Short-wavelength Edge Research Articles

Nonlinear optical crystal BiAlGa 2(BO 3) 4

The new nonlinear optical crystals BiAlGa 2(BO 3) 4 have been grown by spontaneous crystallization with molten flux based on a Bi 2O 3–B 2O 3 solvent. From single crystal X-ray diffraction measurement, BiAlGa 2(BO 3) 4 has been found to crystallize in the trigonal huntite structure type, space group R32, with cell dimensions a = 9.4433(9) and c = 7.4130(10) Å. The diffuse reflectance spectrum on a powder sample indicated that the short-wavelength absorption edge of BiAlGa 2(BO 3) 4 extends to approximately 271 nm. Second-harmonic generation (SHG) on powder samples has been measured using Kurtz and Perry technique, which indicated that BiAlGa 2(BO 3) 4 is a phase-matchable material, and its SHG coefficient is measured to be four times as large as that of KDP.

Tunable one‐dimensional photonic crystal structures based on grooved Si infiltrated with liquid crystal E7

AbstractTuning of a photonic band gap in a composite one‐dimensional photonic crystal structure infiltrated with nematic liquid crystal E7 has been evaluated by simulations. It has been found that a sufficient shift of the short wavelength edge, λ edge of a main photonic band gap can be obtained in structures with a pronounced initial alignment of the liquid crystal director in the grooves of a single crystalline Si matrix. The largest effect is predicted for the reorientation of liquid crystal molecules from planar homogeneous alignment along the grooves to homeotropic alignment with respect to the Si walls due to an electro‐optical effect. The shift in photonic band gap due to thermal tuning, resulting from the transition of liquid crystal from the mesophase to the isotropic phase, is less than for electric tuning. Thermo‐tuning has been demonstrated experimentally. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Four-wave mixing of solitons with radiation and quasi-nondispersive wave packets at the short-wavelength edge of a supercontinuum

We apply the recently developed theory of frequency generation by mixing of solitons and dispersive waves [Phys. Rev. E 72, 016619 (2005)] to explain the observed formation, quasi-trapping and frequency shift of the spectral peaks at the blue edge of supercontinua generated in silica-core photonic crystal fibers.

Fabrication and characterization of an all-solid 2D photonic bandgap fiber with a low-loss region (< 20 dB/km) around 1550 nm

The fabrication and characterization of an all-solid photonic bandgap fiber is reported. The fiber presents a low-loss region (< 20 dB/km) around 1550 nm and can be used as single-mode even for a fiber core diameter as large as 20 microm. The fiber presents a zero dispersion at the short wavelength edge of the bandgap. The measured polarisation mode dispersion is wavelength dependent but remains small (few ps/km1/2). This fiber opens the possibility to realize low-loss large mode area bandgap fiber with a doped core and or Bragg gratings.

Improved supercontinuum generation through UV processing of highly nonlinear fibers

We demonstrate that UV exposure of highly nonlinear, germano-silicate fibers can significantly broaden the infrared supercontinuum generated by femtosecond pulses in these fibers. Both simulations and measurements of the fiber chromatic dispersion show that UV-induced refractive index changes increase the waveguide dispersion by up to 5 ps/(nm-km) at 1570 nm and shift the dispersion zero by over 100 nm. We examine fibers with a range of UV exposure levels and show that the short wavelength edge of the supercontinuum can be continuously changed by more than 100 nm. We also show that the long wavelength edge is extended beyond that of the unexposed fiber. The resulting continuum spans from 0.85 to 2.6 /spl mu/m. Cutback measurements show that the supercontinuum in the exposed fiber is generated in as little as 1 cm of fiber. A nonlinear Schro/spl uml/dinger equation (NLSE) model of the supercontinuum generation in the nonlinear fiber shows that the short wavelength behavior of the continuum is primarily controlled by changes in the fiber dispersion caused by the UV-induced change in refractive index of the fiber core.

Supercontinuum generation in ultraviolet-irradiated fibers

We demonstrate that UV exposure of highly nonlinear, germanosilicate fibers causes a strong change in their chromatic dispersion and can significantly alter the infrared supercontinuum generation in these fibers. By varying the level of UV exposure to the fiber, we show that the dispersion zero and the short-wavelength edge of the supercontinuum can be changed by more than 100 nm. A nonlinear Schrödinger equation model of the continuum generation in the nonlinear fiber shows that the short-wavelength behavior of the continuum is primarily controlled by changes in the fiber dispersion caused by the UV-induced change in the refractive index of the fiber core.

The State of Protoplanetary Material 10 Million years after Stellar Formation: Circumstellar Disks in the TW Hydrae Association

We have used the Spitzer Space Telescope Infrared Spectrograph to observe seven members of the TW Hya association, the nearest stellar association whose age (~10 Myr) is similar to the timescales thought to apply to planet formation and disk dissipation. Only two of the seven targets display infrared excess emission, indicating that substantial amounts of dust still exist closer to the stars than is characteristic of debris disks; however, in both objects we confirm an abrupt short-wavelength edge to the excess, as is seen in disks with cleared-out central regions. The mid-infrared excesses in the spectra of Hen 3-600 and TW Hya include crystalline silicate emission features, indicating that the grains have undergone significant thermal processing. We offer a detailed comparison between the spectra of TW Hya and Hen 3-600, and a model that corroborates the spectral shape and our previous understanding of the radial structure of these protoplanetary disks.

Electronic absorption spectra of symmetric cationic dye in constant electric field

The electronic absorption coefficient of polymer films doped with symmetric cationic polymethine dye external electric field constant changes is researched. This effect is characterised by the short-wavelength band edge intensity increases and it decreases on the long-wavelength edge. The dye cation charge distribution in the model electric field 10 8 V m −1 of point charges was calculated by the method AM1. On the basis of the quantum chemical calculations the spectral regularities in electric field is interpreted by the cation electronic charge changes. The theoretical model, based on the eigenfrequencies value changes of the charged anharmonic oscillators under operation field is offered for the observed effects description. The experimental spectra well correlate with the theoretically calculated.

Dramatic Enhancement of Third-Harmonic Generation in Three-Dimensional Photonic Crystals

We have observed a dramatic enhancement of third-harmonic generation in 3D polystyrene-air photonic crystals pumped by a near infrared laser beam. As the pump wavelength is tuned, the peak of the enhancement occurs when the third-harmonic wavelength approaches the short-wavelength edge of the band gap. We show that the origin of the enhancement is phase matching provided by the periodic structure of the photonic crystals.

Secondary electron emission and self-consistent charge transport and storage in bulk insulators: Application to alumina

The self-consistent charge transport in bulk alumina samples during electron beam irradiation is described by means of an iterative computer simulation. Ballistic electron and hole transport as well as their recombination and trapping are included. As a main result the time-dependent secondary electron emission rate σ(t) and the spatial distributions of currents j(x,t), charges ρ(x,t), the field F(x,t), and the potential slope V(x,t) are obtained. For bulk insulating samples, the time-dependent distributions approach the final stationary state with j(x,t)=const=0 and σ=1. Especially for low electron beam energies E0=1 keV, the incorporation of charges can be controlled by the potential VG of a vacuum electrode in front of the target surface. Finally, for high electron beam energies, the real negative surface potential V0&amp;lt;0 is measured by x-ray bremsstrahlung spectra and the shift of the short wavelength edge. For the initial beam energy E0=30 keV, the experimental value V0=−16 kV is still in good agreement with our simulations.

Spectral macro-bending loss considerations for photonic crystal fibres

Both a scalar method and a fully vectorial method have been applied to modelling the bending losses in photonic crystal fibres (PCFs). Due to a decreasing index difference between the core and the cladding towards short wavelengths, an unusual loss mechanism is seen, that is a short-wavelength loss edge. This is verified by measurements. For large-mode area PCFs, the model gives a massive overestimate, never an underestimate.

Optical properties of new aliphatic–aromatic co-polyimides

Optical transmission of a series of aliphatic–aromatic co-polyimides have been investigated in the range 200–3000 nm. The short wavelength edge of transmission depends on the ratios of diamines (aliphatic to aromatic) and on the type of aliphatic diamine. To obtain the optical parameters the approach proposed by Tauc for amorphous semiconductors has been used, because of the similarity of the absorption edges. The values of pseudogaps were found from 1.01 to above 4 eV, while the Urbach energy changed in the range 75–800 meV. All determined parameters have been found to be related to the influence of the polymer chain structure.

Scanning infrared radiometer for measuring the air–sea temperature difference

We describe a vertically scanning infrared radiometer for measuring the air-sea temperature difference without disturbing the water skin layer. The radiometer operates with a single wavelength channel that is 1.1 mum wide, centered on 14.2 mum, on the short-wavelength edge of a CO(2) atmospheric absorption band. The resulting high atmospheric absorption enables calibration of the horizontal-viewing signal with an in situ air-temperature sensor. The signal at all other scan angles is measured relative to that at the horizontal, providing a differential air-sea temperature measurement that is nearly independent of calibration offsets that can be a problem with independent air- and water-temperature sensors. We show data measured on a ship in the Tropical Western Pacific Ocean during July 1999, which exhibit important discrepancies from in situ data using bulk air-and water-temperature sensors. These discrepancies illustrate important differences between bulk versus skin water temperature.

X-ray spectrum determination from the angular dispersion of radiation in a diamond prism

The X-ray spectrum of hard polychromatic radiation with energies of up to 40 keV was obtained from the angular dispersion of the radiation beam in a diamond prism. In contrast to the classical optical scheme, the beam to be analyzed is passed through the entrance face of the prism without refraction and is directed onto the exit face at a glancing angle ∼(2δ)1/2, where δ is the real part of the decrement of the refractive index for the short-wavelength edge of the spectrum. The spectrum distortions caused by the intersection of the reciprocal lattice points by the Ewald sphere at a fixed number of wavelengths λ are minimized by the use of the angular divergence of the beam of ∼10′ in the plane perpendicular to the refraction plane. In the energy range of 8–9 keV, an energy resolution of less than 100 eV was obtained, which exceeds the corresponding parameter for cooled semiconductor detectors. The measuring scheme suggested makes it possible to solve the problem of the analysis of spectra of directional X-ray beams when studying fast nonstationary generation processes.

Bragg-grating-enhanced polarization instabilities

Experiments demonstrate a dramatic decrease in polarization-instability threshold as an optical pulse is tuned near the short-wavelength edge of the photonic bandgap formed by a fiber Bragg grating. These enhanced nonlinear interactions and birefringent effects are modeled with coupled-mode numerical simulations. Nonlinearities are shown to increase much more rapidly than the effective birefringence as the pulse wavelength approaches the bandgap edge.

Tunable two-dimensional photonic crystals using liquid crystal infiltration

The photonic band gap of a two-dimensional photonic crystal is continuously tuned using the temperature dependent refractive index of a liquid crystal. Liquid crystal $E7$ was infiltrated into the air pores of a macroporous silicon photonic crystal with a triangular lattice pitch of 1.58 $\ensuremath{\mu}$m and a band gap wavelength range of 3.3--5.7 \ensuremath{\mu}m. After infiltration, the band gap for the H polarized field shifted dramatically to 4.4--6.0 \ensuremath{\mu}m while that of the E-polarized field collapsed. As the sample was heated to the nematic-isotropic phase transition temperature of the liquid crystal $(59\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}),$ the short-wavelength band edge of the H gap shifted by as much as 70 nm while the long-wavelength edge was constant within experimental error. Band structure calculations incorporating the temperature dependence of the liquid crystal birefringence can account for our results and also point to an escaped-radial alignment of the liquid crystal in the nematic phase.

Synthesis and optical properties of the Ge–Sb–S:PrCl 3 glass system

The (GeS 2) 80− x (Sb 2S 3) 20 · xPrCl 3 and (GeS 2) 60− x (Sb 2S 3) 40 · xPrCl 3, ( x=0.01, 0.1, 0.5) glass systems were prepared from elements and PrCl 3 by thermal synthesis. The glasses were of orange to red colour, they were optically homogeneous up to 0.5 mol% of PrCl 3, the samples with 1 mol% of PrCl 3 were partly crystalline. The homogeneity was confirmed by optical methods and by electron microscopy. The glass transition temperatures, T g, were between 330°C and 333°C for (GeS 2) 80− x (Sb 2S 3) 20 · xPrCl 3 glasses and between 256°C and 274°C for (GeS 2) 60− x (Sb 2S 3) 40− x · xPrCl 3 glasses, ( x=0.01, 0.1, 0.5). The short-wavelength absorption edge was between 550 and 700 nm for (GeS 2) 60− x (Sb 2S 3) 40 · xPrCl 3 glasses and between 500 and 650 nm for (GeS 2) 80− x (Sb 2S 3) 20 · xPrCl 3 glasses. The long-wavelength absorption edge was between 900 and 1000 cm −1 and is apparently caused by multiphonon absorption of Ge–S and Sb–S vibration modes. Doping with Pr 3+ ions creates new absorption bands near 2100, 4980, 6370, 6640 and 9843 cm −1, which are assigned to the electron transitions from the 3H 4 level of Pr 3+ to the 3H 5, 3H 6, 3F 2, 3F 3, 3F 4, 1G 4 levels, respectively, of the Pr 3+ ion. The absorption bands corresponding to 3H 4 → 3H 6, 3F 2 and to 3H 4 → 3F 3, 3F 4 transitions are overlapped. The Raman spectra were deconvoluted into several bands, which can be assigned as the vibrations of GeS 4, SbS 3 and S 8 structural units. The structure of these glasses can be described as interconnected GeS 4 tetrahedra and SbS 3 pyramids with dissolved S 8 molecules and Pr 3+ ions. The luminescence spectrum consisted of two luminescence bands. The one at ∼1300 nm, was assigned to 1G 4– 3H 5 electron transitions of the Pr 3+ ions. The other luminescence band of smaller amplitude was ∼1600 nm and assigned to transitions between the 3F 3 and 3H 4 electron levels of Pr 3+.

Lasing in the vertical direction in quantum-size InGaN/GaN multilayer heterostructures

Lasing is discovered in the direction perpendicular to the surface in quantum-size InGaN/GaN multilayer heterostructures grown by vapor-phase epitaxy. At high excitation densities one of the modes in the luminescence spectrum, which is modulated by modes of the Fabry-Perot cavity formed by the GaN/air and GaN/sapphire-substrate interfaces, is sharply amplified and begins to dominate the spectrum. The dependence of the luminescence intensity on pump density has a clearly expressed threshold character. The threshold excitation density in the vertical direction is 5–6 times greater than the stimulated-emission threshold for observation from an end surface of the structure. The gain coefficient in the active region at the threshold for surface-emitting lasing is estimated as 2×105 cm−1. The interaction between the cavity modes and the gain spectrum is detected in the form of displacement (by up to 2.6 nm) of modes on the short-wavelength edge of the luminescence spectrum toward higher photon nergies. The characteristic temperature (T0) measured in the range from 16 to 120 K is 480 K. At higher temperatures T0 K.

Comparison between crystal field effects on the fluorescence spectra of crystalline SmP5O14 and vitreous (Sm2O3)(0.248)(P2O5)(0.752)

To investigate how the local symmetry of the Sm3+ ion affects the fluorescence of a samarium metaphosphate glass of composition (Sm2O3)0.248(P2O5)0.752, the temperature and pressure dependences of its laser induced fluorescence spectrum are compared with those of a samarium pentaphosphate crystal (SmP5O14). Findings include: (i) The crystal field splitting of the energy levels responsible for fluorescence in SmP5O14 at room temperature is consistent with the local symmetry of oxygen atoms of the phosphate cage around the Sm3+ ions being quite close to cubic – in accord with crystal structure. At 12 K there is a systematic disappearance of the shortest wavelength lines of each fluorescence band attributable to a decreasing population of higher crystal field levels, which are occupied at ambient temperature. (ii) The (Sm2O3)0.248(P2O5)0.7520.248 glass fluorescence spectrum forms five bands, which can be related to that of the crystal but with inhomogeneous line broadening; the short wavelength edges sharpen at low temperatures, also attributable to a decreasing population of higher crystal field levels at lower temperatures. (iii) The shifts (dλ/dp) in the wavelengths of the fluorescence peaks of the SmP5O14 crystal induced by pressure up to 50 kbar in a diamond anvil cell are small but measurable at room temperature, being about +0.03 nm kbar−1 (+0.3 A kbar−1). Application of pressures up to 50 kbar to the (Sm2O3)0.248(P2O5)0.752 glass did not alter the positions of the bands within the error in the fluorescence wavelength measurements. Neither the SmP5O14 crystal nor the metaphosphate glass showed any indication of undergoing a phase transition up to the highest pressure reached. A low frequency Raman mode has been observed, which softens with reducing temperature, indicating softening of the associated optical mode and suggesting that, like other RP5O14 crystals, SmP5O14 undergoes a ferroelastic phase transition.

On the origin of reversible photodarkening in amorphous As 2S 3 thin films

It is shown that the response of a-As 2S 3 virgin film on the illumination is `two-fold'. One part of the darkening observed is due to irreversible structural changes, the other part is due to reversible photodarkening (RPD) of the illuminated (originally virgin) film since the film is bleached after annealing. The reversible photodarkening is accompanied also by reversible changes in the slope of the short wavelength edge (SWE) and by small reversible changes in the infrared activity around 310 cm −1, the region of stretching motion of As–S bonds. The role of a substrate on the RPD process has not been confirmed. Also local overheating does not seem to be a plausible explanation for RPD. It is assumed that RPD is a multifactorial process involving the changes in the short range order which, however, are not sufficient and the dominant origin of the RPD process.