Pulse properties of a synchronously mode-locked,cavity dumped,picosecond dye laser system

A synchronously mode-locked, cavity-dumped picosecond dye laser is described. The structure and intensity of the picosecond pulses measured under different conditions are reported. It was found that the structure of the pulses from the synchronously pumped dye laser depends critically on the length of the Ar⁺ laser pulses. At the shortest Ar⁺ laser pulses of about 70 ps the dye pulses are as short as 1.1 ps. With Ar⁺ laser pulses of 200 ps the dye laser pulses contains a broad satellite pulse which contains a large fraction of the total intensity. When a cavity dumper is added to the system one gets dye laser pulses 15–20 ps long with a substructure, which indicates incomplete mode-locking. Well mode-locked 1.5–2.0 ps pulses were obtained in the red part of the dye laser action spectrum, i.e. 620–650 nm for R6G, 595–608 nm for R 110 and 657–662 nm for RB, respectively. Addition of mode-locking dyes also improved the pulse quality at some wavelengths.     

Picosecond duration laser pulse generation by simultaneous active-passive mode-locking in Ar+ ion and dye lasers

We use a rate equation model to analyse a technique for the generation of picosecond duration laser pulses in argon pumped dye lasers. The system is based on passive mode-locking of the Ar⁺ laser by saturable absorption of the dye which is inserted in the Ar⁺ laser cavity. Simultaneously, the dye is forced to oscillate in a regime of synchronously pumped mode-locking. The system is relatively simple, does not require the use of an acousto-optical light modulator and can be easily implemented in commercially available lasers. Pulses shorter than 100 ps and having an average power of 20 mW were obtained at a repetition rate of 110 MHz. Good agreement between the model and experimental results was also obtained.     

Photoluminescence detection of impurities introduced in silicon by dry etching processes

We report on a photoluminescence study of silicon samples subjected to different dry etching processes. Several luminescence lines, known from defects produced by high-energy irradiation, manifest damage of the crystalline material. Noble gas ion beam etching (using Ne⁺, Ar⁺, Kr⁺, and Xe⁺) with ion energies as low as 400 eV produces characteristic luminescence lines which correspond to defects within a 200–300 Å thick surface layer. Incorporation of carbon during CF₄ reactive ion etching produces the familiar G-line defect. The G-line photoluminescence intensity in our samples is directly correlated with the substitutional carbon concentration, as determined by infrared absorption measurements before the etch process; we therefore suggest that a simple method to determine the substitutional carbon concentration in a crystalline silicon sample is a standard dry etching process and a comparison of the resulting G-line photoluminescence intensity to a calibrated sample. The sensitivity of this method seems to be better than 10¹⁴ carbon atoms/cm³.     

The self-injected XeCl excimer laser

We present experimental and modelling results of the first self-injected excimer laser. The intracavity losses of a XeCl oscillator are properly modulated by a Pockels cell allowing generation, amplification and extraction of short laser pulses with selectable duration in the range of 1–12 ns, tailored temporal profile and peak power increment up to a factor of three. Longer output laser pulses, up to 100 ns, can be obtained by slicing the intracavity laser radiation without peak power increment. Laser output peak powers in excess of 2 MW have been obtained, with remarkable reproducibility characteristics.ENEA fellow     

Efficient single-mode operation of a standing-wave dye laser

The selection of a single longitudinal mode of a dye laser can be achieved with double or triple Michelson reflectors instead of Fabry-Perot etalons inside the laser cavity. These systems can be adapted to commercial multimode dye lasers. They provide a high efficiency and a good geometrical quality of the output beam. With a 7 watt pump power (λ=514.5 nm line of the Ar⁺ laser), a tunable output power of 1 watt was obtained at the peak of R6G, in the main output beam; the total single-mode power, including the auxiliary output beams, exceeded 1.4 watt. These results correspond to efficiencies of 14% and 20%, respectively (24% and 30% slope efficiencies). We discuss in the text why these efficiencies are comparable to those of ring lasers. In the infrared, with a 4.5 watt pump power (red lines of a Kr⁺ laser), 250 mW of single-mode output power in the main output beam was obtained at 780 nm with Oxazine 750.     

Generation of 49 fs pulses in a CPM laser pumped by a small-frame Ar+ laser

A colliding pulse mode-locked ring dye laser, pumped by an all-line small frame Ar⁺ laser has been used to generate stable pulses as short as 49 fs with pulse duration and stability similar to those obtained with the usual configuration where a single line of the pump is used. The effects of cavity alignment on the absorber saturation are presented and explained in terms of an analysis of stability of the ring cavity.     

Optical bistability in fluorescein dyes

Optical bistability has been observed in highly concentrated fluorescein dye solutions and in thin (1 m) doped polymeric films. At concentrations larger than 10^–5 mole/l dye dimers are formed. For fluorescein dye, the dimer-monomer equilibrium constant is 10⁵ l/mole so that most of the dye species are in the dimer form. At 480 nm the dimer absorption cross section is 10^–18 cm²/molecule, while that for the dye monomer molecule is 7.6×10^–17 cm²/molecule. Upon laser excitation dimers dissociate to form monomers thus providing a highly nonlinear laser induced absorption. This high nonlinear absorption coefficient can be utilized for optically bistable response of the dye system.Optical bistability was observed by placing dye solutions or dye thin films inside a Fabry-Perot resonator and exciting it with 480 nm dye laser pulses of 10 ns duration. The effect is more pronounced in 10^–4 mole/l fluorescein than in 10^–6 mole/l fluorescein in which dimer formation is not that efficient.In disodium fluorescein no significant dimer formation is observed even at 10^–3 mole/l dye concentration. The observed bistability both in solution and in thin films can be explained in terms of recent models for optical bistability in nonlinearly absorbing molecular systems.     

Cluster emission under ion bombardment of metallic targets

Mass spectroscopic studies of the neutral particles sputtered by Ar⁺ ions at 8 keV from polycrystaline samples have been performed, using non-resonant laser ionization and subsequent time-of-flight mass spectroscopy. Besides sputtered atoms, also dimer and trimer contributions in the order of 10^–1 to 10^–2 and 10^–3 to 10^–4, respectively, are found in the sputtered flux. The data obtained here together with previously published data by other groups for different bombarding energies provide strong support for the validity of the recombination model.     

用负温度高能离子束的相对论多普勒效应实现从红外到X射线连续调谐激光器

利用负温度高能离子束的相对论多普勒效应,可能实现从红外到X射线连续调谐激光器。调谐范围△ν_T=2ν₀βγ。离子束从零到c调速时,激光频率为0—∞。本文给出了激光参数的相对论变换式,计算了负温度相对论离子束激光器的增益、阈值和输出特性,提出了用激光束对相对论离子束进行共振激发的方法,并讨论了He⁺离子束及Ar⁺离子束激光器的设计参数。 关键词：     

Isotope selective photoionization of calcium using two-step laser excitation

The isotope specific two-step photoionization of metastable Ca atoms was investigated using a continuous wave dye laser and an Ar⁺ laser. The photoionization was performed via a transition to an autoionizing state. Therefore an ionization probability as high as 10⁻² could be obtained.     

Femtosecond transform-limited Kerr-lens mode-locked dye lasers

Using SF6 glass plates as intracavity Kerr lenses and double-prism pairs for dispersion compensation, we achieve tunable femtosecond passive mode locking in rhodamine 590 (R6G) and 4-dicyanomethylene-2-methyl-16-p-dimethylaminostyryl-4H-pyran (DCM) dye lasers. The R6G laser produces transform limited 240–500 fs pulses between 577 and 606 nm, and the DCM laser produces 150 fs transform-limited pulses between 650 and 671 nm. We use dilute intracavity saturable-absorber jets to make the mode locking self-starting. Characteristics of the pulses and the stability regions of the lasers agree with general theories of passive mode locking.     

Characterization of a Co-Se thin film by scanning Auger microscopy and Raman spectroscopy

Scanning Auger microscopy and micro-Raman spectroscopy are combined to characterize a Co-Se thin film sample, containing 84 at.% Se, which had been modified in localized areas following excitation with an intense focused Ar⁺ laser (514.5 nm). The information obtained helps to establish that a previous assignment for a Co-Se sample of Raman features between 168 and 175 cm⁻¹ actually refers to an oxygenated Co-Se species, and that Co-Se interactions in a Se-rich environment give rise to Raman structure between 181 and 184 cm⁻¹. Comparisons are made for the use of Ar⁺ and HeNe laser sources for Raman measurements in this context; the latter in general gives both better resolution and better signal-to-noise characteristics.     

Preferential sputtering of TIC under keV Ar+ ion bombardment: Simultaneous sputtering-ISS measurement with He+ and Ar+ ions

The composition change of the outermost atom layer of TiC(110) under ion bombardment with 1.5–3 keV He⁺ and He⁺ + Ar⁺ ions has been measured by ion scattering spectroscopy with He⁺ ions at different sample temperatures. It has been found that the preferential sputtering of C atoms takes place for both the He⁺ and Ar⁺ ion bombardment, however the preferred sputtering is more pronounced for Ar⁺ ions than for He⁺ ions. The ion bombardment with He⁺ ions at elevated sample temperatures hardly results in any change in surface composition below ～800°C, while Ar⁺ ion bombardment results in C enrichment for elevated temperatures as reported so far.     

Energy distributions and emission coefficients of secondary ions ejected from AIIIBV group semiconductors

The energy distributions of Ga⁺ and Ga₂ ⁺ secondary ions sputtered from the surface of single crystals of GaAs and GaP by Ar⁺ (E = 18 keV, j = 1·10^–6 A/cm²) ions were studied. Integral emission coefficients for secondary ions from four semiconductors (GaAs, GaP, InAs, and A1_0.35Ga_0.65As) bombarded by Ar⁺ ions (E = 2 keV, j₀ = 1·10^–6 A/cm²) were also measured. It is shown that the energy distribution of the Ga⁺ secondary ions are broad and qualitatively similar to distributions of secondary ions sputtered from metal oxide surfaces. Very high integral emission coefficients (up to 20–30%) for secondary ions are also observed, exceeding analogous coefficients for clean metal surfaces. These facts (the significant number of low energy secondary electrons and high secondary ion emission coefficients) are due to the lower density of free electrons in comparison with metals, which ensures a high degree of survival of sputtered ions, and the partially ionic character of the bonds in the semiconductors under study.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 11, pp. 88–92, November, 1990.     

Xe2Cl fluorescence and absorption in self-sustained discharge XeCl lasers

Fluorescence at 490 nm from the triatomic excimer Xe₂Cl^* has been investigated to determine the 308 nm absorption due to this species in an x-ray preionized, self-sustained gas discharge XeCl laser. The dependence of Xe₂Cl^* density on laser intensity (at 308 nm), buffer gas and Xe and HCl partial pressures has been determined for discharges with a peak electrical power deposition of 2.5 GWl^–1. Xe₂Cl^* absorption is estimated to reach 0.6% cm^–1 under non-lasing conditions but decreases to a non-saturable 0.2% cm^–1 for intracavity laser intensity>1 MW cm^–2. XeCl^* and Xe₂Cl^* fluorescence intensities were found to be a similar for both helium and neon buffer gases but laser output was a factor of two greater with a neon buffer.     

Effect of helium/argon gas ratio in a He-Ar-Cu<Superscript>+</Superscript> IR hollow-cathode discharge laser: modeling study and comparison with experiments

The He-Ar-Cu⁺ IR laser operates in a hollow-cathode discharge, typically in a mixture of helium with a few-% Ar. The population inversion of the Cu⁺ ion levels, responsible for laser action, is attributed to asymmetric charge transfer between He⁺ ions and sputtered Cu atoms. The Ar gas is added to promote sputtering of the Cu cathode. In this paper, a hybrid modeling network consisting of several different models for the various plasma species present in a He-Ar-Cu hollow-cathode discharge is applied to investigate the effect of Ar concentration in the gas mixture on the discharge behavior, and to find the optimum He/Ar gas ratio for laser operation. It is found that the densities of electrons, Ar⁺ ions, Ar_m ^* metastable atoms, sputtered Cu atoms and Cu⁺ ions increase upon the addition of more Ar gas, whereas the densities of He⁺ ions, He₂ ⁺ ions and He_m ^* metastable atoms drop considerably. The product of the calculated Cu atom and He⁺ ion densities, which determines the production rate of the upper laser levels, and hence probably also the laser output power, is found to reach a maximum around 1–5 % Ar addition. This calculation result is compared to experimental measurements, and reasonable agreement has been reached. Received: 14 October 2002 / Revised version: 28 November 2002 / Published online: 19 March 2003 RID="*" ID="*"Corresponding author. Fax: +32-3/820-23-76, E-mail: annemie.bogaerts@ua.ac.be     

Passive mode-locking of an Ar+-laser with rhodamine 6g as saturable absorber and double mode-locking of the pump- and dye laser system

The synchronization of the modes of the Ar⁺-pump laser by using a dye as saturable absorber is proposed. Besides, the passive mode-locking is extended to double mode-locking. In this case the dye simultaneously acts as a synchroneously pumped laser medium.     

Highly excited,normally inaccessible vibrational levels by sub-doppler modulated gain spectroscopy: The Na2 A1Σ+u state

Modulated gain spectroscopy is a sensitive, widely applicable, rovibronically state selective, sub-Doppler, triple resonance method for examining excited vibronic levels which are Franck—Condon inaccessible from thermally populated levels of the electronic ground state. A cw optically pumped molecular laser (OPL) prepares a steady-state population in a selected, vibrationally highly excited, rotation-vibration level of the electronic ground state (the lower level of the OPL transition). An intensity-modulated, single frequency dye laser excites part of this intracavity OPL-prepared population to the level of interest, thereby causing an increase in the OPL population inversion density, and, in turn, its output power. As the frequency of the dye laser is scanned, resonances are selectively detected by the appearance of modulation on the OPL output power; discrimination against dye laser excitations out of levels unconnected with the OPL is nearly perfect. Sub-Doppler (≈300 MHz FWHM) transitions are observed, thereby extending knowledge of the Na₂A¹Σ⁺_u state from v=44 to 62.     

Expansion velocities of 0.5 ps KrF excimer laser induced plasma by Doppler-shift analysis of pump and probe measurements

High gradient laser plasma is formed by focused KrF laser pulses (248.3 nm, 450 fs, 10¹³ W/cm²) on liquids (water, styrene) and solids (silicon, aluminum, and polyimide). The hydrodynamic expansion of the plasma was studied by measuring the blue Doppler-shift of reflected probe pulses which was produced by a delayed dye laser (496.6 nm, 450 fs). The Doppler-shift corresponds to the velocity of the reflecting surface of the plasma which is defined by the critical electron density. Expansion is investigated as a function of delay time and laser intensity. The reflecting surface of the plasma accelerates over 1–2 ps after the onset of the ablating laser pulse. With increasing intensity up to 2×10¹³ W/cm² the maximum average velocities are monotonously increasing up to 1–2×10⁵ m/s. PACS 52.38.Kd; 52.50.Jm, 52.70.Kz     

A tunable frequency stabilized Ar+ laser

The frequency of a single-mode Ar⁺ laser atv=582 THz (=515 nm) is stabilized by means of an external iodine cell to a Doppler broadened absorption profile. The —unmodulated — stabilized laser frequency is reproducible to ±1 MHz or ±1.5×10^–9 v and can be shifted within a 180 MHz wide range.