X. N. He

Enhancement of optical emission from laser-induced plasmas by combined spatial and magnetic confinement

To enhance optical emission in laser-induced breakdown spectroscopy, both a pair of permanent magnets and an aluminum hemispherical cavity (diameter: 11.1 mm) were used simultaneously to magnetically and spatially confine plasmas produced by a KrF excimer laser in air from pure metal and alloyed samples. High enhancement factors of about 22 and 24 in the emission intensity of Co and Cr lines were acquired at a laser fluence of 6.2 J/cm2 using the combined confinement, while enhancement factors of only about 11 and 12 were obtained just with a cavity. The mechanism of enhanced optical emission by combined confinement, including shock wave in the presence of a magnetic field, is discussed. The Si plasmas, however, were not influenced by the presence of magnets as Si is hard to ablate and ionize and hence has less free electrons and positive ions. Images of the laser-induced Cr and Si plasmas show the difference between pure metallic and semiconductor materials in the presence of both a cavity and magnets.

Accuracy improvement of quantitative analysis by spatial confinement in laser-induced breakdown spectroscopy

To improve the accuracy of quantitative analysis in laser-induced breakdown spectroscopy, the plasma produced by a Nd:YAG laser from steel targets was confined by a cavity. A number of elements with low concentrations, such as vanadium (V), chromium (Cr), and manganese (Mn), in the steel samples were investigated. After the optimization of the cavity dimension and laser fluence, significant enhancement factors of 4.2, 3.1, and 2.87 in the emission intensity of V, Cr, and Mn lines, respectively, were achieved at a laser fluence of 42.9 J/cm(2) using a hemispherical cavity (diameter: 5 mm). More importantly, the correlation coefficient of the V I 440.85/Fe I 438.35 nm was increased from 0.946 (without the cavity) to 0.981 (with the cavity); and similar results for Cr I 425.43/Fe I 425.08 nm and Mn I 476.64/Fe I 492.05 nm were also obtained. Therefore, it was demonstrated that the accuracy of quantitative analysis with low concentration elements in steel samples was improved, because the plasma became uniform with spatial confinement. The results of this study provide a new pathway for improving the accuracy of quantitative analysis of LIBS.

Coherent anti-Stokes Raman scattering and spontaneous Raman spectroscopy and microscopy of microalgae with nitrogen depletion

Microalgae are extensively researched as potential feedstocks for biofuel production. Energy-rich compounds in microalgae, such as lipids, require efficient characterization techniques to investigate the metabolic pathways and the environmental factors influencing their accumulation. The model green alga Coccomyxa accumulates significant amounts of triacylglycerols (TAGs) under nitrogen depletion (N-depletion). To monitor the growth of TAGs (lipid) in microalgal cells, a study of microalgal cells (Coccomyxa sp. C169) using both spontaneous Raman and coherent anti-Stokes Raman scattering (CARS) spectroscopy and microscopy were carried out. Spontaneous Raman spectroscopy was conducted to analyze the components in the algal cells, while CARS was carried out to monitor the distribution of lipid droplets in the cells. Raman signals of carotenoid are greater in control microalgae compared to N-depleted cells. Raman signals of lipid droplets appear after N-depletion and its distribution can be clearly observed in the CARS microscopy. Both spontaneous Raman spectroscopy and CARS microscopy were found to be suitable analysis tools for microalgae.

Optimally enhanced optical emission in laser-induced breakdown spectroscopy by combining spatial confinement and dual-pulse irradiation

In laser-induced breakdown spectroscopy (LIBS), a pair of aluminum-plate walls were used to spatially confine the plasmas produced in air by a first laser pulse (KrF excimer laser) from chromium (Cr) targets with a second laser pulse (Nd:YAG laser at 532 nm, 360 mJ/pulse) introduced parallel to the sample surface to re-excite the plasmas. Optical emission enhancement was achieved by combing the spatial confinement and dual-pulse LIBS (DP-LIBS), and then optimized by adjusting the distance between the two walls and the interpulse delay time between both laser pulses. A significant enhancement factor of 168.6 for the emission intensity of the Cr lines was obtained at an excimer laser fluence of 5.6 J/cm(2) using the combined spatial confinement and DP-LIBS, as compared with an enhancement factor of 106.1 was obtained with DP-LIBS only. The enhancement mechanisms based on shock wave theory and reheating in DP-LIBS are discussed.

Surface-enhanced Raman spectroscopy using gold-coated horizontally aligned carbon nanotubes

Gold-coated horizontally aligned carbon nanotube (Au-HA-CNT) substrates were fabricated for surface-enhanced Raman spectroscopy (SERS). The Au-HA-CNT substrates, which are granular in nature, are easy-to-prepare with large SERS-active area. Enhancement factors (EFs) of ∼10(7) were achieved using the Au-HA-CNTs as substrates for rhodamine 6G (R6G) molecules. Maximum enhancement was found when the polarization direction (E-field) of the incident laser beam was parallel to the aligned direction of the HA-CNTs. Simulations using the finite-difference time-domain (FDTD) method were carried out for the granular Au-HA-CNT samples. Enhancement mechanisms and determination of EFs were analyzed. Biological samples, including (13)C- and deuterium (D)-labeled fatty acids and Coccomyxa sp. c-169 microalgae cells, were also measured using this SERS substrate. The limits of detection (LODs) of D- and (13)C-labeled fatty acids on the SERS substrate were measured to be around 10 nM and 20 nM, respectively. Significantly enhanced Raman signals from the microalgae cells were acquired using the SERS substrate.

Resonant excitation of precursor molecules in improving the particle crystallinity, growth rate and optical limiting performance of carbon nano-onions

A catalyst-free and highly efficient synthetic method for growing carbon nano-onions (CNOs) in open air has been developed through the laser resonant excitation of a precursor molecule, ethylene, in a combustion process. Highly concentric CNO particles with improved crystallinity were obtained at a laser wavelength of 10.532 µm through the resonant excitation of the CH(2) wagging mode of the ethylene molecules. A higher growth rate up to 2.1 g h( - 1) was obtained, compared with that without a laser (1.3 g h( - 1)). Formation of the CNOs with ordered graphitic shells is ascribed to the decomposition of polycyclic aromatic hydrocarbons (PAHs) into C(2) species. The optical limiting performances of the CNOs grown by the combustion processes were investigated. CNOs grown at 10.532 µm laser excitation demonstrated improved optical limiting properties due to the improved crystallinity.

Flame-enhanced laser-induced breakdown spectroscopy

Flame-enhanced laser-induced breakdown spectroscopy (LIBS) was investigated to improve the sensitivity of LIBS. It was realized by generating laser-induced plasmas in the blue outer envelope of a neutral oxy-acetylene flame. Fast imaging and temporally resolved spectroscopy of the plasmas were carried out. Enhanced intensity of up to 4 times and narrowed full width at half maximum (FWHM) down to 60% for emission lines were observed. Electron temperatures and densities were calculated to investigate the flame effects on plasma evolution. These calculated electron temperatures and densities showed that high-temperature and low-density plasmas were achieved before 4 µs in the flame environment, which has the potential to improve LIBS sensitivity and spectral resolution.

Plasmonic-enhanced carbon nanotube infrared bolometers

Plasmonic nanoantennas show significant potential in photodetection applications, but the extent to which their full potential can be realized is dictated by the volume and location of the active materials within the plasmonic structure. Carbon nanotubes (CNTs) have been used as a novel material in photodetection application due to their excellent electronic and optoelectronic properties. However, difficulties in the integration of CNTs in the gaps of nanoantennas have limited the investigation of antenna-coupled CNT detectors. Here, we demonstrate a unique plasmonic approach for selectively growing CNTs in the gap of nanoantenna arrays for fabrication of plasmonic infrared bolometers operating at room temperature. Strong concentration of light at the tips of nanoantennas was utilized for localized heating and growth of CNTs. Moreover, interaction of this strong optical field with the small volume of CNTs enhanced the photoresponse of the bolometers. Consequently, a high responsivity of about 800 V W(-1) was achieved at room temperature.

High-performance flexible solid-state supercapacitors based on MnO2-decorated nanocarbon electrodes

Flexible energy storage units are highly desired to meet the ever-increasing demands for flexible electronics. In this paper, highly flexible solid-state supercapacitors are fabricated using MnO2-decorated nanocarbon electrodes and 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide–poly(vinylidene fluoride)-hexafluoropropylene ([EMIM][NTf2]–PVdF(HFP)) gel electrolytes. The flexible electrodes are prepared by electrodepositing MnO2 onto the carbon nanotube/carbon nanoonion (CNT/CNO) films. CNT/CNO films have a large surface area for MnO2 deposition and work as mechanical supports with high flexibility and light weight. CNOs act as spacers to separate CNTs, introducing mesopores inside the CNT/CNO films for preventing pore blocking during MnO2 deposition. The supercapacitor exhibits enhanced electrochemical performance with an energy density of 16.4 W h−1 kg−1 at a power density of 33.3 kW kg−1 by using the [EMIM][NTf2]–PVdF(HFP) gel electrolyte. Moreover, the supercapacitors can exhibit high electrochemical performance under large mechanical stress, making the devices suitable for flexible electronics.

Generation of high-temperature and low-density plasmas for improved spectral resolutions in laser-induced breakdown spectroscopy

Improved spectral resolutions were achieved in laser-induced breakdown spectroscopy (LIBS) through generation of high-temperature and low-density plasmas. A first pulse from a KrF excimer laser was used to produce particles by perpendicularly irradiating targets in air. A second pulse from a 532 nm Nd:YAG laser was introduced parallel to the sample surface to reablate the particles. Optical scattering from the first-pulse plasmas was imaged to elucidate particle formation in the plasmas. Narrower line widths (full width at half maximums: FWHMs) and weaker self-absorption were observed from time-integrated LIBS spectra. Estimation of plasma temperatures and densities indicates that high temperature and low density can be achieved simultaneously in plasmas to improve LIBS resolutions.