Bragg Order Research Articles

Organic dual-wavelength distributed feedback laser empowered by dye-doped holography

An organic dual-wavelength distributed feedback (DFB) laser empowered by a dye-doped holographic polymer dispersed liquid crystal (HPDLC) grating is reported for the first time. The dual-wavelength laser operates at 586.6 nm and 670.2 nm simultaneously in one laser beam via the seventh and eighth Bragg orders of a one dimensional (1D) HPDLC grating with a special period. We theoretically and experimentally demonstrate the mechanism of our dual-wavelength DFB laser and investigate the threshold and spectral properties of the output laser. Our organic dual-wavelength DFB laser shows the advantages of simple configuration, one-step fabrication, cost-effectiveness and the potential application to be integrated in all optical networks.

Contouring of images by means of the Bragg acoustooptical diffraction into two orders

Contouring of a rectangular image by the Bragg acoustooptical diffraction into two symmetric orders is studied. It is proven theoretically and experimentally that, unlike the ordinary diffraction into one Bragg order, the diffraction into two orders is provides for significantly more efficient contouring: the image contour becomes approximately half as wide.

Generation of adaptive coordinates and their use in the Fourier Modal Method

We present an improvement of the standard Fourier Modal Method (FMM) for the analysis of lamellar gratings that is based on the use of automatically generated adaptive coordinates for arbitrarily shaped material profiles in the lateral plane of periodicity. This allows for an accurate resolution of small geometric features and/or large material contrasts within the unit. For dielectric gratings, we obtain considerable convergence accelerations. Similarly, for metallic gratings, our approach allows efficient and accurate computations of transmittance and reflectance coefficients into various Bragg orders, the spectral positions of Rayleigh anomalies, and field enhancement values within the grating structures.

New acoustooptic effect: Constant high diffraction efficiency in wide range of acoustic power

A new effect, viz., acoustooptic Bragg diffraction without the overmodulation mode, in which the efficiency of the Bragg order attains its maximal value (close to 100%) upon an increase in the intensity of an acoustic wave and then remains practically unchanged, is predicted theoretically and observed experimentally. The effect takes place in the case of considerable bending of phase fronts of the acoustic field in the acoustooptic diffraction plane and attains its maximal value at a relatively low frequency of sound, a small width of a piezoelectric transducer, strong acoustic anisotropy of the medium, and a large distance between the light beam and the piezoelectric transducer.

Myelin Structural Integrity in a Model for Human Early-Onset CMT1B

Charcot-Marie-Tooth disease type 1B (CMT1B), a peripheral neuropathy, is caused by mutations in MPZ, the gene encoding protein zero (P0), the major integral protein of PNS myelin. An adhesive protein, P0 plays a significant role during elaboration and maintenance of multilamellar myelin. P0 mutation Arg69Cys (R69C) causes a severe early-onset form of CMT1B. To elucidate the pathogenesis of this neuropathy, an Arg69Cys knock-in mouse was generated by targeting the Arg69Cys mutation to one MPZ allele by homologous recombination in ES cells. Here we report our x-ray diffraction (XRD) measurements on the periodicity, membrane structure, and amount of myelin in unfixed, freshly-dissected nerves from wildtype (WT or +/+), heterozygous (R69C/+), and homozygous (R69C/ R69C) mice. CNS myelin (optic nerve) was also examined. The diffraction patterns showed decreasing strength of scattering intensity from myelin: WT > R69C/+ > R69C/R69C, indicating decreasing relative amounts of myelin. By contrast, optic nerves exhibited no such differences among genotypes. From the positions of the reflections the myelin periods of sciatic but not optic nerves were found to differ among the genotypes: 177.0 ± 0.4 A for WT, 178.4 ± 0.5 A for R69C/+, and 193.1 ± 4.2 A for R69C/R69C. The calculated electron density profiles showed R69C/R69C's wider period derived from ∼20 A-swelling at the extracellular apposition. The extent of membrane packing distortion (Δ/d) in PNS myelin, calculated using Bragg order peak widths, was 25% greater in R69C/+ and doubled in R69C/R69C compared to WT. Differences in amount of myelin, period, and Δ/d among the genotypes were statistically significant at p < 0.001. Finally, comparison of R69C/+ with P0± and R69C/R69C with P0-/- suggested the small amount of mutant P0 that enters the myelin may detrimentally affect myelin-myelin interactions to produce less regular/unstable packing.

Nonlocal mode mixing and surface-plasmon-polariton-mediated enhancement of diffracted terahertz fields by a conductive grating

This work generalizes our previous study of the effect of plasma-wave induced longitudinal fields [D. H. Huang et al. J. Appl. Phys. 100, 113711 (2006)]. We incorporate a conductive grating on top of a conducting sheet for generating and mixing Bragg modes arising from a reflected and/or transmitted electromagnetic field. It also generalizes our recent work on the coupling of electronically modulated two-dimensional plasmons with the surface plasmon [G. Gumbs and D. H. Huang, Phys. Rev. B 75, 115314 (2007)] by including the retardation and longitudinal-field effects in calculating the diffracted near and far electromagnetic fields. The main result of this paper is the prediction of large enhancements at the band edges of a coupled Bloch-surface-plasmon-polariton band in the spectrum of the reflected far electromagnetic field due to anticrossing gaps induced by the strong coupling between the continuous surface-plasmon-polariton mode and discrete Bloch-like modes. The existence of these Bloch-like modes is a direct consequence of the nonlocal mixing of specular and diffraction modes of the reflected electromagnetic field by free-electron induced optical polarization and the interference of a pair of surface optical-polarization waves with opposite Bragg order numbers in the presence of a grating. The interference of these two counterpropagating surface waves leads to the formation of a Wannier-like state with associated electromagnetic fields localized within the grating-gap regions. The effects of the sheet density, plasmon instability, grating period, angle of incidence, and absorption loss on the optical enhancements of both transmitted and reflected electromagnetic fields are investigated.

Quantum phase transitions in magnetism and superconductivity: Emergent spin topology seen with neutrons

Magnetic spins and charges interact strongly in high-temperature superconductors. New physics emerges as layers of copper oxide are tuned towards the boundary of the superconducting (SC) phase. As the pseudogap increases the characteristic spin excitation energy decreases. We show that our well-annealed YBa 2Cu 3O 6+ x (YBCO 6+ x ) single crystals are orthorhombic and superconducting but not antiferromagnetically ordered. Near the critical concentration for superconductivity for x=0.35 the spins fluctuate on two energy scales, one a relaxational spin response at ∼2 meV and the other a slow central mode that is resolution-limited in energy (<0.08 meV) but broad in momentum. The gradual formation on cooling of a central mode over a range of momenta suggests that the spin ground state from which coherent SC pairing emerges may be quantum disordered. We show that YBCO 6.35 adopts a homogeneous state that consists of highly organized frozen sub-critical three-dimensional spin correlations. The continuous spin evolution indicates that a single quantum state occurs in contrast to claims from site-based probes that lightly doped YBCO undergoes a transition to antiferromagnetic (AF) Bragg order followed by a sharp transition to a cluster glass phase. For x=0.35, where T c=18 K is reduced to 1/5 of T cmax, the spin ground state is reached without a sharp transition and consists of short correlations extending over only 8 Å between cells and 42 Å within the planes. Polarized neutrons show the angular spin distribution to be isotropic unlike the AF insulator. Since moment is conserved we interpret this as evidence for hole-induced spin rotations rather than decay.

Ge/Si self-assembled Islands for Photonics Applications

ABSTRACTWe first present an analysis of the band line-up in the case of SiGe/Si quantum wells and in the case of SiGe/Si self-assembled islands. The conduction and valence band diagrams are obtained from a 30 band k.p Hamiltonian which allows to describe simultaneously conduction and valence band states. The strain field is obtained from a microscopic valence force field theory. The band edge alignment is strongly dependent on the input parameters for this heterosystem. We determine the average valence band offset from photoluminescence measurements of heterostructures grown on relaxed SiGe buffer layers. A type II band line-up is calculated for all Ge compositions in the case of two-dimensional quantum wells and SiGe/Si self-assembled islands. The 30-band formalism allows the determination of the near-infrared interband recombination energy as a function of the self-assembled island structural parameters. We then present recent results obtained by embedding SiGe/Si self-assembled islands in two-dimensional photonic crystals. The photoluminescence of GeSi islands acts as an internal probe to characterize the optical properties of silicon-based two-dimensional photonic crystals designed for the near-infrared spectral range. Cavities, defect-free photonic crystals operated at the second Bragg order and two-dimensional photonic crystals fabricated on top of one-dimensional Bragg mirrors (2D + 1D) are described. We show that, in the case of 2D +1D structures, we can control the quality factor of optical modes at the second Bragg order by matching the resonance conditions and controlling the thickness of the layers. Photonic crystals with pure Ge layers are finally described.

A Comparative Study of Higher Order Bragg Gratings: Coupled-Mode Theory Versus Mode Expansion Modeling

The modal reflectivity and loss of lamellar diffraction gratings to be used in distributed-feedback and distributed-Bragg reflector lasers were computed in dependence of wavelength, duty cycle and Bragg order. Different methods based on the approximate coupled-mode theory and and the exact bidirectional mode expansion modeling were compared and a good mutual agreement was found. The slab Green's function needed to compute the coupling coefficients can be approximated by that of a homogeneous unbounded medium with sufficient accuracy.

Micro‐zone optical measurements on GaN based nitride/air distributed Bragg reflector (DBR) mirrors made by focused ion beam milling

Two kinds of three pairs nitride/air distributed Bragg reflector (DBR) with depth over 1.3 μm and vertical sidewall were successfully fabricated by focused Ga ion beam (FIB) milling. One was the combination of 1st Bragg order air gap and 3rd Bragg order semiconductor wall DBR-A for the wavelength 514 nm, the other was the DBR-B facet with 3rd air gap and 5th semiconductor wall at 405 nm. To our best knowledge, the period of about 283 nm of these deep etched nitride/air DBR is the smallest one reported so far. Under a white light illumination, the DBR area shows special color at the expected wavelength. The properties of these DBR were investigated by the micro-zone measurements of reflectivity and photoluminescence (PL) respectively. As a result, the reflectivity of nitride/air DBR mirrors exhibited as high as 3.2–2.4 times of that of cleaved nitride facet and FIB etched facet at the concerned wavelength. The measured reflectivity data were compared with the calculation results from transfer matrix method and discussed. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Volume holographic recording and readout for 90-deg geometry

When a prerecorded cross-beam hologram is reconstructed (so-called edge-lit readout) with a uniform plane wave and a point source, the resulting exact solutions reveal Bessel-function-type dif- fracted beam profiles, which are fundamentally modified under weak propagational diffraction. The case of a profiled beam readout with propagational diffraction may be analyzed using a transfer function ap- proach based on 2-D Laplace transforms. In a second series of investi- gations, dynamic readout from a cross-beam volume hologram recorded with two orthogonal uniform plane waves is considered for various de- pendences of the refractive index modulation with intensity. Typically, refractive index profiles that are proportional to the intensity (as in the case of Kerr-type media or photorefractives with predominantly photovol- taic effect) and to the derivative of the intensity (as in diffusion-dominated photorefractives) are considered. Two-dimensional nonlinear coupled equations are developed for the two (Bragg) orders for both cases. Closed form solutions are obtained for the first case, indicating only non- linearly induced self and cross-phase coupling. A simple experiment in- volving simultaneous recording and readout using photorefractive lithium niobate crystal indicates beam profile distortion, which may be expected in such 90-deg geometries. © 2004 Society of Photo-Optical Instrumentation Engi- neers. (DOI: 10.1117/1.1774195)

Etched facet and semiconductor/air DBR facet of a AlGaInP laser diode prepared by focused ion beam milling

The study of focused ion beam (FIB) milling for making etched facet and semiconductor/air distributed Bragg reflector (DBR) facets of AlGaInP-based red laser diodes (LD) is presented in this letter. For the Ga ion beam current of 100 pA at fixed accelerated voltage 30 kV, FIB milling rate of GaAs was found to be 0.46 μm 3/nC. As a trade-off between high reflectivity and enough technical tolerance, the combination of third Bragg orders of semiconductor wall and air gap was chosen. The deeply etched mirror and distributed Bragg reflector facet consisting of pairs of semiconductor wall/air gap on laser diodes (LD) cavity facets with vertical sidewall on AlGaInP LDs were fabricated by focused Ga ion beam milling. Comparison of the AlGaInP LD with the mirrors between cleaved and FIB made facet was given and discussed.

Polarization and threshold energy variation of distributed feedback lasing of oxazine dye in zirconia waveguides and in solutions

Distributed feedback lasing characteristics of oxazine dye in zirconia waveguides and in solutions were investigated. Oxazine-dye-doped waveguides were characterized by ellipsometry. Intensity modulation and polarization modulation were used to generate laser action. Tunable narrow line width laser action was achieved for the first, second, third and fourth Bragg orders. It was observed that the threshold pump energy decreased with the decrease of the Bragg order. The degree of polarization of the laser output also changed from 0.8 at the third Bragg order to 0.1 at the first Bragg order. Wide-band tuning of the distributed feedback laser output from 700 nm to 870 nm was achieved for the first-order action during intensity modulation.

The fabrication of GaN‐based optical cavity mirrors by focused ion beam milling

The study of focused Ga ion beam milling for making GaN-based cavity mirrors is presented. The FIB etching rate of GaN was found to be in the rang of 0.6 μm3/nC – 0.43 μm3/nC. Three kinds of mirrors including polishing mirror, tilt mirror and nitride/air distributed Bragg reflection (DBR) mirror were fabricated. In particular, by using the transfer matrix method, the dependences of reflectivity and tolerance on the DBR Bragg order combination, number of DBR pair and nitride fill factor were calculated. To take trade-off between high reflectivity and enough tolerance, the combination of 3rd Bragg order of air gap and 5th Bragg order of semiconductor wall and three pairs were chosen. A deeply etched nitide/air DBR with vertical sidewall was obtained by focused Ga ion beam milling. Negative effects of the FIB on the etched GaN-based mirrors were also noticed. (© 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Narrow-stripe distributed reflector lasers with first-order vertical grating and distributed Bragg reflectors

Narrow-stripe 1.55-/spl mu/m wavelength distributed reflector lasers consisting of both distributed Bragg reflectors and vertical grating of the first Bragg order were fabricated. A low threshold current I/sub th/ of 2.8 mA, a differential quantum efficiency /spl eta//sub d/ of 28% from the front facet, and a submode suppression ratio of 44 dB were obtained for structures with a stripe width of 1.3 /spl mu/m and a cavity length of 150 /spl mu/m.

A structural study of effects of NiP seed layer on the magnetic properties of CoCrPt/Ti/NiP perpendicular magnetic films

The CoCrPt/Ti/NiP films for perpendicular magnetic recording were studied using X-ray scattering and anomalous X-ray scattering. When the NiP seed layer was used, the long range order of the texture peak of the magnetic film decreased and less Co was associated with this Bragg order. The structural results were consistent with the observed increased coercivity and decreased magnetization due to the increased magnetic grain isolation caused by the presence of NiP seed layer.

Dispersion relation of guided-mode resonances and Bragg peaks in dielectric diffraction gratings.

We present the dispersion relation of guided-mode resonances in non-dissipative dielectric diffraction gratings, both for s-polarized (TE mode) and p-polarized (TM mode) incident waves. We present a simple approximate theory as well as a rigorous calculation within the so-called coupled-wave theory. We discuss the dependence of the positions and the lifetimes of the resonances on the thickness of the gratings and on the strength of its modulation. We find that the diffraction efficiency of the different orders show peaks at different Bragg orders.

High-resolution X-ray study of specular and diffuse scattering from Ni/C multilayer upon annealing

A high-resolution X-ray diffraction study of specular and diffuse scattering from a Ni/C multilayer upon annealing was performed to examine its optical characteristics and the structural changes of interfacial roughness. The Ni/C multilayer was deposited by the laser ablation technique. The data obtained at the optimal annealing temperature (about 590 K), show a drastic improvement in the reflectivity of the first Bragg order by a factor of 2.4, up to 81%, without any worsening of X-ray optical characteristics and any changes of macroroughness. It supports our previous conclusion that annealing at the optimal temperature causes the spinodal decomposition of the interfacial range between the metal and carbon layers. Both the improvement of the sharpness of the boundaries and an increase of the X-ray optical contrast provide a gain in reflectivity.

Normal-incidence efficiencies of 4800-grooves/mm-ruled replica gratings with multilayer and gold coatings in the 125–325-Å wavelength region

The normal-incidence efficiencies of two 4800-grooves/mm ruled replica gratings, one with a dual-bandpass molybdenum/silicon multilayer coating and the other with a gold coating, were measured by use of synchrotron radiation in the 125-325-A wavelength region. The peak reflectance of the multilayer coating was 22% in the first Bragg order near 235 A and 28% in the second Bragg order near 126 A. The peak efficiency of the multilayer grating was 2.6% in the first diffraction order near 225 A and 0.3% in the second diffraction order near 125 A. The efficiencies of the multilayer grating were much higher than the corresponding efficiencies of the gold grating. The characterization of the surfaces of the gratings by atomic force microscopy indicated rms microroughness values in the 5-18-microm(-1) frequency range of 12-20 A for the multilayer grating and 22-32 A for the gold grating. Both gratings had bumpy surface features larger than the nominal groove height. The rather large surface roughness and groove irregularities had a detrimental effect on the grating efficiencies.

Neutron double multilayer monochromator-polarizer Co/Ti

The proposed neutron multilayer monochromator-polarizer, consisting of the alternating layers of Co and Ti, prepared by the method of thermal vacuum evaporation on glass and silicon substrates is considered. In this monochromator the distance between first Bragg order peak and edge of total reflection plateau are considerably increased for glass substrate compare with Fe/Ag monochromator and for silicon substrate the total reflection region is absent for reflection of the beam from silicon. The curves of intensities of direct beam and for both spin components of the beam of cold neutrons reflected four times from this double monochromator-polarizer together with data from thermal neutron reflectometer are presented.