Ultrafast mode-locked dual-wavelength thulium-doped fiber laser using a Mach-Zehnder interferometric filter

A simple and robust method to generate a dual-wavelength mode-locked laser using a tunable Mach-Zehnder filter (TMZF) and a single-wall carbon nanotube (SWCNT) based saturable absorber (SA) is proposed and demonstrated. The proposed laser uses a thulium-doped fiber for lasing in the two-micron region and exploits the interferometric spectrum of the TMZF to produce dual peaks with nearly equal magnitude. SWCNT based SA enables mode-locking at a threshold value of 150.4?mW with distinct dual-wavelength peaks at 1919.2?nm and 1963.7?nm. The peaks have a calculated pulse width of 1.8?ps and 1.6?ps, respectively with a repetition rate of 9.1?MHz with a relatively high optical-signal-to-noise ratio value of 59.1?dB. The output is also observed to remain unchanged over time, indicating high stability. The proposed laser has a promising application, particularly in ultrafast gas molecular spectroscopy and sensing.     

Mode-locked 1.93 microm thulium fiber laser with a carbon nanotube absorber

We report a ring-cavity thulium fiber laser mode locked with a single-wall carbon nanotube absorber used in transmission. A carboxymethyl cellulose polymer film with incorporated carbon nanotubes synthesized by the arc discharge method has an absorption coinciding with in the amplification bandwidth of a Tm-doped fiber. This laser is pumped by an erbium fiber laser at 1.57 microm wavelength and produces a 37 MHz train of mode-locked 1.32 ps pulses at 1.93 microm wavelength with an average output power of 3.4 mW.     

Vector solitons in a laser passively mode-locked by single-wall carbon nanotubes

Polarization Rotation Locked Vector Solitons (PRLVSs) are experimentally observed for the first time in a fiber ring laser passively mode-locked by a single-wall carbon nanotube (SWCNT) saturable absorber. Period-doubling of these solitons at certain birefringence values has also been observed. We show that fine adjustment to the intracavity birefringence can swing the PRLVSs from period-doubled to period-one state without simultaneous reduction in the pump strength. The timing jitter for both states has also been measured experimentally and discussed analytically using the theoretical framework provided by the Haus model.     

Studies of chemical diffusion in La1-X SrXCoO3-δ

Recent investigations of superconductivity in carbon nanotubes have shown that a single-wall zig-zag nanotube can become superconducting at around 15?K. Theoretical studies of superconductivity in nanotubes using the traditional phonon exchange model, however, give a superconducting transition temperature T _c less than 1?K. To explain the observed higher critical temperature we explore the possibility of the plasmon exchange mechanism for superconductivity in nanotubes. We first calculate the effective interaction between electrons in a nanotube mediated by plasmon exchange and show that this interaction can become attractive. Using this attractive interaction in the modified Eliashberg theory for strong coupling superconductors, we then calculate the critical temperature T _c in a single-wall nanotube. Our theoretical results can explain the observed T _c in a single-wall nanotube. In particular, we find that T _c is sensitively dependent on the dielectric constant of the medium, the effective mass of the electrons and the radius of the nanotube. We then consider superconductivity in a bundle of single-wall nanotubes and find that bundling of nanotubes does not change the critical temperature significantly. Going beyond carbon nanotubes we show that in a metallic hollow nanowire T _c has some sort of oscillatory behaviour as a function of the surface number density of electrons.     

Influence of topological defects on the structure of G and D spectral bands of a single-layer carbon nanotube

A topological defect in a carbon nanotube grown by chemical vapor deposition from methane onto a silicon substrate with thermal oxide has been investigated and visualized (with a resolution of about 1.5 μm) by confocal Raman spectroscopy. Vibrational Raman spectra of molecular fragments of a single-wall carbon nanotube (SWCNT) without a defect and with Stone–Wales defects (two pentagonal and two heptagonal cells) are calculated. The influence of defects on the shape of G-band components (G⁺ and G^–), which makes it possible to determine the nanotube conductivity type, is considered.     

Equilibrium Configurations of Lipid Bilayer Membranes and Carbon Nanostructures

The present article concerns the continuum modelling of the mechanical behaviour and equilibrium shapes of two types of nano-scale objects: fluid lipid bilayer membranes and carbon nanostructures. A unified continuum model is used to handle four different case studies. Two of them consist in representing in analytic form cylindrical and axisymmetric equilibrium configurations of single-wall carbon nanotubes and fluid lipid bilayer membranes subjected to uniform hydrostatic pressure. The third one is concerned with determination of possible shapes of junctions between a single-wall carbon nanotube and a flat graphene sheet or another single-wall carbon nanotube. The last one deals with the mechanical behaviour of closed fluid lipid bilayer membranes (vesicles) adhering onto a flat homogeneous rigid substrate subjected to micro-injection and uniform hydrostatic pressure.     

Graphene-based passively Q-switched 2 μm thulium-doped fiber laser

We demonstrated stable pulses generation at 2 μm in a passively Q-switched thulium-doped fiber laser using a few layer graphene thin film. The maximum output power was 4.5 mW and the single pulse energy was 85 nJ at 53 kHz repetition rate, and the pulse width was about 1.4 μs. The pulse width and the repetition rate of the Q-switched fiber laser can be changed along with the pump power. To the best of our knowledge, this is the first report of graphene saturable absorber for passively Q-switched 2 μm fiber lasers.     

Carbon nanotube grease with enhanced thermal and electrical conductivities

A stable and homogeneous grease based on carbon nanotubes (CNTs, single-wall and multi-wall) in polyalphaolefin oil has been produced without using a chemical surfactant. For example, for a 11 wt% (7 vol%) single-wall CNT (diameter 1–2 nm, length 0.5–40 μm) loading, the thermal conductivity (TC) of the grease shows a 60–70% increase compared to that for no nanotube loading. In addition, the grease is electrically conductive, has a high dropping point, good temperature resistance, and does not react with copper at temperatures up to 177 °C. The performance of carbon nanotube grease could be much better with the improvement of nanotube quality and purity. A possible explanation for these results is that of a high loading of CNTs (>10 wt%), they become associated with each other by van der Waals forces in the grease to form three-dimensional percolation networks. Time-dependent magnetic results demonstrate that, even under the influence of a strong outside magnetic field, the TC value remains constant. This phenomenon can be attributed to the existence of networks that makes magnetic alignment of nanotubes impossible.     

Toward higher-order passive harmonic mode-locking of a soliton fiber laser

We report >13 MHz/mW pump power efficiency in increasing the repetition rate of passive harmonic mode-locking by engineering the soliton pulse energy in Er fiber lasers incorporating carbon nanotube saturable absorber. Stable pulses with a ～5 GHz repetition rate and 40 dB of super-mode suppression are demonstrated with only ～400 mW pump power in a single-clad fiber laser.     

Growth of carbon nanotubes from ring carbon clusters

The possibility of growing single-wall carbon nanotubes from ring carbon clusters that appear at a certain stage of cooling carbon vapor is discussed. Such a technique could allow one to grow single-wall nanotubes without introducing a macroscopic amount of a catalyst and to retain nanotubes open during their growth. An analysis performed using semiempirical quantum-chemical methods shows that, when catalyst atoms interact with the edge of an already formed nanotube surface, the bonds of these atoms with carbon tend to occupy positions normal to the generatrix of the nanotube. This situation is natural for transition-metal atoms, since they favor the destruction of pentagonal cycles at the edge of the surface. The destruction mechanism consists in the fact that pentagons incorporate carbon atoms from the outside and become hexagons. The dependence of this tendency on the type of catalyst atom is considered.     

Mode-locked self-starting Cr:forsterite laser using a single-walled carbon nanotube saturable absorber

We report what we believe to be the first passive mode-locking of Cr:forsterite laser using a single-walled carbon nanotube saturable absorber (SWCNT-SA). The dispersion-compensated Cr:forsterite laser in a self-starting configuration produces nearly Fourier transform-limited pulses as short as 120 fs near 1.25 microm. The maximum average output power of 202 mW obtained with a 5% output coupler at a repetition rate of 79.1 MHz represents, to the best of our knowledge, the highest power level ever reported for SWCNT-SA mode locking of solid-state lasers.     

The dynamics simulation of Ne atom injected into single-wall carbon nanotube

The dynamical processes of Ne atom injected into single-wall carbon nanotube (SWCNT) are modeled with molecular dynamics simulations. The threshold energies to encapsulate rare-gas atoms in SWCNT are presented. The range of tube radius for stable oscillation is revealed, which is independent of the type of carbon nanotubes. And the oscillatory frequency is sensitive to the change in the diameter, the length and chirality of the tube.     

Solid-state Er:Yb:glass laser mode-locked by using single-wall carbon nanotube thin film

We design single-wall carbon nanotube (SWNT) thin-film saturable absorbers (SAs) integrated onto semiconductor distributed Bragg reflectors for mode-locking solid-state Er:Yb:glass lasers. We characterize the low nonsaturable loss, high-damage-threshold SWNT SAs and verify their operation up to a pulse fluence of 2 mJ/cm(2). We demonstrate passive fundamental continuous-wave mode locking with and without group-delay dispersion compensation. Without compensation the laser produces chirped 1.8 ps pulses with a spectral width of 3.8 nm. With compensation, we obtain 261 fs Fourier-transform-limited pulses with a spectral width of 9.6 nm.     

Raman fiber laser pumped by a semiconductor disk laser and mode locked by a semiconductor saturable absorber mirror

A 1.6μm mode-locked Raman fiber laser pumped by a 1480nm semiconductor disk laser is demonstrated. Watt-level core pumping of the single-mode fiber Raman lasers with low-noise disk lasers together with semiconductor saturable absorber mirror mode locking represents a highly practical solution for short-pulse operation.     

Effects of vacancy percentage on the energy gap of zigzag single-wall carbon nanotubes

The influence of vacancy percentage on the energy gap of zigzag single-wall carbon nanotube is investigated by the Green's function method in coherent potential approximation. Our probes for various kinds of zigzag single-wall carbon nanotubes show that by increasing vacancy percentage the energy gap is also increased, so for metallic single-wall carbon nanotubes, a metallic to semi-metallic transition is occurred. However, any transition does not appear for semiconductor carbon nanotubes. So by controlling on concentration of vacancies, one can make a semiconductor SWCNT with a predetermined energy gap which is useful in nanoelectronic devices.     

氧吸附对单壁碳纳米管的电子结构和光学性能的影响

用密度泛函理论计算了氧分子物理吸附在半导体型单壁碳纳米管的束缚能，能带结构和吸收 光谱.计算结果指出氧分子吸附在碳纳米管表面的优先位置，研究发现氧吸附对碳管的电子 输运特性和吸收光谱有着重要的影响，并对光致氧分子解吸附的现象进行了理论分析. 关键词： 单壁碳纳米管 氧物理吸附 能带结构 吸收光谱     

Femtosecond mode-locked holmium fiber laser pumped by semiconductor disk laser

We report on a 2085 nm holmium-doped silica fiber laser passively mode-locked by semiconductor saturable absorber mirror and carbon nanotube absorber. The laser, pumped by a 1.16 μm semiconductor disk laser, produces 890 femtosecond pulses with the average power of 46 mW and the repetition rate of 15.7 MHz.     

Eightfold shell filling in a double-wall carbon nanotube quantum dot

We fabricated a quantum-dot device consisting of an individual double-wall carbon nanotube and studied its electrical transport properties at low temperatures. In the negative bias region, the gate modulation curve exhibited quasiperiodic current oscillations, attributed to the Coulomb blockade of single-electron tunneling. We observed both four- and eightfold shell filling in the Coulomb diamond structures. The observation implies an eightfold degeneracy in the single-particle energy levels, which is higher than the fourfold degeneracy of a single-wall carbon nanotube. We show that the observed eightfold shell filling is a unique characteristic of a double-wall carbon nanotube quantum-dot device.     

Nucleation of single-walled carbon nanotubes

The nucleation pathway for single-wall carbon nanotubes on a metal surface is demonstrated by a series of total energy calculations using density functional theory. Incorporation of pentagons at an early stage of nucleation is energetically favorable as they reduce the number of dangling bonds and facilitate curvature of the structure and bonding to the metal. In the presence of the metal surface, nucleation of a closed cap or a capped single-wall carbon nanotube is overwhelmingly favored compared to any structure with dangling bonds or to a fullerene.     

Real space imaging of one-dimensional standing waves: direct evidence for a Luttinger liquid

Electronic standing waves with two different wavelengths were directly mapped near one end of a single-wall carbon nanotube as a function of the tip position and the sample bias voltage with high-resolution position-resolved scanning tunneling spectroscopy. The observed two standing waves caused by separate spin and charge bosonic excitations are found to constitute direct evidence for a Luttinger liquid. The increased group velocity of the charge excitation, the power-law decay of their amplitudes away from the scattering boundary, and the suppression of the density of states near the Fermi level were also directly observed or calculated from the two different standing waves.