3d D lifetime in laser cooled Ca : Influence of cooling laser power

We have measured the lifetime of the metastable 3D _5/2 level in Ca⁺ using the “quantum jump" technique on a single stored and laser cooled ion in a linear Paul trap. We found a linear dependence of the measured decay rate on the power of the laser which repumps the ions from the long lived 3D _3/2 level. This can be explained by off-resonant depletion of the 3D _5/2 level. The proper lifetime of this level is obtained by a linear extrapolation of the measured lifetime to zero laser power. We obtain 1100(18) ms in agreement with theoretical calculations. The observed systematic change of the decay rate resolves discrepancies between earlier experiments in which this effect had not been considered. Measurements on a linear chain of 10 laser cooled ions showed unexpected frequent coincidences of quantum jumps within our observation time of 20 ms. This indicates a so far unexplained correlation between the ions in the chain at large distances. Received 3 March 1999     

Importance of pre-ionisation for the non-chain discharge-pumped HF laser

The importance of pre-ionisation for the non-chain discharge-pumped HF laser is studied through experiments on an X-ray photo-triggered laser using mixtures of Ne, SF₆, and ethane. The discharge dynamic in Ne/SF₆ mixtures or pure SF₆, as well as the stabilisation effect induced by C₂H₆ and consequences for the laser performance, are investigated for pre-ionisation electron density values, n_eo, ranging from 10⁶ cm^-3 up to 10⁹ cm^-3, as well as for the so-called discharge self-breakdown mode. Without ethane, the minimum n_eo value which is needed to complete 100% homogeneous charge deposition in the plasma is a very sharply increasing function of the SF₆ pressure. This hinders performance optimisation when the molecule used to react with F-atoms, for instance H₂, has no effect on the discharge dynamic. The minimum ethane partial pressure that is needed to stabilise the discharge depends on n_eo, the pumping pulse duration, the deposited electric charge, and the SF₆ pressure. Discharges in Ne/SF₆ can be much more efficiently stabilised by addition of a small amount of ethane than by an increase of n_eo. A pre-ionisation density as low as 10⁶ cm^-3 is sufficient to achieve the maximum laser energy value, but total suppression of the pre-ionisation has a detrimental effect on the active medium homogeneity. Received: 30 May 2000 / Revised version: 9 October 2000 / Published online: 9 February 2001     

Spectral properties of a Tm:Ho:YAG laser in active mirror configuration

3+ :Ho³⁺:YAG laser is operated in active mirror configuration. This laser is characterised by a rather thin crystal (3 mm thickness in our experiment) and a very short resonator (about 3.2 mm). Therefore this laser is well suited for operation in a single longitudinal mode. Single-frequency operation is demonstrated with and without an intracavity etalon. It is further shown that single-frequency emission is reached with a stable cw emission. With a slight detuning of the outcoupling mirror, spiking can be achieved, resulting in an emission on different lines. The assignment of the observed laser lines to the known transitions between Stark sublevels of the ⁵I₇ and the ⁵I₈ level is discussed and compared with predictions in the literature. Received: 2 December 1997     

Tunable frequency-controlled laser source in the near ultraviolet based on doubling of a semiconductor diode laser

Continuously tunable ultraviolet laser radiation at 397 nm was generated by doubling the output of a semiconductor diode laser. The fundamental radiation was provided by a 150 mW AlGaAs laser diode injected by a low-power AlGaAs laser diode which was frequency stabilized by optical feedback using a new scheme of a miniature external cavity. Second-harmonic generation was produced in a lithium-triborate crystal placed in a compact enhancement cavity. The fundamental radiation was used for sub-Doppler spectroscopy of the Ar I 4s ³ P ⁰ ₀–4p ¹ P ₁ transition at 795 nm; the second-harmonic radiation was used for spectroscopy of the Ca II 4² S _1/2–4² P _1/2 transition at 397 nm.     

F2 laser induced oxidation of inorganic material

Transparent SiO₂ thin films were selectively fabricated on Si wafer by 157 nm F₂ laser in N₂/O₂ gas atmosphere. The F₂ laser photochemically produced active O(¹D) atoms from O₂ molecules in the gas atmosphere; strong oxidation reaction could be induced to fabricate SiO₂ thin films only on the irradiated areas of Si wafer. The oxidation reaction was sensitive to the single pulse fluence of F₂ laser. The irradiated areas were swelled and the height was approximately 500-1000 nm at the 205-mJ/cm² single pulse fluence for 60 min laser irradiation. The fabricated thin films were analytically identified to be SiO₂ by the Fourier-transform IR spectroscopy. The SiO₂ thin films could be also removed by subsequent chemical etching to fabricate micro-holes 50 nm in depth on Si wafer for microfabrication.     

High-power diode-pumped Tm:YLF slab laser

A 2% Tm³⁺-doped LiYF₄(Tm:YLF) slab is double-end-pumped by two laser diode stacks. The pumped volume has a rectangular cross section. The Tm:YLF laser produced 148 W of continuos-wave output at 1912 nm in a beam with M _x ²≈199 and M _y ²≈1.7 for 554 W of incident pump power. The slope efficiency with respect to the incident pump power was 32.6%, and the optical-to-optical efficiency was 26.7%.     

Far-infrared laser emission from deuterated ammonia

By optically pumping the deuterated isotopomers of ¹⁴NH₃ and ¹⁵NH₃ using ¹²C¹⁶O₂, ¹³C¹⁶O₂, ¹²C¹⁸O₂, and ¹³C¹⁸O₂ lasers, several new far-infrared (FIR) emission lines between 65 μm and 125 μm have been detected. The existing spectroscopy of ¹⁴N-ammonia isotopomers has been used to identify many of these lines, as well as some previously observed but unidentified. The spectroscopic data have been analyzed to predict over 20 additional FIR laser lines that could be pumped by a more capable CO₂ laser. This effort was motivated by a need for strong laser lines in frequency coincidence with molecular transitions of astrophysical interest. Of particularnote is the measurement of the 2680-GHz line of ¹⁴NHD₂, whose frequency is 4.9 GHz higher than that of the important J=1-0 line of interstellar HD. Received: 25 July 2002 / Published online: 20 December 2002 RID="*" ID="*"Corresponding author. Fax: +1-303/492-5941, E-mail: boreiko@spot.colorado.edu     

Analyses of surface coloration on TiO2 film irradiated with excimer laser

TiO₂ film of around 850 nm in thickness was deposited on a soda-lime glass by PVD sputtering and irradiated using one pulse of krypton-fluorine (KrF) excimer laser (wavelength of 248 nm and pulse duration of 25 ns) with varying fluence. The color of the irradiated area became darker with increasing laser fluence. Irradiated surfaces were characterized using optical microscopy, scanning electron microscopy, Raman spectroscopy and atomic force microscopy. Surface undergoes thermal annealing at low laser fluence of 400 and 590 mJ/cm². Microcracks at medium laser fluence of 1000 mJ/cm² are attributed to surface melting and solidification. Hydrodynamic ablation is proposed to explain the formation of micropores and networks at higher laser fluence of 1100 and 1200 mJ/cm². The darkening effect is explained in terms of trapping of light in the surface defects formed rather than anatase to rutile phase transformation as reported by others. Controlled darkening of TiO₂ film might be used for adjustable filters.     

Continuous-wave broadly tunable Cr:ZnSe laser pumped by a thulium fiber laser

We describe a compact, broadly tunable, continuous-wave (cw) Cr²⁺:ZnSe laser pumped by a thulium fiber laser at 1800 nm. In the experiments, a polycrystalline ZnSe sample with a chromium concentration of 9.5 × 10¹⁸ cm⁻³ was used. Free-running laser output was around 2500 nm. Output couplers with transmissions of 3%, 6%, and 15% were used to characterize the power performance of the laser. Best power performance was obtained with a 15% transmitting output coupler. In this case, as high as 640 mW of output power was obtained with 2.5 W of pump power at a wavelength of 2480 nm. The stimulated emission cross-section values determined from laser threshold data and emission measurements were in good agreement. Finally, broad, continuous tuning of the laser was demonstrated between 2240 and 2900 nm by using an intracavity Brewster cut MgF₂ prism and a single set of optics.     

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     

An argon ion ring laser as a gyroscope

As a prototype gyroscope for a precision measurement of the Earth's rotation _E, we set up an argon ion ring laser with an area of 1.4 m². Different cavity geometries were tested in order to achieve a cancellation of the effect of the plasma flow. In a set-up with two laser tubes of the same type facing towards opposite directions, a stability of the beat frequency of 2–3 _E was measured. In a configuration with a double transition of the laser beam through the tube, the lock-in threshold was too high for the Earth's rotation to be measured, however, no effect of the plasma flow was observed.     

Intensity threshold in vacuum laser acceleration

The dependence of the electron-energy gain on the on-axis laser intensity of a TEM₀₀ light wave in vacuum, called the capture and acceleration scenario (CAS), has been studied. We found that there exists a laser intensity threshold for the CAS scheme to work. The physical meaning of the intensity threshold is that, when the intensity is strong enough, fast electrons injected into the Rayleigh zone where the phase velocity of the light wave is subluminous can be accelerated until they catch up with the phase velocity before they slip out. Thereby these electrons can receive a considerable amount of energy from the laser field. Analytical calculations based on this situation and simulation results show similar features in that the intensity threshold value, (a₀ ^T)², is strongly dependent on the laser-beam width at focus, kw₀. For example, kw₀=40 corresponds to a₀ ^T∼5, which is available by present laser systems. Also, it has been proved that the maximal electron-energy gain in the CAS regime is linearly proportional to the laser intensity as well as to kw₀. Received: 20 January 2003 / Revised version: 6 March 2003 / Published online: 23 May 2003 RID="*" ID="*"Corresponding author. Fax: +86-21/6564-3815, E-mail: hoyk@fudan.ac.cn     

Nanostructures on SiC surface created by laser microablation

Silicon carbide (SiC), as it is well-known, is inaccessible to usual methods of technological processing. Consequently, it is important to search for alternative technologies of processing SiC, including laser processing, and to study the accompanying physical processes. The work deals with the investigation of pulsed laser radiation influence on the surface of 6H-SiC crystal. The calculated temperature profile of SiC under laser irradiation is shown. Structural changes in surface and near-surface layers of SiC were studied by atomic force microscopy images, photoluminescence, Raman spectra and field emission current-voltage characteristics of initial and irradiated surfaces. It is shown that the cone-shaped nanostructures with typical dimension of 100-200 nm height and 5-10 nm width at the edge are formed on SiC surface under nitrogen laser exposure (λ = 0.337 μm, t_p = 7 ns, E_p = 1.5 mJ). The average values of threshold energy density 〈W_thn〉 at which formation of nanostructures starts on the 0 0 0 1 and surfaces of n-type 6H-SiC(N), nitrogen concentration n_N ≅ 2 × 10¹⁸ cm⁻³, are determined to be 3.5 J/cm² and 3.0 J/cm², respectively. The field emission appeared only after laser irradiation of the surface at threshold voltage of 1000 V at currents from 0.7 μA to 0.7 mA. The main role of the thermogradient effect in the processes of mass transfer in prior to ablation stages of nanostructure formation under UV laser irradiation (LI) was determined. We ascertained that the residual tensile stresses appear on SiC surface as a result of laser microablation. The nanostructures obtained could be applied in the field of sensor and emitting extreme electronic devices.     

Hard tissue ablation with a free running Er:YAG and a Q-switched CO2 laser: a comparative study

The Er:YAG and the CO₂ laser are competitors in the field of hard tissue ablation. The use of Er:YAG lasers (2.94 μm, pulse length _L of 100 to 200 μs) show smaller areas of thermal defects then ‘‘superpulsed’’ CO₂ lasers with pulse lengths of approximately 100 μs. Only the development of a Q-switched CO₂ laser (9.6 μm, τ_L=250 ns) allowed for similar results. In this paper new results for the Er:YAG and the Q-switched CO₂ laser under the influence of water spray will be presented. Several parameters are of special interest for these investigations: the specific ablation energy, which shows a minimum for the CO₂ laser at an energy density of 9 J/cm ² and a broad shallow minimum in the range of 10 to 70 J/cm² for the Er:YAG laser, and comparison of the cut-shape and depth. Surface effects and cutting velocity are discussed based on SEM pictures. Received: 19 July 2000 / Revised version: 1 November 2000 / Published online: 30 November 2000     

Efficient and compact intracavity-frequency-doubled Nd:GdVO4/KTP laser end-pumped by a fiber-coupled laser diode

A compact and efficient diode-pumped intracavity-frequency-doubled Nd:GdVO₄/KTP green laser is demonstrated with a flat–flat cavity design. With a 1.3 at. % Nd³⁺-doped GdVO₄ crystal and pumped at the weak-absorption peak of 806 nm, the second-harmonic output power at 532 nm was measured to be 1.95 W at an incident pump power of 8.4 W, corresponding to an optical conversion efficiency of 23.2%. The output characteristic at the fundamental wavelength of 1.063 μm was investigated with two different pump wavelengths. More than 4.5-W output power was generated when the laser was pumped at 806.2 nm. Received: 26 July 2000 / Revised version: 18 September 2000 / Published online: 7 February 2001     

Continuous-wave high-power multi-pass pumped thin-disc Nd:GdVO4 laser

A thin-disc Nd:GdVO₄ laser in multi-pass pumping scheme was developed. Continuous-wave output power of 13.9 W at 1.06 μm for an absorbed power at 808 nm of 22 W was demonstrated from a 250-μm thick, 0.5-at.% Nd:GdVO₄ in a 4-pass pumping; the slope efficiency in absorbed power was 0.65, or 0.47 in input power. Output performances were also investigated under diode laser pumping at 879 nm, directly into the emitting ⁴F_3/2 level: maximum power of 3.6 W was obtained at 6.2 W of absorbed power with 0.69 slope efficiency. Compared with pumping at 808 nm, into the highly absorbing ⁴F_5/2 level, improvements of laser parameter in absorbed power (increase of slope efficiency, decrease of threshold) were obtained, showing the advantages of the pumping into the emitting level. However, the laser performances expressed vs. the incident power were modest owing to the low absorption efficiency at 879 nm. Thus, increased number of passes of the medium would be necessary in order to match the performances in input power obtained under 808-nm pumping.     

UV laser induced desorption of CsI and RbI ion clusters

Experimental results of laser sputtering of cesium and rubidium iodide secondary ions are presented. A TOF mass spectrometer, operating in linear mode, continuous extraction for positive or negative ions, was used for the analysis of (CsI)_nCs⁺, (CsI)_nI⁻, (RbI)_nRb⁺ and (RbI)_nI⁻ ion emission as a function of the laser irradiance. Experimental data show that the cluster ion emission yields decrease exponentially with n, for all the laser irradiances applied. Theoretical analysis of the clusters structure was performed using density functional theory at the B3LYP/LACV3P level, for the positive and negative cluster series. A quasi-equilibrium evolution of the clusters is proposed to extract a parameter characteristic of the cluster recombination process: the effective temperature. The hypothesis of the atomic species’ recombination (during the expansion of a high density highly ionized cloud) leading to cluster formation is confirmed to some extent in a second set of experiments: the UV laser ablation of a mixed and non-mixed cesium iodide and potassium bromide targets. These experiments show that the emission yields contain contributions from both the recombination process and from the sample stoichiometry, even for high laser irradiances.     

Pulse energy optimization of passively Q-switched flash-lamp pumped Er:glass laser

An approach to the energy optimization of the passively Q-switchedEr:glass laser is considered. The optimization procedure is represented in the maximally verifiable and usable graphical form, which is applied to the flash-lamp pumped Er:glass laser passively Q-switchedwith the Co²⁺-doped MgAl₂O₄ and LaMgAl₁₁O₁₉ saturable absorbers. Received: 6 December 2001 / Revised version: 5 March 2002 / Published online: 8 August 2002     

High-power diode-end-pumped Nd:YVO4 laser: thermally induced fracture versus pump-wavelength sensitivity

We report two kinds of compact and efficient diode-end-pumped TEM₀₀ lasers with output power >25 W at ≈50 W of incident pump power. One laser consists of a single 0.3 at. % Nd:YVO₄ crystal in a V-type cavity, the other laser includes two 0.5 at. % Nd:YVO₄ crystals in a linear cavity. Experimental results show that lowering Nd³⁺ concentration can be beneficial in extending the fracture-limited pump power but it also increases the sensitivity of the pump wavelength due to the overlapping efficiency. Received: 19 February 2000 / Revised version: 30 May 2000 / Published online: 20 September 2000     

Continuous-wave Ho:GdVO4 laser pumped by Tm-doped fiber laser with a fiber Bragg grating at room temperature

We reported the Ho:GdVO₄ laser pumped by Tm-doped laser with a fiber Bragg grating. 2.03 W continuous-wave Ho:GdVO₄ laser output power is obtained under 10.5 W incident pump power, with the optical-to-optical conversion efficiency and slope efficiency of 19.3% and 32.3%, respectively, at 7 °C. We can see that, the lower the temperature is, the better the laser output character is. The beam quality factor is M² ∼ 1.29 measured by the traveling knife-edge method.