Spectral phase matching for highly efficient frequency tripling of short-pulse KrF laser radiation in argon

For the efficient frequency tripling of KrF laser radiation, argon is very well suited as a nonlinear medium since a near-three-photon resonance can be used, which enhances the nonlinear susceptibility. According to theoretical predictions a zero crossing of the dispersive phase mismatch for the frequency-tripling process allows nearly perfect phase matching at high densities for the central KrF laser wavelength at 248.5 nm. Experiments that demonstrate the associated spectral sensitivity of the phase matching were performed in a pulsed argon gas jet for controlled input spectra and by varying the particle density up to 6×10¹⁸ cm^-3. The experiments show good agreement with corresponding theoretical model calculations and explain the recently achieved high conversion efficiencies in frequency tripling of short-pulse KrF laser radiation. PACS 42.65.Ky     

A Q-band pulsed ENDOR spectrometer for the study of transition metal ion complexes in solids

We describe the design of a pulsed electron nuclear double resonance (ENDOR) spectrometer operating at Q-band frequencies (35 GHz) for studies of transition metal ion complexes in the temperature range between 4.2 and 297 K. Specific features of the spectrometer are a microwave IMPATT generator, a homebuilt cavity, and a commercial Bruker magnet. Standard Davies and Mims ENDOR sequences have been implemented. The performance of the spectrometer is demonstrated for a broad radio frequency range by¹H,¹⁴N,³¹P,¹³³Cs, and²⁰⁷Pb pulsed ENDOR experiments of Cu²⁺, Cr⁵⁺, and V⁴⁺ transition metal ion complexes in both single crystals and disordered materials.     

Particle diagnostics of a ZnO laser ablation plume for nanostructured material deposition

We report results on the pulsed laser deposition of ZnO obtained with the help of a new apparatus that includes in situ reflectron time-of-flight mass spectrometry, with a view to progress the understanding of the role of clusters in the laser deposition of nanostructured materials. Experiments were carried out using a Nd-YAG laser at its fundamental frequency and frequency tripled, with a fluence on target of ∼7.7 J/cm², in vacuum (10⁻⁴ Pa) or oxygen (1 Pa) atmospheres. The results show that under certain conditions there is preferential clusterisation of the material into certain mass numbers and finally that there exists a correlation between cluster presence in the plume and the deposition of nanostructures.     

New applications of the Mössbauer effect

Results from a Mössbauer experiment to observe acoustic oscillations induced by pulsed laser excitation in MgO:⁵⁷Fe²⁺ crystal are presented. Time-domain spectra are satsifacorily described by the theory of the frequency modulation of Mössbauer radiation transmitted through a vibrating resonance medium. It is proposed that the D _4Ω/D _2Ω ratio of the fourth and second Fourier harmonics of the modulated radiation be used to measure the amplitude of nuclear oscillations.     

High power narrowband pulsed dye laser oscillator-amplifier-system

Construction details and operating characteristics are reported for a pulsed dye laser oscillator followed by a three-stage dye laser amplifier. The system is excited with 220 mJ of the second or120 mJ of the third harmonic radiation of a Nd-YAG laser. With Rhodamine dyes the output energy exceeds 55 mJ (9 MW peak power). Coumarin dyes provide pulses of more than 15 mJ (3 MW). Spectral narrowing to less than 260 MHz or 2.7 × 10 ^-4 nm at 565 nm is achieved by a single intracavity etalon of 37.5 GHz free spectral range. Gas pressure tuning allows a continuous linear variation of the laser frequency over more than 3.9 × 10³ GHz (4 nm at 565 nm). The frequency doubled laser output provides tunable UV light of narrow bandwidth (1.4 × 10^-4 nm) and of peak powers exceeding 3 MW.     

Formation of periodic structures by laser ablation of metals in liquids

Experimental results are presented on ablation of metals (W, Cu, brass and bronze) in a liquid environment (e.g., ethanol or water) by irradiation with either a pulsed copper vapor laser (0.51 μm) or a pulsed Nd:YAG laser (1.06 μm). The target material is ejected into surrounding liquid in the form of nanoparticles. In a certain range of laser parameters (fluence and number of laser shots) the surface of the solid target is composed of micro-cones having a regular structure. The distance between neighboring micro-cones in the structure depends on the laser spot size. The structures allow the observation of up-conversion of the laser frequency due to generation of the second harmonics in the eye retina.     

Modulated resonant versus pulsed resonant photoacoustics in trace gas detection

Modulated resonant photoacoustics is a sensitive technique widely used for trace gas sensing. Generally, a continuous-wave laser is modulated at a frequency corresponding to an acoustic resonance of a photoacoustic cell. Another mode of operation—which we propose to call the pulsed resonant mode—consists in matching the frequency repetition rate of a pulsed laser to an acoustic resonance of the cell. We present a theoretical model to compare the performance of these two configurations. For a given average power of the incoming light inside the cell, the pulsed resonant mode of operation (nanosecond pulses or shorter) produces π/2 times higher photoacoustic signals than the modulated resonant scheme (the latter is optimized for a 50% duty cycle). This result agrees with experiments during which both cases were investigated at 532 nm using the same photoacoustic cell containing trace concentrations of NO₂.     

Resonance THz spectroscopy in high magnetic fields

The employment of the high-magnetic-field resonance spectroscopy to probe magnetic excitations in the THz frequency range is reported. To illustrate the great potential of this technique in solid state physics, we present results of recent electron spin resonance studies of the quantum-tunneling effect in the single-molecule magnet Mn12tba and of the soliton–magnon crossover in Cu-PM, a spin-1/2 Heisenberg chain system with a staggered transverse field. Among others, we report on the successful use of the THz-range time-domain and free electron laser spectroscopy to study magnetic excitation spectra in pulsed magnetic fields.     

Effect of pulsed laser radiation on the structure and electrical properties of ferroelectric ceramics

Ceramic samples of barium titanate and solid solution of barium-strontium titanate subjected to pulsed laser irradiation have been studied by impedance spectroscopy. The measurements have been carried out in the frequency range 10²?C10⁶ Hz and at temperatures in the range from 20 to 450°C. The experimental data are represented by the dispersion of the electric modulus. The activation energy of relaxation processes in the paraelectric phase has been calculated. Scanning electron microscopy has been used to obtain micrographs of the sample surfaces and the data on the elemental composition.     

The effect of potential of impedance in the pulsed-laser-deposited SrBi2Ta2O9 thin film

Ferroelectrics SrBi₂Ta₂O₉ (SBTO) thin films were grown on a highly oriented Pt/Ti/SiO₂/Si substrates using the pulsed laser ablation. The ac impedance of SBTO thin films have been measured at room temperature both in the frequency range from 10⁻¹ to 10⁶ Hz and bias voltage range from −6 to 6 V. The ac impedance dispersion was observed at low frequency with increasing bias voltage, which was interpreted based on a blocked charge. We can explain that the blocking interface gives rise to constant phase element (CPE) response, and we give an impedance model function that can fit data along the low frequency range when such a CPE is found. The low frequency dispersion phenomena of SBTO thin film are related to a charge diffusion process at the surface of thin film.     

High-precision pulsed photoacoustic spectroscopy in NO2-N2

A method for high-precision pulsed photoacoustic spectroscopy applied to a simple system for detection of NO₂ traces in nitrogen is presented. The acoustic signal from a closed cell containing NO₂/N₂ samples irradiated by a pulsed visible laser is analyzed in the frequency domain. A signal-processing method to obtain a high-resolution Fourier spectrum of the signal was developed. An accurate fitting of the resonance peaks with Lorentzian profiles gives high-precision determination of the amplitude and width of the resonance peaks. The resonance maximum is proportional to the absorbed energy; therefore, the choice of the laser wavelength, linewidth and frequency stability are critical for a precise calibration due to the fine structure of the NO₂ optical spectrum. The method also allows high-accuracy measurement of the Q of the acoustic cavity. The dependence of Q on the buffer gas pressure is characteristic of an acoustic cavity where energy losses near the walls predominate. Consequently, an important enhancement of sensitivity takes place at high N₂ pressure. Received: 1 June 2001 / Revised version: 27 July 2001 / Published online: 7 November 2001     

Resonance Ionization Mass Spectrometry (RIMS) with Pulsed and CW-Lasers on Plutonium

The detection of long-lived plutonium isotopes in ultra-trace amounts by resonance ionization mass spectrometry (RIMS) is a well-established routine method. Detection limits of 10⁶ to 10⁷ atoms and precise measurements of the isotopic composition have been achieved. In this work multi-step resonance ionization of plutonium atoms has been performed with tunable lasers having very different output intensities and spectral properties. In order to compare different ways for the resonance ionization of plutonium broadband pulsed dye and titanium:sapphire lasers as well as narrow-band cw-diode and titanium:sapphire lasers have been applied for a number of efficient excitation schemes. It has been shown, that for identical excitation schemes the optical isotope selectivity can be improved by using cw-lasers (bandwidths < 10 MHz) instead of pulsed lasers (bandwidths > 2 GHz). Pulsed and cw-laser systems have been used simultaneously for resonance ionization enabling direct comparisons of pulsed and continuous ionization processes. So far, a three-step, three-color laser excitation scheme has been proven to be most practical in terms of efficiency, selectivity and laser wavelengths. Alternatively a newly discovered three-step, two-color excitation scheme which includes a strong two-photon transition from an excited state into a high-lying autoionizing state yields similar ionization efficiencies. This two-photon transition was characterized with respect to saturation behavior and line width.     

Nanosecond laser ablation for pulsed laser deposition of yttria

A thermal model to describe high-power nanosecond pulsed laser ablation of yttria (Y₂O₃) has been developed. This model simulates ablation of material occurring primarily through vaporization and also accounts for attenuation of the incident laser beam in the evolving vapor plume. Theoretical estimates of process features such as time evolution of target temperature distribution, melt depth and ablation rate and their dependence on laser parameters particularly for laser fluences in the range of 6 to 30 J/cm² are investigated. Calculated maximum surface temperatures when compared with the estimated critical temperature for yttria indicate absence of explosive boiling at typical laser fluxes of 10 to 30 J/cm². Material ejection in large fragments associated with explosive boiling of the target needs to be avoided when depositing thin films via the pulsed laser deposition (PLD) technique as it leads to coatings with high residual porosity and poor compaction restricting the protective quality of such corrosion-resistant yttria coatings. Our model calculations facilitate proper selection of laser parameters to be employed for deposition of PLD yttria corrosion-resistive coatings. Such coatings have been found to be highly effective in handling and containment of liquid uranium.     

Surface plasmons in porous gold films

The surface plasmon resonance effects in porous gold (por-Au) films—nanocomposite porous films containing an ensemble of disordered gold nanoparticles—have been investigated by modulation-polarization spectroscopy. Por-Au films have been obtained by pulsed laser deposition (using a direct particle flow from an erosion torch formed by a YAG:Nd³⁺ laser in argon). The spectral and angular dependences of the polarization difference ρ(λ, θ) of internal-reflection coefficients of s- and p-polarized radiation in the Kretschmann geometry and the spectral dependences of isotropic reflection angles at ρ(θ) = 0 are measured. Two types of surface plasmon resonance are found: one occurs on isolated nanoparticles (dipole and multipole modes), and the other is due to the dipole–dipole interaction of neighboring nanoparticles. The frequency of electron plasma oscillations for the nanoparticle ensemble and the frequencies and decay parameters of resonances are determined. Dispersion relations for the radiative and nonradiative modes are presented. The negative sign of the dispersion branch of nonradiative modes of dipole–dipole interaction is explained by the spatial dispersion of permittivity. The relationships between the formation conditions of the films, their structure, and established resonance parameters (determining the resonant-optical properties of films) are discussed.     

Partly tunable mid-infrared NH3 laser

Using a continously tunable pulsed 20-atmosphere CO₂ laser as a pump source, we generated pulses of mid-infrared radiation, partly tunable in frequency intervals in the range between 200 and 900 cm^–1 via stimulated Raman scattering in gaseous ammonia. As a Raman cell we used a multiple pass cell. We observed for¹⁴NH₃ and¹⁵NH₃ laser lines at 52 different frequencies with 39 lines observed for the first time. Tuning ranges up to 150 GHz and peak powers of several MW were achieved. The quantum efficiency reached 40 %.     

Intense beam production with ECR ion sources

Electron cyclotron resonance ion sources are now able to attain a beam current as high as some milliamperes, which until a few years ago was obtained only for pulsed laser ion sources with a much higher emittance and energy spread, by keeping also the possibility to produce high charge states close to electron beam ion sources, at much higher intensity. They usually operate at frequencies up to 18 GHz, but now many sources are able to operate or will be able, at 28 GHz frequency or more, with plasma density of the order of 10¹³ cm^?3, much higher than any other source of the previous generation. The state-of-the-art technique will be described along with the major characteristics of the upcoming sources.     

Tunable single-mode pulsed Ti3+:Sapphire laser injection-seeded by a continuous-wave Cr3+:LiSrAlF6 laser

Spectrally pure high-power tunable single-mode operation of a pulsed Ti³⁺:sapphire laser by a tunable injection-seeding is reported. The injection laser was a cw diode laser pumped, spectrally narrowed tunable Cr³⁺:LiSrAlF₆ (Cr³⁺:LiSAF) laser with a grating in the auxiliary cavity. Single-mode tunable operation of a pulsed Ti³⁺: sapphire ring oscillator was obtained at different wavelengths in the range between 818 nm and 848 nm with a typical linewidth of 0.006 cm^-1. To extend the applicability of this operation to a differential absorption lidar system, the single-mode Ti³⁺:sapphire oscillator output was amplified and a high energy output of 38 mJ was obtained with the same linewidth.     

Precise determination of the171Yb+ ground state Hyperfine separation

We performed a microwave-optical double resonance experiment on the ground state of¹⁷¹Yb⁺ ions. About 10⁵ particles were confined in a r.f. quadrupole trap for periods of several hours in the presence of He buffer gas. Hyperfine pumping by a pulsed dye laser was followed by microwave transitions, which we observed via changes in the ionic fluorescence intensity. The ground state hyperfine splitting has been determined togD W=12642812124.2±1.4 Hz. The ultimate line width obtained in this experiment was 33 mHz, corresponding to a lineQ of 3.8·10¹¹. The final error ofgD W is mainly determined by the accuracy of the available frequency reference.     

双光子激发钾5d2DJ态的光波混频和参量过程

将一倍频Nd:YAG激光泵浦的染料激光光束聚焦于热管炉中。用双光子将钾激发到5d²D_J能级,研究了钾蒸汽中所产生的光学混频和参量过程。在302—421nm范围内共获得相干辐射谱线28条。用基于波动方程、原子极化率和位相匹配条件建立起来的理论,对参量过程谱线的分裂和频移以及混频过程谱线波长和相对强度进行了分析计算。理论和实验结果相符。 关键词：     

基于减反膜外腔反馈半导体激光器拉曼光的产生

通过直接对减反膜外腔反馈半导体激光器进行电流调制的方法,得到了两束位相锁定且频率差在6.0-9.3GHz范围内连续可调的激光,其中6.835和9.192GHz分别对应Rb⁸⁷和Cs¹³³原子基态超精细能级之间的频率差,激光功率分别可以达到6.87mW和5.09mW. 根据减反膜外腔反馈半导体激光器的特点,通过调整外腔腔长、激光器工作温度、电流以及所加射频调制信号的功率和频率,在调制频率小于等于4.0GHz时可以将载波完全压制. 调制频率大于4.0GHz时,虽不能将载波完全压制,但由于外腔与调制频率共振时对调制的增强也得到了调制深度很高的激光,并对其中的物理机理作了分析. 通过后续滤波等方法处理以后,该拉曼光源可以广泛应用到原子的量子操控中. 关键词： 受激拉曼光 高频调制 调制增强