Estimation of the Refractive Index Change in Glass Induced by Femtosecond Laser Pulses

It is known that local refractive index change occurs when femtosecond laser pulses with extremely high peak power are launched into glass. We focused 130-femtosecond laser pulses of 800 nm into the bulk of glass and examined the shape of the induced refractive index change. We found that the length of the spot of the refractive index change along the optical axis reached about 30 μm despite the diameter being about 2 μm. To estimate the distribution of induced refractive index change, we fabricated Bragg grating by scanning the focused spot and calculated the amount of the change by applying Kogelnik’s coupled mode theory to the measured diffraction efficiencies of the higher order diffracted beams.     

Relaxation Oscillation Enhancement and Coherence Collapse in Semiconductor Lasers with Optical Feedback

The dynamic characteristics of a semiconductor laser with optical feedback are strongly dependent on the injection current and the reflectivity and position of the external feedback reflector. We investigated the relaxation oscillation enhancement and coherence collapse state of the laser oscillation based on the laser rate equations. It is well known that laser output power jumps with increase of the injection current due to external mode transition. But here for the first we time demonstrate the existence of a chaotic scenario within successive laser power jumps. The results calculated by numerical simulations based on the rate equations are compared with those of the experiments and good coincidence between them is found.     

Dynamics of Semiconductor Lasers with Optical Feedback from Photorefractive Phase Conjugate Mirror

The dynamics of semiconductor lasers with photorefractive phase conjugate optical feedback are experimentally studied. Photorefractive fringes considered here are rather static compared with time fluctuations of laser output power. Therefore, it is expected that a semiconductor laser with photorefractive feedback shows similar dynamics to those with conventional optical feedback. We examine relaxation oscillation and external cavity modes of laser output power in the presence of photorefractive phase conjugate feedback. It is proved that the dynamics of photorefractive phase conjugate feedback are fundamentally the same as those of conventional optical feedback.     

Effect of Substrates on the Addition Polymerization of 1,4-Benzenedithiol to 1,4-Diethynylbenzene in the Solid State and the Electronic Properties of the Resulting Polymer

The effect of substrates on the addition polymerization of 1,4-benzenedithiol (BDT) to 1,4-diethynylbenzene (DEB) in the solid state and the electronic properties of the polymers obtained were studied. As the substrate polymer sheets, for instance, PET (poly (ethylene terephthalate)) sheet, ON-6 (oriented nylon-6) sheet and so on having surface free energies Γ_s from 27.4 to 55.0 erg/cm² were used. At the monomer sublimation temperature of 60°C, the S wt% (sulfur content) and the cis content of the polymers were not affected by the kind of polymer sheets. However, the molecular weights, M¯_n of the polymers polymerized on the polymer sheets were 13,000–30,000, and the values were several times higher than the molecular weight of the polymers polymerized on glass plate. On the other hand, at the sublimation temperature of 82°C, the cis content of the polymers apparently increased with decreasing d-value of the polymer sheets. On X-ray diffraction patterns of monomer mixtures sublimed onto polymer sheets, the diffraction intensities and the diffraction peak positions were concerned with the d-value of the polymer sheets. Using polymer sheets, the diffraction peak intensities of the monomer mixture at 7.73 and 7.58 Å decreased compared with those on glass plate. In contrast, the peak at 3.65 Å, which is a negligibly small peak on glass plate, obviously increased. However, as the d-value of the polymer sheets (PET 3.45 Å; OPP (oriented polypropylene) 5.2 Å) increased, the diffraction peak intensities at 7.73 Å and 7.58 Å gradually increased and the diffraction peak intensity at 3.65 Å gradually decreased. The parallel electrical conductivities (σ_||) toward the layered structural polymer on PET, ON-6 and glass plate under air atmosphere were 10⁻⁷, 10⁻⁹ and 10⁻¹¹ S/cm, respectively. Under a reduced pressure of 10⁻³ mmHg, the σ_|| values of each polymer lowered by one or two orders of magnitude. On the other hand, the σ_|| values of the nonlayered structural polymers under air atmosphere were about 10⁻¹¹–10⁻¹² S/cm and were independent of the substrates. Even under a reduced pressure of 10⁻³ mmHg, the σ_|| values hardly changed and remained at 10⁻¹¹–10⁻¹² S/cm. The vertical electrical conductivities (σ_⟂) of the layered structural polymers on conductive PET sheet coated by indium tin oxide or NESA glass plates were independent of the substrates and were 10⁻¹⁴ S/cm under air atmosphere. The σ_⟂ values of the nonlayered structural polymers also exhibited the same values. The reversible change of the amount of the layered structural polymer on PET sheet was also caused by irradiation of the photo-light which is the effective wavelength for the phase transition of the polymers mounted on glass plate. The σ_|| value of the layered structural polymer on ON-6 sheet reversibly changed with the amount of the layer structure controlled by the photo-light, that is, the σ_|| increased up to about one order of magnitude by the photo-light at 545.6 nm. On the other hand, the _|| decreased to about one order of magnitude by the photo-light at 539.6 nm. Anisotropic conductivity with respect to σ_|| and σ_⟂, and oxygen doping mechanisms were discussed in relation to the layer structure of polymers. © 1997 John Wiley & Sons, Ltd.     

Synthesis and crystallographic studies of garnet-related lithium-ion conductors Li6CaLa2Ta2O12 and Li6BaLa2Ta2O12

High-purity specimens of Li₆CaLa₂Ta₂O₁₂ and Li₆BaLa₂Ta₂O₁₂ have been successfully synthesized by solid-state reactions. The analytical chemical compositions of these samples were in good agreement with the nominal compositions of Li₆CaLa₂Ta₂O₁₂ and Li₆BaLa₂Ta₂O₁₂. The Rietveld refinements verified that these compounds have the garnet-type framework structure with the lattice constants of a = 12.725(2) Å for Li₆CaLa₂Ta₂O₁₂ and a = 13.001(4) Å for Li₆BaLa₂Ta₂O₁₂. All of the diffraction peaks of X-ray powder diffraction patterns were well indexed on the basis of cubic symmetry with space group Ia-3d. To make a search for Li sites, the electron density distributions were precisely examined by using the maximum entropy method. Li⁺ ions occupy partially two types of crystallographic site in these compounds: (i) tetrahedral 24d sites, and (ii) distorted octahedral 96h sites, the latter of which are the vacant sites of the ideal garnet-type structure. The present Li₆CaLa₂Ta₂O₁₂ and Li₆BaLa₂Ta₂O₁₂ samples exhibit the conductivity σ = 2.2 × 10^? 6 S cm^? 1 at 27 °C (E_a = 0.50 eV) and σ = 1.3 × 10^? 5 S cm^? 1 at 25 °C (E_a = 0.44 eV), respectively.     

100-femtosecond MeV electron source for ultrafast electron diffraction

A near-relativistic 100-fs MeV electron beam is developed by using a photocathode rf gun for revealing the hidden ultrafast dynamics of intricate molecular and atomic processes in materials through experimentation of ultrafast time-resolved electron diffraction (UED). The transverse and longitudinal dynamics of femtosecond electron beam in the rf gun were studied theoretically by particle simulation. The growths of the emittance, bunch length and energy spread due to the rf and space charge effects were investigated by changing the laser parameters, field gradient and electron charge. The theoretical studies indicate that a 100-fs MeV electron beam with the transverse emittance of 0.1 mm mrad and the relative energy spread of 10⁻³–10⁻⁴ at bunch charge of 0.1–2 pC (10⁶–10⁷ electrons per pulse) is achievable for UED, in which the intensity is three orders of magnitude higher than that produced by the conventional dc or pulsed guns.     

A Chemical System that Mimics Decoding Operations

The chemical information stored in equilibrium mixtures of molecular species is larger than the sum of information carried by the individual molecules. Protonation equilibria in dilute dichloromethane solution of a shape‐persistent macrocycle bearing two 2,2′‐bipyridine units and two Coumarin 2 moieties (see figure) can be exploited to mimic decoding operations.