Fullerenes: The attractive medium for harmonic generation

We review the results of systematic experimental studies of high-order harmonic generation (HHG) in C₆₀-rich laser-produced plasma under different plasma conditions and laser parameters. The morphology of fullerene clusters before and after ablation is analyzed to define the optimal conditions of excitation of C₆₀-containing targets. The enhancement of HHG efficiency in C₆₀-rich plasmas by using the two-color pump technique is discussed. The conversion efficiency for the odd and even harmonics in the vicinity of surface plasmon resonance of the C₆₀-containing plasma (40–70 nm) was estimated to be in the range of 10⁻⁴.     

Polymerization of solid C<Subscript>60</Subscript> under C<Stack><Subscript>60</Subscript><Superscript>+</Superscript></Stack> cluster ion bombardment

The structure transformation occurring in fullerene film under bombardment by 50 keV C₆₀⁺ cluster ions is reported. The Raman spectra of the irradiated C₆₀ films reveal a new peak rising at 1458 cm⁻¹ with an increase in the ion fluence. This feature of the Raman spectra suggests linear polymerization of solid C₆₀ induced by the cluster ion impacts. The aligned C₆₀ polymeric chains composing about 5–10 fullerene molecules have been distinguished on the film surface after the high-fluence irradiation using atomic force microscopy (AFM). The surface profiling analysis of the irradiated films has revealed pronounced sputtering during the treatment. The obtained results indicate that the C₆₀ polymerization occurs in a deep layer situated more than 40 nm below the film surface. The deep location of the C₆₀ polymeric phase indirectly confirms the dominant role of shock waves in the detected C₆₀ phase transformation.     

Low- and high-order nonlinear optical characterization of C60-containing media

Third- and higher-order nonlinear optical processes in fullerenes were studies to define the influence of low-order nonlinearities on the high-order harmonic generation in these media. We measured the nonlinear absorption coefficients of the C₆₀:toluene solution using the 532 and 1064 nm, 50 ps pulses. The high-order harmonic generation was studied during propagation of the 790 nm, 150 fs pulses through the plasmas produced on surfaces containing fullerene powder. These studies have shown that the low-order nonlinearities of fullerenes have no impact on the generation of harmonics in such mediums in the vacuum ultraviolet range at optimal intensity of laser radiation.     

Characterisation of laser-produced tungsten plasma using optical spectroscopy method

This paper describes results of spectroscopic investigation of laser-produced tungsten plasma. The laser intensity on the target surface reached up to 30 GW/cm² depending on the focusing conditions. Optical spectra emitted from plasma plumes which were formed under vacuum conditions in front of the tungsten target due to the interaction of Nd-YAG laser pulses (1.06 μm, 0.5 J), were characterised by means of an optical spectrometer (λ/Δλ= 900) in the wavelength range from 300 to 1100 nm. The spectra were recorded automatically with the use of a CCD detector with exposition time varied from 100 ns to 50 ms. On the basis of WI and WII lines it was possible to estimate electron temperature and electron density which corresponded to the expansion phase of the plasma. T_e and N_e were measured as 1.1 eV and 8×10¹⁶ cm^-3, respectively. The spectra collected by the ion energy analyser showed that the plasma included tungsten ions up to 6+ ion charge. Signals from the ion collector allowed to estimate the average value of ion energy of tungsten as 4.6 keV. Basing on this value the electron temperature corresponding to the initial stage of the plasma formation was estimated to be about 320 eV. Optical microscope investigation showed that laser irradiation caused structural changes on the surface of the target.     

Generation of lower and higher harmonics in different plasma media with small <Emphasis Type="Italic">Z</Emphasis>

The generation of lower (third) and higher harmonics of femtosecond laser radiation in plasmas produced by laser ablation of different targets with a small atomic number Z (B, Be, Li) has been investigated. The high (10⁻³) efficiency of third-harmonic generation was observed in plasma produced on the boron surface. Efficient third-harmonic generation was also observed in beryllium plasma using femtosecond pulses of Ti:sapphire laser radiation (λ = 790 nm) and its second harmonic (395 nm). We could tune the higher harmonics generation spectrum by tuning the crystal converter when using 395-nm radiation to be converted. It is shown that, in plasmas formed on targets with small Z, the conversion efficiency and limiting generated harmonic order depend on the delay between the ablation pulse and the pulse to be converted.     

Synthesis and photophysical properties of polyamides containing in-chain porphyrin and [60]fullerene

Conjugated polyamides containing porphyrin and [60]fullerene (C₆₀) in the main chain were prepared by a direct polycondensation of the 3′H,3″H-dicyclopropa[1, 9:16, 17] [5, 6]fullerene-C₆₀-I _h -3′,3″-dicarboxylic acid and 5,15-bis(4-aminophenyl)-10,20-bis(3,5-dialkoxyphenyl)porphyrin in the presence of triphenyl phosphite and pyridine. Gel permeation chromatography (GPC) analysis of the polyamides showed the weight-average molecular weight was about 23,626–23,736, and the temperature at 5% weight loss determined by thermogravimetric analysis (TGA) was above 216 °C. The transmission electron microscopy (TEM) images displayed the regular one-dimensional linear arrays of the polyamides with lengths exceeded 200 nm. The photoinduced electron transfer from porphyrin to C₆₀ in the polyamides was observed in nanosecond laser-flash photolysis experiments at ambient temperature, which produced a charge-separated state (porphyrin radical cation–C₆₀ radical anion pair) with a lifetime as long as 40 μs. The calculated ratio of k _CS/k _CR was found to be 2.1 × 10⁴. They could have potential applications for photoelectronic devices, organic solar cells and so on.     

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     

CdS plume composition and dynamics of neutral species upon ablation with 532 nm laser light

The neutral species present in CdS ablation plumes upon nanosecond 532 nm laser irradiation at a moderate fluence of 0.5–0.75 J cm⁻² have been studied. Neutral Cd _n S _m clusters have been identified, some as large as (CdS)₃₃₋₃₄ (1–2 nm in diameter). The analysis of the dynamics of neutral species shows an expansion with two components that differ both in composition and dynamics. A fast, high kinetic energy component, dominated by S₂ which acquires free-flow conditions at short distances from the target, is followed by a slower component characterized by similar speeds for all species. This slower component shows dynamic features that are expected to favor aggregation processes leading to effective cluster formation.     

Laser deposition of amorphous diamond films from liquid aromatic hydrocarbons

6 H₅CH₃, C₆H₆, and C₆H₅CH(CH₃)₂) to pulsed visible laser radiation of a copper vapor laser (λ=510.6 nm). The X-ray Auger electron spectroscopy (XAES), reflection high energy electron diffraction (RHEED), and Raman analysis are employed to characterize the deposited films. The sp³ fraction in deposited films amounts to 60–70% and depends on the precursor. The average film thickness on a glass substrate is about 100 nm. The films show excellent adherence, are transparent in the visible and have microhardness of 50–70 GPa, as measured by nanoindentor. Received: 28 September 1998 / Accepted: 13 January 1999     

High-order harmonic generation in nanoparticle-containing laser-produced plasmas

Nanoparticle-containing media can be used for the efficient high-order harmonic generation (HHG) of laser radiation in the extreme ultraviolet range. We review the results of recent studies of the HHG in laser-produced plasmas containing Ag, Au, Pd, Pt, Ru, GaN, BaTiO₃, and SrTiO₃ nanoparticles. The harmonics of femtosecond radiation up to the 55th order were achieved using the nanoparticle-containing plumes, when the femtosecond radiation propagated through the preformed plasma. These results are compared with the high-order harmonics generated from the plasma produced on the surface of bulk targets at different delays between the subnanosecond heating prepulse and femtosecond pulse. We discuss a six-fold enhancement of the HHG yield, which was achieved in the case of nanoparticle-containing plumes with regard to the monoparticle-containing plasmas.     

Wavelength modulation and cavity enhanced absorption spectroscopy using ～1.9?μm radiation produced by difference frequency generation with a MgO doped PPLN crystal

We have demonstrated the production of ∼1.9 μm near-infrared radiation by using difference frequency generation within a 5% MgO doped PPLN crystal by coupling ∼735 nm radiation from a tunable external cavity diode laser with relatively high powered 532 nm radiation from both Nd:YVO₃ and Nd:YAG lasers. The radiation produced is of low power, ∼15 μW, and was used in conjunction with the sensitivity enhancing techniques of wavelength modulation spectroscopy (WMS) and cavity enhanced absorption spectroscopy (CEAS). Experiments were carried out on rotationally resolved transitions in the combination bands of NH₃ and CO₂ in the 1.9 μm region. An α _min value of 3.6×10⁻⁶ cm⁻¹ Hz^−1/2 was achieved for WMS measurements on CO₂. A comparable α _min value of 2.2×10⁻⁶ cm⁻¹ Hz^−1/2 was achieved for NH₃ using CEAS. The low NIR power indicates that despite the level of MgO doping quoted for the crystal, under prolonged exposure photorefractive damage has occurred.     

Generation of strong coherent extreme ultraviolet radiation from the laser plasma produced on the surface of solid targets

We demonstrate the generation of high harmonics (up to the 65th order, λ=12.24 nm) of a Ti:sapphire laser radiation after the propagation of femtosecond laser pulses through the low-excited plasma produced by a picosecond prepulse radiation on the surface of different targets. High-order harmonics generated from the surface plasma of most targets showed a plateau pattern. It is assumed that the harmonic generation in these conditions occurs due to the interaction of the femtosecond pulses with the ions. The conversion efficiencies at the plateau region were varied between 1×10^-7 to 8×10^-6, depending on the target. The main contribution to the limitation of harmonic generation efficiency and cutoff energy was attributed to the self-defocusing of main pulse. A considerable restriction of the 27th harmonic generation was observed at different focusing conditions in the case of chromium plasma. Our observation of the resonance-induced enhancement of a single harmonic (λ=61.2 nm) at a plateau region with the efficiency of 8×10^-5 in the case of In plasma can offer some expectation that analogous processes can be realized in other plasma samples in the shorter wavelength range where the highest harmonics were achieved. PACS 42.65.Ky; 52.35.Mw; 52.38.-r     

Synchrotron and laser excitation of luminescence in PbWO<Subscript>4</Subscript>:Tb crystals at different temperatures

The effect of temperature on the spectral luminescence characteristics of PbWO₄:Tb³⁺ crystals with synchrotron and laser excitation is studied. If PbWO₄:Tb³⁺ is excited by synchrotron radiation with λ = 88 nm at 300 K, a faint recombination luminescence of the impurity terbium is observed against the matrix luminescence. When the temperature is reduced to 8 K, the luminescence intensity of PbWO₄:Tb³⁺ increases by roughly an order of magnitude and the characteristic luminescence of the unactivated crystal is observed. Excitation of PbWO₄:Tb³⁺ by a nitrogen laser at 300 K leads to the appearance of emission from Tb³⁺ ions. At 90 K, a faint matrix luminescence is observed in addition to the activator emission. The formation of the luminescence excitation spectra for wavelengths of 60–320 nm is analyzed and the nature of the emission bands is discussed.     

On the direct formation of HO2 radicals after 248 nm irradiation of benzene C6H6 in the presence of O2

We present in this work the direct observation of HO₂ radicals after irradiation of benzene C₆H₆ at 248 nm in the presence of O₂. HO₂ radicals have been unambiguously identified using the very selective and sensitive detection of continuous wave cavity ring-down spectroscopy (cw-CRDS) coupled to a laser photolysis reactor. HO₂ radicals were detected in the first vibrational overtone of the OH stretch at 6638.20 cm^-1, using a DFB diode laser. This reaction might be important because 248 nm photolysis of H₂O₂ has often been used in the past for studying the OH^•-initiated degradation of C₆H₆, often using a large excess of C₆H₆ over H₂O₂. The possible importance of the title reaction with respect to these former laboratory studies has been quantified through comparison with HO₂ ^• signals obtained from 248 nm photolysis of H₂O₂: one obtains under our conditions (excess O₂ and total pressure of 6.6 kPa helium) from the 248 nm irradiation of identical initial concentrations [C₆H₆]=[H₂O₂] the following relative initial radical concentrations: [HO₂ ^•]=(0.28±0.05)×[OH^•]. Experiments with various O₂ concentrations have revealed that the origin of the HO₂ radicals is not the reaction of H-atoms with O₂, but must originate from the reaction of O₂ with excited C₆H₆ ^*. The quantum yield of C₆H₆ ^* formation has been deduced to ϕ=0.2±0.1. PACS  42.62.Fi; 82.20.Pm; 82.33.Tb     

Electron-induced radiation from C60 fullerene in the gas phase

The formation of radiating particles in the excitation of C₆₀ fullerene molecules by electrons with energies E _e<100 eV is investigated by the method of crossed molecular and electron beams. A quasicontinuous (with a spectral resolution of 3 nm) emission spectrum, close to the Planck emission spectrum of a heated body, is recorded in the wavelength range 300–800 nm. The temperature of the radiation corresponds to an internal energy of the C₆₀ molecule of approximately 40 eV. Pis’ma Zh. éksp. Teor. Fiz. 63, No. 12, 915–919 (25 June 1996)     

Improvement of performance of tetraphenylporphyrin-based red organic light emitting diodes using WO3 and C60 buffer layers

One of the porphyrin derivatives, meso-tetraphenylporphyrin (TPP), has been synthesized and examined as an emitter material (EM) for efficient fluorescent red organic light-emitting diodes (OLEDs). By inserting a tungsten oxide (WO₃) layer into the interface of anode (ITO) and hole transport layer N,N′-Di-[(1-napthyl)-N,N′-diphenyl]-(1,1′-biphenyl)-4,4′-diamine (NPB) and by using fullerene (C₆₀) in contact with a LiF/Al cathode, the performance of devices was markedly improved. The current density–voltage–luminance (J–V–L) characterizations of the samples show that red OLEDs with both WO₃ and C₆₀ as buffer layers have a lower driving voltage and higher luminance compared with the devices without buffer layers. The red OLED with the configuration ITO/WO₃ (3 nm)/NPB (50 nm)/TPP (60 nm)/BPhen (30 nm)/C₆₀ (5 nm)/LiF (0.8 nm)/Al (100 nm) achieved the high luminance of 6359 cd/m² at the low driving voltage of 8 V. At a current density of 20 mA/cm², a pure red emission with CIE coordinates of (0.65; 0.35) is observed for this device. Moreover, a power efficiency of 2.07 lm/W and a current efficiency of 5.17 cd/A at 20 mA/cm² were obtained for the fabricated devices. The study of the energy level diagram of the devices revealed that the improvement in performance of the devices with buffer layers could be attributed to lowering of carrier-injecting barrier and more balanced charge injection and transport properties.     

Optical characteristics of a transverse discharge in neon at atmospheric pressure

We present the results of investigation into radiation of a pulsed transverse discharge in neon at a pressure of 10–200 kPa. Survey spectra of plasma radiation, time characteristics of radiation, and the effect of small impurities of water vapors and air on the optical characteristics of a neon plasma were studied. We show that at a pressure of residual gases at a level of 10 Pa intense OH*, NO*, and N _* ² bands are observed in radiation of the plasma of a nanosecond transverse discharge in Ne against the background of continuous plasma radiation, and in the spectral region with λ>400 nm radiation was observed on the H_β 486.1 nm and NeI 585.3 nm lines, and (when P≥100 kPa) on the line at the 3s–3p-transitions of a Ne atom. The radiation intensity of the third continuum of neon increases with pressure and with energy contribution to plasma, with its maximum being located in the VUV spectral region (λ _max <200 nm). To whom correspondence should be adressed. Uzhgorod State University, 46, Pidgirna St., Uzhgorod, Ukraine. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 66, No. 1, pp. 5–10, January–February, 1999.     

Interfacial electronic structure of molybdenum oxide on the fullerene layer,a potential hole-injecting layer in inverted top-emitting organic light-emitting diodes

The interfacial electronic structures of molybdenum oxide (MoO_x) deposited on fullerene (C₆₀) which could be used as a hole-injecting layer in inverted top-emitting organic light-emitting diodes (TE-OLEDs) were investigated by photoemission spectroscopy. The hole-injecting barrier height (Φ_B^h) at each interface investigated by an ultraviolet photoemission spectroscopy was reduced to from 1.4 to 0.1 eV as the thickness of MoO_x (Θ_MoOx) was increased from 0.1 to 5.0 nm on C₆₀. In these interface system, the sign of vacuum-level shift, highest occupied molecular orbital (HOMO)-level shift, and core-level shifts were all positive indicating that the interface mechanism is attributed to the work-function differences due to a band bending at these interfaces. Moreover, the near-edge X-ray absorption fine structure spectra at carbon K-edge did not show any structural modification as well as any chemical reaction at the MoO_x-on-C₆₀ interfaces when Θ_MoOx was changed on C₆₀. From these results, the inverted TE-OLED with C₆₀ (5.0 nm)/MoO_x (5.0 nm) showed the power efficiency of 1.7 lm/W at a luminance of about 1000 cd/m² and the maximum luminance of about 76.000 cd/m² at the bias voltage of 11.0 V. It exhibited the highest performance among the inverted TE-OLEDs fabricated as a function of MoO_x thickness from 0 to 5.0 nm.     

Synthesis and luminescent characteristics of one-dimensional europium doped Gd2O3 phosphors

One-dimensional (1D) Gd₂O₃:Eu³⁺ nano-rods and micro-rods were prepared using a facile sol-gel precipitation method, without a template and with a post-growth heat treatment in air. Based on scanning electron microscopy (SEM) and X-ray diffraction (XRD) data, hexagonal Gd(OH)₃:Eu³⁺ micro-rods, 60∼90 nm in diameter and 700 nm to 1 μm in length, were synthesized and then transformed by calcining (800°C, 2 hr) to cubic Gd₂O₃:Eu³⁺ with the same morphology and dimensions. Nano-rods of Eu³⁺ doped Gd(OH)₃ and calcined Gd₂O₃, 60∼90 nm diameter and 150∼300 nm length, were prepared by adding polyethylene glycol (PEG) as a capping agent during the sol-gel synthesis. Photoluminescence (PL) spectra exhibited the ⁵D₀–⁷F₂ transitions of Eu³⁺ at 612 and 627 nm from excitation at 280 nm. Photoluminescence excitation (PLE) data showed that a small fraction of PL from Eu³⁺ resulted from direct excitation, but most PL resulted from the oxygen to europium charge-transfer band (CTB) between 250 and 280 nm.     

EUV and soft X-ray photoelectron spectroscopy of isolated atoms and molecules using single-order laser high-harmonics at 42 eV and 91 eV

Photoelectron spectroscopy of isolated atoms and molecules using single-order high-harmonics of Ti:Sapphire laser pulses (800 nm, 12 fs/30 fs) is demonstrated. Dielectric multilayer mirrors, SiC/Mg and Mo/Si, are used to isolate the 27th (42 eV) and 59th (91 eV) order harmonics, respectively. The obtained harmonics are characterized by valence and inner-shell photoelectron spectroscopy of Xe. The applications to two-color two-photon ionization of He and pump-probe spectroscopy of ultrafast photodissociation of Br₂, Br₂(C¹Π_u) → Br(²P_3/2) + Br(²P_3/2), are presented.