Higher-order Bands Research Articles

Generation of a vacuum ultraviolet to visible Raman frequency comb in H2-filled kagomé photonic crystal fiber.

We report on the generation of a purely vibrational Raman comb, extending from the vacuum ultraviolet (184 nm) to the visible (478 nm), in hydrogen-filled kagomé-style photonic crystal fiber pumped at 266 nm. Stimulated Raman scattering and molecular modulation processes are enhanced by higher Raman gain in the ultraviolet. Owing to the pressure-tunable normal dispersion landscape of the "fiber + gas" system in the ultraviolet, higher-order anti-Stokes bands are generated preferentially in higher-order fiber modes. The results pave the way toward tunable fiber-based sources of deep and vacuum ultraviolet light for applications in, e.g., spectroscopy and biomedicine.

Collective motion of self-propelled particles with density-dependent switching effect

We study the effect of density-dependent angular response on large scale collective motion, that particles are more likely to switch their moving direction within lower local density region. We show that the presence of density-dependent angular response leads to three typical phases: polar liquid, micro-phase separation and disordered gas states. In our model, the transition between micro-phase separation and disordered gas is discontinuous. Giant number fluctuation is observed in polar liquid phase with statistically homogeneous order. In the micro-phase separation parameter space, high order and high density bands dominate the dynamics. We also compare our results with Vicsek model and show that the density-dependent directional switching response can stabilize the band state to very low noise condition. This band stripe could recruit almost all the particles in the system, which greatly enhances the coherence of the system. Our results could be helpful for understanding extremely coherent motion in nature and also would have practical implications for designing novel self-organization pattern.

Raman Spectroscopy of Graphene Molecules: Size Dependent and Reaction Time Studies

Graphene molecules, nanostructures with fascinating optical and electronic properties, also referred to as graphene quantum dots, have attracted significant interest for a number of applications, including photovoltaics, electrochromic, and bio-sensing.[1],[2],[3],[4] Raman spectroscopy is a non-destructive, powerful technique, commonly used to characterize graphitic materials and investigate the doping level, the number of layers, edge-type, and the presence/type of defects. Even though, Raman studies of large-area graphene, as well as, nano-graphene sheets, with lateral dimensions larger than ∼3 nm, have been previously reported, an investigation of structures with well-controlled dimensions below 2 nm has been lacking. Raman spectroscopy can be employed to gain important insights on the structure of well-defined graphene, by studying the dependence of the Raman spectral features on the edge-type and the level of defects present on graphene molecules. In this study, we present a detailed Raman analysis of graphene molecules with synthetically controlled lateral dimensions between 0.76-1.39 nm, which correspond to structures with 16-79 ordered carbon rings. Graphene molecules with armchair edges and controlled lateral dimensions smaller than 2 nm were synthesized utilizing a new synthetic approach. [3.4] The samples were excited using laser light at discrete wavelengths (405 nm and 532 nm), and their Raman spectral features, including integrated band area ratios and peak positions, were systematically investigated. Our studies indicated that the integrated area ratios of D and G bands (AD/AG) increase with the size of the molecules. These results are in good agreement with the findings of the Raman studies of structures larger than 2 nm, generated via ion-bombardment.[5]The D and G band positions were also found to depend on the size of the graphene molecules. Additionally, we observed that the intensities of the higher-order bands (e.g., 2D, D+G, and 2G), increase for both wavelengths, as the size of the graphene molecules increases. To elucidate the effect of synthesis conditions on the structure of graphene molecule, we varied the reaction time and probed the crystallinity of the resulting graphene molecules. The integrated area band ratios and the band positions of the graphene molecules (with 79 and 48 carbon rings) were strongly dependent on the reaction time. These results demonstrate the suitability of Raman spectroscopy as a useful diagnostic tool for structural characterization and reaction dynamics of graphene molecules smaller than 2 nm, which is currently missing from the studies of larger graphene structures generated via ion-bombardment or other techniques. [1] S. H. Jin et al., ACSNano, 7 (2), 1239-1245, 2013. [2] H. Razmin et al., Biosensors and Bioelectronics, 41, 498-504, 2013. [3] Z. Ji et al., ACS Appl. Mater. Interfaces, 6, 20473-20478, 2014. [4] Z. Ji et al., ACSNano, 9 (4), 4043-4049, 2015. [5] L. G. Cancado et al., Nano Lett. 11, 3190-3196, 2011.

The ties that bind: Maternal kin bias in a multilevel primate society despite natal dispersal by both sexes.

In many social animals, individuals derive fitness benefits from close social bonds, which are often formed among kin of the philopatric sex. Hamadryas baboons, however, exhibit a hierarchical, multilevel social system where both sexes disperse from their natal one-male-unit (OMU). Although this would seem to hinder maintenance of kin ties, both sexes appear largely philopatric at the higher order band and clan levels, possibly allowing for bonds with same sex kin by both males and females. In order to investigate the possibility of kin bonds in hamadryas baboons, we identified kin dyads in a band without known pedigree information using a large panel of genetic markers: 1 Y-linked, 4 X-linked, and 23 autosomal microsatellites and part of the mitochondrial hypervariable region I. With these data, we performed a kinship analysis while accounting for misclassification rates through simulations and determined kinship among two types of dyads: leader and follower males and female dyads within OMUs. Leader and follower males were maternal relatives more often than expected by chance, suggesting that kinship plays a role in the formation of these relationships. Moreover, maternal female relatives were found in the same OMU more often than expected by chance, indicating that females may be motivated to maintain post-dispersal contact with maternal female kin. Our results suggest that hamadryas baboons can recognize maternal kin and that kin selection has contributed to shaping their complex social system. This implies that an ancestral maternal kin bias has been retained in hamadryas society. Am. J. Primatol. 78:731-744, 2016. © 2016 Wiley Periodicals, Inc.

A Compact Printed Filtering Antenna With Good Suppression of Upper Harmonic Band

In this letter, a printed planar filtering antenna composed of the stepped-impedance dipole (SID), stepped-impedance resonator (SIR), and low-pass filter (LPF) is presented. The SIR is used as a parasitic element in proximity to the SID radiator to improve its upper band-edge selectivity and to widen its operation bandwidth. Our extensive study is at first conducted to exhibit that the upper harmonic band of radiation can be tremendously pushed upward by appropriately replacing the main dipole radiator and the parasitic resonant element with the SID and SIR. Next, a stepped-impedance LPF is employed to further effectively suppress the high-order harmonic band of radiation as quantitatively demonstrated. The measured reflection coefficient has achieved a wide fractional bandwidth up to 27.5%, covering a frequency range of 2.5–3.3 GHz, with $\vert{ S}_{11}\vert . In particular, its first harmonic band for radiation has been significantly extended from 7.2 GHz beyond 14.0 GHz as confirmed in simulation and experiment.

Fermi states and anisotropy of Brillouin zone scattering in the decagonal Al-Ni-Co quasicrystal.

Quasicrystals (QCs) are intermetallic alloys that have excellent long-range order but lack translational symmetry in at least one dimension. The valence band electronic structure near the Fermi energy EF in such materials is of special interest since it has a direct relation to their unusual physical properties. However, the Fermi surface (FS) topology as well as the mechanism of QC structure stabilization are still under debate. Here we report the first observation of the three-dimensional FS and valence band dispersions near EF in decagonal Al70Ni20Co10 (d-AlNiCo) QCs using soft X-ray angle-resolved photoemission spectroscopy. We show that the FS, formed by dispersive Al sp-states, has a multicomponent character due to a large contribution from high-order bands. Moreover, we discover that the magnitude of the gap at the FS related to the interaction with Brillouin zone boundary (Hume–Rothery gap) critically differs for the periodic and quasiperiodic directions.

Spin- and valley-dependent magneto-optical properties of MoS2

We investigate the behavior of low-energy electrons in two-dimensional molybdenum disulfide when submitted to an external magnetic field. Highly degenerate Landau levels form in the material, between which light-induced excitations are possible. The dependence of excitations on light polarization and energy is explicitly determined, and it is shown that it is possible to induce valley and spin polarization, i.e. to excite electrons of selected valley and spin. Whereas the effective low-energy model in terms of massive Dirac fermions yields dipole-type selection rules, higher-order band corrections allow for the observation of additional transitions. Furthermore, inter-Landau-level transitions involving the n=0 levels provide a reliable method for an experimental measurement of the gap and the spin-orbit gap of molybdenum disulfide.

Honeycomb-lattice plasmonic absorbers at NIR: anomalous high-order resonance

We design, fabricate and characterize a plasmonic honeycomb lattice absorber with almost perfect absorption at 1140 nm over a wide incident angle range. This absorber also possesses a narrow-band, angle- and polarization-dependent high-order resonance in the short-wavelength range, with a bandwidth of 19 nm and angle sensitivity of 3 nm per degree. The nature of this high-order absorption band is analyzed through finite-element simulations. We believe it is due to Bragg coupling of the incident light to the backward-propagating surface plasmon polariton through the periodic modulation of the structure.Such fine absorption bands can find applications in plasmonic sensors and spectrally selective thermal emitters.

Ping-pong modes and higher-periodicity multipactor

Multipactor is a vacuum discharge driven by secondary electron emission. Multiple period multipactors have long been known to exist but have been studied less extensively. In a period-n multipactor, electrons undergo multiple impacts in one rf period, with the synchronous phase alternating periodically between multiple values. A novel resonant form is proposed that combines one- and two-surface impacts within a single period, provided the total transit time is an odd number of rf half-periods and the product of secondary yields exceeds unity. For low fD products, the simplest such mode is shown to significantly increase the upper electric field boundary of the multipacting region and lead to overlap of higher-order bands. The results agree nicely with 3-D particle-in-cell code simulations. An alternative, map-based method is introduced for analyzing higher-periodicity multipactor. Practical implications of the findings are discussed, including consequences for multipactor suppression strategies using a dc magnetic field.

Band structures of exchange spin waves in one-dimensional bi-component magnonic crystals

We present the micromagnetic study of magnonic band structures for exchange spin waves propagating in one-dimensional magnonic crystals. The crystals are of laterally patterned periodic arrays of alternating cobalt and nickel stripes. Large magnonic bandgaps with widths of tens of GHz are observed. It is found that the higher-order transmission bands and bandgaps have wider widths than those of the lower-order bands and bandgaps. Another interesting feature is that the widths of the first two bandgaps are independent of the applied field, in contrast with an earlier report of decreasing bandgap widths with increasing applied field observed for dipolar spin waves.

Designing one-dimensional photonic crystal filters by irregularly changing optical thicknesses

We propose a method for designing the filters of one-dimensional photonic crystals (1DPCs) that is based on the traditional transfer matrix method and some fundamental functions. By increasing some design parameters, our method, which can be easily used to design the spectrum shapes of 1DPCs and other types of multilayer structures, can offer designers more choices in selecting the structure forms of 1DPCs. We use our method to design two types of narrow-band reflecting filters based on the high-order band gaps of 1DPCs. These filters, with their peak wavelengths near 1555 nm and filter bandwidths about several nanometers, include a single- and a two-channel filter by adjusting structure parameters. In order to prove our design method effective, we fabricate these filters by lithography and measure their insertion loss spectrums. The experiment and theory results accord very well and prove our method effective.

Refractive index and temperature dependent displacements of resonant peaks of long period grating inscribed in hydrogen loaded SMF-28 fiber

In this paper, the effects of refractive index (RI) of surrounding medium and ambient temperature on the transmission characteristics of a long period grating (LPG) are experimentally analyzed. The spectral behavior of LPG is investigated when the ambient index is higher or lower than that of the cladding material. The results show that the refractive index sensitivity of lower order attenuation bands is very low compared with that of the highest order attenuation band. But in the case of temperature, the lower order attenuation bands of the LPG can also exhibit good sensitivity like the higher-order bands.

Research of band gap properties based on two-dimensional photonic crystal with mixed shapes of rods

Two new structures of photonic crystals were designed. The band gap properties of photonic crystals with square and circular dielectric rods mixed arrangement are researched. The band gap properties of mixed shapes rods photonic crystal are calculated and compared with the crystals with square rods or round rods by using plane wave expansion method. Simulation results show that for the square lattice, mixed shapes of rods make the higher-order bands of TM modes moving toward the low frequency range. The gap bands’ widths and locations are between the parameters of square and round rods photonic crystal. In triangle lattice, a significant band gap is presented in photonic crystal with mixed shapes of rods in TE mode, while it is almost not presented in square and round rods crystals. The phenomenon of bands moving toward the low frequency range is also found in the triangle lattice mixed shapes rods photonic crystal. The reasons of the results in the vision were analyzed.

Ping-Pong Modes: A New Form of Multipactor

The multipactor is a vacuum discharge based on a secondary electron emission. A novel resonant form is proposed that combines one- and two-surface impacts within a single period, provided the total transit time is an odd number of rf half-periods and the product of secondary yields exceeds unity. For low fD products, the simplest such mode is shown to significantly increase the upper electric field boundary of the multipacting region and lead to overlap of higher-order bands. The results agree nicely with 3D particle-in-cell code simulations. Practical implications of the findings are discussed.

Using Crosslinking and Mass Spectrometry to Study Glycine Receptor Allostery

A wealth of high-resolution structural information regarding pentameric ligand-gated ion channels is now available, but less is known of the molecular details underlying complex allosteric mechanisms involved in channel gating and desensitization. Receptor allostery can be studied by identifying state-dependent distance constraints that may be used in molecular modeling of these receptors. Systematically generated single Cys mutations of the human α1 glycine receptor (GlyR) expressed in insect cells were labeled with a clickable methanethiosulfonate-benzophenone crosslinker. After covalent ligation to Cys, crosslinks may then be introduced in the resting, open, or desensitized states by photoactivation. Including an alkyne tag on the crosslinker permits click chemistry addition of biotin, which allows for enrichment by avidin chromatography. Mass spectrometry (MS) fingerprinting of monomeric and higher-order GlyR bands on SDS-PAGE using ESI-QTOF MS/MS then allows for determination of the site of crosslinking. Our initial proof-of-principle studies conducted on purified GlyR have provided state-dependent information on this receptor. This approach may be broadly applicable to studies of any allosteric complex.

Optical properties of high-order band gaps in one-dimensional photonic crystal

Plane wave expansion method and transfer matrix method have been used to study the optical properties of eighty ninth photonic band gaps (PBGs) in one dimensional photonic crystal (1DPCs) with a form of Si (air|Si)8. The results show that high order PBGs (HR-PBGs) in normalize dispersion curves have slower convergence speed than low order PBGs (LR-PBGs) and appear periodic detergency phenomena with the increasing of Λ/λ values (where Λ is the constant of one dimensional lattice and λ is wavelength) and their band widths and central wavelengths are more sensitive to fabrication errors than that of LR-PBG. Finally, some special forms of Si (air|Si)8 structure with input and output rib waveguides are fabricated on Silicon-on-Insulator (SOI) materials by semiconductor micromachining technology. By measuring and analyzing their insertion loss spectra, we demonstrate that the HR-PBGs of 1DPCs also have the potential application in integration optics.

High-order coplanar waveguide (CPW) filters implemented by means of open split ring resonators (OSRRs) and open complementary split ring resonators (OCSRRs)

In this paper, it is demonstrated that transmission lines loaded with a combination of open split ring resonators (OSRRs) and open complementary split ring resonators (OCSRRs) are useful for the design of high-order band pass filters subjected to standard responses. Specifically, we present a 7-pole Chebyshev band pass filter in coplanar waveguide technology. The device is compact and filter performance is good, including a wide stopband response. With this work, we extend our previous work on the applications of OSRR- and OCSRR-loaded lines to the design of highly selective filters.

Linear discrete diffraction and transverse localization of light in two-dimensional backbone lattices

We study the linear discrete diffraction characteristics of light in two-dimensional backbone lattices. It is found that, as the refractive index modulation depth of the backbone lattice increases, high-order band gaps become open and broad in sequence, and the allowed band curves of the Floquet-Bloch modes become flat gradually. As a result, the diffraction pattern at the exit face converges gradually for both the on-site and off-site excitation cases. Particularly, when the refractive index modulation depth of the backbone lattice is high enough, for example, on the order of 0.01 for a square lattice, the light wave propagating in the backbone lattice will be localized in transverse dimension for both the on-site and off-site excitation cases. This is because only the first several allowed bands with nearly flat band curves are excited in the lattice, and the transverse expansion velocities of the Floquet-Bloch modes in these flat allowed bands approach to zero. Such a linear transverse localization of light may have potential applications in navigating light propagation dynamics and optical signal processing.

Phase properties of reflected light in photonic band gap

We find that the phase shifts of reflected light within band gap of two-dimensional photonic crystal (PC) are as follows: with frequency altering from the lower edge to the upper edge of first stop band, the reflection phase shift varies from −π to 0 for the PC’s unit cell with the high-index material near the center, while it varies from 0 to π for that with low-index material near the center. For the higher-order stop band, there exists a certain value of filling fraction, which makes the phase shifts in higher-order stop bands almost the same as that in the first stop band. When the filling fraction is far from that value, the phase shifts are significantly different. The further study on the Bloch modes demonstrates that their distribution of electric field and magnetic field determines the phase shifts. Moreover, we have found that, in the overlap area of transverse magnetic and transverse electric stop band, the phase difference between two polarizations of reflected light can remain invariant in a broad frequency region. Based on this property, the broadband and angle-insensitive phase retarders are designed. These interesting phase characteristics will bring about many potential applications.

Isotope study of far IR absorption of bistable centers in hydrogen-implanted silicon

Far IR study of high-purity crystalline silicon implanted by protons and deuterons at low ion beam current density and room temperature was performed. Isotope analysis shows that a set of new absorption bands at 200–1500 cm−1 are connected to hydrogen-related centers including bistable centers. It is found that the behavior of the bands correlates with that of high-order bands observed in the near infrared absorption spectra of neutron-irradiated silicon. The bands are tentatively associated with nanostructured hydrogen-related defects.