Plasma-based nitrogen tail pulse shutter for CO2-TEA lidar dial systems

This paper presents the construction, use and characterisation of a laser-induced sealed plasma shutter to clip off the nitrogen pulse tail of a CO₂-TEA laser-based lidar dial system. Investigation of the optimum gas filling pressure, temporal profile of the clipped pulse, and the laser threshold power intensities to achieve ionisation growth and breakdown in helium, argon, and nitrogen are also presented. Values of these power density thresholds lie between 3×10¹¹ W cm^-2–5×10¹² W cm^-2, 2×10¹¹ W cm^-2–2×10¹² W cm^-2 and 3×10¹³ W cm^-2–2×10¹⁴ W cm^-2 for helium, argon, and nitrogen, respectively. The range resolution attainable with the present clipped pulses is 15 m, which is 30 times better than that readily obtained with the nitrogen-tailed pulses. Field measurements of the lidar returns with the clipped pulse from a co-operative target are presented. Received: 27 December 1999 / Revised version: 11 February 2000 / Published online: 27 April 2000     

Cantilever-based photoacoustic detection of carbon dioxide using a fiber-amplified diode laser

A compact and sensitive photoacoustic setup has been developed based on a recently demonstrated cantilever technique. A micromechanical cantilever transducer is attached to a cylindrical photoacoustic cell and the cantilever’s deflection is monitored with a compact Michelson interferometer. A commercial 1-Watt optical fiber amplifier was used to enhance the performance of the system. A normalized sensitivity of 1.4×10^-10 cm^-1 W Hz^-1/2 was achieved in the detection of carbon dioxide at 1572 nm wavelength. Using 34 mW optical power from a DFB diode laser, the noise-equivalent detection limit for carbon dioxide at this wavelength is 4.0 ppm. Employing the fiber amplifier, we improved the sensitivity to yield measurement of sub-ppm concentrations. PACS 42.62.Fi; 42.55.Px; 82.80.Ch     

Gas-phase photoacoustic sensor at 8.41 μm using quartz tuning forks and amplitude-modulated quantum cascade lasers

We demonstrate the performance of a novel infrared photoacoustic laser absorbance sensor for gas-phase species using an amplitude-modulated quantum cascade (QC) laser and a quartz tuning fork microphone. The photoacoustic signal was generated by focusing 5.3 mW of a Fabry–Pérot QC laser operating at 8.41 μm between the tines of a quartz tuning fork which served as a transducer for the transient acoustic pressure wave. The sensitivity of this sensor was calibrated using the infrared absorber Freon 134a by performing a simultaneous absorption measurement using a 31-cm absorption cell. The power and bandwidth normalized noise equivalent absorption sensitivity (NEAS) of this sensor was determined to be D=2.0×10^-8 W cm^-1/Hz^1/2. A corresponding theoretical analysis of the instrument sensitivity is presented and is capable of quantitatively reproducing the experimental NEAS, indicating that the fundamental sensitivity of this technique is limited by the noise floor of the tuning fork itself. PACS 43.60.Vx; 43.58.Wc; 43.58.Hp; 84.40.Xb     

Photoacoustic gas sensing employing fundamental and frequency-doubled radiation of a continuously tunable high-pressure CO2 laser

We present the first photoacoustic spectrometer for gas sensing employing both the fundamental and the frequency-doubled radiation of a continuously tunable high-pressure CO₂ laser with room temperature operation. A quasi-phase-matched diffusion-bonded GaAs crystal is used in the system for second-harmonic generation. A pulsed photoacoustic detection scheme with a non-resonant cell, equipped with an 80-microphone array, is employed. The wide continuous tuning range in the fundamental (9.2–10.7 μm) and the frequency-doubled (4.6–5.35 μm) regimes, together with the narrow linewidth of 540 MHz (0.018 cm^-1) for the 10-μm region and of 1050 MHz (0.0315 cm^-1) for the 5-μm region, allow the measurement of gas mixtures, individual species and isotope discrimination. This is illustrated with measurements on NO and CO₂. The measured isotope ratio ¹⁵ NO/¹⁴ NO=(3.58±0.55)×10^-3 agrees well with the literature (3.700×10^-3) and demonstrates the good selectivity of the system. Received: 30 April 2002 / Revised version: 10 June 2002 / Published online: 2 September 2002 RID="*" ID="*"Corresponding author. Fax: +41-1/633-1077, E-mail: sigrist@iqe.phys.ethz.ch     

Search for solar axions emitted in the M1-transition of 7Li* with Borexino CTF

Results of background measurements with a prototype of the Borexino detector were used to search for 478 keV solar axions emitted in the M1-transitions of ⁷Li^*. The Compton conversion of axion to a photon A+e→e+γ, axioelectric effect A+e+Z→e+Z, decay of axion in two photons A→2γ and Primakoff conversion on nuclei A+Z→γ+Z are considered. The upper limit on constants of interaction of axion with electrons, photons and nucleons – g_Aeg_AN≤(1.0–2.4)×10^-10 at m_A≤450 keV and g_Aγg_AN≤5×10^-9 GeV^-1 at m_A≤10 keV are obtained (90%c.l.). For heavy axions with mass at 100<m_A<400 keV the limits g_Ae<(0.7–2.0)×10^-8 and g_Aγ<10^-9–10^-8 are obtained in assumption that g_AN depends on m_A as for KSVZ axion model. These limits are stronger than obtained in previous laboratory-based experiments using nuclear reactor and artificial radioactive sources. PACS 14.80.Mz; 29.40.Mc; 26.65.+t     

Relaxation effects and high sensitivity photoacoustic detection of NO<Subscript>2</Subscript> with a blue laser diode

A detection limit of 200 ppt of NO₂ in N₂ at atmospheric pressure was obtained with a photoacoustic detector and a high power blue laser diode. This corresponds to a normalized noise (1σ) equivalent absorption coefficient of 2×10^-9 cm^-1W/Hz^0.5. Measurements at different laser modulation frequencies showed no frequency dependence of the photoacoustic signal, indicating a relaxation time τ < 4 μs. Mixing O₂ into the NO₂ containing gas results in a decrease of the photoacoustic signal. A simple model shows that this effect can be attributed to an increased vibrational-vibrational relaxation of NO₂ to O₂. PACS 31.70.Hq; 34.50.Ez; 42.55.Px     

A Fe<Superscript>3+</Superscript>/Hg<Superscript>2+</Superscript>-Selective Anthracene-Based Fluorescent PET Sensor with Tridentate Ionophore of Amide/β-Amino Alcohol

A new anthracene-based fluorescent PET sensor 1 with a tridentate ionophore of amide/β-amino alcohol displays very good selectivity and sensitivity for Fe³⁺ (K _a = 1.6 × 10³ M⁻¹) and Hg²⁺ (K _a = 2.1 × 10³ M⁻¹) in CH₃CN–H₂O (3:7, v/v) with detection limit of 1 μM. More fluorescence enhancement was observed when 1 selectively detected Fe³⁺ or Hg²⁺ in CH₃CN and its detection limit was up to 0.03 μM.     

Dielectric breakdown of rubber materials by femtosecond irradiation

We report the reversible micro-structuring of a synthetic rubber polymer (cis1,4-polybutadiene (PB)) by femtosecond laser illumination. Visco-elastic relaxation of the optically damaged region was observed. The recovery time, typically 10²–10⁴ ms, can be varied by changing the irradiation pulse energy. Multi-shot-induced damage recovers on the much longer scale of 10¹–10² s. It was found that the doping of PB by 4 wt. % of pentazadiene ([4-NO₂]–phenyl–N=N–N(C₃H₇)–N=N–phenyl–[4-NO₂]) reduces the threshold of light-induced photo-modification by 20%. This is explained by photo-induced (homolytic) cleavage of the pentazadiene bonds and formation of gaseous N₂, which facilitates material failure at the irradiated spot. The recovery of optical transmission can be applied to optical memory, optical and micro-mechanical applications. The underlying mechanism of the phenomenon is discussed in terms of anelastic α- and β-relaxation (polymer backbone and chains/coils relaxation, respectively). Received: 11 October 2001 / Accepted: 9 July 2002 / Published online: 25 October 2002 RID="*" ID="*"Corresponding author. Fax: +81-88/656-7598, E-mail: misawa@eco.tokushima-u.ac.jp     

Diffusion study of <Superscript>15</Superscript>N implanted into α-Ti using the nuclear resonance technique

The diffusion of ¹⁵N in α-Ti was studied in the 673–1023 K temperature range by using the ion implantation and nuclear resonance techniques. The measurements show that the diffusion coefficients follow an Arrhenius behavior D(T)=D₀ ^-Q/RT, where D₀=(1.1±0.8)×10^-7 m² s^-1 and Q=(183±2) kJ/mol. A comparison with previous results is also given. PACS 66.30.Jt; 85.40.Ry     

Influence of pH on the preparation of dispersed Ag–TiO<Subscript>2</Subscript> nanocomposite

In this paper, data concerning the effect of pH on the morphology of Ag–TiO₂ nanocomposite during photodeposition of Ag on TiO₂ nanoparticles is reported. TiO₂ nanoparticles prepared by sol–gel method were coated with Ag by photodeposition from an aqueous solution of AgNO₃ at various pH levels ranging from 1 to 10 in a titania sol, under UV light. The as-prepared nanocomposite particles were characterized by UV–vis absorption spectroscopy, transmission electron microscopy (TEM), X-ray diffraction (XRD), and N₂ adsorption/desorption method at liquid nitrogen temperature (−196 °C) from Brunauer–Emmett–Teller (BET) measurements. It is shown that at a Ag loading of 1.25 wt.% on TiO₂, a high-surface area nanocomposite morphology corresponding to an average of one Ag nanoparticle per titania nanoparticle was achieved. The diameter of the titania crystallites/particles were in the range of 10–20 nm while the size of Ag particles attached to the larger titania particles were 3 ± 1 nm as deduced from crystallite size by XRD and particle size by TEM. Ag recovery by photo harvesting from the solution was nearly 100%. TEM micrographs revealed that Ag-coated TiO₂ nanoparticles showed a sharp increase in the degree of agglomeration for nanocomposites prepared at basic pH values, with a corresponding sharp decrease in BET surface area especially at pH > 9. The BET surface area of the Ag–TiO₂ nanoparticles was nearly constant at around a value of 140 m² g⁻¹ at all pH from 1–8 with an anomalous maximum of 164 m² g⁻¹ when prepared from a sol at pH of 4, and a sharp decrease to 78 m² g⁻¹ at pH of 10.     

Photoacoustic detection of oxygen using cantilever enhanced technique

A tunable diode laser photoacoustic setup based on a recently demonstrated cantilever technique was used for sensitive detection of oxygen. As light sources, we used a distributed feedback (DFB) diode laser and a vertical-cavity surface-emitting (VCSEL) laser, both operating near 760 nm. With the 30 mW DFB laser a noise-equivalent detection limit of 20 ppm for oxygen was obtained, while a detection limit of 5 ‰ was achieved with the VCSEL having an output power of 0.5 mW. Our results yield a noise-equivalent sensitivity of 4.8×10^-9 cm^-1W Hz^-1/2 and demonstrate the potential of this technique for compact and sensitive measurement of oxygen. PACS 42.62.Fi; 42.55.Px; 82.80.Kq     

Low thermal expansion behavior and transport properties of Ni and Ge co-doped manganese nitride materials at cryogenic temperatures

A series of Ni and Ge co-doped manganese nitride materials were fabricated by mechanical ball milling followed by solid-state sintering. Their thermal expansion properties and electrical and thermal conductivities were investigated in the temperature range of 77–300 K. The results show that Ni and Ge co-doped manganese nitride materials have negative thermal expansion (NTE), and the operation-temperature window of NTE shifts toward the lower temperature region and the variation of linear thermal expansion (ΔL/L _(300K)) in the operation-temperature window of NTE decreases with increasing Ni content. The combination of these two factors results in a low coefficient of thermal expansion (CTE) at cryogenic temperatures. The average CTE of Mn₃(Cu_0.2Ni_0.4Ge_0.4)N drops to ‘zero’ in the temperature range of 190–77 K. The values of electrical and thermal conductivities of the Ni and Ge co-doped manganese nitride materials are in the ranges of 2–3×10³ (ohm cm)⁻¹ and 1.6–3.4 W (m K)⁻¹, respectively.     

Resonant photoacoustic simultaneous detection of methane and ethylene by means. of a 1.63-μm diode laser

A compact multi-component trace-gas detector based on the resonant photoacoustic technique and a NIR external cavity diode laser has been developed. It has been characterized using a mixture of ethylene and methane diluted in ambient air. A spectroscopic investigation of combination bands and overtones between 5900 and 6250 cm^-1, obtained with an IR pulsed laser photoacoustic spectrometer, allowed us to find a wavelength region where the 2ν₃ overtone of CH₄ and the ν₅+ν₉ combination band of C₂H₄ show uncongested rotational lines. Using a single-mode scan of the diode laser in this region, around 6150 cm^-1, the sensitivity for the simultaneous detection of ethylene and methane is 8 ppm/mW and 40 ppm/mW respectively. Factors affecting the sensitivity and selectivity of the detection system and possible improvements suitable to reach the sub-ppm detection limit are discussed. Received: 1 August 2001 / Revised version: 28 November 2001 / Published online: 7 February 2002 An erratum to this article is available at .     

Simultaneous measurement of the piezoelectric and dielectric response of nanoscale ferroelectric capacitors by an atomic force microscopy based approach

We present a sensitive method to simultaneously acquire the C(V ) characteristics and piezoresponse of sub-micron size ferroelectric capacitors using an Atomic Force Microscope (AFM). Model Pt/(La_0.5,Sr_0.5)CoO₃/PbZr_0.4Ti_0.6 O₃/(La_0.5,Sr_0.5)CoO₃/La:SrTiO₃/Si nanocapacitors were fabricated by focused ion beam milling from 100 μm² down to 0.04 μm². With this AFM based capacitance measurement technique we show clear “double-humped” C(V ) for all sizes with no significant change in the peak value of the ε_r down to capacitors with the smallest area of 0.04 μm². The smallest capacitance measured is only of the order a few femtofarads, demonstrating the high sensitivity of the technique. Simultaneously, the piezoelectric response is recorded for each measurement, thus the technique facilitates simultaneous piezoresponse and dielectric characterization of ferroelectric memory devices. PACS 77.55.+f; 77.90.+k; 85.50.-n; 77.84.-s     

Nonlinear transformation of a spectrum of femtosecond laser pulses in a gas-filled microcapillary

Features of light pulse propagation and nonlinear optical transformation of the spectrum generated by titanium-sapphire laser pulses (τ_0.5 = 27 fs, λ₀ = 790 nm) have been studied experimentally in a 50-cm cylindrical hollow waveguide (microcapillary with 280-μm diameter core) filled with gaseous molecular nitrogen and helium. Stable guided propagation of light pulses with an intensity of ~1.5⋅10¹⁴ W/cm² in the fundamental EH₁₁ mode of the gas-filled capillary has been demonstrated. Exact focusing of the laser light made it possible to obtain rather high relative (≥95%) and absolute (~60%) energy transmission efficiencies for the pulses at gas pressures equal to or lower than 760 Torr. A method to determine the nonlinear phase shift of the pulses has been proposed. Values of the nonlinear refractive index n₂ ≈ 4.5⋅10^–23 cm²/(W⋅Torr) (N₂) and n₂ ≈ 2.8⋅10^–23 cm²/(W⋅Torr) (He) have been found. A short-wavelength shift in addition to the Kerr nonlinearity has been shown to be contributed by the generated electron plasma at high pulse intensities (≥10¹⁴ W/cm²).     

Impedance and FTIR studies on plasticized PMMA–LiN(CF<Subscript>3</Subscript>SO<Subscript>2</Subscript>)<Subscript>2</Subscript> nanocomposite polymer electrolytes

Plasticized polymer electrolytes composed of poly(methyl methacrylate) (PMMA) as the host polymer and lithium bis(trifluoromethanesulfonyl)imide LiN(CF₃SO₂)₂ as a salt were prepared by solution casting technique at different ratios. The ionic conductivity varied slightly and exhibited a maximum value of 3.65 × 10⁻⁵ S cm⁻¹ at 85% PMMA and 15% LiN(CF₃SO₂)₂. The complexation effect of salt was investigated using FTIR. It showed some simple overlapping and shift in peaks between PMMA and LiN(CF₃SO₂)₂ salt in the polymer electrolyte. Ethylene carbonate (EC) and propylene carbonate (PC) were added to the PMMA–LiN(CF₃SO₂)₂ polymer electrolyte as plasticizer to enhance the conductivity. The highest conductivities obtained were 1.28 × 10⁻⁴ S cm⁻¹ and 2.00 × 10⁻⁴ S cm⁻¹ for EC and PC mixture system, respectively. In addition, to improve the handling of films, 1% to 5% fumed silica was added to the PMMA–LiN(CF₃SO₂)₂–EC–PC solid polymer electrolyte which showed a maximum value at 6.11 × 10⁻⁵ S cm⁻¹ for 2% SiO₂.     

Synthesis, characterization, DNA binding and cleavage studies of mixed-ligand Cu(II) complexes of 2,6-bis(benzimidazol-2-yl)pyridine

Four novel copper(II) complexes of the composition [CuLX] where L = 2,6-bis(benzimidazole-2yl)pyridine, X = dipyridophenazine (L₁), 1,10-phenanthroline (L₂), hydroxyproline (L₃) and 2,6-pyridine dicarboxylic acid (L₄) were synthesized and characterized by using elemental analysis, FT-IR, UV–vis, ESI-MS, molar conductance and magnetic susceptibility measurements. The complexes [CuLL₁](NO₃)₂ [1], [CuLL₂](NO₃)₂ [2], [CuLL₃](NO₃) [3] and [CuLL₄] (NO₃) [4] are stable at room temperature. In DMSO the complexes [1] and [2] are 1:2 electrolytes, [3] and [4] are 1:1 electrolytes. Based on elemental and spectral studies five coordinated geometry is assigned to all the four complexes. The interaction of four copper ion complexes with calf thymus DNA were carried out by UV–vis titrations, fluorescence spectroscopy, thermal melting and viscosity measurements .The binding constant (K_b) of the above four metal complexes were determined as 5.43 × 10⁴ M_,⁻¹ 2.56 × 10⁴ M⁻¹, 1.21 × 10⁴ M⁻¹ and 1.57 × 10⁴ M⁻¹ respectively. Quenching studies of the four complexes indicates that these complexes strongly bind to DNA, out of all complex 1 is binding more strongly. Viscosity measurements indicate the binding mode of complexes with CT DNA by intercalation through groove. Thermal melting studies also support intercalative binding. The nuclease activity of the above metal complexes shows that 1, 2 and 3 complexes cleave DNA through redox chemistry.     

Miniaturized resonant photoacoustic cell of inclined geometry for trace-gas detection

A photoacoustic cell intended for laser detection of trace gases is represented. The cell is adapted so as to enhance the gas-detection performance and, simultaneously, to reduce the cell size. The cell design provides an efficient cancellation of the window background (a parasite response due to absorption of laser beam in the cell windows) and acoustic isolation from the environment for an acoustic resonance of the cell. The useful photoacoustic response from a detected gas, window background and noise are analyzed in demonstration experiments as functions of the modulation frequency for a prototype photoacoustic cell with the internal volume ∼0.6 cm³. The minimal detectable absorption for the prototype is estimated to be ∼1.2×10⁻⁸ cm⁻¹ W Hz^−1/2.     

Absolute absorber quantification in turbid media at small source–detector separations

The aim of our studies was to develop a method to determine the absorption coefficient of a turbid medium in a reflection geometry with small source–detector separations. Therefore, the time-integrated microscopic Beer–Lambert law (MBL) was modified in order to obtain the absolute absorption coefficient from mean time of flight and dc-intensity measurements. The new technique was evaluated using turbid phantoms having varying scattering (μ_s ^′ between 0.2 and 2.2 mm^-1) and absorbing properties (μ_a between 0.04 and 0.14 mm^-1), comparable to many biological tissues at various source–detector separations between 3 and 11 mm. The measurements were performed at a wavelength of 1064 nm. We found that this new method was able to determine the absolute absorption coefficient of the selected phantoms with a standard error of less than 0.005 mm^-1 over the range of optical properties investigated. Received: 23 November 2001 / Revised version: 28 February 2002 / Published online: 14 May 2002     

Laser-based photoacoustic ammonia sensors for industrial applications

An industrial trace-ammonia sensor based on photoacoustic spectroscopy and CO₂ lasers has been developed for measuring ammonia with a 1σ detection limit of 220 parts-per-trillion (ppt) in an integration time of 30 s. The instrument response time for measuring ammonia was 200 s, limited by adsorption effects due to the polar nature of ammonia. The minimum detectable fractional absorbance was 2.0×10^-7, and the minimum normalized detectable absorption coefficient for this system was 2.4×10^-7 W cm^-1/z. The 9R(30) transition of the CO₂ laser at 9.22 μm with 2 W of output power was used to probe the strong sR(5,K) multiplet of ammonia at the same wavelength. This sensor was demonstrated with an optically multiplexed configuration for simultaneous measurement in four cells. Received: 3 April 2002 / Revised version: 31 May 2002 / Published online: 21 August 2002 RID="*" ID="*"Corresponding author. Fax: +1-310/458-0171, E-mail: webber@pranalytica.com