Laser action in an argon vacuum arc discharge

A transverse vacuum arc discharge has been developed. The anode cathode distance is 4 mm, the discharge length 100 mm. Gain up to 0.55% cm^?1 and a maximum laser power of 20 mW have been observed in argon at 488 nm.     

Laser action in the anode region of a transverse argon discharge

A transverse low pressure high current argon discharge has been developed. Gaun up to 0.63% cm^?1 has been observed at 488 nm. Quasi cw laser action was achieved with a discharge length of 85 mm and threshold currents of 70 A.     

Magnetic Field Enhancement of the Optogalvanic Signal in a Hollow Cathode Discharge

An optogalvanic signal enhancement was obtained in a magnetic field applied on a hollow cathode discharge-detector. In this magnetic field a mutual correlation was observed between optogalvanic signal behaviour, discharge current change and lower level population of the transition corresponding to the 632,8 nm laser irradiation spectral line.     

Nonlinear Hanle effect in the active discharge of an argon laser

 We present experimental evidence of the NonLinear Hanle Effect (NLHE) on the 488 nm transition in an active discharge of an argon laser. Experimental data are used to estimate the homogeneous linewidth of the upper level of the transition. The role of the NLHE in the laser power enhancement is also discussed. Received: 22 February 1996 / Revised version: 17 June 1996     

Plasmon resonance modulated photoluminescence and Raman spectroscopy of diindenoperylene organic semiconductor thin film

In a comparison between a bare diindenoperylene (DIP) film and a DIP film spin-coated with a layer of gold nanoparticles, we have investigated the influence of plasmon resonances in the gold particles on spectroscopic properties of the molecular film. Under off-resonant excitation with a laser at 633 nm, the bare DIP film showed only weak photoluminescence (PL) and Raman signals, but after spin-coating gold nanoparticles on such a DIP film, we found an enhancement of both the PL and Raman signals by a factor of about 3, whereas no enhancement could be observed when the same sample was excited with laser light of 488 nm. This difference reveals that at 633 nm, plasmon resonances in the gold nanoparticles are excited, leading in turn to an enhancement of PL and Raman signals of the weakly absorbing DIP film via coupling between plasmons in the gold particles and exciton-polaritons in the molecular film. For the laser at 488 nm, due to a much larger absorption coefficient of DIP, excitons in the molecular film are directly excited, out-weighing the influence of an off-resonant coupling to the plasmon resonances in the gold particles occurring at much lower energy.     

Spectroscopic study of a brush cathode plasma in a magnetic field

The brush cathode helium discharge in the magnetic field has been operated stably at discharge currents larger than those without magnetic field. The diameter of the plasma column has been determined by the configuration of the magnetic field. The measurements of the spectral intensities of the recombination continuum followed by the 2³S-n³P series reveals that the electron density is 1·8 × 10¹³ cm^-3 and the electron temperature is 0·17 eV at a discharge current of 500 mA and a pressure of 0·9 torr for a magnetic flux density of 1·3 kG. The principal quantum number for line merging is 20.     

Intensity enhancement of the C5+ Balmer radiation excited by capillary discharge pumping

The intensity of the 18.2 nm Balmer α-transition in C⁵⁺ excited in a capillary discharge using alumina and polyacetal tubes was studied. For discharge currents of up to 80 kA in tubes filled with C₂H₂, intense radiation from the excitation of C⁵⁺ ions and from the recombination of C⁶⁺ ions was observed. With increasing length of the discharge, the intensity in the falling edge of the recombination pulse rises faster than proportional. In contrast to previous investigations, gain by stimulated emission is excluded. The enhancement is ascribed to an optical guiding of the XUV radiation in the dense plasma created by ablation from the tube walls. Received: 1 April 1999 / Revised version: 22 July 1999 / Published online: 30 November 1999     

Cold-hollow-cathode arc discharge in crossed electric and magnetic fields

A crossed-field cold-hollow-cathode arc is stable at low working gas pressures of 10⁻²–10⁻¹ Pa, magnetic-field-and gas-dependent arcing voltages of 20–50 V, and discharge currents of 20–200 A. This is because electrons come from a cathode spot produced on the inner cathode surface by a discharge over the dielectric surface. The magnetic field influences the arcing voltage and discharge current most significantly. When the plasma conductivity in the cathode region decreases in the electric field direction, the magnetic field increases, causing the discharge current to decline and the discharge voltage to rise. The discharge is quenched when a critical magnetic field depending on the type of gas is reached. Because of the absence of heated elements, the hollow cathode remains efficient for long when an arc is initiated in both inert and chemically active gases.     

Nonlinear beam clean-up using resonantly enhanced sum-frequency mixing

We investigate the possibility of improving the beam quality and obtaining high conversion efficiency in nonlinear sum-frequency generation. A 765 nm beam from an external cavity tapered diode laser is single-passed through a nonlinear crystal situated in the high intracavity field of a 1342 nm Nd:YVO₄ laser, generating a SFG beam at 488 nm. The ECDL have M _H ² =1.9 and M _V ² =2.4 and the solid-state laser has M ²<1.05. Varying the focusing of the 765 nm beam, the conversion efficiency and the beam quality of the generated 488 nm beam change correspondingly. We show that it is possible to improve the M ² of the 488 nm beam to less than 1.3 while preserving a high conversion efficiency of the SFG process.     

Photodissociation effects on pulsed laser deposited silver oxide thin films: surface‐enhanced resonance Raman scattering

Temporal Raman scattering measurements with 488, 532 and 632 nm excitation wavelengths and normal Raman studies by varying the power (from 30 W/cm² to 2 MW/cm²) at 488 nm were performed on silver oxide thin films prepared by pulsed‐laser deposition. Initially, silver oxide Raman spectra were observed with all three excitation wavelengths. With further increase in time and power, silver oxide photodissociated into silver nanostructures. High‐intensity spectral lines were observed at 1336 ± 25 and 1596 ± 10 cm⁻¹ with 488 nm excitation. No spectral features were observed with 633 nm excitation. Surface‐enhanced resonance Raman scattering theory is used to explain the complex behavior in the intensity of the 1336/1596 cm⁻¹ lines with varying power of 488 nm excitation. Copyright © 2011 John Wiley & Sons, Ltd.     

磁场约束下激光诱导等离子体光谱强度演化研究

对磁场约束下激光诱导铜等离子体光谱强度演化进行了实验研究,分析了在磁场约束环境下的等离子体光谱强度演化过程以及激光能量对光谱增强的影响.实验结果表明:在磁场约束下铜等离子体内原子光谱和离子光谱均有所增强,在磁场约束下Cu I 510.55 nm谱线强度时间演化过程中在1.2—5.7μs时间范围内附近出现双峰结构,在距离靶材表面0—1.4 mm空间范围内磁场约束Cu I 510.55 nm光谱增强明显.Cu I510.55 nm和Cu I 515.32 nm光谱增强因子随激光能量的增加呈单调递减变化,激光能量20 mJ时增强因子最大分别为11和8.对磁场约束下等离子体发射光谱强度增强的物理原因进行了探讨.     

Optical losses and gain in silicon-rich silica waveguides containing Er ions

Rib-loaded waveguides containing Er³⁺ coupled to Si-nc have been produced by magnetron sputtering and successive thermal annealing to investigate optical gain at 1535 nm. It has been shown that all Er ions are optically active, whereas the fraction that can be excited at high pump rates under non-resonant excitation is strongly limited by confined carrier absorption (CA), up-conversion processes, and mainly by the lack of coupling to the Si-nc. Er³⁺ absorption cross-section is found comparable to that of Er³⁺ in SiO₂, but a dependence with the effective refractive index has been found. Although the presence of Si-nc strongly improves the efficiency of Er³⁺ excitation, it introduces additional optical loss mechanisms, such as CA. These Si-nc losses affect the possibility of obtaining net optical gain. In the present study, they have been minimized by lowering the annealing time of the Er-doped Si-rich oxide. In pump-probe measurements it is shown that signal enhancement of the transmitted signal can be achieved at low pumping rate when the detrimental role of confined CA is attenuated by reducing the annealing time. A maximum signal enhancement of about 1.30 at 1535 nm was observed.     

Measurement of electric field strengths in a waveguide by means of the dynamic Stark effect in hydrogen and neutral helium spectral lines

The dynamic Stark effect of the spectral lines H_β and of the neutral helium lines λ=402.6 nm (2³ P ⁰−5³ D) and λ=438.8 nm (2¹ P ⁰−5¹ D) emitted from a discharge tube was used for probing rf electric fields in a transverse waveguide. Calculations accounting for the pertubation of the atomic states by strong unidirectional fields prove to be suitable in order to interprete the main experimental results. If the waveguide is terminated with a metallic reflector and the plasma in the discharge tube becomes overdense—then representing a slightly permeable mirror—a resonant enhancement of the electric field strength may be achieved by tuning. This enhancement is well recognizable in the spectral line contours.     

CW HCN laser gain in a hollow dielectric rectangular cross-section discharge tube

This paper describes a 1 m-long CW HCN discharge laser with a hollow dielectric rectangular discharge tube of 5×20 cm² cross-section. For optimum working conditions, the characteristics of the amplifying medium have been made constant over the cross-section, by using an additional magnetic field. Then, the unsaturated gain for the 337 μm line is 6.8% m⁻¹. It is shown to be dependent on the smaller dimension of the cross-section rather than on the other dimension. By using the tube as a waveguide, laser output power of 1 Watt might be obtained with a discharge length of 4 m, instead of the 12m-long discharge required to get the same output power from a cylindrical waveguide laser.     

Investigation of L(+)-Ascorbic acid with Raman spectroscopy in visible and UV light

Abstract: Raman spectroscopy investigations of l(+)-ascorbic acid and its mono- and di-deprotonated anions (AH^? and A^2?) are reviewed and new measurements reported with several wavelengths, 229, 244, 266, 488, and 532 nm. Results are interpreted, assisted by new DFT/B3LYP quantum chemical calculations with 6-311++G(d,p) basis sets for several conformations of ascorbic acid and the anions. Raman spectra were measured during titration with NaOH base in an oxygen-poor environment to avoid fluorescence when solutions were alkaline. The ultraviolet (UV) absorption band for ascorbic acid in aqueous solution at ～247 nm was found to cause strong resonance enhancement for the ring C?C stretching mode (called B) at ～1692 cm^?1. The ascorbate mono-anion absorbs at ～264.8 nm giving Raman resonance enhancement for the same ring C–C bond stretching, downshifted to ～1591 cm^?1. Finally, for the ascorbate di-anion, absorption was found at ～298.4 nm with molar absorptivity of ～7,000 L mol^?1 cm^?1 and below ～220 nm. With UV light (244 and 266 nm), strongly basic solutions gave pronounced Raman resonance enhancement at ～1556 cm^?1. Relatively weak preresonance enhancement was seen for A^2? when excitation was done with 229 nm UV light, allowing water bands to become observable as for normal visible light Raman spectra.     

Study of magnetically enhanced corona pre-charger

In this paper, experimental investigations of the discharge characteristics of magnetically enhanced corona discharges, for the purpose of capturing fine aerosol particles, are presented. The discharge mechanism during such a process is analyzed as well. The effects of magnetic enhancement under different magnet flux densities, and in positive- or negative-corona discharges, were experimentally compared. The magnetically enhanced effects in different inter-electrode regions were studied. Experimental results demonstrated that the magnetic field could efficiently increase the concentrations of both the negative ions and the free electrons during negative-corona discharge. The dominant mechanism of magnetic enhancement in a corona discharge involves the Larmor precessions of free electrons which enhance ionization of the gas molecules near the discharge electrode. A convenient configuration for enhancing corona discharge was formed by placing permanent magnets with a local strong magnetic field near the discharge electrode. A magnetically enhanced negative-corona (MNC) pre-charger was assembled in front of an electrostatic enhancement filter. The influence of the MNC pre-charger on the efficiencies of an electrostatic enhancement filter was measured and compared with that of a conventional corona pre-charger. The free-electron-charging mechanism of the MNC pre-charger was preliminarily analyzed. Our results show that the new pre-charging technique is promising for capturing fine aerosol particles in electrostatic enhancement filters or electrostatic precipitators.     

Sensitizing of Sm<Superscript>3+</Superscript> fluorescence by silver dopant in the TiO<Subscript>2</Subscript> films

Composite material based on a TiO₂ matrix doped with Sm³⁺ ions and co-doped with silver was investigated. Samarium ions together with nano- and micro-aggregates of silver were incorporated into the titanium alkoxide during the sol-gel process. Samarium ions were excited either directly (λ _exc = 488 nm) or through the TiO₂ host (λ _exc = 355 nm). It was revealed that samarium fluorescence (λ _exc = 488 nm) in gelled TiO₂ films is enhanced by up to 20 times in the vicinity of silver inclusions. Sensitizing and plasmonic mechanisms of enhancement in Sm³⁺ fluorescence are discussed.     

金纳米与磁场作用下LIBS检测灵敏度改善研究

针对激光诱导击穿光谱技术（LIBS）中等离子体的发射光谱增强问题,提出一种磁场增强LIBS与纳米颗粒增强LIBS（NELIBS）相结合的方法。采用热蒸发法在样品表面沉积一层直径20 nm的金纳米颗粒。利用波长为1 064 nm,最大能量为200 mJ的Nd∶YAG脉冲激光器在室温,一个标准大气压下对纯铜和黄铜进行诱导击穿。调整激光能量为30~110 mJ,分别使用传统LIBS、磁场增强LIBS、NELIBS以及两种方法结合对纯铜进行激光诱导击穿,得到特征谱线(Cu Ⅰ 521.8 nm)的强度增强因子和信噪比,并对其增强机理进行分析。在相同环境下使用四种方式对黄铜和纯铜进行诱导击穿以探测样品中的微量元素。当在样品表面沉淀金纳米颗粒或者将沉淀有金纳米颗粒的样品放在磁场中进行诱导击穿时,发现纯铜样品的光谱中存在Mg元素的特征谱线Mg Ⅱ 279.569 nm,黄铜样品的光谱中存在Si元素的特征谱线 Si Ⅰ 251.611 nm。实验结果表明：单独施加磁场约束或增加纳米金颗粒均可以有效增强等离子体光谱强度,但增强效果弱于两种方法结合,磁场约束对光谱的增强效果弱于NELIBS的增强效果。当结合NELIBS与磁场约束LIBS时,谱线增强因子最高可达14.3(Cu Ⅰ 521.8 nm),相比于磁场增强LIBS和NELIBS,最大增强因子分别提高了28%和59%。四种情况中当激光脉冲能量逐渐增大时,等离子体向外膨胀的强度增大,磁场产生的洛伦兹力束缚等离子的能力相对减弱,同时纳米金颗粒对等离子体发射光谱的增强作用被削弱,谱线强度降低,等离子体的增强因子逐渐减小后趋于稳定。通过NELIBS与磁场约束LIBS结合方式,不仅可以有效提高等离子体的发射谱线强度,改善光谱信号信噪比,而且传统LIBS方法中由于谱线强度低、背景噪声大而无法探测的微量元素可以被探测到,LIBS技术对微量元素的探测能力得到显著提高,微量元素的探测下限变得更低。NELIBS与磁场约束LIBS结合的方法具有更高的灵敏度和准确度,为激光诱导击穿光谱技术的谱线增强方法提供了新的思路,在该领域具有广阔的应用前景。     

A dynamic nuclear polarization strategy for multi-dimensional Earth's field NMR spectroscopy

Dynamic nuclear polarization (DNP) is introduced as a powerful tool for polarization enhancement in multi-dimensional Earth’s field NMR spectroscopy. Maximum polarization enhancements, relative to thermal equilibrium in the Earth’s magnetic field, are calculated theoretically and compared to the more traditional prepolarization approach for NMR sensitivity enhancement at ultra-low fields. Signal enhancement factors on the order of 3000 are demonstrated experimentally using DNP with a nitroxide free radical, TEMPO, which contains an unpaired electron which is strongly coupled to a neighboring ¹⁴N nucleus via the hyperfine interaction. A high-quality 2D ¹⁹F–¹H COSY spectrum acquired in the Earth’s magnetic field with DNP enhancement is presented and compared to simulation.     

Enhancement of soft X-ray emission from a pinch plasma using a rotating plasma for pre-ionization

An experimental study on the enhancement of soft X-ray emission from a pinch plasma has been carried out by using a rotating plasma for pre-ionization. The rotating plasma is produced by a pulsed J×B cross-field discharge between coaxial electrodes, and subjected to a pinch discharge. Under an optimized discharge condition, it has been observed that UV emission of 251.6 nm (Ar III) soft X-ray intensity increases with increasing applied axial magnetic field B. At 400 G, the X-ray intensity is enhanced by a factor of 3 compared to that without the rotating plasma. It is considered that this result is due to the improved characteristics of the pre-ionization rotating plasma, which influences the uniformity and stability of subsequent pinch plasma.