Mechanism of large optical nonlinearity in gold nanoparticle films

The Z-scan technique, using femtosecond (fs) laser pulses at 1480 nm laser pulses, was used to measure the nonlinear optical properties of gold (Au) nanoparticle (NP) films made by both nanosecond (ns) and fs pulsed laser deposition (PLD) in vacuum. At irradiance levels of 1×10¹²  Wm^-2, the ns-PLD films displayed induced absorption with β=4×10^-5  mW^-1, and a negative lensing effect with n₂=-4.7×10^-11  m² W^-1 with somewhat smaller values for the fs-PLD films. These values of n₂ imply an unphysically large change in the real part of the refractive index, demonstrating the need to take account of nonlinear changes of the Fresnel coefficients and multiple beam interference in Z-scan measurements on nanoscale films. Following this approach, the Z-scan observations were analyzed to determine the effective complex refractive index of the NP film at high irradiance. It appears that at high irradiance the NP film behaves as a metal, while at low irradiance it behaves as a low-loss dielectric. Thus, it is conjectured that, for high irradiance near the waist of the Z-scan laser beam, laser driven electron tunneling between NPs gives rise to metal-like optical behavior.

Ferromagnetic behaviour of nickel contacted multiwalled carbon nanotubes

A novel method for the contact of ferromagnetic nano-nickel onto multiwalled carbon nanotubes (MWCNTs) is proposed in this work. The process involves the decomposition of the precursor: nickel carbonyl-Ni(CO)4 into nickel and CO by laser chemical vapour deposition at 150-200 degrees C and the deposition of nano-nickel onto MWCNTs. Field emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM), HR (High resolution) TEM, and Raman spectroscopy were employed for the detailed analysis of the nickel contacted MWCNTs. The ferro-magnetic nature of the sample was confirmed by Super-conducting Quantum Interference Device (SQUID) analysis.

Hydrogen in chemical vapour deposited carbon nanotubes: an active site for functionalization

The presence of hydrogen in as-grown carbon nanotubes (CNTs) synthesized by microwave plasma (MP) chemical vapour deposition (CVD) technique is demonstrated. Our results showed that the MPCVD, as-grown CNTs were hydrogenated consisting of C-H bonds; whereas, the tubes synthesized by arc discharge consisted of non-hydrogenated multi-walled CNTs. Fourier transform infrared spectroscopy (FTIR) and micro-Raman spectroscopy techniques were used to detect C-H bonding in the as-grown CNTs. The effective functionalization of as-grown hydrogenated CNTs grown using a microwave CVD process is first time demonstrated by laser assisted CVD process. It was found that the laser-assisted CVD process resulted in the termination of hydrogen and the oxidation of as-grown CNT structure leading to the carboxylic group attachment. The FTIR results show the presence of -OH and C=O bonds in the functionalized samples. However, the non-hydrogenated CNTs could not be effectively functionalized by the same process, probably due to the fact that it did not contain active sites pre-requisite for functionalization, as did the CVD grown samples. The functionalization of CVD grown tubes is believed to take place at the 'active' hydrogen-terminated sites on the CNT surfaces.

Mesoporous silicas impregnated with cobalt and nickel oxide nanoparticles and the growth of carbon nanotubes there from

Metal oxide-based nanoparticles of cobalt or nickel were deposited inside the pores and on the surface of hexagonal mesoporous silicas by a direct synthesis technique using Pluronic P85 and P123 surfactants as structure directing agents with the appropriate metal phthalocyanine as a metal precursor. Metal loadings were between 0.4-3.2 wt.%. XPS studies showed that the initial form of the metal oxide nanoparticles were [CoO] and [NiO] respectively. Samples of these materials formed from the P85 surfactant and 3.0 wt.% were used to grow carbon nanotubes (CNTs) from acetylene feedstock in a catalytic chemical vapour deposition (CCVD) reactor at 800 degrees C. CNT growth appeared to be random and the CNTs had diameters ranging from < 10 to > 90 nm. Treatment of the metal impregnated silicas with nitric acid produced materials which, under the same CNT growth conditions, afforded more uniform CNTs with diameters between 5-15 nm. No significant loss in mesophase ordering was seen in the TEM, PXRD or nitrogen physisorption analysis of the acid washed samples. CNTs grown with cobalt impregnated silicas formed with the P123 surfactant had diameters in the range 15-25 nm. Raman spectroscopy of the CNT products showed the nanotubes were highly graphitised and of good quality.

Blue amplified spontaneous emission from a stilbenoid-compound-doped polymer optical fiber

We report on the fabrication and characteristics of a step-index glass-clad polymer optical fiber that uses a novel fluorescent stilbenoid compound for lasing and amplification applications. The compound, 1, 4-bis(4-diphenyl-amino-styryl)-benzene, is specifically designed for the blue region of the spectrum and has a very high quantum yield of 0.85 in a solid-state polymer host and a large Stokes shift. Significant spectral narrowing and superlinear increase of output intensity are observed under photoexcitation at 355 nm, which are indicative of the occurrence of amplified spontaneous emission. By means of gain spectroscopy, a large optical gain of up to 36 cm(-1) at 494 nm has been obtained for the fiber when it is transversely photoexcited at 12 mJ/cm(2) . The waveguide loss has been measured to be 0.7 cm(-1) at 494 nm. The demonstration of high gain and low waveguide loss has favorable implications for the construction of a very compact, tunable coherent light source.