPA band for these compounds can be tuned by choosing different electron donors. The maximum TPA cross-section σ2 values for the three metal ion-free chromophores are variable from 0.58 to 2.26 × 10−20 cm4 GW−1 n (±15%). The metal ion-free chromophore-containing dihexylamino groups (9c) has a larger TPA cross-section σ2 value compared to the chromophores containing alkyloxy groups or alkylthio groups. When the metal ion-free chromophores coordinate with the nickel(II) ion, the resulting complexes do not lose their excellent two-photon-absorbing ability. The nickel(II) chelated complexes display red-shifted two-photon absorption bands compared with their metal ion-free chromophores.

Synthesis, two- and three-photon absorption, and optical limiting properties of fluorene-containing ferrocene derivatives

The synthesis and characterization of two novel fluorene-containing ferrocene derivatives were reported. The two-photon absorption spectra and three-photon absorption cross-section values in the IR region for these two chromophores were studied. Together with their thermal stabilities, their linear absorption and emission properties were also investigated. The results show that the ferrocene derivatives have large two-photon and three-photon absorption in IR region as well as excellent thermal stabilities. The three-photon absorption based optical limiting properties of these two ferrocene derivatives were investigated by using sub-picosecond IR laser pulses.

Optical microfabrication of highly reflective volume Bragg gratings

An approach for fabricating wide-bandwidth and highly reflective Bragg grating structures with the technique of holographic photopolymerization of a liquid crystal (LC) polymer composite is presented. The key to this fabrication method that distinguishes it from previous methods is the use of a nonreactive solvent, acetone, to dissolve the photoinitiator and coinitiator in an acrylate monomer/LC mixture. The addition of acetone results in the creation of controllable periodic voids inside the thin film after the acetone evaporates. Peak reflectivity as high as 80% and a broad reflection bandwidth of 80nm were observed in the reflection gratings formed with acetone present in the starting mixture. It was estimated from a fit to the experimental data that the resulting index mismatch was approximately 0.2; consistent with the presence of air voids. It is determined that tunable wavelength, diffraction efficiency, and bandwidth of reflection notches can be achieved by backfilling with fluids of different ref...

Two-, three-, and four-photon-pumped stimulated cavityless lasing properties of ten stilbazolium-dyes solutions

Two-, three-, and four-photon-pumped stimulated emission (cavityless lasing) properties of ten novel stilbazolium dyes in solution phase have been comprehensively studied. These newly synthesized multiphoton active dye compounds have the same molecular backbone but differ either in their electron donors or in their electron acceptors and can be utilized to generate highly directional stimulated emissions over a broad visible spectral range (from 490 to 618 nm) under multiphoton-pump conditions. The pump source was a powerful Ti:sapphire oscillator-amplifier system associated with an optical-parametric generator, which could specifically provide ∼160 fs duration and ∼775, 1320, and 1890 nm laser pulses for two-, three-, and four-photon excitation, respectively. The spectral, spatial, and temporal properties as well as the efficiency of multiphoton-pumped lasing output from different dye-solution samples have been studied. Based on the measured results, two salient features have been found: (i) the threshold pump-energy values for two-, three-, and four-photon-pumped lasing were quite close (within a factor of 3-4); and (ii) there was an obvious wavelength difference (10-30 nm) between the forward and backward lasing output under three- and four-photon-pump conditions.

Quasi-reversible photoluminescence quenching of stable dispersions of silicon nanoparticles

Optically clear and stable dispersions of brightly photoluminescent Si nanoparticles were obtained by covalent attachment of alkenoic compounds to the particles. Quenching of photoluminescence by ethylamine, diethylamine, triethylamine, pyrazine, and piperazine was investigated. The photoluminescence was quenched by the action of these nitrogenous species, but in some cases could be partially restored by the addition of trifluoroacetic acid. The extent of restoration of photoluminescence, after equilibrium is reached, was independent of the sequence of addition of the amine and the acid. The photoluminescence quenching and recovery are influenced by a combination of basicity, polarity, and steric factors of the quencher molecules. The quenching and subsequent restoration occurs gradually at room temperature and it takes several minutes to reach equilibrium.

Two- and three-photon absorption based optical limiting and stabilization using a liquid dye

It is experimentally shown that both two- and three-photon absorption in a high-molar-concentration chromophore system can be more efficiently utilized to accomplish optical power limiting and stabilization at laser wavelengths of 1.064 and /spl sim/1.3 /spl mu/m, respectively. The nonlinear absorbing medium is a novel liquid dye system, trans-4-[4-(dihexylamino)styryl]-N-(2-{2-[2-(2-hydroxy-ethoxy) -ethoxy]-ethoxy}ethyl)-pyridini um p-tosylate (abbreviated as ASEPT), consisting of chromophore molecules capable of multiphoton absorption in the near IR range. The nonlinear transmission property and output/input characteristics have been studied based on this liquid dye system, using nanoseconds 1.064-/spl mu/m laser pulses for two-photon excitation and /spl sim/1.3-/spl mu/m subpicoseconds laser pulses for three-photon excitation. A fairly good optical stabilization capability of this new material has been demonstrated at both laser wavelengths. The relative intensity fluctuation of the laser pulses can be remarkably reduced by simply passing through this multiphoton absorbing medium.

Enhancement of third-harmonic generation in a polymer-dispersed liquid-crystal grating

We report the observation of significant enhancement of one-step third-harmonic generation in a one-dimensional photonic crystal pumped by a near-infrared laser beam tuned to the low-frequency edge of the first photonic band gap. The third-harmonic phase matching can be controlled by changing the angle of incidence of the fundamental radiation, allowing tunability of the third-harmonic wavelength. The observed phenomenon was modeled theoretically using the transfer-matrix method. The enhancement is attributed to the combined action of phase-matching between the pump and harmonic waves and pump-field localization within the photonic crystal.

DNA-based materials for electro-optic applications : current status

Purified deoxyribonucleic acid (DNA), derived from salmon milt and roe sacs, waste products of the Japanese fishing industry in Hokkaido, has been processed into a promising, optical waveguide quality, biopolymer material suitable for both passive and active optical and electro-optic applications. Intercalation of aromatic compounds into stacked layers within the double helix of DNA molecules has rendered active optical waveguide materials with excellent nonlinear optical properties.

Ab initio studies of two-photon absorption of some stilbenoid chromophores

Two-photon absorption of a series of donor-acceptor trans-stilbene derivatives is studied by means of density functional theory applied to second-order response function. Several important issues in modeling are highlighted which must be addressed for a reliable reproduction of the experimental results. It is evident that the correct order of magnitude of calculated two-photon absorption cross sections can only be obtained if proper account is taken of vibrational broadening of the absorption profiles. A comparison of the theoretical results with the experimental ones indicates that the computed two-photon absorption cross sections are in rough agreement with our previous report, although the observed systematic increase of the cross sections with the electron acceptor strength is not well reproduced. It is suggested that this disagreement may be due not only to the deficiencies of the computations but also to a variety of factors contributing to the experimental value of the effective two-photon absorption cross section, which are not taken into account in the ab initio calculations.